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) 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
Page 2 and 3: Section -1Guide for Design of Outdo
Page 4 and 5: The terms atmospheric overvoltages
Page 6 and 7: Table 2.31. Basic Data Example 1.Hi
Page 8 and 9: Spark gaps specified for fitting to
Page 10 and 11: 2.4.2 Working Safety Clearances125
Page 12 and 13: Highest System Voltage rating kV Sp
Page 14 and 15: 2.6 Insulator TypeTypes of insulato
Page 16 and 17: Bending .. .. 1265 kg/sq.cm.Shear .
Page 18 and 19: v) Installations in difficult site
Page 20 and 21: During maintenance, it is essential
Page 22 and 23: Areva Compact Air Insulated Substat
Page 24 and 25: The bus bar sizes should meet the e
Page 26 and 27: disconnect switch contacts in such
Page 28 and 29: (iv)Electro magnetic type interlock
Page 32 and 33: 4.6 TestsThe CTs shall be provided
Page 34 and 35: 10. 1.2/50 micro seconds impulse 32
Page 36: 7.0 Lightning ArrestorsLightning ar
Page 39 and 40: and this aspect has to be kept in m
Page 41 and 42: Fig. 12.1: Layout of 12 kV Agnoor S
Page 43 and 44: Fig. 12.4 Layout of 72.5 kV Switchy
Page 45 and 46: 464006800 7600 3000LA30002000 2000X
Page 47 and 48: Fig. 12.6:145 kV Outdoor Switchyard
Page 49 and 50: Fig. 12.7: 245 kV Outdoor Switchyar
Page 51 and 52: 2-20
Page 53 and 54: 3.0 Transformer RatingThe full load
Page 55 and 56: OFAFOil Immersed with forced oil ci
Page 57 and 58: The transformer may be operated wit
Page 59 and 60: 8.10 Parallel OperationThe transfor
Page 61 and 62: (a)(b)33 kV Transformers (MVA)1.001
Page 63 and 64: (e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p
Page 65 and 66: 18.0 Oil Containment and Fire Prote
Page 67 and 68: ) The limits of temperature rise me
Page 69 and 70: Example Calculation -1Generator Tra
Page 71 and 72: TRANSFORMER RATING AND CHARACTERSTI
Page 73 and 74: 3.0 Type of Circuit BreakerFollowin
Page 75 and 76: iii) during arcing of SF6 breaker,
Page 77 and 78: 3.4 Protection Classes for Switchge
Page 79 and 80: The rated voltage is expressed in k
Page 81 and 82:
5.7 Rated Short Circuit Making Curr
Page 83 and 84:
Effect of circuit conditions on rec
Page 85 and 86:
5.17 Rated time quantities and oper
Page 87 and 88:
25405017.5 812.516254024 812.516254
Page 89 and 90:
component of fault current in addit
Page 91 and 92:
Steps involved in applying method a
Page 93 and 94:
to earthxvii. Difference in the ins
Page 95 and 96:
Datai) Sub transient reactance of g
Page 97 and 98:
Annexure-1Current waveAA = envelop
Page 99:
Annexure-3Fig. 3.3 Arc extinction4-
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