standards / manuals / guidelines for small hydro development - AHEC

Section –1Guide for Design of Outdoor Step up Sub-Stationand Selection of Equipment1 IntroductionOutdoor step up substation at hydroelectric stations are provided to step up power atgenerated voltage generally for interconnection with the grid to evacuate power.Generation voltage in SHP varies from 415 volts to 11 kV and step up voltage of smallhydro upto 25 MW capacity may not exceed 145 kV. Guidelines for design and selectionof comprises of main equipment, ancillary equipment, switchyard structures and substation layout.2. Design RequirementsReferences and CodesLatest edition of the following shall apply.IS: 9920 Part I to IV – Alternating current switches for rated voltages above 1000 voltsand less than 52 kVIS: 9921Part 1 to 5 – Alternating currents disconnectors (isolators) and earthing switchesrating, design, construction, tests etc.IS: 1893 – Criteria for Earthquake resistance design of structuresIS: 2705 Part 1 to 4 – Current transformerIS: 3156 Part 1 to 4 – voltage transformerIS: 3070 part 1 to 3 – Lightning arrestorsIS: 2544 – Porcelain insulators for system above 1000 VIS: 5350 – Part III – post insulator units for systems greater than 1000 VIS: 5621 – Hollow Insulators for use in electrical equipmentIS: 5556 – Serrated lock washers – specificationIS: 3716 – Application guide for insulation co-ordinationIS: 2165 – Phase to earth insulation co-ordinationRural electrification Corporation (REC) specification and standardsPower Engineers Hand Book - Tamil Nadu Engineer’s AssociationCentral Board of Irrigation and Power - Manual on Sub-Station LayoutUPSEB - Construction Manual for Rural Electrification and secondary system planning2.1 GeneralThe equipment shall be designed and manufactured to provide most optimum functionalvalue and neat appearance. All major assemblies or equipment shall be designedto facilitate easy and quick surveillance, maintenance and optimum operation. Allcontrol sequences shall be simple and rational.1-1

All live, moving and rotating parts shall be adequately secured in order to avoid dangerto the operating staff. All electrical components shall be electrically earthed.Suitable lifting eyes and forcing off bolts shall be provided where required or wherethey will be useful for erection and dismantling.2.2 Seismic ConsiderationForces caused by earthquake which may occur for the seismic intensity of the zoneconcerned should be taken into account. Stresses resulting after including these loadsshould not exceed permissible stresses. For Himalayan region projects it may be specifiedas under:-Switchyard equipment and structure be designed to safely withstand earthquakeacceleration force 0.3g both in the vertical and horizontal direction.For other regions refer IS: 1893.2.3 Basic Insulation Level and Insulation Co-ordinationInsulation coordination is the correlation of the insulation of electrical equipment andsystem with the characteristics of protective devices such that the insulation is protectedfrom excessive over voltages. Thus in a substation the insulation of transformer, circuitbreakers, bus supports, etc. should have insulation strength in excess of the voltage levelsthat can be provided by protective equipment such as lightning arrestors and gaps.According to International Electro Technical Commission Technical Committee No. 28on Insulation Coordination the same is defined as follows by IEC:“Insulation coordination comprises the selection of the electricstrength of equipment and its application in relation to the voltageswhich can appear on the system for which the equipment is intendedand taking into account the characteristics of available protectivedevices, so as to reduce to an economically and operationallyacceptable level the probability that the resulting voltage stresses willcause damage to equipment insulation or affect the continuity ofservice”.2.3.1 Over-voltages: The selection of basic insulation level for various components ofswitchyard equipment and its coordination is based on the extent of different types ofover voltages and dielectric stresses on insulation of the equipment. Overvoltages areclassified as follows (IS: 3716).(i)(ii)(iii)(iv)Power frequency voltages under normal operating conditionsTemporary OvervoltagesSwitching overvoltagesAtmospheric or Lightning overvoltages1-2

The terms atmospheric overvoltages and switching overvoltages are defined bythemselves. The term temporary overvoltages means overvoltages essentially of powerfrequency or a frequency close to it. Switching overvoltages are of consequence only atlevels above 220 kV and not applicable to system under consideration. The protectionagainst overvoltages is essentially made by Surge diverters (lighting arrestors). Lightningimpulse wave is defined as time in microsecond for the wave to reach crest (1.2 microsecond) followed by the time in microsecond for the wave to reach half magnitude (50micro second). This has been standardized in the test forms to establish insulation levelon a common basis.2.3.2 Selection of basic Impulse Insulation Level (BIL): Equipment insulation mustwithstand temporary overvoltages and protected against lightning by suitable lightningarrestor. The basic impulse insulation level should be selected which can be protectedwith a suitable lightning protective device. The best protection is provided by moderntype (gapless) lightning arrestors. The spread margin between the BIL and the protectivedevice, allowing for manufacturing tolerance, is an economic consideration that mustbalance the chances of insulation failure against the cost of greater insulation strength.When using lightning arrestors the economic factor may be one of greater risk to thearrestor than to the equipment insulation. The arrestor can be applied so that it willprotect the insulation but may under certain extreme conditions, usually unlikely, besubjected to sustained rms temporary over voltages against which it cannot recover.Practice has been to apply arrestors so that they have an rms voltage rating above themaximum possible rms line-to-neutral power frequency voltage under any normal orexpected fault condition with sufficient margin. The BIL of the equipment insulationmust therefore be higher than the maximum expected surge voltage across the selectedarrestors selected to withstand highest credible temporary overvotage.2.3.3 Station Design for Lightning and Standardisation of Insulation Levels: Stationdesign for lightning involves in general, provisions of an adequate insulation level for allequipment and protective measures to prevent, as for as possible lightning overvoltagesapproaching that level from appearing on station lines or on equipment. These levels aregiven in table 2.1 & 2.2 as per the Indian Standard IS: 2165. In this standard, table 2.1covers the standard insulation levels highest system voltages of 52 kV and below andTable 2.2 for highest system voltages of more than 52 kV and less than 300 kV.Table 2.1: Standard Insulation Levels for (equipment in range A 1 kV < U m < 52 kV)clause (4) for preferred valueHighestVoltage forEquipmentU mRated Lightning ImpulseWithstand Voltage (Peak)Rated Short Duration PowerFrequency Withstand Voltage (rms)List 1List2kV kV kV kV3.6 20 40 107.2 40 60 201-3

12 60 75 2824 85 125 5035 145 170 70Note: Insulation levels as per list 2 are recommended.For SHP application where temporary overvoltages are high due to speed rise on loadthrow off equipment insulation as per list 2 of table 2.1 should be used.Standard Phase-to-Phase Insulation Levels for 52 kV ≤ U m < 145 kV (IS: 2165)Table 2.2Highest Voltage Base for P.U. Rated Lightning Rated Shortfor Equipment Values Impulse Withstand Duration Power-FrequencyWithstand VoltageU m U m 2 / 3(rms) (peak) (peak) (rms)kV kV kV kV72.5 59 325 140123 100 450550185230145 118 (450)*550650(230)*230275245 200 650*750*8509501050360*360*360395460Generator transformer/step up transformer in SHP are liable to be subjected to hightemporary overvoltages due to load rejection, as well as line capacitance which mayremain connected on interconnecting tie line in case of receiving end breaker opening.Basic insulation level on power frequency overvoltages of 1.5 per unit for transformers isworked out in table 2.3 for a typical 145 kV transformer.In case of generator transformer in SHP 90 – 95 % lightning arrestors are recommended.An increase of impulse level of 15% above the withstand level to earth is recommendedfor disconnecting switches between the terminals of each pole in the open condition. Acorresponding increase of distance may be applied for distances between phases for busbars and connections, or between connections which may be located on opposite sides ofan open disconnecting switch.1-4

Table 2.31. Basic Data Example 1.Highest voltage for equipment Um k V rms 145Corresponding line to (ground) neutralU m voltage k V rms 83.72 3 Corresponding Temporary over voltage(derived from system studies)This voltage is high in step up sub stationsdue to load rejection i.e. 1.5 per unitk V rms 104.652.Minimum safety factorFor lightning overvoltagesRepresentative Characteristics ofSurge Diverter (obtain frommanufacturer)Rated Voltage (nearest available above125 x 104.65)Maximum lightning impulse spark overvoltageMaximum front-of-wave impulse sparkover voltageMaximum residual impulse spark overvoltage3 Protection Levels1.25kV rms 140kV 415kV 470kV 405To lightning impulse kV 4154 Insulation level (phase to earth)Lightning ImpulseMinimum lightning impulse voltage kV 415Rated lightning impulse voltage as perIS:Ratio of the rated lightning impulsevoltage to the lightning impulseprotective levelkV 550kV 1.321-5

2.3.4 Existing Insulation Practice of Substation EquipmentsThe substation equipments namely the power transformers circuit breakers anddisconnecting switches are considered for detailing the existing practice.Existing PracticeThe commonly adopted insulation levels at present for the above mentioned equipmentsare given in table 2.4 with reference to Karnataka and Tamil Nadu (66 kV, 110 kV and220 kV) and Punjab (132 kV Bhakra System).A commonly adopted practice is to locate lightning arrester as near to the transformer aspossible. In large substations additional arresters could be required at suitable locations toprotect circuit breakers, isolators and other equipments. Since each of these equipmentscannot be provided with arrester individually, it is the normal practice to adopt higherinsulation to provide the equipment with as good protection as is economically justified.Insulation level for circuit breaker and other equipments connected to the busbar togetherwith the bubars themselves are designed for insulation class about 10% higher than theinsulation level for the transformers (one class higher). Insulation level across the openpoles of the isolating switches is kept about 10 to 15 % higher than that provided betweenthe poles and the earth, so that in the event of a surge at an open isolating switch, theflashover should pass to earth and not across open poles.Table 2.4: Insulation Levels of the terminal Equipments RecommendedS. DescriptionNo.1. Highest systemvoltage kV (rms)2. Power transformerinsulation levels kV(Crest)3. Circuit breaker kV(Crest)4. Disconnectingswitches between poleand earth kV (Crest)Nominal voltage in kV220 132 110 66 33 11245 145 123 72.5 35 12900 550 450 325 170 751050 650 550 350 170 751050 650 550 380 170 752.3.5 Protection with Spark GapsThe spark gap is among the cheapest protective devices used for diverting the surgesfrom line to earth. After the break down of the spark gap the circuit breaker alwaysoperates to interrupt the fault of power frequency current in the circuit. Thus theoperation of the gap generally results in the circuit outage and interruption of supply ofthe power system. It is therefore used as a back up to surge arrestor (lightning arrestor).1-6

Spark gaps specified for fitting to the bushings of power transformers, potential andcurrent transformers, rated 66 kV and above.The spark gaps are to conform to the following specification, to prevent any damage tothe bushing due to the flashover gazing the petticoats of the bushing:a) The rods are to be circular not less than 12 mm diameterb) The rods should overhang their supports at-least one half of the gap spacing.c) The rods should be mounted so as to give a height of 1.3 times the gap spacingplus 100 mm (4 inches) above the ground plane as shown in fig. 2.3.5.Fig. 2.3.5The gap setting furnished below are adopted for all stations whether lightning arrestersare provided or not.Spacing for standard rod-gaps is given in table 2.5 (TNEB Practice).Highest systemvoltage kV rmsU mTable 2.5Critical Flashovervoltage 1/50 microsecondmkV Peak 23 Spacing of Standard Rod GapU Positive Polarity Negative Polarity72.5 59.1 7.7 6.5123 100.4 12.4 10145 118.3 15.4 13245 200 28.0 23Note: - The spacing given above are for the standard atmospheric conditions viz:1-7

Barometric pressure = 760 mm Hg.Temperature = 20 0 CHumidity= 11 grams of water vapour per cubic meter.For non-standard atmospheric conditions the spacing to give the critical flash overvoltage should be modified by dividing the above spacing by ‘d’ where:d =P0 .386 273 tP = barometric pressure in mm Hg.and t = temperature in 0 C.When the humidity differs the standard spacing should be increased by 1% for eachgramme per cubic meter below the standard value and decreased by 1 % each grammeper cubic meter above the standard.2.4 Electrical Clearances for Installing Equipment in the FieldSpace requirements and layout of electrical equipment in switchyard depends uponvarious types of air clearances required to be provided for laying the equipment ofdifferent rated voltages. Following basic clearances govern the sub-station design.(i) Earth clearance i.e. phase to ground clearance.(ii) Phase clearance i.e. phase to phase clearance.(iii) Safety clearance i.e. (a) Ground clearance.(b) Section clearance.2.4.1 Co-relation between insulation Level and minimum Phase to earth ClearancesMinimum clearances in air between live conductive parts and earthed structures to securea specified impulse withstand voltage for dry conditions as per IS 3716-1978 are given intable 2.6.These minimum clearances are valid for altitudes not exceeding 1000 m and do notinclude any addition for construction tolerances, effect of short circuits, safety ofpersonnel etc. these clearances are suitable for general application, providing as firstapproximation.Table 2.6: Correlations between Insulation Levels and Minimum Phase-to-Air Clearances as per IS: 3716 - 1978EarthRated Lightning Impulse WithstandVoltage (kV)Minimum Phase-to-Earth Air Clearances(mm)45 6060 9075 12095 1601-8

2.4.2 Working Safety Clearances125 220170 320325 630380 750450 900550 1100650 1300750 1500850 1600950 19001050 2400Safety clearance consists of ground clearance and section clearance. The groundclearance is the minimum clearance from any point on or about the permanent equipmentwhere a man may be required to stand (measured from the position of feet) to the nearestpart not at earth potential of an insulator supporting a line conductor and the same hasbeen taken as 2.59 meters (i.e. 8.5 feet), which is the dimensions for a tall man with armsoutstretched below the conductor.The section clearance is the minimum clearance from any point on or about thepermanent equipment where a man may be required to stand (measured from the positionof feet) to the nearest unscreened live conductor in the air. The section clearance systemupto 132 kV 650 kV BIL may be determined by adding 2.5 meters to minimum phase toground clearance of 1.3 which works to 3.8 meters for 132 kV system.Height of Bus Bars Above Ground Within Sub-Station PromisesThe minimum conductor clearance from ground is obtained by adding ground clearance,earth clearance and height of bus bar supporting clamps on the post insulator. Inconsideration to it, minimum height of bus bar for 132 kV may be about 365 mm whichmay be raised to about 450 mm to correspond to the terminal height of the 132 kV circuitbreakers.Conductor Clearance from Roadways Within Sub-Station PromisesMinimum clearance between overhead conductors and roadways within sub-stationpremises is computed to be as “Ground clearance plus 625 mm. This dimension providesfor a truck with a man standing on its top 130 + 625 meter = 755 meters app.2.4.3 Minimum and Safety clearances recommended in Central Board of Irrigation andPower manualClearances from the point of view of system reliability and safety of operating personnelrecommended for sub station upto 245 kv are given in table 2.7. These include the1-9

minimum clearances from live parts to earth, between two live parts of different phasesand sectional clearances between live parts of different phases and sectional clearancesbetween live parts and work section required for maintenance of an equipment. Besides,it is also necessary that sufficient clearance to ground is also available within thesubstation so as to ensure safety of the personnel moving about within the switchyard.HighestSystemVoltage (kV)Lightningimpulse voltage(kVp)Table 2.7Minimum clearancesBetween phase& earth (mm)Betweenphase (mm)36 170 320 320 280072.5 325 630 630 3100123 4505509001100900110034003700145 550650110013001100130037003800245 9501050190021001900210043004600Notes:Safetyclearances (mm)i) Safety clearances are based on the insulation height of 2.44 m which is the heightof lowest point on the insulator where it meets the earthed metal.ii) The distances indicated above are not applicable to equipment which has beensubjected to impulse test since mandatory clearances might hamper the design ofthe equipment, increase its cost.iii) The values in table refer to an attitude not exceeding 1000 m and take intoaccount the most unfavorable conditions which may result from the atmosphericpressure variation, temperature and moisture. A correction factor of 1.25 % per100 m is to be applied for increasing the air clearance for altitude more than 1000m and upto 3000 m.iv) No safety clearance is required between the bus-bar isolator or the bus-barinsulator. However, safety clearance is necessary between the section isolator orthe bus-bar itself and the circuit breaker.v) For the purpose of computing the vertical clearance of an overhead strungconductor the maximum sag of any conductor shall be calculated on the basis ofthe maximum sag in still and the maximum temperature as specified.vi) As an alternative to maintain safety clearances in some substation earthedbarriers are used to ensure safety of the maintenance personnel. The use ofearthed barriers is quite common at lower voltages of 36 kV 72.5 kV. In case ofpaucity of space and if 2.44 m clearance is not available then localized earthedfencing with clearance can be considered by the designer.Following are the normally adopted spacing for the strung bus:1-10

Highest System Voltage rating kV Spacing between phases in mm245 kV 4500145 kV 360072.5 kV 220036 kV 130012 kV 1300 or 920The spacings for the equipment in a sub-station depend upon the manufacturerspractice.The minimum clearance of live parts to ground in an outdoor sub-station are as follows(Tamil Nadu Practice):Highest System VoltageClearances in mm245 kV 5500145 kV 460072.5 kV 460036 kV 370012 kV 3700The bottom most portion of any insulator or bushing in service should be at anabsolute minimum height of 2500 mm above ground level.2.4.4 Section ClearancesA station which can not be shunt down entirely for maintained purpose must be split intosections so arranged that any one section can be isolated from its neighbour withadequate clearances as given below. Where it is impossible to obtain the required safetyclearances, earthed screens may be provided.The following table gives the sectional clearances for persons to enable inspectioncleaning, repairs; painting and general maintenance works to be carried out in a substation.Highest SystemVoltageSection Clearances145 kV 3500 mm72 kV 3000 mm36 kV 2800 mm12 kV 2600 mmThe following minimum clearances should be adopted for enclosed indoor busbarsand connections in air which are not filled with any insulating medium like compoundetc.Highest System voltage betweenphases or polesMinimum clearances in Air1-11

Between phases or Phase/pole to earthpoles36 kV 356 mm 222 mm12 kV 127 mm 76 mmIn indoor kiosks in power stations and main receiving stations, the busbar andconnections should also be taped but the fact of taping should however, be taken intoconsideration in deciding the clearances. In addition indoor kiosks etc. should besubjected to a flashover test at works to prove that clearances are adequate so as toprevent flashovers during surge conditions.2.4.5 Standard Bay Widths (in meters) as TNEB Practices66 kV - 733kV - 4.622 kV - 3.811 kV - 3.52.5 Insulators – Creepage DistanceProvision of adequate insulation in a substation is of primary importance from the pointof view of reliability of supply and safety of personnel. However, the station designshould be so evolved that the quantity of isolators required is minimum commensuratewith the expected security of supply. An important consideration in determining theinsulation in a sub-station, particularly if it is located near sea or a thermal powergenerating station or an industrial plant is the level of pollution. As a first step to combatthis problem, special insulators with higher creepage distance should be used.The creepage distances for the different pollution levels are provided according to table2.8.Table 2.8: Creepage distance for different pollution levelsPollution LevelCreepage distance (mm/kV ofhighest system voltage)Recommendedfor adoptedLight 16 25 x highestMedium 20system voltageHeavy 25i.e. 1813 mmVery heavy 31for 72.5 kVFor determining the creepage distance requirement, the highest line-to-line voltage of thesystem forms the basis.1-12

2.6 Insulator TypeTypes of insulators used:A) Bus Support Insulatorsi) Cap and Pin typeii) Solidcore typeiii) Polycone typeB) Strain Insulatorsi) Disc insulatorsii) Long rod porcelain insulatorsiii) Polymer insulators2.7 Switchyard StructuresThe cost of structures also is a major consideration while deciding the selection of asubstation. For instance, in the case of the strain/flexible bus-bar arrangement, cost ofstructures is much higher than in the case of rigid bus type. Similarly the form ofstructures also plays an important part and the choice is usually between using a fewheavy structures or a large number of smaller structures. While finalizing the design, sizeand single line diagram of structures, safety clearance requirements should be ensured.Steel is the most commonly used in India for substation structures. Normally the steelstructures are hot-dip galvanized so as to protect them against corrosion. However,galvanizing sometimes has not proved effective, particularly in substations located incoastal or industrial areas and in such cases painting also becomes essential. In othercountries special paints have developed which are applied within the shop and thesepaints have quite effective.2.7.1 Design Data for Design of Switchyard Structures(Based mostly on Tamil Nadu Electricity Board Practice)Design Loadsi) Wind Pressure on Structures (Refer table 7)Maximum for the area on 1.5 times the projected area of one face for latticedstructures and on single projected area in the case of other structures.In coastal regions the wind pressure may be assumed as 170 kg/sq.m.ii)Wind Pressure on Conductor1-13

…… kg/sq.m. (according to area see table 2.9) on two-thirds projected area.iii)iv)Maximum tension of transmission line conductors strung from terminal tower tostation structures or of strung buses for lines 33 kV and above … 226.8 kg.(Tamil Nadu Electricity Board Practice - TNEB Practice).Maximum spans adjacent to stations:a) Lines rated 66 kV and above …. 152.40 mb) Lines rated 33 kV and below … 60.96 mv) Uplift on adjacent spans:Maximum slope (mean of the 3 – phase) at the point of attachment 1 : 8 abovehorizontal.Table 2.9: Wind Pressure & Temperature DataThe table below gives the values of wind pressure and maximum and minimum temperaturesspecified in different states, as per REC for design of structure.State Wind Pressure Zones Max.Temp.Min.Temp.ICELoadingKg/m 20 C0 CAndhra - 75 100 - 60 10 NilPradeshAssam - - 97.8 - 50 4.44 NilBihar - - 97 - 60 4 NilGujarat - 75 100 - 50 10 NilHaryana - - - 150 50 (-) 2.5 NilKerala - 75 - - 55 10 -Madhya - 75 - - 60 4.4 NilPradeshMaharashtra 50 75 100 150 65 5 NilKarnataka 50 70 - - 54.4 10 NilOrissa - 75 100 150 60 5 NilPunjab - 100 - - 64.5 (-) 2.5 NilRajasthan - - 100 - 50 (-) 2.5 NilTamil Nadu - 73.25 87.8 122 65.5 (-) 5 NilUttar Pradesh - 75 - 150 60 4.44 NilWest Bengal - 75 100 150 60 0 Nil2.7.2 Working Stressesa) for steel:1-14

Bending .. .. 1265 kg/sq.cm.Shear .. .. 1265 kg/sq.cm.b) for concrete – 1 : 2 : 4Bending .. .. 52.7 kg/sq.cm.Shear .. .. 5.27 kg/sq.cm.Bend .. .. 7.03 kg/sq.cm.2.7.3 Factor of SafetyIndian Adopted by (TNEB)ElectricityRulesa. Fopr steel 2.0 2.5 based on maximumloading conditions (onelastic limit for tensionmembers and cripplingload for compressionmembers).b. For R. C. 2.5 3.5 on ultimate breakingc. For hand 3.0loadMoulded R. C.RecommendedAs per TNEBPracticeFactory of safety against overturning:a) Steel 2.5b) R. C. 2.02.7.4 Slenderness Ratio (L/R)Ratio of unsupported length (l) to radius of gyration (r) should not exceed;a) 140 for leg membersb) 200 for other members having calculated stresses only and 250 for membershaving nominal stress only.2.7.5 Minimum Thickness for Steel Members2.7.6 MaterialSteel employed for structures – open hearth steel with a high yield point and an ultimatestrength of not than 3867 kg/sq.cm.The following maximum stresses in lbs. per square inch are assumed for outdoorstructures, fabricated out of steel sections manufactured in India;1-15

i. Tension 18,000ii. Compression 18,000 – 76 l/r where l/r is less than 150 and 13,000 – 48l/r where l/r is more than 150iii. Shear on 13,500boltsiv. Bearing on 27,000bolts2.8 GIS SubstationsAdvancement in the use of SF6 as an insulating and interrupting medium have resulted inthe development of gas insulated substations. Environment and/or space limitations mayrequire the consideration of GIS (gas insulated substation) equipment. This equipmentutilizes SF6 as an insulating and interrupting medium and permits very compactinstallations. GIS substation are preferable to air insulated system ((AIS) because offollowing reasons:i) Compact design reduces space requirementsii) Higher reliabilityiii) Life cycle costs and safety are better because GIS is maintenance freeiv) Location advantage especially in areas (town) where space costs are highv) Environmental advantage as rain, dust, snow, ice, salt etc. do not affect thehermetically sealed metal clad GISThree-phase or single-phase bus configurations are normally available up to 145 kVclass, and single phase bus to 500 kV and higher, and all equipment (disconnect/isolatingswitches, grounding switches, circuit breakers, metering current, and potentialtransformers, etc.) are enclosed within an atmosphere of SF6 insulating gas. The superiorinsulating properties of SF6 allow very compact installations.GIS installations are also used in contaminated environments and as a means of deterringanimal intrusions. Although initial costs are higher than conventional substations, asmaller substation footprint can offset the increased initial costs by reducing the land areanecessary for the substation.2.8.1 GIS Compact SwitchgearCompact sub-station with gas insulated switchgear may be considered in following cases.i) Installations in areas with high risk of pollution and corrosion from industrialplants or by marine and desert climates.ii) Applications involving use of metal clad switchgear with components ofconventional design to minimize area requirement.iii) Underground substationsiv) Outdoor installation where space is not easily available1-16

v) Installations in difficult site conditions (e.g. seismically active areas, high altitudeareas etc.).2.8.2 Metal Clad GIS SwitchgearSF6 – insulated metal enclosed high voltage switchgear upto 145 kV are now availableand may be used where space may be provided. The data of siemens GIS sub station asper Siemens Power Engineers Guide is given in table 2.10 Feeder control and protectionare inbuilt.Table 2.10Rated voltage (kV) Upto 145Rated power frequency Upto 275withstand voltage (kV)Rated lightning impulse Upto 650withstand voltage (kV)Rated normal current bus Upto 3150bar (A)Rated normal current Upto 2500feeder (A)Rated breaking current Upto 40(kA)Rated short-time Upto 40withstand current (kA)Rated peak withstand Upto 108current (kA)Inspection (years) > 25Bay width (mm) 8002.8.3 Compact Air Insulated Substation (CAIS)A compact station is mounted on common base frame with integrated current transformerand with SF6 insulated dead tank interrupter assembly. Compact air insulated sub-station(CAIS) factory assembled with dead tank SF 6 design is being offered for such-station at66 kV and 132 kV. Technical particulars of Areva sub station are given in table 2.11space saving upto 60% is claimed for H type (single sectionalized bus) substation withtwo incoming generator transformers and two outgoing feeders configuration.Extracts of Siemens Power Engineering guide regarding metal clad switchgear substationupto 145 kV is enclosed as Annexure 1.1.Extracts from Areva regarding compact substation are at annexure 1.2.1-17

Table 2.11Technical CharacteristicsRated voltage kV 72.5 123 145Rated frequency Hz 50/60Rated power-frequency withstand kV 140 230 275voltageRated lightning impulse withstand kV 325 550 650voltageRated normal current A 2500Rated short-circuit breaking current kA 40Rated short-circuit making current kA 100Rated duration of short circuit s 3Circuit Breaker Specific Technical CharacteristicsOpening Time ms 38 38 38Break time ms 50 60 60Closing time ms 85 106 1062.9 Power Line Carrier EquipmentThe carrier equipment required for communication, relaying and telemetering isconnected to line through high frequency cable, coupling capacitor and wave trap. Thewave trap is installed at the line capacitor. The coupling capacitors are installed on theline side of the wave trap and are normally base mounted. The wave traps for voltagelevels upto 145 kV can be mounted on the gantry structure on which the line isterminated at the substation or mounted on top of the capacitor voltage transformer.However, the wave traps for voltage levels of 245 kV and above generally requireseparate supporting insulator stacks mounted on structures of appropriate heights.2.10 Substation Auxiliary FacilitiesAuxiliary facilities for step up sub station in SHP are designed in conjunction with powerhouse auxiliaries and are discussed in a separate guideline.2.11 Bus Bar SchemesSimple single bus bar schemes or single sectionalized bus bars schemes are generallyprovided in small hydro scheme.2.12 Inspection and MaintenanceAdequate facilities must be provided in the substation for inspection and maintenance ofvarious equipment and at the same time to ensure safety of personnel and maintain properand other clearances.1-18

During maintenance, it is essential that the equipment is isolated and earthed. One of theessential requirements of earthing is that earthing must be actually visible from the pointof working in the substation. Where this is not possible, provision of temporary earthingis made near the equipment. Besides the permanent illumination, provision should also bemade for portable lights for which purpose power outlets should be provided inmarshalling boxes or equipment cubicles.1-19

Metal Clad SF 6 Insulated Switchgear up to 145 kVAnnexure-1.1Three-phase enclosures are used for type 8D9N switchgear in order to achieve extremely lowcomponent dimensions. The low bay weight ensures minimal floorloading and eliminates theneed for complex foundations. Its compact dimensions and low weight enable it to be installedalmost anywhere. This means that capital costs can be reduced by using smaller buildings, or bymaking use of existing ones, for instance when medium voltage switchgear is replaced by 145kV GIS.The bay is based on a circuit breaker mounted on a supporting frame. A special multifunctionalcross-coupling module combines the functions of the disconnector and earthing device. It can beused as:An active busbar with integrated disconnector and work-in-progress earthing switchOutgoing feeder module with integrated disconnector and work-in-progress earthingswitch.Busbar sectionaliser with busbar earthing. For cable termination, a cable terminationmodule can be equipped with either conventional sealing ends or the latest plug-inconnectors. Flexible single pole modules are used to connect overhead lines andtransformers by using a splitting module which links the 3-phase encapsulated switchgearto the single pole connections.The feeder control and protection can be located in a bay-integrated local control cubicle,mounted in the front of each bay. It goes without saying that we supply our gas-insulatedswitchgear with all types of currently available bay control systems - ranging from contactorcircuit controls to digital processor bus-capable bay control systems, for example the modernSICAM HV system based on serial bus communication. This system offers:Online diagnosis and trend analysis enabling early warning, fault recognition andcondition monitoringIndividual parameterization, ensuring the best possible incorporation of customizedcontrol facilities.Use of modern current and voltage sensors. This results in a longer service life and loweroperating costs, in turn attaining a considerable reduction in life cycle costs.1-20

Areva Compact Air Insulated SubstationAnnexure-1.2All components are mounted together on a common base frame. The compact is optimized bycombining single components.The circuit breaker with an integrated current transformer is the main component of the module.In addition, the bushings of the breaker also function as insulators for the fixed contacts of thebus and feeder disconnectrors.The horizontal interrupter chamber is the determining factor for the low overall height of theentire module, allowing all energized components to be located on one level.The three column disconnectors installed in the module allow for a small clearance betweenphases and a reduced bay width.The integrated voltage transformer may substitute the support insulator of the feederdisconnector.Add-on bus and feeder earthing switches can provide the entire module with additionalfunctions.1-21

Section –2Selection of Switchyard Equipment1.0 Bus barsThe out door bus-bars are either of the rigid type or the strain type.In the rigid type, pipes are used for bus-bars and also for making connections among thevarious equipments wherever required. The bus-bars and the connections are supportedon pedestal insulators. This leads to a low level type of switchyard wherein equipment aswell as the bus-bars are spread out. Since the bus-bars are rigid. The clearances remainconstant. However as the bus-bars and connections are not very high from the ground, themaintenance is easy. Due to large diameter of the pipes, the corona loss is alsosubstantially less. It is also claimed that this system is more reliable than the strain bus.This type is however not suitable for earthquake prone area due to rigidity.The strain type bus bars are an overhead system of wires strung between two supportingstructures and supported by strain type insulators. The stringing tension may be limited to500-900 kg. depending upon the size of the conductor used. These type of busbras aresuitable for earthquake prone areas.1.1 Bus bar Material – The materials in common use for bus bars and connections of thestrain type are ACSR and all aluminum conductor. The following sizes are commonlyused.Code NameRemarks12 kV = 6 x 4.72 + 7 x 1.76 Dog upto 10 MVA in case line conductor is36 kV = 6 x 4.72 + 7 x 1.76 Dog upto 10 MVA of higher sizes same be72.5 kV = 30 x 2.79 + 7 x 2.79 ACSR Panther adopted as bus bar145 kV = 30 x 4.27 + 7 x 4.27 ACSR Zebra material245 kV = 54 x 3.53 + 7 x 3.53 ACSR MooseIn the case of rigid bus arrangement, aluminum pipes of Grade 63401 WP confirming toIS: 5082 are commonly used. The sizes of pipes commonly used for various voltages aregiven below:External Dia. Internal Dia System RemarksVoltages42 mm 36 mm Upto 36 kV Tamil Nadu Uses 50 mm IPSAluminium Tube upto 72.5 kV60 mm 52 mm Upto 72.5 kV Tamil Nadu Uses 75 mm IPS80 mm Upto 145 kV Aluminium Tube for 110 & 230 kVSince aluminum oxides rapidly great care is necessary in making connections. In the caseof long spans expansion joints should be provided to avoid strain on the supportinginsulators due to thermal expansion or contraction of pipe.2-1

The bus bar sizes should meet the electrical and mechanical requirements of thespecific application for which they are chosen.2.0 Circuit BreakersFor selection of circuit breakers refer section 4.Mounting and supporting structureThe circuit breakers should be self supporting type. However, if necessary for thepurpose of minimum ground clearance the circuit breakers should be mounted on raisedsteel structures which should be included in the scope of supply of circuit breaker.Information and data for design of foundations from the supplier of the circuit breaker beobtained.3.0 IsolatorsIsolating switches are used to isolate equipment for maintenance. Isolating switches online side are provided earthing blade for connection to earth in off position for safety.Transfer of load from one bus to another by isolators is not recommended. The isolatingswitches are designed for no load operation. Inadvertent operation of the isolating switchon load will damage the switch. Although a variety of disconnect switches are available,the factor which has the maximum influence on the station layout is whether thedisconnect switch is of the vertical break type or horizontal break type. Horizontal breaktype normally occupies more space than the verticalIsolators for 12 kV and 36 kV normal system voltage conform to IS: 9920 (Part I to IV)and for voltage 66 kV and above as per IS: 9921.Earthing switches is a mechanical switching device for earthing parts of a circuit, capableof withstanding fir a specified time short-circuit currents, but not required to carry normalrated currents of the circuit.Disconnecting switches may be motorized or operated manually it is recommended thatkV and above should be motorized. Earthing switches may be manually operated.In case of double circuit lines the earthing switches shall be capable of switchinginductive (electromagnetically) and capacitive currents (electrostatically induced) as perthe values specified in IEC 62271 – 102 when parallel circuit is energized. Thedisconnector must also be capable of interrupting and making parallel circuits whentransferring load between main and reserve bus bars according to IEC requirements.3.1 Temperature RiseMaximum temperature attained by any part of the isolating switch/ isolating cum-earthswitches when in service at site under continuous full load conditions and exposedcontinuously to the direct rays of the sun and the air has to be evaluated carefully and2-2

depends upon site conditions e.g. for 2 x 10 M Mukerian SHP 72.5 kV switchyard(Punjab Plains), it was specified as follows and is recommended for similar breakers.i) Reference ambient temperature in shade = 50 0 Cii) Reference temperature under direct rays = 60 0 Cof the sun for limiting temperature riseas per IS: 99213.2 RatingEach isolating switch should have the following particulars (table 2.3.2) under the siteconditions for the system under design.Table 2.3.21. Highest system voltage 72.5 kV 36 kV 12 kV2. Rated frequency (cycle/second) 50 c/s 50 c/s 50 c/s3. Rated lightning impulse withstand voltage (withoutarcing horn)i) To earth and between poles (kV Peak)ii) Across the isolating distance (kV peak)4. Rated one-minute power frequency wet withstandvoltagei) To earth and between poles (kV rms)ii) Across the isolating distance (kV rms)Voltage against ground and between poles2-3325 kV(+ ve & - ve waveto earth &between poles)140 kV(against ground &between poles)170 kV195 kV70 kV80 kV75 kV85 kV28 kV32 kV5. Continuous rated current (Amps) 1600 A 630 A 400 A6. Short time current ratingsi) For one second not less than kA (rms) 20 kA16 kA 16 kAii) For 3 secondsTo be stated To be To be7. Rated peak withstand current kA (peak) in closedpositionstated statedTo be stated 40 kA 40 kA8. Transformer off-load breaking capacity A (rms) To be stated 6.3 kA 6.3 kA9. Line charging capacity A (rms) To be stated 6.3 A 2.5 A10. Rated DC voltage for auxiliary circuits A (rms) To be stated11. Rated supply frequency and voltage of AC operatingdevicesIsolators 36 kV and 12 kV: should conform to IS: 9920Isolated 72.5 kV & above should conform to IS: 99213 phase 415 volts and single phase 220V ACThe location of disconnect switches in substations affects substation layouts.Maintenance of the disconnect contacts is also a consideration in the layout. In somesubstations, the disconnects are mounted at high positions either vertically orhorizontally. Although such substations occupy smaller areas, the maintenance of

disconnect switch contacts in such substations is more difficult as the contacts are noteasily accessible.3.3 Isolator InsulationInsulation to ground, insulation between open contacts and the insulation between phasesof the completely assembled isolating switch should be capable of withstanding thedielectric test voltages specified as per IS: 2026. Insulation between open contacts of apole should at least 15% more than the insulation between the live parts of a pole toground so that if any flashover occurs when switch is open, it should be to the ground.The post insulators should consist of no. of stack units conforming to IS: 2544. Theinsulators selected should be suitable for use in the type of normally polluted atmosphereof the area as per relevant IS and should be specifically suited to meet the particularrequirements of ultimate torosional strength and cantilever loads which they will becalled upon to resist during service at the rated voltages. The guaranteed data andparticulars of the insulators adopted for the equipment should be obtained from thesupplier.The porcelain should be homogeneous and free from all cavities and flaws.Design of the insulators should ensure ample insulation, mechanical strength and rigidityfor satisfactory operation under site conditions. The design should also ensure that thelosses caused by capacitive currents or conduction through dielectric are minimum andthat the leakage due to moist and dirty insulator surface is least.3.4 Arcing Horn & Arcing ContactsA set adjustable arcing horns should be mounted on each insulator stack of the isolatingswitch.Besides above adjustable arcing horns which are required for the purposes of insulationco-ordination, the isolators may be provided make before and break after arcing contactsif considered necessary by the manufacturers.A graph showing impulse and power frequency spark over voltages for various gapsettings of the arcing horns be obtained from supplier.3.5 Load Break SwitchesLoad break switches for sectionalizing or for selection of bus if required may be used asper following specifications.i) 12 kV REC Specification 43; IS: 9920 Part I to IVii) 36 kV REC Specification 54; IS: 9920 Part I to IV2-4

iii) 72.5 kV & above IS: 99213.6 Terminal ConnectorsEach isolator connected with outgoing lines should be provided with appropriate numberof bimetallic, solderless clamp type of connectors suitable for the transmission lineconductor. Each terminal clamp should be suitable for both vertical & horizontalconnection of station bus bars and jumpers. Each isolator should also be provided withappropriate number of grounding terminals and clamps for receiving groundingconnections. The maximum length of the jumper that may be safely connected or anyspecial instructions considered necessary to avoid undue loads on the post insulatorsshould be avoided.3.7 Interlocks“For the purpose of making the operation of the isolator dependent upon the position ofthe associated circuit breaker or other equipment as may be required at site, a suitableelectrical interlock should be provided on each isolator. The interlocks should be ofrobust design of some reputed make and contained in a weather proof and dust tighthousing.Besides the electrical interlocks, the earthing switches should be provided withmechanically operated interlock so as to ensure that: -(a)(b)(c)It should be possible to close the earthing switch only when the isolating switch is in thefully open position.It should be possible to close the isolating switch only when the earthing switch is in thefully open position.The earth switch should not open automatically while attempting to close the isolator.The operation of the earth switches should also be interlocked with the CVTs/CTssupplies from the transmission line i.e. it should be possible to close the earth switch onlywhen the line is dead from the feeding end, and there is no supply from the secondariesof the line CVTs/CTs.(d)The operation of earth/isolating switch should not take place when the correspondingisolator/earth switch is in operating stroke.In addition to the above, the line and the bus isolators should fulfil the followingrequirements:-(i)(ii)(iii)The circuit breaker of corresponding bay is open.The bus isolator of the bus coupler bay should close only when the bus coupler circuitbreaker is open.The line isolator should close only when the corresponding circuit breaker and theearthing switch of the corresponding line are open.2-5

(iv)Electro magnetic type interlocking should also be provided to avoid wrong localoperation of the isolator (manual or motor) when the corresponding circuit breaker is inclosed position. Operation of isolator may be categorized manual or motorized withremote facility according to facilities provided in the system.Isolators and earth switches should be so designed that the above noted requirements canbe conveniently met.3.8 Supporting Structures3.9 TestsAll isolators and earthing switches should be mounted rigidly in an upright position ontheir own galvanised steel supporting structure and not on the bus-bar structures.Each isolator and earth switch should strictly comply with the requirements of all the typetests and should be subjected to all routine tests stipulated in the latest edition of relevantIndian standard.Copies of the type tests already performed on similar type of isolators must be obtainedand scrutinized for adequacy.4.0 Current Transformersa) 12 kV and AboveCurrent transformers may be either of the bushing type or wound type. The bushing typesare normally accommodated within the transformer bushings and the wound types areinvariably separately mounted. The location of the current transformer with respect toassociated circuit breaker has an important bearing upon the protection scheme as well aslayout of, substation. Current transformer class and ratio is determined by electricalprotection, metering consideration.Outdoor Type: The outdoor CTs shall be either oil filled type or of resin cast type whichshall be enclosed in a sealed housing to avoid direct exposure to sun and otheratmospheric effects.Indoor Type: The CTs shall be of resin-cast type suitable for indoor installation.Current ratings, design, Temperature rise and testing etc. should be in accordance withIS: 2705 (Part I to IV). Unless otherwise specified in these specification.12 kV current transformers should conform to REC specification 59/19932-6

4.1 Type and RatingThe current transformer should be of outdoor type, single phase, oil immersed, selfcooled and suitable for operation in 3 phase solidly grounded system (11 kV CTs will beas per Para 1.6 above).Each current transformers should have the following (table 2.4.1) particulars under thesite conditions for the system under design (typical values upto 72.5 kV system aregiven).Table 2.4.1i) Nominal system voltage 66 kV 33 kV 11 kVii) Highest system voltage 72.5 kV 36 kV 12 kViii) Frequency 50 Hz 50 Hz 50 Hziv)Insulation level (kV Peak)(based on system insulation coordination)Impulse withstand test voltage with 1.2/50micro-second, + ve and – ve wave to earthand between polesv) One minute power frequency (wet) withstandvoltage against ground and between poles.vi)vii)viii)Short time current rating(based on system studies)Rated dynamic current peak(based on system studies)Total minimum creepage of CTs bushings(based on environment)4.2 Details of Current Transformer325 kV 170 kV 75 kV140 kV(rms)70 kV (rms) 28 kV (rms)31.5 kA 31.5 kA 12.5 kA78.75 kA 2.5 time of short time currentrating viAs per Para 2.5 of section 1Details of current transformer i.e. current, number, ratio, no. of cores andprotection/metering class based on metering and relaying scheme be specified.4.2.1 Temperature Risea) 36 kV and aboveThe maximum temperature attained by any part of the equipment in service at site undercontinuous overload capacity conditions and exposed continuously to the direct rays ofsun should not exceed the permissible limit fixed by the applicable standard, whencorrected for the difference between the ambient temperature at site and the ambienttemperature specified by the standard.2-7

) Temperature rise of 11 kV Class CTsThe maximum temperature rise of windings shall not exceed the following (table 2.4.2):Table 2.4.2Indoor TypeOutdoor TypeMaximum ambient temp. 45 0 C 60 0 CPermissible temp. rise forClass E insulationClass B insulationClass F insulation70 0 C80 0 C105 0 C50 0 C60 0 C85 0 CNote: The supplier shall furnish evidence to the satisfaction of the purchaser about theclass of insulation used.4.3 General Requirementsa) 36 kV & AboveCurrent transformers should be of robust design, tested quality and reliable in operation.Only pure high grade paper, wound evenly under controlled conditions and impregnatedwith mineral oil under high vacuum should be used for the main insulation. The assemblyof each CT should be dried, filled with appropriate quality of insulating oil under highvacuum and hermetically sealed with or without inert gas to eliminate undesirable effectof moisture and oxygen on the internal insulation. No breathers and/or drying chemicalsshould be used in the design and construction of CTs.The shape of the external metal parts should ensure that rain water runs off and it doesnot accumulate. All external surfaces should be resistant to atmospheric corrosion eitherby the selection of suitable materials or by proper treatment such as hot dip galvanisation,zinc coating and suitable enamel painted over rust inhibitive coat of zinc chrome primeretc. Likewise, the internal metal surfaces coming in contact with oil should be givenproper treatment unless the material used itself is oil resistant. Bolts, nuts and washers tobe used as fastners should be heavily hot dip galvanised throughout. The galvanisingshould conform to IS: 2629-1966. All CTs should have an oil level gauge marked withthe maximum and minimum levels. Although no oil samples may be required to be takenfor analysis nor any filter connections made for reconditioning of oil at site but a fillingplug at the top and a drain at the bottom of the lower tank should be provided on each CTfor use during initial assembly or any subsequent repair.The current transformers should be with dead/live tank design. The current transformersshould be of single phase oil immersed, self cooled and suitable for services indicated,complete in all respects conforming to the latest edition of relevant standardspecification. The cores should be of high grade, non-ageing silicon laminated steel oflow hysteresis loss and high permeability to ensure high accuracy at both normal andfault currents. The CTs should be hermetically sealed with or without inert gas toeliminate breathing and prevent air and moisture from entering into the tank. To take care2-8

of volumetric variation of oil due to temperature changes-stainless steel bellows/Nitrogenshould be provided. In case Nitrogen is used the supplier should ensure that gas is filledat suitable pressure to take care of the expansion & compression of nitrogen gas. Theequipment should be provided with oil level gauge and pressure relieving device capableof releasing abnormal internal pressures. The secondary terminals should be brought outin a compartment on one side of the equipment for easy access. The secondary tapsshould be adequately reinforced to withstand normal handling without damage.Equipment should be provided with power factor terminals for testing loss angle (Tandelta). The equipment should also be provided with drain valve, sampling plug to checkdeterioration of oil characteristics and replacement of oil at site. Means adopted forsealing the CTs hermetically and to absorb the variation in volume of oil due totemperature variation by way of provision of stainless steel volume adjustable bellows orother means should be clearly brought out in the tender. Rubber or PVC/syntheticbellows for the purpose should not be accepted. The secondary terminal of CTs should beprovided with short circuiting arrangement.b) 11 kV ClassWindings: Change in the CT ratio shal be obtained by providing tapings in the secondarywinding. The primary bar and secondary windings shall be of copper.Core: The core of the CT shall invariably be of torroidal type. The magnetic circuit shallbe of high grade, non-ageing electrical silicon laminated steel of low hysterias loss andhigh permeability to ensure high accuracy at both normal and over currents.4.4 Terminal Connectors 33 kV and AboveAll current transformers should be provided with appropriate number of solderless clamptype primary connectors suitable for ACSR conductor and should be suitable forhorizontal as well as vertical take off with single conductor as per actual requirement.4.5 Type of Mountinga) 12 kV & AboveThe current transformers should be suitable for mounting on steel structures. Thenecessary flanged, bolts etc. for the base of CTs should be galvanized.b) 11 kV Outdoor TypeThe CT shall be supported on a suitable post insulator to be mounted on a pedestal/steelstructure. Mounting flanges, bolts, etc. shall be hot dip galvanized and shall be suppliedalong with the CT. Suitable mounting holes shall be provided at the base for clamping tothe structures.2-9

4.6 TestsThe CTs shall be provided with bolted type terminals to receive ACRS conductors upto15 mm dia (without requiring use of lugs) both in vertical and horizontal directions. Theterminals shall be such as to avoid bimetallic actions.Each current transformer should comply with type and routine test including short timecurrent test as stipulated in relevant Indian Standard specification.4.7 External Insulation (12 kV & Above)The external insulation should comprise of a hollow porcelain, which will also serve as ahousing for the main insulation or other internal parts of the CTs. Insulators should be ofhigh grade and homogeneous procelain made by the wet process. The poreclain shouldhave hard glazing and should comply with the requirements of IS 5621 in all respects.The skirt forms should be carefully selected to achieve the necessary flashover distanceand total / protected creepage distances as required.4.8 Fittings and Accessories (12 kV & Above)1. Primary terminals2. High frequency current surge divertors3. Terminal connectors for connections from line to the CT primary4. Oil level gauge5. Pressure relief device6. Expansion chamber or other suitable type of device for absorbing variations in thevolume of oil due to change of temperature.7. Weather proof secondary terminal box fitted with door and complete withterminals and shorting links.8. Lifting lugs9. Fixing lugs with bolts, nuts and washers for holding down the CTs on thesupporting steel structures.10. Rating and diagram plates11. First filling of oil12. Oil filling plug and drain valve13. Earthing terminals5.0 Potential Transformer and Coupling Voltage Transformera) 33 kV & AboveThe voltage transformer may be either of the electro-magnetic type or the capacitor type.The electro-magnetic type VTs are costlier than the capacitor type and are commonlyused where higher accuracy is required as in the case of revenue metering. For otherapplications capacitor type is preferred particularly at high voltages due to lower cost andit serves the purpose of a coupling capacitor also for the carrier equipment. For groundfault relaying an additional core or a winding is required in the Voltage transformers2-10

which can be connected in open delta. The voltage transformers are connected on thefeeder side of the circuit breaker. However, another set of voltage transformer is normallyrequired on the bus-bars for purpose of synchronization. Potential transformer class andratio is determined by electrical protection, metering consideration.b) 12 kVThe voltage transformer shall of outdoor, 3 phase either oil filled or resin cast type, whichThe voltage transformers shall be of outdoor, 3 phase either oil filled or resin cast type,which shall be enclosed in a weather-proof housing to avoid direct exposure to sun andother atmospheric influences. The incoming and outgoing terminals shall be brought outthrough suitable porcelain bushings. The voltage transformer shall be suitable foroperation in a solidly grounded system.5.1 Type and Rating of Potential TransformerPotential transformer, design, Temperature rise and testing etc. should be in accordancewith IEC: 186, IS: 3156 (Part I & II).The PTs should be single phase oil immersed self cooled type suitable for outdoorinstallation of kV class required. The core should be of high grade non ageing electricalsilicon laminated steel of high permeability. The PTs sealed hermetically scaled toeliminate breathing and prevent air and moisture entering the tank. Oil level and pressurereleasing device etc. should be provided.Each potential transformers should have the following (table 2.5.1) particulars under thesite conditions for the system under design (typical values for 72.5 kV system are given).1. Rated voltage 72.5 kV 36 kV 12 kV2. Rated frequency 50 Hz 50 Hz 50 Hz3. Accuracy class of winding 1.0 1.0 1.04. Voltage ratio 66 kV/3/110V/3 33 kV/3/110V/3 11 kV/3/110V/35. Grade of oil As per IS: 3356. Maximum phase angle error 40 min. 40 min. 40 min.with 25% and 110% of ratedburden at 0.8 p.f. lagging atany voltage between 80% and120%7. Temperature rise at 1-1 times As per IS: 3156 As per IS: 3156 As per IS: 3156rated voltage with rated burden(OC)8. Rated voltage factor & time Continuous 1.2 Continuous 1.2 Continuous 1.29. 1 minute power frequency(wet/dry) withstand testvoltage30 sec. – 1.5 30 sec. – 1.5 30 sec. – 1.5140 kV r.m.s. 75 kV r.m.s. 28 kV r.m.s.2-11

10. 1.2/50 micro seconds impulse 325 kV (Peak) 170 kV (Peak) 75 kV (Peak)wave withstand test voltage11. One minute power frequency 2 kV 2 kV 2 kVwithstand test voltage onsecondary12. 3 second short time current As per IS: 3156 As per IS: 3156 12.5 kArelay13. Dynamic Rating As per IS: 3156 As per IS: 3156 2.5 times14. Minimum creepage distance ofbushings (based onAs per Para 2.5 of section 1environment)5.2 Temperature Risea) 36 kV & AboveThe maximum temperature of the windings, cores etc. should not exceed 45C overambient, while max. temperature of oil at top should not exceed 35C over ambient. ThePTs should be suitable for mounting on steel structures. All nuts, bolts, flanges and baseshould be hot dip galvanized. The terminal connectors should be such as to give intimatecontact between conductor & terminal and offer protection against and effects ofelectrolytic and atmospheric corrosion and should also have sufficient mechanicalstrength. The connectors should conform IS 5556: 1970. The junction boxes should besuitable for terminating all the connections of the PTs secondaries with other equipmentsof the power station 400V grade terminal connectors of 15 Amp (continuous) currentrating should be provided.b) 12 kVWhen tested in accordance with IS: 3156, the temperature rise of the windings shall notexceed the following *table 2.5.2) limits:Class E insulationClass B insulationClass F insulation50 0 C600 0 C85 0 CNote: Maximum ambient temperature shall be taken as 65 0 C5.3 11 kV Voltage TransformerThe tank shall be given three coats of rust preventing paint. The other iron parts shall behot dip galvanized. The tank shall be provided with lifting lugs either welded on the sidesor top cover plate of the tank.The dimensions and electrical characteristics of the 11 kV bushings shall be inaccordance with IS: 2099-1986 or its latest version.2-12

The tank shall be provided with two separate earthing terminals.The unit shall have rating and diagram plate and will have suitable base channels tofacilitate mounting of the equipment on the structure.Terminals: The voltage transformers shall be provided with bolted type terminals on the11 kV side to receive ACSR conductors upto 8 mm dia (without requiring use of lugs)both in vertical and horizontal directions. The terminals shall be such as to avoidbimetallic action.Indoor TypeThe voltage transformer shall be of resin-cast type suitable for indoor installation andshall be normally mounted on one of the 12 kV incoming circuit breakers.5.4 Coupling Voltage Transformer (36 kV & Above)These transformers should be suitable for use on transmission line to pass through thecarrier frequencies for communication and low voltage for protection and metering. Thesingle phase CVTs should be of suitable ratio (say 66 kV/√3/110V/√3 for 66 kV line)suitable for outdoor installation on steel structures. The equipment should be suppliedwith terminal connectors suitable for vertical take off from line conductor and hot dipgalvanized base fasteners. Other details should be in accordance with the specificationsfor potential transformers. The secondary terminals should be provided duly marked forabove requirements.The wave traps should hanged underneath feeder bay structure. The carrier frequenciesand wave trap capacity should be decided in accordance with the other ends of thetransmission lines terminating at sub station.6.0 TransformersSelection of power transformer is discussed in section 3. Layout of transformer isdiscussed as it is the largest piece of equipment in a substation and it is, therefore,important from the point of view of handling and station layout. In small hydro stationstransformer are installed in the switchyard and the bay width is determined bytransformer dimensions. Handling of transformer is normally done by the powerhousecrane and for large transformer rails are laid from powerhouse to the site of installation inswitchyard. For this purpose bi-directional rollers are provided on the transformers.Arrangement for removal of transformer in case of repair/maintenance without disturbingother equipment is required and also affects layout. In order to reduce the chances ofspread of fire, transformers are provided with a soaking pit of adequate capacity tocontain the total quantity of oil. Sometimes where feasible drainage arrangements areprovided to drain the oil away from the transformers in case of fire. Besides, separationwall are provided in between the transformers and also between the transformer androads within the substation.2-13

7.0 Lightning ArrestorsLightning arrestors are the basis of insulation co-ordination (Para 2.3 of section 1) in thesystem and are installed at outdoor transformer terminals for direct protection againstlightning impulse overvoltage spark over (1.2/50 micro second wave) and are capable ofwithstanding dissipation of energy associated with lightning impulse only. This impliesthat temporary overvoltages (at or near power frequency) which are of the order of milisecondmust be withstood to avoid damage. Taking into consideration high temporaryover voltages expected on load throw off 90- 95 % lightning arrestors should beprovided.Metal oxide (gapless) lightning arrestor confirming to following standards are now beingspecified.IEC: 99-4 - Specification part – 4 for surge arrestor without gap for AC systemIS: 3070 - Specification for lightning arrestorsTypical parameters for a 66 kV system are given (table 2.7.0) below.Table 2.7.01. Nominal system voltage (kV rms) 66 kV 33 kV 11 kV2. Highest system voltage (kV rms) 72.5 kV 36 kV 12 kV3. 1.2/50 microsecond impulse voltagewithstand levela) Transformer (kVp)325 170 75b) Other equipment and lines (kVp)4. Minimum prospective symmetrical faultcurrent for 1 second at Arrestor location(kA rms) (based on system studies)325 17031.5 kA To be stated5. Anticipated levels of temporary overvoltage and its duration(based on systemstudies)a > Voltage (p. u.)b > Duration (seconds)1.5/1.2 1.5/1.2 1.5/1.26. System frequency(Hz) 50 2.5 c/s 50 2.5 c/s 50 2.5 c/s7. Neutral Grounding EffectivelyearthedEffectivelyearthedEffectivelyearthed8. Number of Phases Three Three ThreeGeneral Technical Requirements1 The Surge Arrestors should conform to the technical requirements given in table2.7.1.2 The energy handling capability of the Arrestor offered, supported by calculationsshould be obtained with offer.3 The Lightning Arrestor should be fitted with pressure relief devices and arc2-14

Note 1Terminal Arrangement: The tope metal cap and the base of the lightning arrestors shallbe galvanized. The line terminal shall have a built-in-clamping device which can beadjusted for both horizontal and vertical take off to suit ACSR (conductor size to bespecified by the purchaser). The base of the lightning arrestors shall be provided with twoseparate terminals distinctly marked for connection to earth.Sealing: The arrestors shall be hermetically sealed to avoid ingress of moisture. Suitablerubber gaskets with effective sealing system should be used. Manufacturers should devicea suitable routine production testing to verify the efficiency of sealing.Disconnective Device: The arrestors for 11 kV system may be provided with a suitabledisconnecting device. This shall be connected in series with the ground lead and shouldnot affect the sealing system of the arrestors. The disconnecting device shall conform tothe requirements specified in IS: 3070 (Part – 2) – 1985.Pressure Relief Device: The arrestors for 33 kV and 66 kV system should have asuitable pressure relief system in order to avoid damage to its porcelain housing.7.2 Lightning ProtectionA substation has to be shielded against direct lightning strokes by provision of overheadearth wires or spikes. This equipment is essential irrespective of the isoceraunic level ofthe area due to .serious consequences and damage to costly equipment in case substationis hit by a direct stroke. The choice between these two methods depends upon severalfactors economy being the most important consideration. Both the methods have beenused sometimes even in the same station. Generally, the spikes method involves tallerstructures than the alternative of using earth wires. Another method' comprises the use ofseparate lightning masts which are provided at location determined on the basis of substation area and height of bus-bars. - Besides providing lightning protection, these mastsserve as supports for luminaires required for switchyard illumination. Spikes and theearth-wire .have to be suitably placed so as to provide coverage to the entire substationequipment. Generally an angle of shield of about 45° for the area between ground wiresand, 30° for other areas is considered adequate for the design of lightning protectionsystem.7.3 InsulatorsProvision of adequate insulation in a substation is of primary importance from the pointof view of reliability of supply and safety of personnel. However, the station designshould be so evolved that the quantity of insulators required is the minimumcommensurate with the security of supply. An important consideration in determining theinsulation in a substation, particularly if it is located near sea or a thermal powergenerating station or an industrial plant is the level of pollution. As a first step to combatthis problem, special insulators with higher leakage distance should be used. In case thisdoes not suffice, washing the insulators by using live line equipment has to be resorted to2-16

and this aspect has to be kept in mind while deciding the layout of the substation.Another method which has proved to be successful in other countries involves theapplication of suitable type of greases or compounds on the surface of the insulators.This, however, also requires cleaning of insulation, the frequency depending upon thedegree and the type of pollution.8.0 Equipment for Communication, Relaying and Tele Metering and Off-site ControlFollowing types of equipments may be used for the purpose.i) Carrier Equipmentii) Microwaveiii) VHF wirelessiv) Dedicated fibre optic cableVHF equipment is normally recommended for 33 kV systems. Fibre optic cable isrecommended when offsite control is provided.The carrier equipment required for communication, relaying and Tele metering isconnected to line through coupling capacitor and wave trap. The wave trap installed atthe line entrance. The coupling capacitors are installed on the line side of the wave trapand are normally base mounted.Economic study for Microwave transmission for the purpose is required.9.0 AuxiliariesBesides the main equipment a number of auxiliary facilities and system as enumeratedbelow have to be provided. These are discussed alongwith auxiliaries for the powerhouse.In step-up substations most of the facilities are provided in the powerhouse.a) Earthing and Grounding – Steel grounding system is provided for earthmat andinterconnectionb) Oil Handling System – portable oil purification system is providedc) Illumination and lighting system – illumination system is discussed withauxiliaries systemd) Compressed air system – is required for cleaning etc. and provided in thepowerhousee) Fire protection system – All substations should be equipped with fire lightingsystems conforming to the requirements given in latest IS: 1646 and Fireprotection manual Part –I issued by Tariff Advisory Committee of InsuranceCompanies.f) AC Auxiliary power system – is provided in the power houseg) DC system is provided in the powerhouseh) Cables – discussed in powerhouse electrical auxiliaries2-17

10.0 Repair/Inspection FacilitiesLarge substations sometimes require the facilities of repair bay alongwith a crane ofadequate capacity for handling the heaviest equipment, which is usually the transformer.In hydropower station powerhouse crane is generally used for this purpose.Repair/service bay of powerhouse is used for repair of transformer.Provision of a rail track should be made for movement of transformer from switchyard tothe repair bay. Points for jacking, winching should be provided at the transformerfoundations and 90 0 turn on the rail track for changing the direction of the wheels.11.0 Pale FencingPale fencing around switchyard consists of 75 mm wide and 2500 mm high pales fixedon two members 45 x 45 x 6 mm angle horizontal runners. Vertical supports may be of50 x 50 x 6 mm angle. Two meter gates of approximately 4000 mm width (2000 mmwide each leaf) is normally required for entry/exist of transformers etc.12.0 Switchyard LayoutLow level layout of the switchyard of step up station should be provided. Layout ofswitchyard may be generally designed in accordance with Central Board of irrigation andpower manual on Sub-Station layout for 36 kV and above. Rural electrification standardbe adopted for 12 kV substations. Typical layouts of substations are attached as follows:12.1 12 kV outdoor switchyard with Lattice type structure recommended for hilly areas – 2 x500 kVA Agnoor SHP12.2 12 kV outdoor switchyard with pole structure – REC standard layout12.3 36 kV outdoor switchyard – single sectionalized bus (H-Type) 2 x 3.5 MW SikasarProject – 2 sheets12.4 36 kV outdoor switchyard – single bus (CBI & P Manual) – 2 sheets12.5 72.5 kV outdoor switchyard – single sectionalized bus – proposed 2 x 10 MW Mukerianproject stage –II – 2 sheets.12.6 145 kV outdoor switchyard single bus (CBI & P) manual for single sectionalized bus – 2sheets.12.7 245 kV outdoor switchyard single bus (CBI & P) manual for single sectionalized bus – 2sheets.2-18

Fig. 12.1: Layout of 12 kV Agnoor SHP SubStation2-19

11kV LA11kV LINELAS1.2.3.NOTESTHE LAYOUT AS SHOWN ENV ISAGES THE USE OF 11kVVACUUM CIRCUIT BREAKERS (REC SPEC. 22/1983).WHICH SHALL NOT REQUIRE ISOLATING SWITCHES ASTHEIR INTEGRAL PART AS PARTICULARLY NOM AINTENANCE IS NEEDED ON THESE BREAKERS AND ITWILL ALSO ECONOMISE IN THE COST.CONDUCTORS USED FOR 11k V JUM PERS AND BUSBARSSHALL NOT BE LESS THAN 50 SQ.MM. (C.E.) ACSR.THE SUPPORTS SHALL NOT BE CYDED BUT MAY BESUITABLY CONCRETED33/11kV P.Tf VCB FDR VCB125004000 450011kV LA11 kVVCB11kV11kV11kV/0.433kVSTATION TRANSFORMERFig. 12.2: 12 kV Outdoor Switchyard with Pole Structure – REC Standard layout2-20

Fig. 12.4 Layout of 72.5 kV Switchyard (Plan) (Sheet 1 of 2)-CBI & P Manual2-21

Fig. 12.4 Layout of 72.5 kV Switchyard (Plan) (Sheet 2 of 2)-CBI & P Manual2-22

464006800 7600 3000LA30002000 2000XCVTWT397502250 35002000 2250 2250 95008500250030001800 2000 2000 180020002500120015001000 35002500 2000750 2500 7502550 2000 2000 2550 20001800 2000ZYLZ70001500 15001800X2000 2000 1800RYB2000LSINGLE LINE DIAGRAMLEGEND1 SF 6 CIRCUIT BREAKER2 ISOLATOR3 CURRENT TRANSFORMER4 WAVE TRAP5 COUPLING VOLTAGE TRANSFORMER6 LIGHTNING ARRESTOR7 POST INSULATOR8 POTENTIAL TRANSFORMER9 PALE FENCING10 LIGHTING MAST11 LIGHTING AND SHIELDING MAST66 kV12 MVA 11/66 KVTRANSFORMERLICTCBPTBIBICTCBLA4250BY5000RBYRWATERTANK500020008800 8800 8800 8800COM PRESSOR ROOMYC.L. OF TRACKPLANFig. 12.5 Layout of 72.5 kV Switchyard (Plan) (Sheet 1 of 2)-Mukerain Stage-II (2 x 10MW)2-23

11600060001800200020001800767500 25001000086000400075002500200050015001500 380022502500 2000 30002250425030001800 180020550SECTION Z-ZSECTION Y - Y5000107500920005001800180023001500305020002500 25502000 200020002000 1800750 18002500 750 20001500 20007600910070009100760040400SECTION L - L2000LEGEND1 CIRCUIT BREAKER100007500 25006542311800200020001800860002 ISOLATOR3 CURRENT TRANSFORMER4 WAVE TRAP5 COUPLING VOLTAGE TRANSFORMER6 LIGHTING ARRESTOR7 POST INSULATOR8 POTENTIAL TRANSFORMER9 PALE FENCING10 LIGHTING MAST11 LGHTING AND SHIELDING MAST3000 250025002000 2750225020001800150015001800 1800SECTION Z-Z20550SECTION X -XFig. 12.5 Layout of 72.5 kV Switchyard (sections) (Sheet 2 of 2) – Mukerian Stage II(2 x 10 MW)2-24

Fig. 12.6:145 kV Outdoor Switchyard Single Bus (CBI & P) Manual for Single Sectionalized bus(Sheet 1 of 2)2-25

Fig.12.6: 145 kV Outdoor Switchyard Single Bus (CBI & P) Manual for Single Sectionalized bus (Sheet 2 of 2)2-31

Fig. 12.7: 245 kV Outdoor Switchyard Single Bus (CBI & P) Manual for Single Sectionalizedbus (Sheet 1 of 2)2-32

Fig. 12.7: 245 kV Outdoor Switchyard Single Bus (CBI & P) Manual for Single Sectionalized bus (Sheet 2 of 2)2-1

2-20

Section –3Selection of Power Transformer1.0 GeneralPower transformers function is to convert electric power from one voltage level toanother. In hydroelectric plants, step up transformer perform the task of delivering powerproduced by the generators to the transmission system. Most of these transformers areunit connected i.e. directly connected to generators with or without a generator breaker.These power transformers are generator transformers. Power transformers are liquidimmersed. Power transformers are located outside preferably in the switchyard /ortransformer deck in powerhouse. These guidelines are for generators/power transformersused in SHP for outdoor switching i.e. 11 kV to 220 kV.1.1 Relevant national and international standards in this connection are as follows:IS: 2026 (Part 1 to 4) - Specifications for Power TransformerIS: 2099 - Bushings for alternating voltage above 1000VIS: 3639 - Fittings and accessories for power transformerIEC: 60076 (Part 1 to 5) - Specifications for Power TransformerCBI & P - Manual on transformer (oil immersed)(Publication No. 295-2007)IS: 11802.0 Generator TransformersPower transformer which step up the power produced by hydroelectric generating units(0.415 to 11 kV) to a level which matches the sub transmission/transmission systemvoltage (typically 12 kV to 245 kV class) for range of power houses under considerationsare 2 winding oil immersed transformers.Three Phase Versus Single Phase TransformerThree phase generator transformers should be used unless transport limitations or otherspecial reasons require use of single-phase transformer because of the following reasons.a) Higher efficiency than three single-phase units of equivalent capacity.b) Smaller space requirements and reduction in weight and dimensions.c) Lower installed cost.d) Lower probability of failure when properly protected by surge arrestors, thermaldevices and oil cooling and preservation system.3-1

3.0 Transformer RatingThe full load kVA rating of the generator transformer should be at least equal to themaximum kVA rating of the generator or generators with which they are associated.Where transformers with auxiliary cooling facilities have dual kVA ratings. Themaximum transformer rating should match the maximum generator rating.4.0 Standard RatingStandard rating for power transformer of voltage class commonly used are given in table1 (A, B, C & D) may be used if possible.Table 1 (A) 11 kV Class TransformersThree phase powerrating kVAVoltage ratio kV (Nominal)Cooling200 11/0.433 ONAN315 11/0.433 ONAN630 11/0.433 ONAN1000 11/0.433 ONAN1600 11/0.433 ONANTable 1 (B) 33 kV Class TransformersThree phase powerrating MVAVoltage ratio kVCooling1.0 33/11 ONAN1.6 33/11 ONAN3.15 33/11 ONAN4.0 33/11 ONAN5.0 33/11 ONAN6.3 33/11 ONAN8.0 33/11 ONAN10.0 33/11 ONANTable 1 (C) 66 kV Class TransformersThree phase powerrating MVAVoltage ratio kVCooling6.3 66/11 ONAN8.0 66/11 ONAN10.0 66/11 ONAN3-2

5.0 Cooling12.5 66/11 ONAN/ONAF20.0 66/11 ONAN/ONAFTable 1 (D) 132 kV Class Two Winding TransformersThrees phase power rating Voltage ratio kV CoolingMVA16 132/11 ONAN/ONAF25 132/11 ONAN/ONAF31.5 132/11 ONAN/ONAFTransformer cooling system for generator transformers specified table 2.4 (A,B & C) inaccordance with IS: 2026 (part II) are identified according to the cooling methodemployed. Letter symbols used in the table are as follows:.(i) Cooling Medium Symbola. Mineral oil or equivalent flammable Osynthetic insulating liquidb. Air A(ii)Kind of Circulationa. Natural Nb. Forced (oil not directed) FTransformer is identified by four symbols for each cooling method for which a rating isassigned by the manufacturer.1 st Letter 2 nd Letter 3 rd Letter 4 th LetterKind of cooling Kind of circulation Kind of cooling Kind of circulationmedium indicatingmedium indicatingthe cooling that isthe coolingin contact with thewindingsmedium that is incontact with theexternal coolingsystemsFollowing cooling systems are used in hydroelectric stations upto 25 MVA capacitySymbolONANONAFOil Immersed Natural Air CooledOil Immersed Forced Air Cooled3-3

OFAFOil Immersed with forced oil circulation Forced Air CooledTransformers when located in powerhouse should be sited so that unrestricted ambient aircirculation is allowed. The maximum transformer rating should match maximumgenerator rating with forced cooling in dual rating transformers, which are commonlyemployed. The rating of these dual rated transformers is usually as follows:ONAN/OFAFONAN - 60%OFAF - 100%The rating under ONAF condition although not guaranteed should be about 80%.Standby cooling capacity should be provided for different type of forced cooling asfollows as per Central Board of Irrigation and Power Manual on Transformer.ONAN/OFAF-2 – 50% group2 – 100% pump of which one will be standby,2 – Standby fans one in each 50% groupOr3 – 50% group with independent pump andfans out of which one group to act as standby.6.0 Temperature Rise, Overload Capacity And Continuous RatingConservative value of temperature rise, overload capacity and continuous rating oftransformer located in the switchyard should be specified. For the purpose ofstandardization of maximum temperature rise of oil and windings, the following ambienttemperatures are recommended by CBI & P.Cooling mediumMaximum ambient temperatureMaximum daily average ambient temperatureMaximum yearly weighted average temperature: Air: 50 0 C: 40 0 C: 32 0 CWith the above ambient temperature condition the temperature rises for powertransformers as per CBI & P are as given below:Oil 0 CWinding 0 C50 55However more conservative temperature rise are specified for generator, transformers.Reference ambient temperatures and climatic conditions and temperature rise specifiedfor generators transformer at Mukerian stage II in Punjab given in Annexure 3.1.3-4

7.0 Transformer StudiesIt is recommended that following studies be carried out for transformers 72.5 kV andabove from economic considerations.a) Type of coolingb) Insulation levelc) Departure from normal design impedance8.0 Electrical Characteristics8.1 Type of Transformer and Operating ConditionsAll transformers, should be oil immersed and may be either core or shell type and shouldbe suitable for outdoor installation. Normally oil immersed transformer should beprovided with conservator vessels. Where sealed transformers are specified, there will beno conservator but adequate space will be provided for expansion of oil withoutdeveloping undue pressure.Transformers designed for mixed cooling should be capable of operating under thenatural cooled condition upto the specified load. The forced cooling equipment shouldlcome into operation by pre-set contacts in WTI and the transformer will operate as forcedcooled unit.Transformer should be capable of remaining in operation at full load for 10 minutes afterfailure of blowers without the calculated winding hot-spot temperature exceeding 150 0 C.Transformer fitted with two coolers each capable of dissipating 50% of the losses atcontinuous maximum rating (CMR) should be capable of remaining in operation for 20minutes in the event of failure of the blower associated with one cooler without thecalculated winding hot-spot temperature exceeding 150 0 C.8.2 Continuous Maximum Rating and OverloadsTransformers provided with mixed cooling should comply, as regards its ratingtemperature rise and overloads, with the appropriate requirements of IS: 2026 whenoperating with natural cooling and with mixed cooling.All transformers, except where stated should be capable of operation continuously, inaccordance with IS: loading guide at their CMR and at any ratio. In case bi-directionalflow of power is required, that should be specifically stated by the purchaser.Temperature rise test should be performed at the tapping as desired by the purchaser. Ifnothing has been stated by the purchaser, the test should be carried out at the tapping withthe highest load losses.3-5

The transformer may be operated without danger an any particular tapping at the ratedkVA provided that the voltage does not vary by more than + 10% of the voltagecorresponding to the tapping.The transformer should be suitable for continuous operation with a frequency variation of+3 % from normal 50 Hz. Combined voltage and frequency variation should not exceedthe rated V/f ratio by 10%.8.3 Voltage RatioThe high voltage rating should be suitable for the voltage of the transmission system towhich it will be connected. The low voltage rating should be suitable for the generatorvoltage (if unit connected) or generator bus. Generator transformers are generallyprovided with Off-circuit taps on HV side for HV variation from +2.5 to – 7.5 % in stepsof 2.5 %. On load taps if provided should have tapping range of +5% to – 10 % in stepsof 1.25%.For interconnecting 2 transmission voltage system say 66 kV and 132 kV size, autotransformerswith standard ratings as per IS should be provided. For interconnectingauto-transformers, use of either regulating transformer or on-load tap changer may bemade. Interconnecting transformer are generally star-star connected with tertiary deltawinding for flow of 3 rd harmonic current.8.4 Duty Under fault ConditionGenerator transformer should be designed for exceptional circumstances arising due tosudden disconnection of the load and should be capable of operating at approximately 25% above normal rated voltage for a period not exceeding one minute and 40 % abovenormal rated voltage for a period of 5 seconds. All transformers above 5 MVA should beprovided with overfluxing protection device.8.5 Electrical ConnectionsTransformers shall be connected in accordance with IS vector symbol specified inordering schedule of the requirements. Vector symbol specified for generatortransformers is normally YND11.Auto connected and star/star connected transformers shall have delta connectedstabilizing windings if specified in the order. Two leads from one open corner of the deltaconnection shall be brought out to separate bushings. Links shall be provided for joiningtogether the two terminals so as to complete the delta connection and earthing it externalto the tank.3-6

8.6 Flux densityThe maximum flux density in any part of the core and yokes, of each transformer atnormal voltage and frequency should be such that the flux density in following overvoltage conditions does not exceeds 1.9 tesla/19000 lines per cm 2 .i) The above flux density has been specified to meet with theover fluxing of the core due to temporary over voltage ofthe order of 31% for 1 min., 44% for 5 sec. That mayappear in abnormal conditions such as those followingsudden loss of large loads/ tripping of Generator breaker.ii) Yoke bolt area and flitch plate areas shall not be counted inthe net core area, if these are provided for fastening of core.iii) The design of limb and yoke shall be so coordinated thatthere is no cross fluxing at the joints.The flux density at normal voltage frequency and ratio should not exceed 1.57 tesla.8.7 Current DensityThe transformer shall be so designed that the current density of HV windings at thelowest tap should not exceed 250 A/cm 2 . And that of LV winding should not exceed 200A/cm 2 .8.8 Short Circuit StrengthTransformers shall be designed and constructed to withstand without damage the thermaland dynamic effects on external short circuits for 5 seconds under condition specified inIS: 2026 (Part-I)- 1977.The transformers shall be provided with separate tapping coil to limit the short circuitforces.The position of the tapping coil shall be so arranged that at extreme negative tap, thepercentage regulation is less than at normal tap.The bidders shall submit test certificates of the short circuit test, if already done on theoffered design and rating. However, the thermal and dynamic ability to withstand shortcircuitforces shall be demonstrated by calculations.Manufacturers shall supply calculation for Thermal & Dynamic withstand capacity of thetransformer as per their design alongwith the tenders.8.9 Frequency and System VoltageThe transformers shall be suitable for continuous operation with a frequency variation of3% from normal 50 cycles per second without exceeding the specified temperature rise.3-7

8.10 Parallel OperationThe transformer shall be capable of parallel operation with each other and with existinggrid.8.11 Vibration and NoiseEvery care shall be taken to ensure that the design and manufacture of all transformersand auxiliary plant shall be such as to have minimum noise and vibration levels followinggood modern manufacturing practices.The manufacturers will ensure that the noise level shall not exceed the figures as perNEMA Pub. No. TR-1.8.12 Basic Insulation Levels (BIL)Transformers are the starting point for insulation co-ordination and are as such directlyprotected by lightning arrestor. Refer Para 2.3 of section -1.9.0 ImpedanceImpedance of the transformers has a material effect on system stability, short circuitcurrents, and transmission line regulation, and it is usually desirable to keep theimpedance at the lower limit of normal impedance design values. Detailed study shouldbe made if reduced short circuit level or line regulation consideration are materials andspecific feasible impedance values are required.Typical values of impedance voltage for transformers with two separate windings (atrated current, given as a % of the rated voltage of the winding to which the voltage isapplied) as per IS 2026 part I – 1977 and for generator transformers as per CBI & PManuals on Transformers are given in table 2.Table - 2Rated Power (kVA)ImpedanceVoltage (%) asper IS 2026Impedance Voltage (%) as per CBI & PmanualThree phase power Impedanceratingvoltage (%)11 kV Trans. (kVA)5Upto 1600 kVA3-8

Up to 630631 to 12501251 to 31503151 to 63006301 to 1250012501 to 25000Above 250014.05.06.257.158.3510.012.533 kV Trans. (MVA)1.001.603.154.005.006.308.0010.0066 kV Trans. (MVA)6.38.010.012.520.056.256.257.157.157.158.358.358.358.358.358.3510.00162531.5101012.5Transformers with lower or higher values of impedance are normally furnished withdifference in cost. The value of transformer impedance should be determined givingconsideration to impacts on selection of interrupting capacities of station breakers and onthe ability of the generators to aid in regulating transmission line voltage. Transformerimpedances should be selected based on system and plant fault study. Impedances shownare subject to a tolerance of plus or minus 10% as per IS:10.0 Transformer EfficiencyTransformer losses represent a considerable economic loss over the life of the powerplant. Standard losses as per CBI & P manual on the basis of optimized design ofmanufacturer is given in table 3 for 11 kV to 66 kV class transformers. Based on theselosses Capitalization for transformer losses should be carried out in accordance with CBI& P manual on transformer Section L enclosed at Annexure 3.3.Table 3: STANDARD LOSSES AT 75 0 CSl.No.(a)Three-phase power rating No-load loss (kW) Load loss (kW)11 kV Transformers (kVA)20031563010001600570 (W)800 (W)1200 (W)1800 (W)2400 (W)3300 (W)4600 (W)7500 (W)11000 (W)15500 (W)3-9

(a)(b)33 kV Transformers (MVA)1.001.603.154.005.006.308.0010.0066 kV Transformers (MVA)6.38.010.012.520.01.82.13.24.04.65.46.17.26.07.18.49.713.08142224273344534048577010211.0 Terminal BushingsConnections for the generator transformers are mostly by power cables for small hydrostations upto 10 MVA rating from generator terminals to power transformer inswitchyard. Bus ducts which could be isolated phase for large units or segregated phasebus ducts for smaller units may be used. Accordingly terminal for the generatortransformers should be as follows:LV Side: LV bushings should be mounted on turrets suitable for connection to busbar in bus ducts. For SHP cable boxes may be provided, if cables are used.HV Side: Solid Porcelain/Oil Communicating and other type bushings upto 36 kVvoltage class in accordance with IS: 3347. The dimensional parameters of the bushingsupto and including 36 kV voltage class should be in accordance with IS: 3347. The ratedcurrent, voltage, Basic Insulation Levels should be in accordance with IS: 2099.66 kV to 220 kV bushings are oil impregnated paper (OIP) type condenser bushings inaccordance with IS 2099 and IEC 137. Dimensions interchange capability current,insulation level and creepage distance for various classes of the bushings should be inaccordance with CBI & P Manual on transformers Section P.The electrical characteristics of bushing insulator shall be in accordance with IS: 2099 asamended from time to time. All type and routine tests shall be carried out in accordancewith IS: 2099. The test voltage for various tests as stipulated in IS: 2099 – 1986 are asfollows:NominalsystemvoltageRated voltage of thebushingOne minute wet anddry power frequencyvoltage withstand testLightning impulsewithstand test 12/50micro sec. kV peak3-10

12.0 TankskV kV kV kV11 12 28 7533 36 70 170The main tank body excluding tap-changing compartments, radiators and coolers shall becapable of withstanding vacuum given in the following table:Highest systemvoltage kVUp to 72 kVMVA rating Vacuum gaugepressure kN/m 2Up to 16 above 34.71.6 & up to 2068.0(mm of Hg)250500Above 72 kVFor all MVAratings100.6476013.0 Pressure Relief DeviceThe pressure relief device shall be provided of sufficient sizes for rapid release of anypressure that may be generated within the tank, and which might result in damage to theequipment. The device shall operate at a static pressure of less than the hydraulic testpressure for transformer tank. Means shall be provided to prevent the ingress of rainwater.Unless otherwise approved the relief device shall be mounted on the main tank, and, if onthe cover, shall be fitted with skirt projecting 25 mm inside the tank and of such a designto prevent gas accumulation.14.0 Anti Earthquake Clamping DeviceTo prevent transformer movement during earthquake, a claming device shall be providedfor fixing the transformer to the foundations. The contractor shall supply necessary boltsfor embedding in the concrete. The arrangement shall be such that the transformer can befixed to or unfastened from these bolts as desired. The fixing of transformer to thefoundation shall be designed to withstand seismic events to the extents that a staticcoefficient of 0.3g applied in the direction of least resistance to that of loading will notcause the transformer or clamping device as well as bolts to be over stressed.15.0 Fittings and Accessories(a)(b)(c)(d)Rating and diagram plate2 Nos. earthing terminalsCover lifting lugs.Skids and pulling eyes on both directions.3-11

(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p)(q)Oil-filling valve with flangeJacking pads.Pocket on tank cover for thermometer.Air release devices.Conservator with oil filling hole, cap and drain plug-size 19 mm nominal pipe(3/4 in. BSP/M 20).1. Plain oil level gauge for all transformers upto and including 1.6 MVA.2. Magnetic type oil gauge for transformers above 1.6 MVA, with low oillevel alarm contact.Silica gel breather with oil seal.Pressure relief device.Valves:1. Drain valve with plug or blanking flanges. The same can be used forfiltering purpose.2. A sampling device or sampling facility on drain valve.3. 1 No. filter valve on upper side of transformer tank.Buchholz relay with alarm and trip contacts with one shut-off valve onconservator side.1. Size of Buchholz relay up to 10 MVA-50 mm2. 10 MVA and above-80 mmOil temperature indicator with one electrical contact shall be provided with antivibrationmounting.Winding temperature indicator with two electrical contacts for alarm and trippurposes. Switching of fans shall be done by winding temperature indicator for alltransformers having ONAF rating. The winding temperature indicator shall beprovided with anti-vibration mounting.Tank mounted weather-proof marshalling box for housing control equipment andterminal connectors. Wiring up to marshalling box with PVC SWA PVC coppercables 660/1100 volts grade.Rollers-4 Nos.3-12

Sl. Rating Type GaugeNo.Shorter axis Longer axis1. Up to 5 MVA Flat, uni-directional As per manufacturer’s practice,however, not to exceed 1000 mm2. 6.3 MVA Flanged, bi-directional 1435 mm 1435 mm3. 10 MVA and Flanged, bi-directional 1676 mm 1676 mmabove(r) Inspection cover.(s) Cooling accessoriesONAN/ONAF cooling1. Radiators with shut-off valves and air release plugs.2. Fans.3. Filter valves.4. Drain and sampling device.5. Air release device.16.0 Dielectric Tests(a)220/132 kV windingi) Lighting impulse on all the line terminals (routine test)ii) Induced over-voltage with partial discharge indication (routine test)(b)33 kV windingi) Separate source AC on the all line terminal (routine test)ii) Lightning impulse on all the line terminals (routine test)17.0 AccessoriesNormal accessories are arcing horns, oil flow alarm, fans and pumps, dissolved gasmonitoring system, Temperature detectors, Lifting devices.Provision of Following Oil preservation system are preferred for generator step-uptransformer.(i)(ii)Inert gas pressure system. Positive nitrogen gas pressure is maintained in the spacebetween the top of the oil and the tank cover from a cylinder through a pressure-reducingvalve.Air-cell, constant-pressure, reservoir tank system. A system of one or more oil reservoirs,each containing an air cell arranged to prevent direct contact between the oil and the air.3-13

18.0 Oil Containment and Fire Protection SystemIf any oil filled transformers are used in the power plant, provisions should be made tocontain any oil leakage or spillage resulting from a ruptured tank or a broken drain valve.Physical separation in the use of fire wall/barriers is also provided in power plants.Specifications for fire protection of power transformers may be provided in accordancewith CBI & P Manual on Transformer in Section ‘O’ or specification refer chapter onmechanical auxiliaries.19.0 Factory and Field TestingTransformer specifications must contain complete and exhaustive section for qualitycontrol, Inspection, factory and field test. Provision for witness testing of factory test andmethod for type test should be specified in detail. Various routine, type and special testsare detailed in IS 2026 part I. For explanations, details on the methods and procedure forcorrections when ideal test conditions cannot be achieved reference may be made to testmanual for transformers issued by CBI & P.20.0 Erection, Maintenance Testing and CommissioningRefer CBI & P manual on transformers section K entitle Erection, Maintenance andCommissioning manual.21.0 Typical Transformer rating and characteristicsTransformer rating and characteristics for a 11/66 kV transformer for Mukerain Stage-IIHEP ( 2 x 10 MW) is enclosed as Annexure 3.4.3-14

Annexure-3.1Ambient temperature & temperature rise for Mukerian Stage II Generatortransformer 11/66 kV class rated 10/12.5 MVA and 6.6/33 kVclass transformer for Sikasar Project Generator Transformer3.3/33 kV class rated 2 x 3.5 MWA. Reference Ambient TemperaturesThe reference ambient temperatures for which the transformers are to be designed are asunder:-i) Maximum ambient temperature 50 degree Cii) Maximum daily average ambient temp : 40 degree Ciii) Maximum yearly weighted average ambient temp : 40 degree Civ) Minimum ambient air temperature : ;(Cooling Minus 5 degreemedium shall be Air)CB. CLIMATIC CONDITIONS :i) Maximum relative humidity 100%ii) Yearly average number of thunder storms _______varies from 30 to 50 .iii) Average no. of rainy days per annum 60 daysiv) Fog : The atmosphere is subject to fog for two monthin winter.v) Number of months during which tropical monsoon 3 monthsconditions prevailvi) Dust storms occur at frequent intervalsvii) Average annual rainfall 60 cmsviii) Maximum wind pressure150 kg/sq.m.B. ALTITUDEAltitude above M.S. level not exceeding 1000 mtrs.C. TEMPERATURE RISE , OVER LOAD CAPACITY & CONTINUOUS RATINGa) With the above service conditions, given in clause-6.4 each transformer shall be capableof operating continuously on any tap at normal rating without exceeding followingtemperature rises, over maximum ambient temperature of 50 deg. C.i) 30 deg. C in oil by thermometerii) 45 deg. C in winding by resistanceiii) The temp. of hot spot in the winding not to exceed 90 deg. C when calculatedover max. annual weighted average temp. of 40 deg. C & 105 deg. C at worstambient of 50 deg. C.3-15

) The limits of temperature rise mentioned above and over load capacity as per IEC-354(1993) will have to be satisfied by the manufacturer by carrying out the heat run test atthe lowest negative tap. This test shall be carried out by feeding the following losses: -(Total max. losses at 75 deg. C at highest current tap) x 1.1c) The safe overload capacity of the transformer and the duration of overload for each typeof cooling (ONAN/ONAF/ ) under maximum temperature conditions (Clause 6.5 above )without any damage to the winding or harmful effects on the insulation shall be clearlystated in the tender, which must be as per IEC-354 (1993) – Guide for loading of oilimmersed transformers, suitable for climatic conditions given in clause-6.4 above.d) The transformer may be operated without exceeding temperature rises, winding gradientsand hot spot at any particular tapping at the rated MVA provided that the voltage does notvary by more than 10% of the voltage corresponding to that tappings. Transformer shallbe able to withstand for 30 minutes after achieving steady state at full load rating withoutinjurious heating to winding/insulation etc. under auxiliary failure condition.3-16

Rationalization of capitalization Formula for Transformer LossesAnnexure-3.2The rated capitalization of transformer losses depends upon the rate of interest, rate of electricalenergy per kWh, life of transformer and average annual loss f actor. The annual loss factor takesinto account the loading of the transformer during the year. In computing the rate ofcapitalization of iron losses, copper losses and auxiliary losses. Following assumptions arerecommended.(i)(ii)(iii)(iv)(v)(vi)Rate of interest (r):Rate of electrical energy (EC): It is the cost of energy per kWh at the bus to which thetransformer to be connected.Life of the transformer (n): It is taken 25 years.Life transformer is in service for a period of 350 days in a year (allowing 15 days formaintenance, breakdown, etc.).The cooling pumps remain in service for 40% of the time, the transformer is in service.Annual loss factor: The annual loss factor may be worked out on the basis of the formulagiven below.LS = 0.3LF + 0.7 (LF) 2Where:LS is the annual loss factorLF is the annual load factorAssuming annual load factor as 60%, annual loss f actor works out to 0.432.Capitalization Formula SuggestedCapitalised Cost of Transformer = Initial cost + Capitalised cost of annual iron losses +Capitalised cost of annual copper losses + Capitalisedcost of annual auxiliary losses.Capitalised cost of iron losses per kW = 8400EC n1r 1r1 r nCapitalised cost of copper losses per kW = 8400EC n1r 1LSr1 r nCapitalised cost of iron losses per kW = 0.48400EC n1r 1r1 r nSubstituting the values, the capitalized cost of transformer.Actual value can be worked out by the purchaser by considering appropriate value of r, E L , L F ,and L C .3-17

Example Calculation -1Generator TransformerDifference of loss = 2.381 kWLoad factor = 80% = 0.8Rate of interest (r) = 10% = 0.1Rate of electrical energy (EC) = 2.5 RsLife of transformer (n) = 35 yearLS = 0.3(LF)+0.7(LF) 2LS = 0.3x0.8+0.7x(0.8) 2= 0.688Capitalised Cost of Additional Copper loss per kW= 8400EC n1rr1 r= 84002.5 10.1 0.11 1 LS1 0.688350.1= Rs. 139338.81Capitalised cost of additional transformer losses= 139338.81 x 2.381= Rs. 331766n353-18

Loss Capitalization formula (Loading factor)Annexure-3.3Method for calculating the loading factors for evaluation of loss capitalization to be specified bythe purchaser as per CBI & P is as follows:The values indicated are typical values and the utility may adopt values different from thoseindicated in case the rates of interest, cost of energy and the number of hours of operation aredifferent from those indicated in the example below. A life expectancy of less than 25 years isnot recommended.Loading factor for no load loss nnA = H E 1 r1/ r 1r = 1rCnn 1/r1 rRate of Expect Number of Cost of 1 +interest ed life hours of energy to r r nr n h E cin perunitoperation ina yearthe utilityat 11 kVlevel0.08 25 8400 E c 1.08 6.8484751 1 nnr 1 r1 n / r 1r 1 A =loadingfactor perkW of ironloss5.84848 10.97477619 Rs.8966xE cLoading factor for copper lossLLF = 0.2 * LF + 0.8 *LF*LF= 0.3B = A x LIF= Rs. 89668 x E c x 0.3 = Rs. 26900 x E cWhere,A = Loading factor in Rs. Per kW of no load lossB = Loading factor in rs. Per kW for load lossH = No. of hours, the transformer will remain charged in a year i.e. no. of operation (taken as8400 hrs.)E c = Cost of energy to the utility at 11 kV levelr = Rate of ineterst per unit (taken as 8 %)n = expected life of the transformer (taken as 25 years)LLF = Loss load factor (where LLF = 0.2 x Load factor + 0.8 x L.F. 2 )LF = Load factor (taken as 0.5)E c = Cost of energy (in rupees per unit at 11 kV feeder level)Note: In case of non-availability of E c (energy cost per unit) at 11 kV feeder level, utilityshould consider the Bulk Rate Tariff plus 5% as the cost of energy at 11 kV feeder level.Load factor considered for B factor is 0.5, higher load factor may be considered for urbanareas.3-19

TRANSFORMER RATING AND CHARACTERSTICSAnnexure-3.4The rating and electrical characteristics of the transformers shall be as under:S.No. Particulars 10/12.5 MVA (Outdoor type)1. Continuous kVA ratings 10/12.5 MVA ONAN/ONAF2. Type Oil immersd3. Frequency 50 C/s4. Type of cooling ONAF5. No. of phases Three6. Rating voltage on H.V. side 72.5 kV r.m.s.7. Rated voltage on L.V. side 11 kV r.m.s.8. Vector symbol YND 119. Connectionsa) H.V. Windingb) L.V. windingStar with neutral earthedDelta10. Off load taps on H.V. side (for H.V. + 2.5 to –7.5 % (in steps of 2.5%)Variation)11. H.V. and L.V. bushings suitability L.V. suitable for cable box.H.V. condenser bushings with plain shedsINSULATION LEVELS1. Insulating material to be used, shall be of class”A” as specified in the latest edition of IS:123712. The dielectric strength of winding insulation and of the bushings shall conform to valuesgiven in IS: 2026/1981 part-III amended upto date except for the changes made in thisspecification.3. The following impulse test and power frequency test voltage must be offered.Rated Highest 1.2/50 Sec. One minute power frequencySystem system positive impulse withstand voltageVoltage voltage with stand voltage -------------------------------------(kV) (kV) of line end Line Neutral(kV peak) end(kV) end(KV)------------------------------------------------------------------------------------------------------11 12 95 28 -33 36 170 70 -66 72.5 325 140 38132 145 550 230 ------------------------------------------------------------------------------------------------------The provision of note under clause 5.4 IS: 2026 (Part-III) – 1981 should be kept in viewwhile offering this parameter. The star connected windings of the transformers shall havegraded insulation. All windings for system voltage lower than 66 KV shall have uniforminsulation.3-20

Section –4Selection of Circuit Breaker1.0 IntroductionCircuit breaker is a mechanical switching device capable of making, carrying andbreaking current under normal circuit condition as well as under specified abnormalcircuit condition such as short circuit etc. Circuit breakers are generally classifiedaccording to interrupting medium used to cool and elongate electrical arc permittinginterruption. Selection of outdoor circuit breakers for switchyards 12 kV and above upto245 kV (highest system voltage) as regards types, rating, performance requirements andtests for AC high voltage circuit breakers that are installed in SHP outdoor switchyardafter the step up transformer on outgoing transmission line feeders. Special requirementfor rating of AC high voltage generator circuit breakers between the generator andtransformer terminals are also discussed.1.1 ReferencesRelevant National and important international standard in this connection are as follows:-1. IS: 13118 - Specification for high-voltage alternating current circuitbreakers2. IEC: 56 - High voltage alternating current circuit breakers3. IEEE: 37010 - IEEE Application Guide for AC high voltage circuit breakers4. IEEE 37013 - AC high voltage generator circuit breaker rated onsymmetrical current basis2.0 Classification2.1 Following types of circuit breakers formerly used in high voltage outdoor substationsare no longer in use and are being phased out.i) Bulk oil circuit breakers (Dead Tank Design) – In these circuit breakers oil contentsis used for arc extinction and also for insulating live parts from the tank which is deadand generally earthed (ground).ii)iii)Minimum oil breakers (Live Tank Design) – In these circuit breakers oil isprimarily used for arc extinction and not necessarily for insulating live parts fromearth (ground). The tank of these circuit breakers are insulated from earth ground.The circuit breakers are phase separated. These circuit breaker were widely used upto72 kV level and are being phased out from existing installation.Air blast circuit breaker – circuit breaking in these circuit breakers occurs in a blastof air under pressure. These circuit breakers were widely used upto 765 kV system.These circuits breakers are being phased out.4-1

3.0 Type of Circuit BreakerFollowing types of circuit breakers are in use now-a-days for max. voltage class used for25 MW hydro station.i) SF-6 – Sulphur Hexa Flouride Breakers 36 kV to 220 kV classii) Vacuum circuit breakers upto 36 kV classiii) Air circuit breaker upto 12 kV (Generator circuit breaker)Sulphar Hexafluoride as an Arc Quenching Agent:- Pure sulphar hexafluoride gas isinert and thermally stable. It possesses very good arc quenching as well as insulatingproperties which make it ideally suitable for use in a circuit breaker. Sulphar hexafluorideremains in a gaseous state upto a temperature of 9 0 C at 15 kg/cm 2 pressure its density isabout five times of air and the free heat convection is 1.6 times as much as that of air.Apart from being a gas, it is non-inflammable, non-poisonous and odourless. Whenarcing takes place through the gas, some by-products are produced due to breakdown ofthe gas. These by-products are a hazard to the health of the maintenance personneltherefore should be properly taken care of.At a pressure of three atmospheres the dielectric strength of sulphar hexafluoride is about2.4 times that of air and compares very well with that of oil. Even when gas is exposed toelectric arcs for fairly long periods, it has been found that decomposition effects are smalland the dielectric strength is not materially affected. On the other hand the metallicfluorides at the temperatures of the arc are good insulators and the arc is therefore, not atall harmful to the breaker.Gas circuit breaker generally employ SF-6 (sulphar hexafluoride) as an interruptingmedium and sometimes as an insulating medium. In “single puffer” mechanisms, theinterrupter is designed to compress the gas during the opening stroke and use thecompressed gas as a transfer mechanism to cool the arc and to elongate the arc through agrid (arc chutes), allowing extinguisher of the arc when the current passes through zero.In other designs, the arc heats the SF6 gas and the resulting pressure is used forelongating and interrupting the arc. Some older low-pressure SF6 breakers employed apump to provide the high pressure SF6 gas for arc interruption.Gas circuit breakers typically operate at pressures between six and seven atmospheres.The dielectric strength of SF6 gas reduce significantly at lower pressures, normally as aresult of lower ambient temperatures. Monitoring of the density of the SF6 gas is criticaland some designs will block operation of the circuit breaker in the event of low gasdensity.Circuit breakers are available as live-tank or dead-tank designs. Dead –tank designs putthe interrupter in a grounded metal enclosure. Interrupter maintenance is at ground leveland seismic withstand is improved versus the live-tank designs. Bushings are used forline and load connections which permit installation of bushing current transformers for4-2

elaying. The dead-tank breaker does require additional insulating gas to provide theinsulation between the interrupter and the grounded tank enclosure.Live-tank circuit breakers consist of an interrupter that is mounted on insulators and is atline potential. This approach allows a modular design as interrupters can be connected inseries to operate at higher voltage levels. Operation of the contacts is usually through aninsulated operating rod or rotation of a porcelain insulator assembly by an operator atground level. This design minimizes the quantity of gas used for interrupting the arc as noadditional quantity is required for insulation of a dead-tank enclosure. The design alsoreadily adapts to the addition of pre-insertion resistors or grading capacitors when theyare required. Seismic capability requires special consideration due to the high center ofgravity of the interrupting chamber assembly.Interrupting times are usually quoted in cycles and are defined as the maximum possibledelay between energizing the trip circuit at rated control voltage and the interruption ofthe main contacts in all poles. This applies to all currents from 25 to 100% of the ratedshort circuit current.Breaker ratings need to be checked for some specific application. Applications requiringreclosing operation should be reviewed to be sure that the duty cycle of the circuitbreaker is not being exceeded. Some applications for out –of- phase switching or back-tobackswitching of capacitor banks also require review and may require specific dutycircuit breakers to insure proper operation of the circuit breaker during fault interruption.3.1 Vacuum Circuit BreakerVacuum circuit breakers use an interrupter that is a smaller cylinder enclosing themoving contacts under a high vacuum. When the contacts part, is a formed from contacterosion. The arc products are immediately forced to and deposited on a metallic shieldsurrounding the contacts. Without anything to sustain the arc, it is quickly extinguished.Vacuum circuit breaker are widely employed for metal-clad switchgear up to 36 kV class.The small size of the breaker allows significant savings in space and material comparedto earlier designs employing air magnetic technology. When used in out door circuitbreaker designs, the vacuum cylinder is housed in a metal cabinet or oil filled tank fordead tank construction.3.2 Advantages and DisadvantagesAdvantages: Advantages of SF6 breakers over the conventional breakers is given below:i) due to outstanding arc quenching property of SF6, the arcing time is very small.This reduces contact erosion.ii) using SF6 gas at low pressure and low velocity; the current chopping can beminimized.4-3

iii) during arcing of SF6 breaker, no carbon dioxide is formed and hence no reductionof dielectric strength.iv) SF6 breaker is silent in operation and moisture ingression into the gas cycle isalmost nil.v) SF6 breaker performance is not affected due to variation in atmosphericconditions.vi) SF6 breaker is compact in size and electrical clearances are drastically reduced.Disadvantages: The only disadvantage of using SF6 to some extent is suffocation. Incase of leakage in the breaker tank, this gas, being heavier than air settles in thesurroundings and may lead to suffocation of the operating personnel. However, it isnon-poisonous.3.3 Evaluation of SF6 and Vacuum Switching TechnologiesSI. No.1Criteria2SF 6 circuit breaker3Vacuum circuit breaker4I. SWITCHING DUTIES1. Summated breakingcurrent capacity2. Critical switchingi. Motor, Reactors smallinductive current.ii. Capacitorsiii. Over head and cablefeeders.iv. Switching of arcfurnaces.3. Suitability of single andmulti shot auto reclosecycles.To 50 times rated short circuitbreaking current To 10,000 timescontinuous rated current.Very well suited Over voltagegenerally under 2.5 p.u. Normallyno action necessary to limit overvoltages.Well suited. Restrike free. In specialcases reactors may be necessary tolimit in rush current - Low overvoltage.Very well suited-Restrike free.Only suitable in applicable withcomparatively low number ofoperations per day.Very well suitedTo 100 times rated short circuitbreaking current To 20,000times continuous rated current.Well suited. Under certaincircumstances steps may benecessary to limit over voltagesbecause of possibility of virtualcurrent chopping. Use of surgearresters recommendedSuited. Generally Restrike free.In special cases reactors may benecessary to limit in rushcurrent-Reignition and restrikespossible in certain cases due tostatistical scatter of breakdownvoltage in vaccum.Very well suited Restrike free.Suitable also for applicationswith very high number ofoperations (over 100 co-per day.Very well suited.SI. No.1Criteria2SF 6 circuit breaker 3Vacuum circuit breaker4II. MAINTAINABILITY4-4

SI. No.1Criteria2SF 6 circuit breaker 3Vacuum circuit breaker41. Number of operationsbetween servicing,referred to operatingmechanism.500 to 20,000 C-O Operations 10,000 to 20,000 C-O operation2. Service life ofinterrupters5000 to 20,000 Cooperations(Between over hauls)20,000 to 30,000 Cooperations or(manufacturers guidelines)3. Service interval Lubrication of operatingmechanism after 5 to 10 years(if limiting number ofoperations not reached)Lubrication of operatingmechanism after 10 years (iflimiting number of operations notreached.4. Expenditure onoverhaul ofinterrupters.5. Supervision of Circuitbreaker condition.6. Refilling of arcquenching mediaOverhaul involves completedismantling of interrupter.Labour costs high, materialcosts low.Supervision of SF6 gas pressurepossible (Pressure guage withcontacts for remote signalling.)PossibleTester used to checkvacuum level. If necessaryreplace interrupter. Lowlabour costs. High material costs.Checking the insulation andquench media not easily possible-But generally supervision ofvacuum level not necessary(sealed for life)Not possibleIII. RELIABILITY1. Failure rate Lowest (7-13 per 1000 CB-Yrs) Lowest(7-13 per 1000 CB-Yrs)2. Mechanical life Good Excellent3. Fire Hazard Zero Zero4. Interrupting capacity incase of leakage.Switching of rated current stillpossible.Not possibleIV. COST1. Cost of production Sightly higher than VCB -2. Maintenance cost Lowest negligible cost forminimum 10 years.Lowest negligible cost forminimum 10 years.4-5

3.4 Protection Classes for Switchgear InstallationThe protection classes are identified by a compound symbol made up of the two code letters'EP, which always remain the same two digits denoting the degree of protection and, if]required, the supplementary code letter B. The supplementary code letter must be stated if inthe case of switchgear and distribution equipment the protection class is attained onlythrough taking certain measures when the apparatus is installed. The term 'Production class*denotes the complete compound symbol (code letters, code digits and the supplementarycode letter if applicable)Protection ClassIP 4 3 BCodeFirst DigitSecond DigitSupplementaryDegree of protection against contactAnd ingress of foreign bodiesDegree of protection against ingressOf waterSealing material between code letterfloor, ceiling or wallIf in addition to the code letters IP only one code digit for the degree of protection is used, the missingdigits a to be replaced by a dash.eg. IP - 4 (Protectionclass : Splash proof)Degree of protection againstContact and foreign bodies:Degree of protection against waterFirst Description SecondDescriptionDigitDigit0 No Protection 0 No Protection1 Protection against largeforeign bodies1 Protection against vertically gallingwater droplet2 Protection againstmedium sized foreign2 Protection against obliquely fallingwater dropletbodies3 Protection against smallforeign bodies3 Protection against sprayed water(spray proof)4 Protection againstgranular foreign bodies4 Protection against splashing (splashproof)5 Protection against dust 5 Protection against water jet (Hotdeposit6 Protection against ingressof dustproof)6 Protection against inundation7 Protection against immersion in4-6

water8 Protection against indefineimmersion in water4.0 Rated CharacteristicsThe main characteristics of a power circuit breaker including its operating devices andauxiliary equipment used to determine the rating are as follows:-Rated voltageRated insulation levelRated frequencyRated normal currentRated short-time withstand currentRated short-circuit breaking currentRated short-circuit making currentRated operating sequence (duty cycle)Rated transient recovery voltage (TRV) for terminal faultTotal breaking time (maximum)Rated characteristics for short-time faults, for three pole circuit breakers designedfor direct connection to overhead transmission lines and rated at 52 kV and aboveand at more than 12.5 kA rated short breaking currentIn addition, the following characteristics are necessity for specific application.Rated line charging breaking currentRated inductive breaking currentRated capacitor breaking currentRated out of phase breaking current5.0 Standard Ratings of Circuit Breakers5.1 Rated Voltage: Voltage rating of the power circuit breaker is in terms of threephase line to line voltage of the system. The rated voltage of the circuit breaker should beof standard rating chosen so as to be at least equal to the highest voltage of the system atthe point where the circuit breaker is to be installed. The operating voltage and the powerfrequency recovery voltage should not exceed the rated maximum values because thismaximum is upper limit for continuous operation.It is however considered that operation at altitude above 1000 meters should be givenspecial considerations and certification from manufacturer be obtained because ofpossible influence of altitude on interrupting capacity.In case of generator circuit breakers the rated maximum should be equal to the maximumoperating voltage of the generator, which is usually equal to 1.05 times rated voltage.4-7

The rated voltage is expressed in kV (rms) and refer to phase to phase voltage.Nominal system voltagekV rmsRated voltage of circuit breakerkV rms11 1222 2433 36132 145220 2455.2 Rated Insulation Level: - Insulation level of power circuit breakers should beselected from standard insulation level listed in IS: 13118. Refer table 2.1 (section-1) forvoltage upto 36 kV and table 2.2 (section-1) for voltages upto 245 kV. For insulation coordinationrefer (Para 2.3.4. of section1).The surge protection of the system should be coordinated with the impulse strength of thebreaker, both across the open contacts and to ground. Attention should also be given toincrease in surge voltage because of reflections which occur at breakers when theircontacts are open, especially where cables are involved.Surge arrestors are generally installed on the bus or on transformers and not on eachcircuit breaker, the surge voltage at the breaker can exceed that at the arrestors. Theamount of the excess depends upon the steepness of the wave front and the distance fromthe circuit breaker to the surge arrestors. When the circuit breaker is in the open position,either intentionally left open or during operation, an incoming surge voltage may bedoubled by reflection at the open contacts. Selection of too low an insulation level forcircuit breakers, if not individually protected by arrestors, may result in line-to-ground, oropen gap insulation failure of the circuit breaker. Use of individual line entrance surgearrestors may be required if the lightning trip-out rate of the line exceeds 1 per year (referIEEE std. 37.010-1999). Refer section 1 for more details.5.3 Rated Frequency: standard power circuit breakers are rated at 50 cycles. Serviceat other frequencies requires special consideration.5.4 Rated Normal Current: rated normal current of a circuit breaker is the rms valueof the current which the circuit breaker shall be able to carry continuously at ratedfrequency. With the rise in temperature of its different parts not exceeding specifiedvalues. Values of rated normal currents should be selected from standard value of normalcurrents as per IS: 13118 which are 400A; 630A; 800A; 1250A; 1600A; 2000A; 2500A;3500A; 4000A; 5000A; 6300A (if required higher values can be selected). These ratingsare based on operation of the circuit breaker or switchgear assembly where the ambienttemperature (measured outside the enclosure) does not exceed 40 0 C and the altitude doesnot exceed 1000 m. Standard equipment may be operated at higher altitude by reducingthe continuous current rating in accordance with the following tables (based on AmericanPractice).4-8

Altitude in meters (approx.) Insulation level Rated continuouscurrent1000 m 1.00 1.001500 m 0.95 Refer to manufacturer3000 m 0.80 Refer to manufacturerThe rated continuous current is based on the maximum permissible total temperaturelimitations of the various parts of the circuit breaker when it is carrying rated current atan ambient temperature of 40 0 C.When the ambient temperature is greater than 40 0 C, the current must be reduced to lessthan rated continuous current to keep temperatures within allowable limits.5.5 Rated short-time withstand currentRated short-time withstand current is equal to the rated short circuit breaking current. Therated peak withstand current is equal to rated short circuit making current. Rated durationof short circuit should be as per IS 13118.5.6 Rated Short Circuit Breaking CurrentThe rated short circuit breaking current is the highest short circuit current which thecircuit breaker will be capable of breaking in a circuit having power frequency recoveryvoltage corresponding to rated voltage and transient recovery voltage (refer Para 3.5.7)equal to the rated value of the circuit breaker as specified in IS. For three pole circuitbreakers the AC component relates to three phase short circuit including short line fault.Rated short circuit breaking current is characterized by two values Fig. 1 (Annexure-1).i) rms value of AC component and is termed rated short circuit currentii) Percentage DC componentrms value of AC component of the rated short-circuit breaking current should be selectedfrom standard values given in IS 13118. Percentage DC component is dependant upon thetime from initiation of short circuit current and standard values are given in IS 13118.The standard values practice of breaking current or being 6.3 kA, 8 kA, 10 kA,12.5 kAkA,16 kA, 20 kA, 25 kA, 31.5 kA, 40 kA, 50 kA, 63 kA, 80 kA, 100 kA. The earlierpractice was to express the rated breaking capacity in MVA.MVA : 3 kV kAMVA : breaking capacity of circuit breakerkV : rated voltagekA ; rated breaking current4-9

5.7 Rated Short Circuit Making CurrentThe circuit breakers some times, close on to a existing fault. In such cases, the currentincreases to the maximum values as the peak of first current loop. The circuit breakermust be able to close without hesitation as contacts touch. The circuit breaker must beable to withstand the high mechanical forces during such closure.As per IS: 13118 rated short circuit making current should be at least 2.5 times the rmsvalue of the A. C. component of its rated short circuit breaking current.5.8 Transient Recovery Voltage and Restriking Voltage and First Pole to clear FactorThe instantaneous value of the recovery voltage at the Instant of arc extinction is calledthe active recovery voltage figure 3.3 (Annexure-3).5.9 Rated Transient Recovery Voltage (TRV) for terminal faults:- The rated transientrecovery voltage (TRV) for terminal faults relating to the rated short-circuit breakingcurrent is the reference voltage. This constitutes the limit of the prospectivetransient recovery voltage of circuits, which the circuit breaker will be capable ofbreaking in the event of a short circuit at its terminals.Wave form of transient recovery voltage varies according to the arrangement ofcircuit.actualStandard value of rated TRV for 3 pole circuit breaker as per in IS: 13118 for the circuitbreakers used in the outdoor substations under consideration are given below table 3.2.Table 3.2RatedvoltageFirst pole toclear factorTimeTRVPeakvalueRaterise(kV) (s) (kV) (kV/μs)3.3 1.5 40 6.2 0.157.2 1.5 52 12.3 0.2412 1.5 60 20.6 0.3436 1.5 108 62 0.5772.5 1.5 166 124 0.75145 1.3 77 215 2.0145 1.5 89 249 2.0245 1.3 130 364 2.0ofRemarks4-10

5.10 First Pole to Clear FactorsThe ratio of transient voltage that appears across the contacts at the instant of arcextinction to service frequency recovery voltage is called the restriking voltage first poleto clear factor.In three-phase circuits the restriking voltage refers to the voltage across the first pole toclear because this voltage is generally higher than that appears across each of the othertwo.5.11 Recovery voltageThe active recovery voltage depends upon the following factors.i) the power factorii) the armature reactioniii) the circuit conditionsEffect of power factor on recovery voltage:- With low p.f., for example when the ratio ofreactance to resistance of the circuit is high, the active recovery voltage will be high;whereas with high p.f. i.e. when the resistance is high as compared to the reactance, theactive recovery voltage would be correspondingly lower. This is illustrated in fig. 3.4. Infig. (a) the p.f. is zero and at the instant of arc extinction A the active recovery voltage isat peak value equaling to AB while in Fig (b) the p.f. is 0.5 and the active recoveryvoltage at the instant of arc extinction is CD which is less than AB.In general the active recovery voltage equals the maximum value of the system voltagemultiplied by sin θ where θ is the power factor angle.(a)Fig. 3.4 Effect of p.f. on recovery voltage(b)Effect of armature reaction on recovery voltage: The recovery voltage is less than thenormal system voltage because of demagnetizing effect of armature reaction. The faultcurrents flowing in the generator winding are of lagging power factor. They have ademagnetizing armature reaction. Hence they reduce the terminal voltage.4-11

Effect of circuit conditions on recovery voltage: Another factor that influences therecovery voltage is the circuit conditions e.g. three phase faults that are insulated fromearth, either at neutral or at fault, produce recovery voltages in the first phase to clearwhich are normally more severe than those produced by single phase or three phase faultson systems with earthed neutrals. This is explained below.Consider an unearthed three phase fault, on a three-phase system with the neutral earthed,being cleared by a circuit-breaker. When the breaker opens, it draws out an arc in eachphase. Assume that the arc in R phase is the first to be cleared (fig. 3.4 a). At the instantof this extinction the Y and B phases are still acting and have the same instantaneousphase voltage – 0.5v, where v is the phase to neutral value of the system. Now theresistance of the arcs in the Y and B phases at this instant are negligible, which meansthat the fault itself is momentarily at the potential – 0.5V. Since the fault is common to allthree phases the momentary value of the recovery voltage component in phase R must beV + 0.5V i.e., 1.5V (fig. 3.4 b). This means that the recovery voltage component in thefirst phase to clear on a 3-phase unearthed fault is 1.5 times that on an earth faultassuming an earthed neutral.5.12 Rated Characteristics for Short-Line faultsRated characteristics for short line faults are required for three generator pole circuitbreakers designed for direct connection to overhead transmission lines and having a ratedvoltage of 52 kV and above and a rated short-circuit breaking current exceeding 12.5 kA.These characteristics relate to the breaking of a single phase earth fault in a system withearthed neutral.The short line fault circuit is taken as composed of a supply circuit on the source side ofthe circuit breaker and a short line on its load side (fig. 3.5) with the following ratedcharacteristics.X sI LX LLCBZGVG = source of power X s = reactance on source sideV = voltage, phase-to-neutral value: V/3 X L = reactance on line sideI L = short-line fault current Z = surge impedance of the lineCB = circuit breaker L = length of line to fault4-12

Fig. 3.5 Short-line fault circuit5.13 Rated Supply Circuit characteristicsVoltage equal to the phase-to-earth voltage V/3 corresponding to the rated voltage V ofthe circuit breaker.Short circuit current, in case of terminal fault, equal to the rated short circuit breakingcurrent of the circuit breaker.Prospective transient recovery voltage, in case of terminal fault, given by the standardvalues in IS: 13118.5.14 Rated line CharacteristicsStandard values of rated surge impedance rated peak factor and time should be takenfrom IS: 13118.5.15 Rated out –of-phase breaking currentRated out –of-phase breaking current is required to be specified for generator breaker andas per IS 13118. This provision will provide with following.i) The power frequency recovery voltage should be 2.0/3 times the rated voltagefor earthed neutral systems and 2.5/3 times the rated voltage for other systems.ii) The rated out-of-phase breaking current should be 25% of the rated short-circuitbreaking current.iii) Transient recovery voltage as pre IS: 13118.5.16 Rated Line Charging Breaking CurrentRated Line Charging Breaking Current is required to be specified for feeder circuitbreakers. Standard value of line charging capacity of circuit breakers for thecommonly used voltages are as follows:Table 3.3Rated voltage (kV) Rated line-charging breaking current (A)12 2.536 4072.5 40145 50245 125This implies for overhead line length equal to 1.2 times the rated voltage of the circuitbreaker in kilovolts. In case line length is longer than it may be necessary to specify ahigher value of line charging capacity.4-13

5.17 Rated time quantities and operating sequenceRated values to be assigned to the following time quantities (fig. 3.6) will depend uponrated supply voltage reclosing operations etc.Opening time;Break time;Closing time;Open-close time;Reclosing time;Close-open time;Rated operating sequence is defined as follows:-o – t – co - t´ - coWhereo - opening operationco - closing operation followed immediately by an opening operation(without internal time delay)t,t´ - time interval between successive operation/ t& t˝ are in minutes orsecondsPower circuit breakers are rated for interrupting ability on the basis of a standardoperating duty.4-14

Fig. 3.6 Reclosing – Close Open Reclose TimeThe rated interrupting time of a circuit breaker is the maximum permissible intervalbetween the energization of the trip circuit at rated control voltage and rated mechanismpressure and the interruption of the current in the main circuit in all poles. It is used toclassify breakers of different speed.For line-to-ground faults, the interrupting time is estimated to exceed the ratedinterrupting time by 0.1 cycle. For asymmetrical faults, it is estimated that theinterrupting time may exceed rated time by an additional 0.2 cycle. Hence, for groundedasymmetrical faults, the last phase to clear is estimated to be 0.3 cycle slower than therated interrupting time. Additionally, rated interrupting time may be exceeded duringextreme cold weather or when the breaker has been closed for an extended period of time.Also, the breaker may be slower at the lower limits of control voltage and/or mechanismstored energy. These interrupting times are in the range of several milliseconds havesystem stability implications.The rated interrupting time may be exceeded for close-open operations. The increase ininterrupting time on close-open operation may be important from the standpoint ofpossible system instability. For low values of current, these considerations are lessimportant.6.0 Co-ordination of rated valuesCo-ordination of rated voltages, short circuit breaking current and rated normal currentfor guidance as per IS 13118 for rated voltage 33 kV and above as commonly used are asfollows. (Table 3.4).Table 3.4Ratedvoltage(kV)3.6 101625407.2 812.516256012 812.516Rated shortcircuitbreakingcurrent (kA)400400400 630630630400400 630630630 125012501250Rated normal current (A)12501250125012501600160016001600160012501250 160025002500 400025002500 40004-15

25405017.5 812.516254024 812.5162540400 630630630630 12501250125063063063016001600160025002500250012501250125012501250 1600 250012501250125016001600160025002500 400036 812.516254052 812.52072.5 12.5162031.5145 12.5202531.54050245 2031.54050630630630800 125012501250125012501250800800800160016001600160016001600160012501250125016001600160012501250 1600 2000125012501250 1600 20001250 1600 20002000200020002000200020002000200020003150315031503150315025002500 40007.0 Tests7.1 Type TestFollowing type tests as applicable in accordance with IS 13118 and IEC 56 arerecommended to determine adequacy of the circuit breaker.i) Dielectric tests (1.2/50 micro second lightning impulse withstand) and 1 minutepower frequency voltage with stand (dry & wet) test4-16

ii) Radio interface voltage (r.i.v.) testsiii) Temperature rise testsiv) Measurement of the resistance of the main circuitv) Short-time withstand current and peak withstand current testsvi) Mechanical and environmental testsvii) Miscellaneous provisions for making and breaking testsviii) Short circuit making and breaking testsix) Basic short circuit test dutiesx) Critical current testsxi) Single phase short making and breaking testsxii) Capacitive current switching testsxiii) Magnetizing and small inductive current switching tests7.2 Routine testsi) Power frequency voltage withstand dry tests on the main circuitii) Voltage withstand tests on control and auxiliary circuitsiii) Measurement of the resistance of the main circuitiv) Mechanical operating testsv) Design and visual checks8.0 Fault CalculationIn order to determine interrupting duty of circuit breakers it is necessity to determine faultcurrent at each circuit breaker location. Determination of maximum short circuit currentis the most important requirement of circuit breaker application. Rigorous determinationof short circuit current as a function of time involves complex calculations. Growth ininterconnecting power system as systems expand will increase short circuit duty.Accordingly some approximation and degree of judgment should be used.Different published methods of determining short circuit currents are available.Reference may be made to the following for details and selection.IEEE application guide for AC high voltage circuit breakers rated on a symmetricalcurrent basis IEEE std. C37010-1999.First step in carrying out short circuit studies is to determine system impedances withreference to the point of fault and current distribution for different kind of faults. Forpresent day large interconnected system this becomes time consuming laborious study.AC or DC network analyzer for calculation of fault current was previously frequentlyused for short circuit studies. In the analyzer all the essential elements of the powersystem were represented in a miniature replica and fault currents determined fromcalculation readings. In this method constant voltage behind sub transient/synchronousreactance were used as required. Accordingly real time studies to determine DC4-17

component of fault current in addition to AC component to determine the critical currentvalue existing at the time of primary arcing contact parting can not be calculated. Furthernetwork analyzers is a fixed place study. Now-a-day computer studies are carried out forsuch application.8.1 Staged Short Circuit TestsStaged fault short circuit tests adequately controlled on actual systems have been carriedout mostly on new equipments and systems to determine circuit breaker capability. Theseare accurate, costly and not always possible as selection of circuit breaker precedes powersystem installation.8.2 Circuit Breaker Rating for Short Circuit DutySteps involved in fixing circuit breaker short circuit rating are as follows:1. Determine normal current duty of the circuit breaker and select higher available ratedcurrent from standard values as per IEC/IS 13118 clause 4.4.2. Short Circuit studies be carried for following types of faults which are considered worstaccording to IEEE std. C37010-1999.a. Three phase ungrounded faultsb. Phase to ground faultMore severe of the short circuit faults be taken for selecting the short circuit rating.3. Determine short circuit currents for the required accuracy by a suitable method. For lineto ground faults, the required symmetrical interrupting capability is 15% higher.4. Circuit breaker having the rms value of the ac component of the short circuit higher thenshort circuit duty as calculated from table X A of IS 13118 (table 2.4 for commonly usedvoltages) be selected.5. A circuit breaker having adequate symmetrical interrupting capability will normally haveadequate capability to meet normal asymmetrical requirement. Maximum symmetricalinterrupting capacity of new circuit breaker is as follows:Rated short current x Rated maximum voltage operating voltage For higher X/R ratio or other special conditions refer the detail methods given in IEEEstd. C37010-1999.4-18

4.8.3 Simplified Methods for calculation short circuit currentSimplified methods calculating fault current to fix short circuit rating of circuitbreakers have been recommended by standardizing agencies over years as given below.Simplified conservative method of calculation were recommended by protective devicescommittee of AIEEE for general use of the industry. It was recommended that rigorousmethods be used when specifically required. The method is based upon determination ofan initial value of rms symmetrical current (ac component) to which followingmultiflying factor are applied for application purposes (table 3.5).Table 3.5General Generator current breakers/shortcircuit more than 500,000 kVAA. 8 cycle breaker 1.0 1.15 cycle breaker 1.1 1.23 cycle breakers 1.2 1.32 cycle breakers 1.4 1.5B. Mechanical stressesand mandatory duty 1.6Accordingly the steps involved for determining short circuit rating of circuit breakersare as follows as per the AIEE simplified method:1. Determine highest value of rms symmetrical current for any type of fault equalE/X 1 phase fault or 3E/2X 1 +X 0 for ground fault whichever is greater.2. Multiply this current by appropriate factors from table 3.5.3. The resulting interrupting and momentary current should be used to select theavailable normal rated circuit breaker.8.4 E/X Simplified method as per IEEE std. C37010-1999The AIEEE simplified method was further referred in IEEE std. 37010-1999. Thissimplified method is now recommended. For short circuit duty of circuit breakers unlesscomplex more accurate studies are warranted.In these studies generating station and transmission lines interconnected with the systemare represented in detail and the system is represented by equivalent system.For small hydro say 5 MW unit size connected to regional grids, the grid size can beassumed to be infinite size and calculation carried out accordingly.Calculation based on simplified method of calculating short circuit current for theapplication of recommended by IEEE std. 37-01-1999 are given as an example andcompared with calculation made according AIEE committee report.4-19

Steps involved in applying method are as follows:1. Calculate E/X 1 for 3 phase faults where X 1 = positive sequencePositive sequence X 1 is assumed equal to negative sequence X 2 and obtained fromdesign date or test.2. Calculate ground fault current 3E/2 X 1 + X 0X 0 = zero sequence reactance obtained from design date or testE = phase to neutral voltage3. If phase fault current does not exceed 80% of 100% symmetrical circuit breakerinterrupting capacity or 70% ground fault current then the circuit breaker selection isadequate.More exact procedure of calculation with adjustment for AC and DC decrements shouldbe used if the criteria is not fulfilled. In this method multiplying factors to initial value ofsymmetrical short circuit current are given in the factor of curves for 2 cycle to 8 cycle.8.5 Simplified MethodBreakers to include effect of ac and dc components for the following types of faults.i) Three phase faultii) Line-to-ground faultiii) Three phase to ground fault8.6 Characteristics specified for 72.5 kV Mukerian Stage II (2 x 10 MW) in Punjab and 36kV Sikasar HE Project (Chattisgarh 2 x 3.5 MW) are given in table 3.6.Table 3.6S. Description Mukerain Stage –II Sikasar HEPNo.HEP (2 x 10 MW) (2 x 3.5 MW)Type and circuit breaker SF 6 SF 6i. Number of poles 3 3ii. Class Outdoor Outdooriii. Rated frequency 50 c/s 50 c/siv. Rated voltage of breaker 72.5 kV 36 kVv. Rated insulation level :a). 1.2/ 50 micro sec. Lightning impulsewithstand voltage for complete C.B.i. to earth (with C.B. closed) 325 kVp 170 kVpii. across terminals of open circuit4-20

eakera. one minute dry and wet powerfrequency withstand voltagei. to earth (breaker closed)ii. across terminals of open circuitbreaker325 kVp140 kV rms140 kV rmsvi. Rated normal current at site conditions 1600 A 630 Avii. Rated line charging breaking current Not less than 10A. 100 Acorresponding switchingover voltage values online side & supply sideto be intimated by thetendererviii.Rated short circuit breaking currenta) rms value of AC component ( ratedshort circuit current )b) percentage D.C. component31. 5 KA at 72. 5 KVAs per IEC-56 (Latestedition)ix. First pole to clear factor 1.5x. Rated transient recovery voltage for As per IEC –56 (latestterminal faultsedition)a) corresponding to rated short circuitbreaking current (Symmetrical & -do-Asymmetrical)b) Corresponding to currents below therated & short circuit currentxi.Breaking capacity under short line faultconditions with rated supply side andline side characteristics-do-xii. Rated short circuit making current 78. 75 KA peakat 72. 5 kVxiii. Rated operating sequence Break dead Time-Make-Break (Minimum deadtime should not be morethan 15 cycles at 50 c/sinclusive of the time forauto reading relay. Unitof adjustment of deadtime shall be 15 to 35cycles.xiv. Total break time for any current upto Not more than 60 msrated breaking currentxv. Min. short time current rating and its 31.5 kA for 3 sec.durationxvi Minimum total creepage distance phase 1700 mm12.5 kA at 36 kV(750 MVA) as perIS: 131182.5 times the rmsvalue of Accomponent4-21

to earthxvii. Difference in the instant ofclosing/opening of contacts of all the 3polea) Openingb) Closingxviii Small inductive current interruptingcapacityxix.Whether breaker suitable for single poleoperation or gang operation of threepolesNot more than 3.33 ms.Not more than 5 ms.Any value upto 10Awith out switching overvoltage exceeding 2.0p.u.Gang operation of threepoles throughmechanical linkagesxx. Number of trip coils Two Nos. per breaker4-22

Example :- Circuit Breaker Rating for hydro electric power system shown in fig. 3.5 isproposed to be determined.100MVA;11kV WATER WHEELGENERATOR WITH DAMPER WINDINGAND HIGH RESISTANCE GROUNDINGGGGEN. CIRCUIT BREAKER100MVA;11/132kVTRANSFORMERX = 12%A132kV BUSM60km 132kV DOUBLE CIRCUIT LINESBX 1 = 8%OPENEQUIVALENT SYSTEMX 1 = 0.03X 1 = 8%132kV BUSFig. 3.54-23

Datai) Sub transient reactance of generator on own base - 24%ii) 100 MVA transformer impedance - 12%iii) 132 KVA transmission lines impedance - 8%iv) Power house is interconnected with grid at bus B.v) Equivalent system X 1 = 0.03 of X 0 = 0.041. Consider the fault at M which is fed by the grid system and the hydrogeneratorswhich is considered worst.2. Three phase fault calculations:- In the system shown in figure 4.5 per unitreactances are indicated.Apparent power is 100 MVA base and nominal voltage 132 kV is used as base at alllevels. 0.24 0.12 x0.08 0.032 2X 1 = 0.24 0.12 0.08 0.03 2 2 =0.18x0.110.18 0.11= 0.068X 1 =0.24X 1 =0.12X 1 =0.24X 1 =0.12X =0.08 1OPENX 1=0.03X 1 =0.08Base voltage 132 kVBase current 437The value of operating voltage corresponding to the highest operating voltage at faults is145 kV or 1.098/unit1.098Isc = x 437 = 7056A0.0683. Single line to ground fault current4-24

Zero sequence impedance would be that of transformers above as 12 % on 100 MVA.Impedance X of diagram is shown in figure.X 0 = 0.04 0.24 x0.120.04 0.240. 12=0.28x0.120.28 0.12= 0.0840.240.12X 0 = 0.040.24Since X 0 is greater than X 1 , single line to ground fault need not be considered.4. Selection of Breaker rating as per IEEE std. 37010-1999.Load current =437 x 2 = 874ARated normal continuous current =As per table Xc of IS 131181250ARated short breaking current = 20,000A at 145 kVThree phase short circuit current is 7056A is less than 80% of symmetrical interruptingcapability ( 16,000A) and large growth margin exists; the 132 kV breakers at step upsub station are rated 1250A; with rated short circuit breaking current of 20,000A.Rating as per earlier AIEEE Committee methodMaximum initial symmetrical current =7050ARating for 5 cycle breakers & factor = 1.1and mandatory duty factor = 1.6short circuit rating = 7050 x 1.1 x 1.6= 12419Aand for 3 cycle breakers = 134547.5According 20,000 kA; 1250 A breakers as O. K.4-25

Annexure-1Current waveAA = envelop of current waveBB´BX = normal zero lineCC´ = displacement of current wave zero line at any instantDD´ = r.m.s. value of the ac component of current at any instant, measuredfrom CC´EE´ = instant of contact separation (initiation of the arc)I AC = peak value of ac component of current at instant EE´I AC /2 =r.m.s. value of ac component of current at instant EE´I Dc = d.c. component of current at instant EE´IDCx100IAC= percentage value of the d.c. componentFig. 1 short-circuit making and breaking currents, and of percentage d. c. component(IS: 13118).4-26

Annexure-2GVrVrcLINEVr is recovery voltage stated in terms of voltage between phases at service frequencyVrc is recovery voltage component existing across the breaks of each pole.Fig. 2 Recovery voltage across poles4-27

Annexure-3Fig. 3.3 Arc extinction4-28