instruction manual - BHEL Bhopal

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POWER POWER TRANSFORMERS TRANSFORMERS & 

& 

REACTORS 

REACTORS 

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INSTRUCTION INSTRUCTION MANUAL 

MANUAL 

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(JÉÆb÷ - 2) 

INSTRUCTIONS FOR INSTALLATION, 

COMMISSIONING, OPERATION AND 

MAINTENANCE 

(VOLUME - II) 

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INSTRUCTIONS FOR INSTALLATION, 

COMMISSIONING, OPERATION AND 

MAINTENANCE OF 

POWER TRANSFORMERS 

& 

REACTORS 

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JÉÆb÷ - II 

(EÖò±É JÉÆb÷ - 2) 

INSTALLATION, COMMISSIONING, 

OPERATION AND MAINTENANCE 

INSTRUCTIONS 

VOL-II of II 

Prepared By Samvet Lahari, Bhopal. Phone- 5277412

PREFACE 

BHEL Transformers and Reactors are designed, manufactured and 

tested with care. With proper attention during installation and use, the user 

should receive from it the maximum expected service. 

Before installing the transformers/ reactor read these instructions 

carefully. 

These instructions have been prepared to provide information on 

assembly, installation, commissioning and regular maintenance of the 

transformers/reactors and shall form part of Instruction Manual. 

These instructions do not intend to cover operation and maintenance of 

the transformer under abnormal conditions. 

Should further information be needed or any problem arises which is 

not covered by these instructions, please ask BHEL for further information. 

In operating the Transformer/Reactor, Care should be taken that loading 

limits as specified are strictly followed. For instructions regarding, general 

information on accessories such as OLTC, WTI, OTI, Buchholz relay, fan, 

pump, etc. Vol. I may please be referred. 


CONTENTS 

Page No. 

SECTION 1 GENERAL INFORMATION 

1.1 INTRODUCTION 11 

1.2 TRANSPORT OF TRANSFORMER / REACTOR 13 

SECTION 2 INSTALLATION 

2.1 RECEPTION AND ASSEMBLING OF TRANSFORMERS 

/ REACTORS DESPATCHED PARTLY DISMANTLED 

AND FILLED WITH NITROGEN 21 

2.2 STORING OF TRANSFORMER / REACTOR DESPATCHED 

FILLED WITH NITROGEN 33 

2.3 ASSEMBLING OF EXTERNAL PIPES 35 

2.4 INSULATING OIL, QUALITY AND TREATMENT 37 

2.5 OIL FILLING UNDER VACUUM 41 

2.6 OIL FILLING INSTRUCTIONS FOR CONSERVATORS 

WITH AIR CELL 47 

2.7 UNTANKING OF ACTIVE PART 53 

2.8 MOUNTING OF GASKETS 55 

2.9 WELDED COVER (IF APPLICABLE) 57 

2.10 EARTHING OF ACTIVE PART AND CORE 

INSULATION TEST 59 

2.11 TOUCH-UP PAINTING 69 

SECTION 3 COMMISSIONING 

3.1 TESTING AFTER ASSEMBLY OF THE TRANSFORMER 

/ REACTOR AT SITE 77 

3.2 COMMISSIONING CHECKS - TRANSFORMER 79 

3.3 COMMISSIONING CHECKS - REACTOR 93 

SECTION 4 MAINTENANCE AND OPERATION 

4.1 SUPERVISION OF TRANSFORMER / REACTOR 101 

4.2 SUPERVISION AND CONTROL OF OIL 109 

4.3 TROUBLE SHOUTING 109 

4.4 PARALLEL OPERATION 119 

4.5 DO'S AND DON'TS 137 

4.6 DISPOSAL 145 


¦ÉÉMÉ-1 

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SECTION-1 

GENERAL INFORMATION 


1.1 INTRODUCTION 

SECTION 1 

GENERAL INFORMATION 

1.1.1 POWER TRANSFORMERS 

Unlike shunt reactor and Neutral Grounding Reactors where standard 

specification is generally followed by utilities, requirements of power 

transformers vary depending upon the system design. Hence a tailor made 

product becomes the answer. Depending upon the requirements two winding/ 

three winding/ auto connection/ split winding arrangement with ON LOAD or 

OFF CIRCUIT tap changer, five limbs/ three limbs core construction, welded 

or bolted tank construction are adopted. 

Variation in capacity, impedance and transport profile result in different 

sizes of transformer. Refer VoI I for specific requirements of the equipment. 

1.1.2 SHUNT REACTORS 

Shunt Reactors are used in high voltage systems to compensate 

capacitive generation from long lightly loaded overhead lines or extended cable 

systems and also for control of dynamic over voltages. 

Gapped core construction is preferred for high system voltages over 

coreless construction due to the high energy density that can be achieved in 

gapped core construction. The core sections between consecutive air gaps 

are moulded in epoxy resin to prevent movement between individual laminations. 

The spacers forming the air gaps are blocks of ceramics with a high modules 

of elasticity and the whole stacking of core modules is cemented together 

during the assembly to form a solid column without possibility of rocking , or 

rubbing between individual parts. 

The core segments are of radial laminated configuration. The radial 

laminations prevent fringing flux from entering flat surfaces of core steel which 

would result in eddy current overheating and hot spots. 

Five limbed core construction is adopted to achieve high zero sequence 

impedance. In addition to the three gapped core limbs with windings, there 

are two continuous outer return limbs. The two unwound side limbs help in 

achieving zero sequence impedance approximately equal to the positive 

sequence impedance. Other construction type can be a three limbed 

construction. For single phase reactors e.g. 800 KV class middle leg wound 

with two return legs type of construction is adopted. 

Interleaved disc winding has been used for rated voltages 220 KV and 

above. This type of winding configuration provides better impulse voltage 

distribution. For lower voltage classes a continuous disc winding or a multi 

layer helical winding are used. The tank is rectangular in construction with flat 

cover welded to the tank rim at top. The associated cooling control equipment 

is housed in a tank mounted weather proof marshalling box. The reactors are 

equipped with all standard measuring and controlling fittings and accessories 

as described in Volume I. 


Reactors have been designed for resting on concrete foundation as shown 

in the foundation plan drawing. Rollers have been provided only for movement 

of the reactor at site. 

1.1.3 NEUTRAL GROUNDING REACTOR 

When single pole reclosing is used on lines, the Neutral Grounding Reactor 

is used. 

The Neutral Grounding Reactors are connected between neutral point 

of 400 kV/800 KV reactors and earth, where the neutral of shunt Reactor is 

suitable for 145 kV class insulation. 

The reactors are oil immersed type ONAN cooled (oil immersed with 

natural air cooling) with continuous rating of 10/15 Amps and a 10 sec. rating 

as indicated in the Rating Data sheet. As continuous losses of the reactors are 

negligible, reactor tank surface is adequate for dissipation of these losses. 

Hence radiators are not provided. 

Considering linear impedance characteristic requirement upto rated short 

time current a core less design with magnetic shielding of CRGO sheet has 

been made. The magnetic circuit, therefore consists of rectangular frame of 

CRGO steel lamination packets of adequate area. Area of magnetic circuit is 

selected such that saturation does not take place under short time current. A 

rigid clamping structure is provided for clamping of magnetic frame and a very 

low flux density is used to minimize the vibration and achieve linear impedance 

characteristic. 

The graded disc type concentric winding suitable for a rated voltage of 

145 kV and basic insulation level of 550 kVp has been provided without core 

inside. Winding is held in position by way of special insulation structure inside 

winding and finally kept under pressure between top and bottom yokes of 

magnetic frame. Return path of flux is provided by way of two return limbs. 

The line terminal is taken out through the tank cover via 145 kV, 800 

Amps OIP condenser bushing provided with suitable terminal connector. The 

neutral lead is taken out through the tank cover via a 36kV, 630 Amps porcelain 

bushing. 

The tank is of welded mild steel plate construction shot blasted on the 

inside and outside to remove scales before painting. The tank is painted on the 

inside with yellow paint and its outside surface is painted with two coats of 

primer paint and finishing coat, of light grey paint to shade 631 of IS:5. This 

is a standard painting scheme followed for all equipment . 

The fittings include a conservator with a magnetic oil gauge, pressure 

relief device, drain and filter valves, sampling valves, thermometer pockets, 

Buchholz relay, temperature indicator for oil and silicagel breather . 

These reactors are designed for mounting directly on plinth. 

1.2 TRANSPORT OF TRANSFORMER/REACTOR 

Power Transformers and Reactors hereinafter referred as "Transformers," 

depending upon the restriction imposed by transport weight and/or other 

considerations, have to be transported either filled with oil or nitrogen as per 

the description given below. The transformer is loaded on wagon as per loading 

gauge drawing prepared in each case. 


1.2.1 WITH OIL 

The Transformer is filled with oil such that about 10% of the tank 

volume is left clear for expansion to limit the excess internal air pressure to 

0.35 kg/cm 2 . Fittings dismantled before transport are packed in packing cases 

in line with shipping list. 

All openings resulting from removal of fittings for shipment are sealed 

with suitable gasketted blanking plates during transport. 

The balance oil quantity required for the complete filling of the reactor is 

supplied separately. 

A transformer despatched according to this method can be stored at 

site or elsewhere for one year provided, that all insulating materials are covered 

with oil and silicagel breather is mounted and quality of oil is maintained as per 

IS: 1866. Measures to be taken on reception at site are given clause 2.1 

1.2.2 WITH NITROGEN 

Large oil filled transformers/reactors are not normally oil filled during 

shipping due to weight limitations. To protect the active parts against moisture 

the transformer tank is filled with dry Nitrogen at a positive pressure of 0.175 

kg/cm 2 (2.5 psi) at BHEL works before despatch. The internal pressure at the 

time of shipment is painted on the tank. 

All openings resulting from the removal of fittings for shipment are 

sealed with suitable gasketted blanking plates during transport. 

A Transformer despatched according to this method should normally be 

stored upto 3 months after arrival at site or elsewhere. 

For control of gas pressure and maintaining the pressure during transport 

and possible storage before assembling, the transformer is equipped with an 

automatic device. This device is called two stage N 2 regulator and is kept in a 

steel box and mounted on the frame provided on tank side for keeping two 

back up Nitrogen cylinders. Nitrogen regulator is connected to two cylinders 

by copper tubes and to tank by a rubber hose as shown in the Fig. 1.1. 

N 2 regulator reduces cylinder high pressure of 120 to 140 kg/cm 2 to 

required low pressure of 0.175 kg/cm 2 in two stages. One gauge provided on 

the first stage indicates the cylinder pressure, whereas other gauge provided 

on second stage indicates the tank pressure. Regulator is fixed at factory. 

When tank pressure falls below 0.15 kg/cm 2 , due to leakage/fall of 

ambient temperature, regulator automatically feeds the gas from cylinders, to 

build up required pressure in the tank. 

The gas consumption during transport and possible storage is difficult 

to estimate as it depends on ambient temperature variation, possible leakages 

as well as on the duration of transport and storage. However, two back up 

cylinders are provided to meet the gas requirement. Normally one cylinder is 

kept open and the other shut. When the pressure of first cylinder falls to 10 

kg/cm 2 this should be considered as empty and immediately its valve should 

be closed and the valve of second cylinder should be opened. 

When cylinder is required to be removed for refilling, disconnect copper 

tubing from cylinder valve. 


Use only dry Nitrogen gas to IS:1747 with 50 ppm moisture and 1% 

oxygen by volume. 

If the storage time exceeds 3 months, the transformer is filled with oil 

according to directions given in clause 2.5. If for some reason oil filling is not 

possible, then nitrogen must be continuously maintained at a positive pressure. 

Measures to be taken on receipt at site are given in clause 2.1. 

1.2.3 INSTRUCTIONS FOR FILLING DRY PURE NITROGEN GAS WITH 

BACK UP CYLINDERS (AT WORKS) 

Lower the oil level to the minimum necessary to dismantle the items 

(such as bushing and turrets) which must be removed for shipping. 

Pull 500 mm mercury vacuum after blanking off all openings. 

Break the vacuum by admitting dry nitrogen through a convenient valve 

at the top of the tank and drain the oil completely. 

Continue to supply nitrogen until it maintains a steady pressure of 0.17 

± 0.02 kg/cm 2 above the atmosphere. 

Pressure of nitrogen gas shall be maintained at 0.17 ± 0.02 kg/cm 2 at 

same reference temperature. Pressure would be monitored by taking three 

readings within 24 hours to ensure that there is no leakage of gas. 

Shut off the gas supply valve and fit dry nitrogen back up cylinders 

through nitrogen regulator valve. 


¦ÉÉMÉ-2 

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SECTION-2 

INSTALLATION 


2. INSTALLATION 

SECTION 2 

2.1 RECEPTION AND ASSEMBLING OF TRANSFORMER/REACTOR 

DESPATCHED PARTLY DISMANTLED AND FILLED WITH NITROGEN. 

To ensure that a Transformer will function satisfactorily it is important 

that handling, lifting, storing and assembling are carried out with great care 

and cleanliness by experienced personnel who know the various working 

operations very well. 

This section gives instructions how handling, lifting, storing and 

assembling should be carried out. For large Transformers it is recommended 

that the work is done by BHEL or is under supervision by experts from BHEL. 

2.1.1 INSPECTION 

In connection with receiving and unloading at site, and at the final storing 

place before assembling, the transformers shall be inspected carefully. External 

visible damages as dents, paint damages etc. may imply that the transformer 

has been subjected to careless handling during transport and/or re-loadings, 

and a careful investigation is therefore justified. 

After the arrival of the material at receiving points, the customer should, 

in case of possible damage/loss of any component, make the necessary claims 

with the contractors representatives under intimation to supplier so that such 

claims can be registered with the transport agents. Before unloading, the 

condition of packing and of the visible parts should be checked and possible 

traces of leaks verified (condenser bushings). If necessary, appropriate 

statements and claims should be made. 

Drums containing oil which have been despatched separately should be 

examined carefully for leaks or any sign of tampering. All drums are despatched 

filled up to their capacity and any shortage should be reported. 

In order to protect the active part against moisture, the transformer 

tank is filled with nitrogen during transport at an over pressure of 0.17 kg/ 

sq.cm (2.5 psi) approximately at room temperature. 

Check immediately the gas pressure at the arrival. A positive pressure 

indicates that the tank and the transformer components respectively are tight, 

and that the active part including the insulation materials is dry. 

If there is no positive gas-pressure, transformer should be immediately 

filled with dry Nitrogen gas at a pressure of 0.17 kg/cm 2 (2.5 psi) without loss 

of time as per instructions given para 1.2.3 

Otherwise, it should be checked if the core isolation is satisfactory and 

that accessories packed separately have not been damaged during the transport. 

Instructions for checking of the core isolation are given in clause 2.10 

2.1.2 UNLOADING 

Typical unloading arrangement of the transformer is shown in fig 2.1. 

Whenever rollers/trolleys are supplied with transformer, movement of 

transformer at site is carried out by mounting these rollers/trolleys. For mounting 

of rollers refer roller mounting drawing included in Vol.I. 


Alternatively for movement of transformer from loading bay to actual 

site of the equipment, skidding on greased rails etc. can also be resorted to. 

2.1.3 STORING 

Dismantled equipment and components are packed to be protected 

against normal handling and transport stresses. The instructions for lifting 

given on the packages, must be complied with to avoid damages. 

Goods stored outdoors must not be placed directly on the ground, and 

should be covered carefully with tarpaulin or similar material. 

Oil drums should be stored in horizontal (lying) position with both the 

bungs also in horizontal position. 

2.1.4 LIFTING 

Lifting devices on the transformer tank are dimensioned for lifting of the 

complete transformer filled with oil. The positioning of the lifting devices, 

permissible lifting angles, minimum height to crane hook and transformer 

weight, appear from the OGA drawings. Check at lifting of complete transformer 

that the lifting wires/ropes are not in contact with bushing or other components 

on the cover. 

For lifting with hydraulic jacks, the transformer is provided with jacking 

pads dimensioned for lifting of complete transformer filled with oil. The position 

of the pads appear on the OGA drawings. If active part is to be lifted refer 

instructions given in clause 2.7. 

2.1.5 LOCATION AND SITE PREPARATION 

a) Reactor shall always be placed on concrete plinth without rollers as 

per foundation plan drawing whereas transformer may be even placed 

with rollers. Therefore it is very important to refer foundation drawing 

before placing the transformer/reactor on final location. 

b) Transformer/reactor should be placed on the foundation so that easy 

access is available all around and diagram plates, thermometers, valves, 

oil gauges, etc. can be easily reached or read. Adequate electrical 

clearances are also to be provided from various live points of the 

transformer to earthed parts. 

c) ONAN type transformers/reactors depend entirely upon the surrounding 

air for carrying away the heat generated due to losses. For indoor 

installation, therefore, the room must be well ventilated so that the 

heated air can escape readily and be replaced by cool air . Air inlets 

and outlets should be of sufficient size and number to pass adequate 

air to cool the transformer. The inlets should be as near the floor as 

possible and outlets as high as the building will allow. Where necessary, 

exhaust fans can be installed for the purpose. 

d) The transformers should always be separated from one another and 

from all walls and partitions to permit free circulation of air. In this 

connection reference is also drawn to IS: 10028 (Part II). 


e) Where rollers are not fitted, level concrete plinth with bearing plates 

of sufficient size and strength can be adopted for outdoor transformers. 

To prevent the formation of rust, it is essential to avoid presence of 

air and water in the space between the plinth and the base of the 

transformer by use of cretex or similar bituminous compound. 

f) Where rollers are fitted, suitable rails or tracks should be used and 

the wheels locked to prevent accidental movement of the transformer. 

Where walls are provided, it should be ensured that the transformer 

gets a good ventilation as mentioned above for indoor transformers. 

Provision should be made for the emergency drainage of the oil from 

the transformers (e.g. in case of fire in neighbouring apparatus or 

bushing or the transformer tank), by surrounding the transformer plinth 

with sump filled with small pebbles. 

2.1.6 INTERNAL INSPECTION AND CHECK POINTS FOR ASSEMBLING 

THE TRANSFORMER/REACTOR 

(a) Check-points before starting assembly: 

1. Conditions of leads. 

2. Bracing, clamping of leads. 

3. Connections. 

4. Tapchanger checks. 

5. General conditions of insulation. 

6. Core check that it has not moved in transit. 

7. Core-ground; this is checked with the megger after removing earth 

connection. 

8. CTs, including the secondary leads and their passage through metal 

parts. 

9. Check that shipping frame for bushings have been removed. 

10. Check that coil position has not moved in transit. 

11. Check for dirt, metal swarf, moisture. 

12. Check that the bushing leads set without being too close to ground 

or other points of different potential. 

(b) Check-points during Assembly 

By means of the Part list and the transformer/reactor OGA, the assembling 

of a fully completed transformer is carried out according to the following 

instructions. The following precautions are to be taken: 

(i) Fire-fighting equipment shall be available at the oil-treatment 

equipment as well as at work on and adjacent to the transformer . 

(ii) Welding work on or adjacent to the transformer shall be avoided, but 

if this is not possible, the work shall be supervised by fire-protection 

personnel. 

(iii) Smoking on or near the transformer shall not be allowed. 

(iv) Transformer tank, control cabinet etc. as well as assembling and oiltreatment 

equipment shall be connected with the permanent earthing 

system of the station. 


(v) Check that there is no overpressure in the transformer when blanking 

plates or connection lids are to be opened. 

(vi) All loose objects, tools, screws, nuts etc., shall be removed from the 

transformer cover before opening the connection and blanking lids. 

(vii) All loose objects (tools, pencils, spectacles etc.,) shall be removed 

from the boiler-suit pockets etc. before starting the work through 

man holes. 

(viii) Tools to be used inside the transformer/reactor -e.g. for tightening of 

screw-joints- shall be fastened to the wrist or another fixed point by 

means of cotton tape or string. 

(ix) Tools with loose sleeves and tools with catches must not be used at 

work inside the transformer. 

(x) Greatest possible cleanliness shall be observed at work inside the 

transformer/reactor, and at handling of parts to be mounted inside 

the transformer. 

(xi) Fibrous cleaning material should not be used as it can deterioate oil 

when mixed with it. 

(xii) All components despatched separately should be cleaned inside and 

outside before being fitted. 

(xiii) A transformer/reactor is best protected from damp hazard by 

circulating warm, dry, deaerated oil through it until it temperature is 

5° C to 10 0 C above ambient. This should be done before allowing 

external excess to the interior of the tank. The warm oil should be 

circulated all the time transformer is open to atmosphere, 

(xiv) Oil pump & all joints in the oil pipe work should be airtight to avoid 

entrance of air through leakage joints. 

(xv) The active part (core and winding) should be exposed to the 

surrounding air as short time as possible. Open therefore only one 

blanking plate or connection lid at a time for remounting of bushings, 

valves etc. 

(xvi) Objects which-despite all precaution are dropped inside transformer/ 

reactor, must absolutely be brought up from the equipment. 

(xvii) Check that the oxygen content inside the transformer tank is minimum 

20% if a person is to enter the tank. 

2.1.7 ASSEMBLY OF WHEELS 

Mounting of wheels under reactor/transformer is to be done as per roller 

arrangement drawing. The reactor however in service, is to be placed on 

plinth with anti-earthquake fastening without rollers. Transformer placement 

can be with or without rollers as per applicable OGA/foundation drawing. 

2.1.8 ASSEMBLY OF BUSHINGS 

In case the bushings are mounted on turrets on the transformer/reactor 

cover, they are either delivered mounted on their turret or -incase of large 

bushings dismantled from the turrets. 


The turrets are often individually adapted; check therefore that they are 

re-mounted in correct places, which appear from the OGA drawing and ,part 

list. 

Assembling of bushings is carried out according to the detailed 

information documents available. 

2.1.9 ASSEMBLY OF VALVES 

Valves which may affect the loading gauge, or will be subjected to 

damages, should be dismounted before delivery. Re-mount the valves -the 

positionings appear from the OGA drawing and part list. The gasketting surfaces 

shall be cleaned well and new gaskets fitted. Check that all valves are closed. 

2.1.10 ASSEMBLY OF COOLING DEVICES 

Valves which are not dismantled like shut-off valves for radiators, coolers 

and possible headers shall be provided with blanking plates during the transport. 

Remove the blanking plates when the assembling of the coolers is to be 

started. Check first that the valves are closed. 

Check that each radiator and possible header are assembled in the correct 

positions according to OGA drawing. In case of OFAF or OFWF cooler mounting 

shall be done as per the relevant leaflet given in Vol.I 

The shut-off valves against the transformer tank shall be closed until 

the oil-filling is started. 

2.1.11 ASSEMBLY OF OIL CONSERVATOR 

The conservator, which may be with or without aircell is assembled 

either on the transformer, or on a separate frame. Before the conservator is 

assembled it shall be checked that belonging equipment -e.g. Oil-level indicator 

-functions satisfactorily. 

The breather is connected to the oil conservator, and it is very important 

that joints and couplings in the pipe between breather and conservator are air 

tight. Refer Clause 2.6 for detailed instruction for oil filling. 

2.1.12 ASSEMBLY OF PIPE WORK 

Pipes with flanges for connection of conservator, radiators, as well as 

pipes for equalising of turrets etc. are mostly delivered completely ready for 

assembling according to OGA & part list. 

In certain cases -e.g. at a separately assembled oil conservator -certain 

fitting and welding of pipes and flanges on site is however required. Instruction 

for such assembly of external pipes is given in Clause 2.3 

2.1.13 FLANGES, BLANKING PLATES 

When re-mounting blanking plates, connection flanges etc., the gasketting 

surface shall be cleaned well and new gaskets fitted. 


2.1.14 ACCESSORIES 

Accessories like cooling fans, pumps, OLTC and components for 

supervision and control, oil-level indicator, flow indicators, gauges, Buchnolz 

relay, PRV, thermometers etc. are assembled according to leaflet/description 

valid for the components (refer Vol. I). 

2.1.15 CONTROL CABLING 

Re-assemble the control cables according to the drawing of wiring system 

and connect the cable ends to terminal blocks in instruments, terminal boxes, 

junction boxes and control cabinets according to valid connection diagram. 

2.1.16 GASKETS 

The sealing system normally used against oil and gas in BHEL's 

transformers and belonging components has rubber gaskets in grooves and 

nitrile rubber bonded cork gaskets at other places. Refer clause 2.8 for general 

information and assembly instructions for gasket mounting. 

2.1.17 OIL FILLING 

The completely mounted Transformer is oil-filled according to directions 

in Clause 2.5 & 2.6. The oil shall be treated according to Clause 2.4. 

The lower and upper shut-off valves for radiators/coolers and possible 

headers shall be open during evacuation and oil-filling. 

If coolers are placed on suspension beams, which are mounted at right 

angle to the tank, the suspension beams shall be supported against the ground 

during the evacuation. Also radiators mounted on the tank wall shall be 

supported in a similar way. 

The hose for filling of oil is connected to the bottom valve of the 

transformer which must not be opened until the hose has been deaerated and 

completely filled with oil. 

2.1.18 CLEANING & PAINTING 

The transformer and its equipment are cleaned carefully from dirt, oil, 

lubricating grease, and damaged surfaces are touch-up painted with the primer 

paints and finish paints delivered as per clause 2.11. 

2.1.19 EXCHANGE OF BUSHING 

When exchanging a damaged bushing or other component on the cover 

of a fully oil- filled transformer, we should proceed according to the following: 

Close the valve in the pipe between the transformer and the oil 

conservator. Pump oil from the transformer tank into the oil conservator so 

that the oil quantity which need be drained off to lower the oil-level enough 

below the transformer cover is pushed to the conservator. As the oil is being 

transferred from tank to conservator, fill up the tank with dry nitrogen via a 

suitable valve on the cover or in turrets. 

Exchange the faulty bushing and transfer then the oil from the conservator 

to the tank by opening the valve in the pipe between conservator and main 

tank. De-air the bushings and turrets. 


2.2 STORING OF TRANSFORMER/REACTOR DESPATCHED FILLED WITH 

NITROGEN 

2.2.1 STORING BEFORE COMPLETE ASSEMBLING 

The storing place should be easily accessible for inspection and 

maintenance of the transformer. The bedding for the equipment should be 

larger than its bottom surface and dimensioned for the load. The transformer 

is placed on boardings or beams so that good ventilation is obtained underneath 

the transformer bottom. 

Before storing, the transformer is inspected according to directions in 

Clause 2.1 "Reception and assembling of transformer/reactors despatched 

partly dismantled and filled with nitrogen". 

A transformer without remarks may be stored up to 3 months after 

arrival at the site without oil-filling. During the storing time, the inert gas 

(nitrogen) filling shall be maintained and pressure regulated, so that exposure 

of active part to atmosphere is avoided. 

If the storage time is judged to exceed 3 months, the transformer should 

be provided with oil conservator including oil-level indicator and breather, and 

oil-filled according to Clause 2.5. "Oil filling under vacuum". Certain valves 

must be re-mounted to enable the oil-filling being carried out. Furthermore, at 

the time at oil-filling of a completely assembled transformer, certain rules in 

Clause 2.5 & 2.6 must be complied with absolutely. 

Wherever possible it would be most desirable to keep the transformer 

energized even at a low voltage so that its temperature is higher than the 

ambient temperature. The low voltage may be applied under open circuit or 

short circuit conditions. It may be ascertained whether partial cooling is required 

in such a case. The oil quality should also be periodically monitored. If for 

some reason, oil filling cannot be carried out after a storage period longer than 

3 months, the nitrogen pressure shall be maintained and supervised carefully. 

If the storage time without oil exceeds 18 months BHEL should be consulted 

about measures to be taken. 

2.2.2 STORING OF COMPONENTS AND ACCESSORIES BEFORE 

COMPLETE ASSEMBLING 

Independent of the duration of the storing time, the directions below 

apply for dismounted components and accessories, as well as for material to 

be used in connection with the assembling work. 

(a) Storing indoors 

In such a room, the following articles should be stored: 

(i) Insulation material as paper, pressboard, bakelite, wood, cotton tape 

etc. 

(ii) Insulated details as paper-insulated conductors, pressboard insulated 

shielding bodies etc. 

(iii) Chemicals as solvents, glues, varnishes, hardeners etc. 

(iv) Breathers, drying agents. 

(v) Terminal boxes, connection boxes, control cabinets. 

(vi) Gas relays, oil-level indicators, thermometers, pressure valves etc. 


(vii) MBox, OLTC motor drive, Fans, Pumps, Instruments & fittings. 

(Heating elements provided shall be connected to supply) 

(b) Storing outdoors 

The below components may be stored outdoors. They should be placed 

above ground and covered with tarpaulin etc. 

(i) Oil-conservator with blanking plates for all openings. 

(ii) Radiators and coolers with blanking plates for all openings. 

(iii) Structures, A frames, Pipe supports, Supports for oil-conservators, 

radiators, control cabinets etc. 

2.2.3 SUPERVISION 

During the storing time, the storing place, transformer/reactor 

components and accessories are inspected regularly. Tap changer if provided 

should be operated at 6 monthly intervals. Two or three runs from one end of 

the range to the other and back are sufficient. Observations, readings, measures 

and dates should be noted and BHEL should be contacted for directions about 

possible measures. Check at even intervals -and further more at weather 

changes as rain, storm, frost or thawing the foundation material (boardings, 

beams etc.) and the condition of the ground. 

Inspect the transformer/reactor periodically with regard to possible 

external faults and/or rust-damage. 

Check also that screws and nuts in sealing joints (covers, lids etc.) are 

tightened. Check every second week that the connected-in heating elements 

in control cabinets function. 

If the Transformer is inert gas filled, one shall check every second week 

the overpressure or inert gas consumption. 

If the reactor is oil-filled, one shall take oil-samples after 6 months, and 

then min. once a year. The oil is investigated with regard to Electric Strength 

(BDV) and moisture content. 

Inspect periodically stored components and accessories with regard to 

possible , external damages and/or rust damages. If any touching up of paint 

is required it should be carried out as per clause 2.11. Check at the same time 

that insulation material and insulated details are not damaged or have become 

dirty. 

2.3 ASSEMBLING OF EXTERNAL PIPES 

2.3.1 This technical description shall be applied at assembling of 

transformers, when external pipes, for some reasons, are not fully completed 

at delivery but require some kind of machining or fitting at site. 

At the design occasion it is judged which details that can be made fully 

completed in the workshop and which details that must be fitted into position 

at site. Fully completed details are delivered with complete surface treatment. 

Details of the pipe work despatched can be seen at shipping list. 

2.3.2 BREATHER PIPE 

The pipe is normally made of 15 NB painted steel pipe. The pipe shall be 

cleaned carefully after machining. Supplementary surface treatment is, not 

required normally. 


It is very important that the pipe is tight. Possible leakage will cause 

moisture on the transformer with the service disturbances. 

Mounting of the breather pipe shall be as per OGA drawing. 

2.3.3. OIL CONSERVATOR 

For position of conservator along with necessary supports, refer OGA 

drawing. 

2.3.4 OIL CONSERVATOR PIPE 

The pipe has dimension 80 mm usually and has welded flanges. The 

pipe shall be painted internally with a yellow, oil-resistant paint, and externally 

with a paint specified on the drawing. The pipes shall always incline upwards 

minimum 2/3 degrees from the transformer. 

2.3.5 EQUALISING PIPES 

The pipe system in connection with the transformer cover is as a rule 

manufactured in workshop. The pipe between transformer and conservator 

may be pre-assembled. 

The pipe is provided with weld-flanges. No pipe part is allowed to be so 

long that internal inspection and cleaning is difficult to carry out. Besides 

straight extensions, each pipe part may contain one bend. 

The pipes shall always be placed with minimum 2/3 degrees inclining 

upwards from the transformer. Internal surface treatment is with yellow oil 

resistant paint. External surface treatment is made as specified on the drawing 

as per specification. Cleanliness is extremely important. 

2.3.6 COOLERS 

The positioning of the radiators/coolers is indicated on OGA drawing. 

These are positioned so that the highest point of the oil-pipe system is always 

positioned below the bottom level of the conservator. 

The pipe system is provided with filter valves at the start and finish to 

enable pumping the oil through the system. 

All places where air may be collected are provided with air release plugs/ 

valves. 

2.3.7 COOLER PIPES 

The pipe having dimensions as per OGA drawing and may be provided 

with weld- flanges, expansion joints. No pipe part is allowed to be so long that 

internal inspection and cleaning will be difficult. 

The pipes shall be placed so that air release plugs will be positioned at 

the highest point of the pipe part. The pipes shall be painted internally with a 

yellow, oil-resistant paint and externally according to the paint as per 

specification. 

2.4 INSULATING OIL, QUALITY AND TREATMENT 

2.4.1 QUALITY 

The oil to be filled in transformer generally complies with the 

requirements of the customer's specification. The characteristics of fresh oil 

as per IS: 335 "Specification for New Insulating Oil" are given below: 


CHARACTERISTICS AND PARAMETERS OF NEW 

INSULATING OIL 

Sr.No. Characteristics Unit Requirement 

1. Appearance The oil shall be clear, 

transparent & free from 

suspended matter or 

sediments. 

2. Density at 29.5°C gm/cc 0.89 

3. Kinematic Viscosity 

at 27° C (max.) Cst 27 

4. Interfacial tension 

at 27oC (Min) N/m 0.04 

5. Flash Point (Min) oC 140 

6. Pour Point (Max) oC -6 

7. Neutralisation value 

(Total acidity), (Max) mg/KOH/g 0.03 

8. Corrosive sulphur : Non-corrosive 

9. Electric strength 

(breakdown voltage) 

(Min) 

(a) New untreated oil, kV (rms) 30 

(If the above value is not 

obtained the oil shall be 

filtered in laboratory). 

(b) After filtration kV (rms) 60 

10. Dielectric dissipation 

factor (tan δ) at 90°C 

(Max) 0.002 

11. Water content (Max) ppm 50 

12. Specific resistance (Min) 

at900C ohm-cm 35x1012 at27OC ohm-cm 1500 x 1012 13. Oxidation stability 

a) Neutralisation value 

(Max) mg/KOH/g 0.4 

b) Total sludge after 

oxidation (Max) mg/KOH/g 0.10% 

14. Ageing characteristics 

after accelerated ageing 

(open beaker method 

with copper catalyst). 

a) Resistivity (Min) 

at 900C ohm-cm 0.2x 1012 at 270C ohm-cm 2.5 x 1012 b) Tan δ at 90°C (Max) 0.2 

c) Total acidity (Max) mg/KOH/g 0.05 

d) Sludge content by weight (Max) 0.05% 

15 Presence of oxidation Oil shall not contain 

inhibitor oxidation inhibitor 

Note: Methods of test shall be as per IS: 335 


2.4.2 SUPPLY AND TESTING 

The transformer oil is despatched directly to site from oil refinery. The 

oil is thoroughly inspected and tested at oil supplier's works in presence of 

BHEL and/or customer's representative or their authorised representatives. 

Test certificate giving values obtained for all characteristics is issued. 

2.4.3 HANDLING 

The oil drums should be carefully handled at site, details of which are 

given in Clause 9.1of IS:1866. 

2.4.4 RECONDITIONING 

Transformer oil is usually contaminated during handling, transport and 

storage due to ingress of moisture and solid impurities. Hence, oil shall be 

vacuum filtered separately at 50 0 C to 60 °C using a suitable filtration machine 

and a spare clean tank before filling in the transformer. Details of filtration are 

given in Clause 9.2 of IS:1866. Oil treatment shall be terminated when the 

following parameters are attained. 

TABLE 2.1 

kV Class of transformer Recommended Permissible limit 

Electric Strength Moisture Content 

(BDV) in kV (Min) ppm (Max) 

Upto 145 60 20 

245 65 15 

420 70 15 

Storage tank shall be as per typical arrangement shown in Fig 2.2 

Electric strength and moisture content shall be determined following 

the test procedure of IS: 335. 

2.5 OIL FILLING UNDER VACUUM 

2.5.1 APPLICATION 

Transformers and Reactors with vacuum-proof tanks shall be filled with 

oil according to this method. 

2.5.2 STORING TIME 

Generally it applies that Transformers and reactors which during transport 

are filled with dry nitrogen on arrival at the site shall be evacuated and oil 

filled. Before storing, the equipment shall be inspected according to the 

directions given in Clause 2.1 "Reception and assembling of transformer/reactor 

despatched partly dissembled and filled with nitrogen". During the storing 

time, overpressure shall be maintained, and nitrogen consumption checked 

according Clause 2.2. 


2.5.3 FLOW CHART FOR HANDLING 

In Annexure 2.1 is given a flow chart of handling procedures. The pressure 

should be measured at different times. The pressure is OK if it is maintained 

according to instructions given in Clause 2.2. Reference is also drawn to para 

9.0 of IS: 1866 regarding handling and filling of oil. 

2.5.4 UNITS 

Annexure 2.1 gives the relation between different units. 

2.5.5 EVACUATION 

The transformer tank (excluding cooler bank & conservator) shall be 

evacuated to a pressure of 1.00 torr. max. The pressure shall be maintained 

for the time given in Table 2.2. 

TABLE 2.2 

VACUUM TREATMENT 

System Voltage Evacuate and hold Standing time after oil 

vacuum for circulation before 

application of voltage 

kV hours hours 

Upto 145 12 12 

145 & upto 420 24 48 

Above 420 36 120 

The equipment required for vacuum treatment and oil-filling under vacuum 

should generally be as per Annexure 2.2. The transformer tank and electrical 

terminals shall be earthed for safety reasons. 

No electrical test on the Transformer is permitted during the evacuation. 

Fig. 2.3 shows a typical example of pipe work and valve positioning. 

Oil conservators are not evacuated. Valve 7 shall be closed. Valve 6 

shall be open unless a breather is fitted. 


2.5.6.1 OIL QUALITY 

The transformer/reactor shall be filled under vacuum with oil which has 

been purified and degassed according to Clause 2.4 

2.5.6.2 OIL FILLING IN MAIN TANK 

The oil shall be heated to a temperature of 50 0 - 60°C measured at the 

filter outlet valve. The pressure during the filling shall be max. 1 torr. During 

the oil-filling, a transparent plastic tube (5) can be used as an oil-level gauge. 

The tube which should be a wall thickness of 5-8 mm, may be connected to a 

top and a bottom valve on the transformer. 


Oil-filling of the tank is done through valve (12) at a low level on the 

transformer and at a maximum rate of 4-5 kL/hour. The pressure in the oil 

pipes shall be kept positive and shall be checked by a manometer 13 (if 

provided). When the oil-level has reached about 50 mm below the cover/the 

vacuum pump is stopped it is preferred that dry nitrogen is introduced in the 

tank by opening valve (8). Valve (7) is then opened .and valve (6) closed and 

the filling is continued in tank and conservator until the correct oil- level has 

been reached in the conservator. 

Conservator supplied with rubber air cell shall be filled according Clause 

2.6. For oil filling in diverter switch assembly of a OLTC refer leaflet (Vol.1). 

Separate vacuum-proof cooler system/radiators can be evacuated for 

about 1-2 hours and filled separately with purified and degassed oil. The oil is 

then circulated through the vacuum filter at least twice via drain valves as 

near as possible to the transformer. When the oil circulation has been completed 

the valves between coolers and main tank shall be opened. 

Care should be taken to keep all air release plugs and valves open to 

allow escape of trapped air during oil filling operation. These valves/plugs 

should be closed after completion of oil filling. 

2.5.7 HOT OIL CIRCULATION 

To facilitate oil-penetration and absorption of possible gas bubbles, the 

temperature of transformer shall-after completed oil-filling-be increased by 

circulating the oil through the vacuum filter and with circulation direction 

according to Fig. 2.4. 

The oil will be circulated through a vacuum filtration machine till the 

parameters are attained as per table 2.3 below. 

TABLE 2.3 

kV Class of 

Transformer 

Recommended Permissible Limits 

Electric Moisture Resistivity * Tan delta* 

Strength Content at 90oC at 90oC (BDV) in 

kV (min) 

ppm (Max) (Ohm-cm) 

Upto 72.5 40 25 1x1012 0.05 

145 50 20 1x1012 0.05 

245 and 420 60 15 1x1012 0.05 

* Subject to availability of testing facility at site. 

Method of test for Electric Strength and moisture content shall be as 

per IS:335. 

CAUTION: 

The temperature during oil circulation should not increase beyond 70 o C 

otherwise this may cause oxidation of oil. 


2.5.7.1 SAMPLING 

Oil sampling at various stages shall be done in accordance with IS: 

6855. when samples are taken from transformer tank, oil will be drawn from 

bottom of the tank. When it is desired to know gas content and composition 

of dissolved gases in transformer oil before commissioning for reference 

purposes (required for interpretation of Dissolved Gas Analysis results during 

service), sampling shall be done as per IS 9434. 

2.5.8 STANDING TIME 

Standing time is the time between 'finished oil circulation' and 

'energisation'. The time appears form table 2.2. 

2.5.9 FINAL OIL FILLING OF TRANSFORMERS/REACTORS DESPACTCHED 

OIL FILLED. 

Smaller transformers/reactors are often factory-filled with degassed oil 

up to about 10% below the cover and transported in this condition. The final 

filling up to the correct level in the conservator is made at site. 

2.5.9.1 TRANSFORMERS/REACTORS WITH SYSTEM VOL TAGE < 36 KV 

Previously degassed oil (e.g., at the factory) stored in tight drums may 

be used for the filling. Check the dielectric strength of the oil which should be 

as per Cluase 2.4.4. If accepted, the oil is pumped into the conservator and in 

this way fed into the tansformer. To prevent any free water in the drums from 

entering the reactor the opening of the suction tube must lie 0.1 m above the 

lowest point in the drum. A suitable valve on the cover and/or valves or upper 

tightening nuts at the bushings have to be opened for complete removal of air 

below the cover and in the bushings. When the oil is seeping out at these 

points, shut the valves and tighten the nuts at the bushings. 

2.5.9.2 TRANSFORMERS/REACTORS WITH SYSTEM VOLTAGE > 36 KV 

The filling is performed as described above, but at least a paper filter 

must be used for drying the oil. 

2.6 OIL FILLING INSTRUCTIONS FOR CONSERVATORS WITH AIR CELL 

2.6.1 INTRODUCTION 

In all transformers specially in high voltage class, maintenance of 

insulating oil notably its dielectric property forms one of the determining factors 

of reliability of equipment in service. Oxidation and contamination of 

transformers/reactor oil can be avoided in a simple and effective way by use 

of above oil preservation system. The complete system is known as 

"Conservator with Air Cell". 

In this oil preservation system a flexible air cell made of oil resistant 

nitrile rubber is placed inside the conservator and floats on the oil surface. The 

air cell inflates or deflates as the oil level in the conservator falls or rises 

depending on the ambient temperature and load on the reactor. The inside of 

the rubber bag (Air Cell) is put into communication with atmosphere by means 

of a silica gel breather which ensures dry atmosphere inside the air cell. In 

addition to the above this system provides following advantages. 

(i) It avoids saturation of absorbed gases. 


(ii) As no gas is used in this system, which operates at constant 

pressure, this formation of gas bubbles at low ambient temperature 

and load is eliminated. The system thus preserves the oil quality 

particularly its dielectric properties. 

The conservator with air cell is provided with a magnetic oil gauge 

having one electrical contact. The indication shown on the dial physically 

corresponds to the oil level in the conservator which is due to balance of 

static pressure between the oil of the conservator and the atmospheric air 

inside the air cell. This system is also provided sometimes with a set of pressure 

and vacuum valves. These valves operate to pass either oil or air in the event 

of over filling or under filling the conservator during installation. 

2.6.2 DESCRIPTION OF OPERATION 

Figure 2.5 indicates the general arrangement of oil preservation system, 

The oil connection between conservator and transformer tank is made through 

Buchbolz relay and valves are provided in between. 

The flexible air cell is connected to the top of the conservator through 

gasket ted joint. Under normal condition air cell is completely surrounded by 

oil and floats in the conservator. The air cell inflates/deflates as the oil volume 

changes. The float of the MOG which is always in contact with under side of 

the air cell moves up and down and indicates the oil level. The cell will sink in 

the remote event if it is damaged and MOG alarm will operate. The conservator 

then functions as a conventional conservator without effecting the performance 

of the transformers/reactor. 

2.6.3 INSTALLATION 

This system is shipped separately from main tank. The air cell is shipped 

fitted in the conservator. A low positive pressure of less than .07 kg/sq.cm (1 

psi) is maintained to avoid excessive movement of air cell in the conservator 

during transit. MOG is also shipped fitted on the conservator. Install the 

conservator and associate parts except breather as per transformers/reactor 

outline drawings and assemble oil pipe work. 


The following procedure is recommended. 

(i) Close and blank the valve (14) to isolate the conservator from main 

tank. Fill the oil in transformer under vacuum upto Buchholz level as 

per instructions given else where. ' 

(ii) After filling the oil in transformer and breaking the vacuum, oil can 

be filled in the conservator either through reactor or by drain valve 

(4). 

(iii) Remove the inspection cover (11) provided on the side of the 

conservator and check the air cell assuring that it is inflated. The air 

cell must remain in fully inflated condition during oil filling operation. 

If the air cell is found deflated fit the inspection cover and inflate the 

air cell with dry air/nitrogen gas to 0.035 kg/sq.cm max. through 

connection (8). A gauge may be put by removing plug (10). After 

filling close these connections. 


(iv) Remove air release plugs (5) (6) and (7) provided on top of the 

conservator. 

(v) Slowly pump the oil through the main reactor/drain valve (4). 

Temporarily stop filling operation when oil starts coming from opening 

(5) and (6) after ensuring that no air bubbles come out through these 

air release holes. Fit the two air release plugs. 

(vi) Continue oil filling till oil start coming from air release plug (7) stop 

oil after ensuring that no air bubbles come out. Fit the plug (7). 

(vii) Now release the air pressure held inside the air cell from point (8) 

and continue oil filling until magnetic oil gauge (3) indicates 35°Clevel. 

(viii) Remove oil pump and connect air cell to breather (9) from point (8). 

Also remove pressure gauge and put plug (10). 

(ix) The system is now properly filled. Air release plugs (5), (6) and (7) 

are fitted in normal operation. 

2.6.5 PRECAUTIONS 

(i) Oil filling in the conservator and also draining whenever required 

must be done very slowly. During oil filling, pressure in the air ce]l 

should not exceed 0.1kg/sq.cm (1.5 psi). 

(ii) If a pressure or vacuum is ever applied to the main reactor tank the 

conservator must be disconnected and a blanking plate fitted on 

shut off valve. 

(iii) Do not weld on conservator to avoid damage to the air cell. 

(iv) Once all the air has been driven out during oil filling in the conservator 

do not remove air release plugs (5), (6) and (7). Otherwise air will be 

sucked inside the conservator. 

2.6.6 MAINTENANCE 

Little maintenance work will normally be required except routine visual 

inspection. However, it is desirable to check the breather opening to ensure it 

is not blocked. Further silicagel should be regenerated/replaced when its colour 

changes from blue to pink. 

2.6.7 AIR CELL 

Air cell is made from Nylon fabric coated with Nitrile rubber, In the 

event it becomes necessary to replace or test the air cell for leaks the following 

method is recommended. 

(i) De-energise the transformers/reactor. 

(ii) Isolate the conservator by closing the valves (14). 

(iii) Drain the oil from the conservator through the valve (4) by removing 

air release plugs (5) and (6). 


(iv) Remove inspection cover (11) if necessary, Install the pressure gauge 

on point (10). 

(v) Pressurise the air cell (2) by dry air/nitrogen to a max. pressure of 

1.5 psi and seal. Check the pressure for 6 hours. 

(vi) If leaks are found, air cell to be repaired by patching or replaced by a 

new air cell. 

(vii) For taking out the air cell from conservator, remove the air cell flange 

and loops from hooks provided on inside of the conservator top. 

Collapse the air cell slowly and fold, remove it from conservator very 

carefully. 

(viii) In the event air cell is not available immediately, conservator may be 

used as a conventional conservator. 

2.6.8 OIL PRESSURE TEST 

Oil pressure test on fully erected Transformer/Reactor to be conducted 

as per annexure 2.3 before hot oil circulation. 

2.7 UNTANKING OF ACTIVE PART 

2.7.1 GENERAL 

If for some reason it becomes necessary to untank the active part (core 

and windings) of a large reactor/transformer, it ought to be done under 

supervision by BHEL erectors. Universal instruction for the untanking procedure 

cannot be given, as the design practices of large equipment vary. The following 

general directions are, however, applicable in most cases. 

The untanking must be done indoors. If there is no suitable hall available, 

lifting can be done in the temporarily arranged room. e.g. a tent. The oil & the 

internal parts of the transformer must not in any case be exposed to rain or 

humidity. 

In case of bell shaped transformer, only bell cover is lifted for access to 

active part. 

2.7.2 OIL DRAINAGE 

Drain off the oil from the transformers/reactor either partly or completely. 

2.7.3 DISCONNECTING 

Disconnect leads to bushings, current transformers, winding temperature 

devices and earthing leads between active part and cover or tank side. 

Disconnections mentioned above can usually be made through handholes 

in cover or tank side. 

2.7.4 REMOVAL 

Remove all large bushings, cover mounted conservator and lightning 

arrestors (if provided). Break all connections between cover and tank-piping to 

oil conservator and cooler thermometers etc. 

Remove the cover bolts. If the cover is welded to the tank flange, free 

the cover according to directions given in Clause 2.9. 


2.7.4.1 COVER 

When lifting the cover use the cover-lifting eyes which are designed for 

a minimum angle of 60 degrees between sling branches and the horizontal 

plane. The length of the sling should therefore be sufficient for at least this 

angle. 

Place the cover on suitable wooden supports and in such a way that 

thermometer pockets, small bushing etc. underneath the cover are not damaged. 

No grinding or cleaning up of the tank flange is to be carried out before 

the active part is lifted out of the tank. 

2.7.4.2 UNTANKING 

Loosen locking devices, if any, between top core clamps and tank side. 

Lift the active part by means of lifting eyes or lifting lugs provided on 

the top core clamps. The untanking height is shown on the outline drawing. 

To avoid damages on the active part it is important that it is centered carefully 

in the tank during lifting procedure. 

After untanking, place the active part on a horizontal foundation. 

2.7.5 RETANKING 

Grinding or/and cleaning up of the tank flange has to be done before the 

active part is lowered into the tank. Check that the tank inside is free from 

contaminations. 

Retanking is then done in the reverse order that is outlined above. Note 

that guiding pins or blocks are welded to the tank bottom to prevent the 

active part from moving in the tank. When lowering the active part, check that 

it fits exactly the guiding pins or blocks. 

2.7.6 REASSEMBLING 

Wipe the underside of the cover free from any dirt or foreign matter 

before lifting it into correct position above the reactor tank. Lower the cover 

the last few inches exactly into position without sliding on the gaskets. 

Reassemble bolts nuts of supporting devices. Weld the cover when the 

welded construction is used. Reassemble Bushings, conservator etc., and 

reconnect pipings, leads to bushings, current transformers, etc. Reassemble 

inspection covers. 

After the transformer is completely assembled, it may be necessary to 

dry it before oil fining- see clause 2.5. 

2.8 MOUNTING OF GASKETS 

2.8.1 The gaskets have a circular/flat cross-section and are made of oil 

and heat- resistant synthetic nitrile rubber/nitrile rubber bonded cork. For small 

gaskets, O-rings are used with diameter 3,5.0 or 8.0 mm, while round rubber 

cords with diameters 8,12 or 19 mm are used for large gaskets. 


The groove is normally milled or turned, but for large flanges the groove 

is created by means of steel bars welded on top of the flange. The width of 

the groove is slightly smaller than the cross-section diameter of the gasket in 

order to keep the gasket in position during the assembling work. 

Opened sealing joints may be sealed again using the same gasket provided 

that the same has not been damaged. If the gasket has become hard or got 

small cracks on the surface, it is recommended to change the gasket by a new 

one. 

When handling and lifting flange, cover etc. with gasket grooves, care 

should be taken when using tools and lifting devices to avoid that the grooves 

getting damaged or deformed. 

Before assembling of groove gaskets, it is checked that the grooves and 

contact-surface in the joint are free from foreign particles and that the paint is 

free from thick coatings, trickles and drops. 

When assembling the rubber cord in the groove, the cord length shall be 

continuous . The gasket is given a small surplus length to compensate for 

shrinkage. 

The gasket is pressed down into the groove without stretching or 

slackening. 

To prevent the gasket from falling from the groove on vertical surfaces, 

the gasket may be spot-glued to the bottom of the groove. 

The screws in the sealing joint shall be tightened so that an even pressure 

is obtained on the gasket. This is obtained preferably by means of a moment 

spanner. Rubber gaskets in grooves need not normally be re-tightened. 

Above description is followed generally for turrets, inspection cover 

etc. For main tank rim joint, LV turrets of Generator Transformers where metallic 

stops are provided, nitrile rubber bonded cork is used, for which following 

instructions shall apply. 

2.8.2 (i) Gaskets when supplied loose, have no bolt holes in them. They 

are usually cut to the size and shape required, although they may 

be supplied as straight, angled pieces from which complete gaskets 

can be built up. 

(ii) Scarfed joints should be used. A 40 mm scarf in 5 mm thick 

material is recommended. Joints should be located away from 

comers and bolt holes, and should be well bonded, smooth and 

free from local thickening. Neoprene solution is used as an adhesive 

for joints. 

(iii) Gaskets are best stored in hermetically sealed containers in a cool 

place. They must be protected from damp, oil and grease- 

(iv) To make a gasket joint, first clean the metal surfaces ensuring 

that they are free from oil, rust, scale etc. Using one of the flanges 

as a template, punch the necessary bolt holes. Insert the bolts 

and tighten the bolts sequentially, a little every time so that uniform 

pressure is exerted on the gasket until the gasket is compressed 

to about 2/3 of its original thickness. Joints should not be subjected 

to pressure until tightening is complete. If care is taken in making 

joints, and in handling the gasket, it is possible to break and remake 

a joint several times, using the same gasket. 


(v) For making leak proof and good gasket joints, it is necessary that 

uniform pressure is achieved all over the gasket after matching 

rims/ flanges have been clamped with bolts/studs. 

Following instructions be followed for proper tightening of bolts/studs. 

a) Tighten lightly the bolts/studs diagonally in the sequence as shown 

in the fig. 2.6. 

b) Tighten again bolts/studs in the same sequence with the torque 

given below : 

Bolt/Stud Max. Torque 

Size (Kg-M) 

M 10 1 

M 12 2 

M 16 6 

M 20 12 

M 24 20 

M 30 30 

In case of metallic stoppers tighten until metal to metal contact is achieved. 

c) Do not overtighten, otherwise gasket will get crushed. 

2.9 WELDED COVER (IF APPLICABLE) 

In order to obtain a good sealing between the transformer tank and the 

cover a welded joint is recommended. The welding is performed in a certain 

way to permit opening and new welding repeated a number of times. The 

chiselling up and re-welding will take about the same time as dismantling and 

reassembling of bolted cover . 

If the welded cover for any reason has to be removed proceed as follows: 

2.9.1 OPENING THE COVER 

When opening the cover the welding joint should be removed by a suitable 

grinding wheel. The cover should be clamped to the frame by means of Gclamps 

to prevent iron chips from penetrating into the tank. Any parts of the 

weld which may possibly remain on the tank flange should be removed by a 

chisel to enable a good result of the re-welding. 

2.9.2 REASSEMBLY 

When fitting the cover again cork-rubber gaskets, 25 x 5 or as specified 

are fitted on the tank flange, see fig. 2.7. 

The gaskets are kept in the correct position with glue base on rubber 

base. The cover should be clamped to the tank flange by means of G-clamps 

evenly distributed along the flange with about 600 mm spacing. Tack welding 

is carried out with about 100 mm spacing. An extra G-clamp is used during 

the tack welding and is moved along the flange during the progress of the 

work. The continuous weld is then applied. Finally the weld should be cleaned 

and painted. 

WARNING 

When welding, a fire-extinguishing equipment should be available, and 

the work supervised by fire-protection personnel. 


2.10 EARTHING OF ACTIVE PART AND CORE INSULATION TEST 

2.10.1 REACTORS 

The ground-connection terminals for the reactor active part are located 

in a box at the tank end, close to the bottom. Please see fig. 2.8. 

The terminal are protected by a cover. The cover can be removed with 

the tank oil- filled. 

The terminal box contains a terminal block with three terminals. 

-The terminal marked CL is connected to the core laminations. 

-The terminal marked CC is connected to the core clamps. 

-The terminal marked G is connected to ground (the tank). 

For the core-insulation test, remove the cover. Disconnect the closing 

link that connects the two terminals CL-G. Use perferably 3500 V direct voltage 

between CL and CC + G. The tank shall be grounded during the test. The 

insulation value after 1 min. test- time shall be minimum 1000 kohms. There 

is no general requirement on the insulation level CC-G . 

2.10.2 TRANSFORMERS 

For checking core insulation incase of transformers refer Fig. 2.9 for 

connection details. 

2.11 TOUCH-UP PAINTING 

2.11.1 PURPOSE 

A basic principle at touch-up painting should be to restore a damaged 

paint coat on a surface to the same quality and finish as of the surrounding 

surface. The touch-up painting should be limited to a surface as small as 

possible. 

2.11.2 CLEANING 

Both damaged and surrounding surface should be cleaned so that all 

grease, dust and other impurities will be removed. 

2.11.3 GRINDING OF DAMAGES 

Large damages and defects are ground by means of a coarse abrasive 

paper, e.g. No.100. The surface is then ground with a finer paper in connection 

with the damage, e.g. 150 or 180. Damages that are limited to the paint coat 

only, should be ground off completely at which glazing can be avoided. 

2.11.4 PRIMER PAINTING 

Damages on the primer paint and grinding down to the steel surface 

should be painted with two coats of anti-corrosive priming paint. 


2.11.5 FINISHING PAINT 

Two coats of finishing paint should be applied as per specification which 

is generally light grey shade No.631 of lS: 5. Please refer Table 2.4 for suppliers 

reference. 

TABLE 2.4 

SI Suppliers Suppliers Reference 

No. Name Anti corrosive High Quality full Oil resistant air 

priming paint for gloss outdoor drying synthetic 

outside painting finishing paint enamel 

1. Addison IS 2907 Shade 631 of IS 5 jasmine yellow 

shade no.397 

of IS 5 for 

inside painting 

2. Asian 32/F 3257 Apcolite synthetic - 

3. Berger BPL Red oxide Luxol 3H1 Glass - 

Zinc Chromate synthetic enamel - 

4. Alkali 32-781 Dulux synthetic - 

Chemicals enamel 

5. Garware PR-4460 - - 

6. Shalimar Sample No. 5999/1 Superlac synthetic 7377/1 

7. Goodlass 205/023 500 Series 548/101 


ANNEXURE 2.1 

FLOW CHART FOR HANDLING 

ARRIVAL AT 

SITE 

MEASUREMENT OF 

PRESSURE 

ASSEMBLY 

EVACUATION AND OIL 

FILLING 

OIL-CIRCULATION 

THROUGH FILTER 

STANDING TIME 

ACCORDING TO TABLE 2.2. 

VOLTAGE APPLICATION 

Relationship between different units 

1 bar = 10 5 Pa = 750 Torr = 14.5 psi = 1.02 kg/sq.cm 

1 Torr = 1.33 mbar = 0.133 kPa 

1 kPa = 10 3 Pa = 10 mbar = 7.501 Torr 

1 MPa = 10 6 Pa 

Force Volume 

1 kp = 9.807 N 11itre = 0.26 US gallons 

1 US gallon = 3.781itres 

11itre = 0.22 Imp gallons 

1 Imp = 4.551itres gallon 

Temperature 

C = 5 x (F-32)/9 

F = 9 x (C+32)/5 


ANNEXURE 2.2 

EQUIPMENT FOR OIL-FILLING UNDER VACUUM 

(i) High-vacuum 2 stage oil filtration plant provided with thermostatcontrolled 

oil heaters and vacuum-proof hoses with independant 

vacuum pumping system for tank evacuation. Capacity: 6000 lph. 

(ii) Oil-storage tanks provided with silica-gel breathers and inlet/outlet 

valves for oil circulation. Recommended capacity 20 kL -30 kL (Clause 

2.4) 

(iii) Vacuum gauges provided in filtration plant. 

(iv) Equipment for measurement of electric strength (BDV) of oil- 100 kV 

set. 

(v) Equipment for moisture content of oil. 

(vi) Equipment for measurement of Resistivity and Tan delta at 90o C. 

(vii) Oil-sampling cans or bottles. 

(viii) Transparent vacuum-proof tubes for checking of oil-level during oil 

filling. 

(ix) Valves, fittings, gaskets etc. 

(x) Dry nitrogen cylinders. 


Prepared By Samvet Lahari, Bhopal. Phone- 5277412 

Fig. 2.1 Typical Unloading Arrangement of the Transformer 

ÊSÉjÉ 2.1 ]ÅõÉÆºÉ¡òÉ¨ÉÇ®ú EòÉä =iÉÉ®úxÉä EòÒ ´ªÉ´ÉºlÉÉ EòÉ xÉ¨ÉÚxÉÉ 

(1) HAULAGE HOLE- JÉÓSÉxÉä Eäò Ê±ÉªÉä ÊUôpù 

(2) JACKING LUG- VÉäËEòMÉ ±ÉMÉ 

(3) JACK- VÉäEò 

(4) RLY. WELL WAGON- ®äú±É´Éä ´Éä±É ´ÉäMÉxÉ 

(5) 90 LB. RAIL - 90 B±É ¤ÉÒ ®äú±É 

(6) SUPPORTS TO PREVENT SLIPPING- Ê¡òºÉ±ÉxÉ ºÉä ¤ÉSÉxÉä Eäò Ê±ÉªÉä +ÉvÉÉ®ú 

(7) GROUND LEVEL- ¦ÉÚÊ¨É ºÉiÉ½þ 

(8) THESE SLEEPERS WILL BE PUT AT SITE AT THE TIME OF UNLOADING-

Prepared By Samvet Lahari, Bhopal. Phone- 5277412 

1. OIL SAMPLING VALVE 15 mm 2 Nos. iÉä±É ºÉä¨{ÉË±ÉMÉ ´ÉÉ±´É 15 Ê¨É¨ÉÒ. 2 xÉÆ. 

2. FILLTER VALVE 50 mm 2 Nos. Ê¡ò±]õ®ú ´ÉÉ±´É 50 Ê¨É¨ÉÒ. 2 xÉÆ. 

3. DRAIN VALVE 80 mm 1 No. bÅä÷xÉ ´ÉÉ±´É 80 Ê¨É¨ÉÒ. 1 xÉÆ. 

4. DRAIN PLUG 15 mm 1 No. bÅä÷xÉ {±ÉMÉ 15 Ê¨É¨ÉÒ. 1 xÉÆ. 

5. MAN HOLE 1 No. ¨ÉèxÉ ½þÉä±É 1 xÉÆ. 

6. VACUUM APPLICATION VALVE 25 mm 1 No. ´ÉäCªÉÚ¨É ´ÉÉ±´É 25 Ê¨É¨ÉÒ. 1 xÉÆ. 

7. BUSHING MOUNTING HOLES WITH BLANKING FLANGE 3 Nos. 

¤ÉÖËÉMÉ ±ÉMÉÉxÉä Eäò Ê±ÉªÉä ÊUôpù ¤±ÉåËEòMÉ }±ÉéVÉ ºÉÊ½þiÉ 3 xÉÆ. 

Fig. 2.2 Typical Arrangement of oil Storage tank 

ÊSÉjÉ 2.2 iÉä±É ºÉÆOÉ½þ Eò®úxÉä EòÒ ]ÆõEòÒ EòÉ xÉ¨ÉÚxÉÉ

1. Oil-filtering plant with heater 

2. Oil-storage tank 

3. High-vacuum pump 

4. Conservator 

5. Oil-level indicator 

6. Valve 

7. Valve 

8. Valve 

9. Valve 

10. Valve 

11. Dry-N2 container 

12. Valve 

13. Manometer 

Fig. 2.3 

ÊSÉjÉ 2.3 

(1) ½þÒ]õ®ú Eäò ºÉÉlÉ iÉä±É Ê¡ò±]õË®úMÉ {±ÉÉÆ]õ 

(2) iÉä±É ºÉÆOÉ½þ Eò®úxÉä Eäò Ê±ÉªÉä ]åõEò 

(3) =SSÉ ÊxÉ´ÉÉÇiÉ {É¨{É 

(4) EòxVÉ®ú´Éä]õ®ú 

(5) iÉä±É-ºiÉ®ú ºÉÚSÉEò 

(6) ´ÉÉ±´É 

(7) ´ÉÉ±´É 

(8) ´ÉÉ±´É 

(9) ´ÉÉ±´É 

(10) ´ÉÉ±´É 

(11) ÉÖ¹Eò xÉÉ

TRANSFORMER 

REACTOR 

]ÅõÉÆºÉ¡òÉ¨ÉÇ®ú/Ê®úBC]õ®ú ]ÆõEòÒ 

Fig. 2.4 

ÊSÉjÉ 2.4 

VACUUM 

FILTER 

´ÉèCªÉÚ¨É Ê¡ò±]õ®ú 


OIL PRESERVATION SYSTEM WITH AIR CELL 

+É

Fig. 2.6 

ÊSÉjÉ 2.6 

Fig. 2.7 

ÊSÉjÉ 2.7 


Fig. 2.8 

ÊSÉjÉ 2.8 


NOTE: 

Fig. 2.9 

ÊSÉjÉ 2.9 

1. WHILE DISCONNECTING/EARTHING CONNECTION BETWEEN CORE/YOKE 

CLAMP & TANK, REMOVE THE LINK. 

1. EòÉä®ú/ªÉÉäEò C±Éä¨{É B´ÉÆ ]ÆõEòÒ EòÉ ¦ÉÚºÉÆ{ÉEÇò Ê´ÉªÉÖHò Eò®úxÉä Eäò Ê±ÉB ªÉ½þ Eòb÷Ò ½þ]õÉ nåù* 

2. WHILE PUTTING IN SERVICE PLACE LINKS IN POSITION. 

2. {ÉÊ®úSÉÉ±ÉxÉ Eäò ºÉ¨ÉªÉ ªÉ½þ Eòb÷Ò VÉMÉ½þ {É®ú ±ÉMÉÉ nåù* 

Earthing Connections from core and top yoke clamp. 

EòÉä®ú B´ÉÆ ]õÉì{É ªÉÉäEò C±Éä¨{É ºÉä ¦ÉÚºÉÆ{ÉEÇò ªÉÖÊHò 


ANNEXURE 2.3 

PRESSURE TESTING OF TRANSFORMER / REACTORS AT SITE 

1. After completion of oil filling, lock PRV and pressurise the air cell by 

filling nitrogen/ dry air to 5.0 p.s.i. (0.35 Kg/cm.sq.). 

2. Maintain above pressure for at least 12 hours. 

3. Inspect all joints for leakage, if any. 

4. Record pressure testing as given below. 

5. After pressure testing unlock PRV and release the nitrogen/ dry air 

pressure of air cell. 

NOTE: In case of conventional oil preservation system (without air cell) 

pressurise conservator by filling nitrogen/dry air in conservator . 

RECORD OF PRESSURE TESTING AT SITE 

DATE TIME 

1. Date & Time of testing (a) Started on ......... ......... 

(b) Finished on ......... ......... 

2. Initial pressure .................................... 

3. Pressure after 12 hours .................................... 

4. Leakages observed if any YES /NO 

location of leakages .................................... 

5. If leakages are observed. Same to be arrested and pressure testing 

is to be repeated. 


¦ÉÉMÉ-3 

Eò¨ÉÒÉËxÉMÉ 

SECTION-3 

COMMISSIONING 


3 COMMISSIONING 

SECTION 3 

3.1 TESTING AFTER ASSEMBLY OF THE TRANSFORMER/ REACTOR 

AT SITE. 

After the transformer/reactor has been assembled at site, it shall be 

tested in order to check that it has not been damaged during transport and 

assembly to such an extent that its future operation will be at risk. Regarding 

the performance of the test, refer to the testing method as per standards. The 

results of the test shall be documented as detailed in Clause 3.2 "commissioning 

checks -Transformers" and Clause 3.3 commissioning checks - Reactors. 

3.1.1 APPLICATION OF VOLTAGE 

If possible, the voltage should be increased step by step up to rated 

voltage. This may be done by connecting the transformer/reactor to a generator 

whose voltage is raised slowly. The higher the rated voltage of transformer/ 

reactor the more important it becomes that the application of the voltage is 

done as described. If it is not possible to use a generator, the transformer/ 

reactor may be connected directly to a live line. 

After the first application of voltage, the transformer/reactor shall be 

checked carefully (gas relay, temperatures, leakages). 

When the transformer/reactor has been is service for some weeks with 

normal working temperature, all sealing joints shall be re-tightened. 

3.1.2 ENERGISING 

The initial magnetising current at the time of switching will be very 

high. This depends on the particular moment in the cycle. The transformer 

should always be soaked for few hours under constant care i.e. keep it 

energized. If the breaker trips on differential, Buchholz or any other device, 

the cause must be investigated before re-energising the transformer or reactor. 

After successful charging, performance of the transformer/reactor , OTI, WTI 

readings should be monitored for 24 hours and ensured that they are as per 

loading. 

3.1.3 ANALYSIS OF GAS 

The gas conected in the relay will help to identify the nature of the fault. 

It is suggested that the following checks are made at convenient intervals or 

following the indication of accumulation of gas through alarm signal. 

1. Rate of gas collection- The greater the rate of gas collection, the more 

severe is the nature of the developing fault. 

2. Colour of the gas helps in finding the affected material as follows. 


Colour Identification 

White Gas of decomposed paper and cloth insulation. 

Yellow Gas of decomposed wood insulation. 

Grey Gas of overheated oil due to burning of iron portion 

Black Gas of decomposed oil due to electric arc. 

3. Combustibility of Gas- A small amount of gas drawn through the top 

pet cock if brightens the test flame, then the gas is combustible. 

Incombustible gas indicates air. 

4. Chemical Analysis of the Gas -The principles of working gas analyzing 

equipment is illustrated in the figure 3.1. 

Two solutions are prepared as detailed below and best results are obtained 

with freshly prepared solution. 

Solution 1: 5 gms of silver nitrate (AgNO ) dissolved in 100 ml 

3 

distilled water. 

Solution 2: A week solution of ammonia in water is slowly added to 

100 ml of solution 1, until a white curdled precipitate 

which forms first disappears in the mixture. 

The gas analyser loaded with these solution is then connected to the 

top pet cock. Small quantities of gas collected in the relay is allowed to pass 

through the two solutions as illustrated. 

The results are identified as follows: 

1. Both solutions are clear : Gas is air . 

2. Solution 1 -White precipitate 

turning brown on exposure to 

sunlight : Gas of oil decomposition. 

3. Solution 2 -turning to 

Dark brown precipitate : Gas of decomposed paper, 

cotton or wood insulation. 

Note: THE ABOVE INFORMATION ON GAS ANALYSIS ARE AS 

DETAILED IN IS:3638-1966. PLEASE REFER IS:3638-1966 FOR 

ALTERNATIVE METHOD OF CHEMICAL ANALYSIS. 

3.2 COMMISSIONING CHECKS -TRANSFORMER 

3.2.0 SCOPE 

This schedule covers the tests to be done at site after transformer has 

been installed and dried out. 


3.2.1 GENERAL 

Check the following: 

Sl.No. Description 

1. Breather Silicagel (Blue when dry) 

2. Oil in the Breather housing cup. 

3. All valves for their correct opening and closing sequence. 

4. Oil level in conservator tank. 

5. Oil in cooling system. 

6. Oil level in bushings. 

7. Release air, wherever necessary. 

8. Cooling accessories (Pump motors, Fan motors etc.) for direction and O/L 

setting. 

9. Buchholz, oil level indicator, pressure gauges, thermometer,Temp. indicators 

etc. for operation. 

10. Earthing of main tank M.Box T/C driving gear, diverter, Pump Fan motor etc. 

11. Neutral earthing. 

12. Earth Resistance of Electrodes. 

13. Earthing of bushing test tap. 

14. Check oil leakage for 24 hrs. 

15. Check Auxiliary circuit voltage (415 V). 

16. Calibration of OTI/WTI with hot oil. 

17. Check Working of WTI/RTD repeaters at control room. 

18. IR of core to earth. 

3.2.2 INSULATION RESISTANCE TEST 

Sl. Description Date Time Megger IR Temp. Remarks 

No. in Hrs used Value o C 

1. Control wiring 

2. Tap Changer 

a) Motor 

b) Control 

3. Cooling system 

a) Motor Fan 

b) Motor pump 

c) Control wiring 

4. Main winding 

a) HV/E+LV 

b) IV/E+HV 

c) LV/E+HV+IV 

d) HV/IV 

e) IV/LV 

f) HV/LV 

(Not less 

than 500 V 

megger) 

(Not less 

than 1000V 

megger) 

NOTE : 

(1) While checking these values no external line, lightning arrestors etc. should 

be in circuit. 

(2) Special care should always be taken while meggering the transformer 

winding to ensure that there is no leakage in the leads. 


3.2.3 OIL CHARACTERISTICS 

Take necessary precaution (regarding rinsing the bottle, cleaning hand, 

air bubble etc.) while withdrawing the samples (Please refer IS:6855). Each 

sample should be free of air bubbles and should not be tested when it is hot 

The sample should satisfy IS:1866. 

Sl. Oil Sample Time & Condition Moisture Break Resisti Tan- Rem 

No. From Date Time Weather in PPM down vity at 90 o C arks 

Voltage at 90 o C 

in kV (in ohm 

-cm) 

1. Tank 

Top Sample 

Bottom Sample 


Top Sample 

Bottom Sample 

3. OLTC Diverter 

(each phase) 

3.2.4 TESTS ON CT 

1. Ratio 

2. Polarity 

3. Magnetising current 

4. IR value. 

3.2.5 ON LOAD TAP CHANGER 

Sl. 

No. 

Description Date Observation Remarks 

1. Visual Inspection of equipment. 

2. Hand operation on all taps. 

3. Complete wiring of the circuits. 

4. Limit Switch 

5. Over running device. 

6. Remote Panel wiring. 

7. Over load Device of Driving Motor. 

8. Local Operation (Electrical). 

9. Remote Operation (Electrical). 

10. Tap Position Indicator. 

11. Step by step contractor. 

12. Out of Step Relay. 

Note : While operating the mechanism on Electrical Control, check once again 

limit switches, step by step contractor, over running device etc. for 

their actual operation and ensure that they are functioning properly. 


3.2.6 OFF CIRCUIT TAP SWITCH 

Tap switch handle should not be left halfway and/or unlocked. 

3.2.7 CONTINUITY TEST 

Continuity between line and neutral (for neutral end TC) or line to line 

(for line end TC) with multimeter on all taps and phases to be confirmed. 

3.2.8 MEASUREMENT OF WINDING RESISTANCE: 

Winding Tap No. Phase U Phase V Phase W Temp Remarks 

HV 1 

Winding 2 

IV 

Winding 

LV 

Winding 

3 

4 

5 

6 

7 

8 

9(a) 

9(b) 

9(c) 

10 

11 

12 

13 

14 

15 

16 

17 


3.2.9 VOLTAGE RATIO TEST 

Ratio on all taps is to be checked by hand operation only. This can be 

done any time, once the tap changer is completely erected. Start the tap 

changer from lowest tap and then go on increasing tap by tap while checking 

the value instead of starting from normal tap. 

Tap Voltage applied Voltage Measured Ratio obtained Remarks 

Nos. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. (a) 

9. (b) 

9. (c) 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

3.2.10 MAGNETISATION CURRENT 

a) Apply 3 phase 415 V on HV Terminals and keep LV open. 

Voltage Applied Current measured Remarks 

U-V Volts U phase m Amps 

V-W Volts V phase m Amps 

U-W Volts W phase m Amps 

Note: In case of single phase transformer, apply 230 V, 1 phase supply 

between line terminal and earth. 

b) Apply 3 phase 415 V on LV terminals and keep HV open. 

u-v volts u phase m Amps 

v-w volts v phase m Amps 

u-w volts w phase m Amps 

Note: In case of single phase transformer apply 230 V, 1 phase between 

line and earth terminal. 


3.2.11 VECTOR GROUP 

Check the vector group of the transformer by connecting in a suitable 

manner. 

Vector Diagram of Diagram as connected Remarks 

group connection for test 

CONDITIONS TO BE PROVED VOLTAGE MEASURED 

1. 1. 

2. 2. 

3. 3. 

3.2.12 FINAL IR CHECKS 

The megger readings finally after the transformer is connected to the 

system 

Date ......................... Time ........................Temp ....................... 

Windings Megger Value obtained in Remarks 

used Mega ohms 

HV/E+LV 

IV/E+HV+LV 

LV/E+HV+IV 

HV/IV 

IV/LV 

HV/LV 

3.2.13 OTHER TEST 

WTI Setting Set for Proved OTI Setting Set for Proved 

Alarm Alarm 

Trip Trip 

Fan Start 

Fan Stop 

Pump Start 

Pump Stop 


3.2.14 PROTECTION AND ALARMS 

Prove the tripping of associated breakers by actual operation of the 

various devices and relays. 

UNDER NO CIRCUMSTANCES SHORTING OF ELECTRICAL 

CONNECTIONS SHOULD BE DONE. 

Sl.No. Device Set for Proved Remarks 

Alarm Trip Alarm Trip 

1. Buchholz 

2. Excessive Winding 

temperature 

3. Excessive Oil Temp. 

4. Oil flow failure 

5. Water flow failure 

6. Differential pressure 

(OFWF cooling system) 

7. Fan failure 

8. Low oil level 

(conservator tank) 

9. Pressure relief valve 

10. Differential relay 

11. Over current relay 

12. Earth fault (REF) 

13. Instt. Earth fault 

14. Inter trip, if any 

15. Trip free check. 

3.2.15 RECORD THE FOLLOWING AFTER CHARGING 

a) No load current at relay terminal 

U phase A 

V phase A CT Ratio 

W phase A 

b) Temperature (at the time of charging) 

O.T.I. 

oC W.T.I. 

oC Ambient oC c) Maximum temperature after 24 hours. o C 


3.3 COMMISSIONING CHECKS REACTOR 

Customer W.O. No. 

Site Sl.No. 

Rating Voltage class 

Scope 

This schedule covers the tests to be done at site after reactor has been 

installed and dried out. 

3.3.1 GENERAL 

Check the following : 

Sl.No. Description Remarks 

1. Breather Silicagel (Blue When Dry) 

2. Oil in the Breather Cup. 

3. All valves for their correct opening and closing sequence. 

4. Oil level in conservator tank. 

5. Oil in cooling system. 

6. Oil level in bushings. 

7. Release air, wherever necessary. 

8. Buchholz, oil level indicator, thermometer, temp. indicators etc. 

for operation. 

9. Earthing of main tank, M. Box. 

10. Neutral earthing. 

11. Earth Resistance of Electrodes. 

12. Earthing of bushing test tap. 

13. Check oil leakage for 24 hours. 

14. Check auxiliary circuit voltage (415 V). 

15. Calibration of OTI/WTI with hot oil. 

16. Check Working of WTI/RTD repeaters at control room. 

17. IR of core to earth., 

3.3.2 INSULATION RESISTANCE TEST 

Sl. Description Date Time Megger IR Temp. Remarks 

No. in hrs. used Value 

1. Control Wiring 500 V 

2. Main Winding 

a) HV/E >1000V 


Note: 

(i) While checking these values no external lines, lightning arresstors 

etc. should be in circuit. 

(ii) Special care should always be taken while meggering the reactor 

winding to ensure that there is no leakage in the leads. 

3.3.3 OIL CHARACTERISTICS 

Take necessary precaution (regarding rinsing the bottle, cleaning hand, 

air bubble etc.) while-withdrawing the samples. Each sample should be free of 

air bubbles and should be tested when it is hot. The sample should satisfy 

IS:1866. 

SI. Oil Sample Time & Condition Moisture Break Resisti Tan- Rem 

No. From Date Time Weather in PPM down vity at at arks 

Voltage 90oC 90oC in kV (in ohmcm) 

1. Tank 

Top Sample 

Bottom Sample 


Top Sample 

Bottom Sample 

3.3.4 TESTS ON CT 

1. Ratio 

2. Polarity 

3. Magnetising current 

4. IR value 

3.3.5 MEASUREMENT OF WINDING RESISTANCE 

Winding 

Phase U Phase V Phase W Temp. Remarks 

3.3.6 CURRENT MEASUREMENT 

a) Apply 3 phase 415 volts on HV Terminals. 

Voltage Applied Current measured Remarks 

U-V Volts U Phase m A 

V-W Volts V Phase m A 

W-U Volts W Phase m A 

Note: In case of single phase reactors apply 230 V, 1 phase between 

line terminal and earth. 


3.3.7 FINAL CHECKS 

The megger readings finally after the reactor is connected to the system. 

Date Time Temperature 

Winding 

HV/E 

Megger used Value Obtained 

in megaohms 

Remarks 

3.3.8 OTHER TEST 

WTI Setting Set for Proved OTI Setting Set for Proved 

Alarm 

Trip 

3.3.9 PROTECTION AND ALARMS 

Prove the tripping of associated breakers by actual operation of the 

various devices and relays. 

Note: Under no circumstances shorting of electrical connections be done. 

Sl.No. Device Set for Proved Remarks 

Alarm Trip Alarm Trip 

1. Buchholz 

2. Excessive Winding 

Temperature 

3. Excessive Oil Temp. 

4. Oil Level 

(Low & Max.) 

5. Pressure Relief Valve. 

3.3.10 RECORD THE FOLLOWING AFTER CHARGING 

a) No load current at relay terminal 

U phase m A 

V phase m A CT Ratio 

W phase m A 

b) Temperature (at the time of charging) 

O.T.I. 

oC W.T.I. 

oC Ambient 

oC c) Maximum temperature 

after 24 hours. 

o C 



¦ÉÉMÉ-4 

+xÉÖ®úIÉhÉ B´ÉÆ {ÉÊ®úSÉÉ±ÉxÉ 

SECTION-4 

MAINTENANCE AND OPERATION 


SECTION 4 

4. MAINTENANCE AND OPERATION 

4.1 SUPERVISION OF TRANSFORMER/REACTOR 

4.1.1 GENERAL 

In order to avoid faults and disturbances, it is important that a careful 

and regular supervision and control of the reactor and its components is planned 

and carried out. 

The frequency and extent of such a supervision and control is dependent 

on climate, environment and service conditions etc. 

The directions for a certain transformer/reactor are therefore preferably 

based on experiences from comparable transformer/ reactor installations. 

A supervision and maintenance program according to schedule in 

ANNEXURE 4.1 is recommended. 

Spare transformers/reactors are supervised and maintained according 

to the same schedule as transformers/ reactors in service. 

4.1.2 GENERAL MAINTENANCE 

Dirt/Dust 

The external transformer surfaces shall be inspected regularly and when 

required cleaned from dust, insects, leaves and other airborne dirt. 

Possible Leakage 

After energising of the transformer, a certain settling may appear in 

sealing joints. These should therefore be retightened according to schedule in 

ANNEXURE 4.1. This applies especially to sealing joints with plain gaskets 

that are not placed in grooves. 

Rust damages, touch-up painting 

A regular inspection of the external surface treatment of the reactor 

should be carried out. Possible rust damages are removed and the surface 

treatment restored to original state by means of primer and finish paints. 

4.1.3 OIL PRESERVATION SYSTEM, MOISTURE ABSORPTION 

Transformer oil absorbs easily moisture from the surrounding air. The 

moisture absorption has been prevented by means of rubber sack in 

convservator which acts as a separating wall between the oil and the 

surrounding air. 

The moisture absorption is further prevented by a breather connected 

to the conservator. The active drying agent (silica gel) should be exchanged or 

regenerated when 2/3 of the agent has been red-coloured by absorbed moisture. 


The breather contains also an oil-seal that prevents spontaneous air 

circulation. This seal should be filled with oil to the level specified. 

4.1.3.1 OIL LEVEL INDICATOR FOR OIL CONSERVATOR 

The Transformer oil conservator is provided with an oil-level indicator. 

At an oil temperature of 45 o C, the conservator should be half filled. If the level 

exceeds the value “full”, oil must be drained off. If the value is “low”, oil must 

be filled in, Normal oil level should be at 35 o C mark. 

4.1.3.2 GAS-OPERATED RELAY (BUCHHOLZ RELAY) 

The use of gas-operated relay as protection for oil-immersed transformers 

is based on the fact that faults as flash over, short-circuit and local overheating 

normally result in gas- generation. The gas- bubbles gathering in the gasoperated 

relay affect a float- controlled contact which gives an alarm signal. 

4.1.3.3 MOISTURE 

If the oil has been subjected to moist air, the moisture content and the 

electric strength of the oil shall be checked. 

4.1.3.4 SLUDGE, ACID 

Normally the BHEL transformers have uninhibitated oil. 

Sometimes inhibited oil is used. The inhibitor works so that it breaks the 

chain reaction by which sludge and acid are produced. This retards the ageing 

process and extends the utilization time of the oil. 

If sludge is being produced in the oil, the oil changes colour and becomes 

darker and turbid. The sludge can be removed by means of filtering, but if the 

sludge formation has started, it will increase with time. The oil should therefore 

be exchanged if the neutralisation value according to IS:1866 exceeds 0.5 mg 

KOH/g or precipitable soluble sludge is produced. 

The oil exchange should preferably be carried out when the transformer 

is warm and the oil viscosity is low. 

The exchange should be as complete as possible, because old oil will 

affect the new one, speeding up the ageing. 

Oil which is not too much oxidized may in certain cases be regenerated, 

but this is economical only for large oil quantities. 

4.1.3.5 OIL, MISCELLANEOUS 

For supplementary information about transformer oil, see clause 4.2 

“Supervision and control of oil”. 

4.1.4 TEMPERATURE SUPERVISION 

The service life of a transformer is highly dependent on the temperature 

prevailing in the core and windings under operation. It is thus important to 

keep the oil and winding temperatures under observation continuously. The 

temperatures should be read regularly and the measured values registered. 

These values will give guidance for judging the service life of the 

transformer, cooling system functions etc. 


4.1.4.1 OVERLOAD 

A Transformer can normally be overloaded at a low ambient temperature. 

However, the permissible overload must not be judged only with regard to the 

oil temperature. We must also consider that the temperature difference between 

winding and oil will increase with the load. Therefore, also the winding 

temperature must be supervised during overloading. 

For determination of the overload capacity for modern transformers refer 

IS:6600. 

If the temperature in a transformer shows a tendency of rising without 

a corresponding increase of the load, this may be caused by a reduction of the 

cooling ability of the cooling equipment (dirt, dust). The thermometer should 

also be checked in this case. 

4.1.4.2 THERMOMETER FOR MEASUREMENT OF TOP-OIL TEMPERATURE 

The thermometer consists of a cylindrical sensing body with a flange, a 

capillary tube, and a thermometer housing with dial and contact-device. The 

measuring system is filled with a liquid, which changes its volume at 

temperature variations, and affects spring bellows. The movements of the 

bellows are transferred to the pointer and signal contacts via a link system. 

The thermometer is provided with two signal contacts of mercury switches. 

The contacts can be set independently of each other. 

For control and adjustment of the thermometer, see OTI leaflet. 

4.1.4.3 WINDING-TEMPERATURE INDICATOR 

The thermometer system consists of a sensitive body with assembling 

details, a capillary tube and a thermometer housing, which contains a scale 

with an indicating pointer and a max. pointer, heating and adjusting resistor, 

as well as contact- device for four switches. 

For further information about function, control and adjustment of winding 

temperature indicator, see WTI Leaflet. 

4.1.4.4 REMOTE CONTROL OF WINDING TEMPERATURE 

For remote control of winding temperature a resistance temperature 

device has been used with a heating element built into the same housing. The 

heating element is fed by the secondary current of a current transformer. 

For setting of the heating element there is a parallel connected adjusted 

resistor installed in the control cabinet. 

For further information about function and calibration, see RTD Leaflet. 

4.1.4.5 RADIATORS 

The external cooling surfaces shall be inspected regularly and when 

required cleaned from dust, insects, leaves or other airborne dirt. This is 

especially important in case of fan cooling. The cleaning is suitably carried out 

by means of water flushing at high pressure. 


Normally, no measures are necessary for keeping the internal cooling 

surfaces clean as long as the oil is in good condition. If, however, sludge 

formation has set in, the sludge may deposit on horizontal surfaces in radiators. 

In such a case, the radiator should be flushed internally with clean oil. 

If the sludge does not loosen, we can firstly flush with petrol etc. and 

then with oil. 

4.1.4.6 CONTROL CABINET 

The operation of the control cabinet and devices normally included, are 

described in Vol. I. 

4.1.4.7 BUSHINGS 

Bushing porcelains shall be cleaned from dust and dirt regularly. In areas 

where the air contains impurities as salt, cement dust, smoke or chemical 

substances, shorter intervals are required. See also the Instructions in the 

special information documents about bushings that are included in vol.I. 

4.1.4.8 CONNECTIONS 

In order to avoid prohibited temperature rises in the electrical connections 

of the transformers/reactor, all screw-joints should be checked and re-tightened 

according to schedule enclosed. 

4.1.4.9 ACCESSORIES 

Separate leaflet/instructions should be followed for various fittings (as 

applicable) for Fans, Pumps, Flow indicators, Pressure Gauges, Oil Gauge, 

Pressure reducing valve, OLTC, Off circuit tap switch, OFAF/OFWF Coolers 

etc. 

These leaflet are enclosed with Vol I. 

4.1.5 MAINTENANCE SCHEDULE 

Maintenance schedule given at Annexure 4.1 should be followed. With 

proper maintenance as per this schedule normal life of Transformer or reactor 

can be expected. 

4.1.6 IR TEMPERATURE RELATION 

It has always been the question from our customers as to how the IR 

value can be converted at different temperatures. We have drawn a curve 

(Fig. 4.1) for guidance of the operating staff on the basis of our experience. 

So that they can judge the IR Value at any temperature. This curve cannot be 

taken for absolute value but will serve as a good guide for conversions of IR 

values at different temperatures. A simple example is given as to how to 

calculate the value at different temperature. 

Suppose IR value of 70 o C is 300 M Ohms and we want to convert at 

40 o C, the curve will give you a factor K for difference in temperature i.e. 

(70 o C-40 o C)=30 o C. 

K for 30 o C temperature difference = 4.2 

So the value at 40 o C = 4.2x300 = 1260 M. Ohms. 

Value thus converted fairly tally with the actual 

Value when cooling the transformer. The factors are tabulated as below: 


Difference 

in temp. 10 o C 20 o C 30 o C 40 o C 50 o C 

Factor K 1.65 2.6 4.2 6.6 10.5 

4.2 SUPERVISION AND CONTROL OF OIL 

4.2.1 GENERAL 

The oil in a reactor provides an electrically insulating and a cooling 

medium. The service reliability of an oil-immersed transformer/reactor is 

therefore mainly, depending on the oil quality. 

In service, oil is subjected or normal deterioration due to the conditions 

of use mainly due to air, water and solid particles/sediment. 

4.2.2 SCHEDULE 

The oil should be maintained as per the guidelines of IS:1866. Periodicity 

of test and permissible limits for important parameters of oil - electric strength, 

water content, resistivity, tan-delta, neutralization value, sediment and 

precipitable sludge, flash point and interfacial tension are given in Annexure 

4.2. Health of reactors can be effectively monitored by Dissolved Gas Analysis 

(DGA) technique described in detail in IS:9434 and IS:10593. This technique 

helps in detection of incipient faults in reactors. 

4.3 TROUBLE SHOOTING 

General measures for trouble shooting are described in this section. 

4.3.1 Items to be informed to BHEL in case of trouble. 

Followings are items to be informed to BHEL. 

1. Transformer or Reactor specifications 

Capacity 

Serial number & work order No. 

Year of manufacture. 

2. Load current (A) or load (kW) 

3. Operating tap position (In case of Transformer) 

4. Conditions at time of trouble. 

a) Date, time 

b) Trouble phenomena 

c) Checked items and results 

4.3.2 Transformer troubles and check items. Troubles, their possible causes 

and items to be checked are given in Table 4.1 & 4.2. 

4.3.3 Detective devices and their functions. Refer to table 4.3 

4.3.4 Check items and judging standard Refer to table 4.4. 

4.3.5 Detection method of oil or gas leaks. Refer to table 4.5. 




Table 4.3: Detective devices and their functions 

No. Detective 

device 

Description 

1. Buchholz This relay detects the gas evolution due to internal trouble and 

relay 

gives alarm, and is suited to the detection of minor or slowly 

(1st stage) developing faults. The nature of the detected fault can be judged 

to an extent by analyzing the gas and oil. In case of nitrogen gas 

sealed transformer, part of dissolved nitrogen gas comes out due 

to the sudden temperature drop, which sometimes results in the 

actuation of this relay. 

2. Buchholz When a major fault suddenly takes place inside the transformer, 

relay 

oil flows suddenly from the transformer tank to the conservator. 

(2nd stage) The second stage of Buchholz relay is actuated by this oil flow. If 

the second stage of the relay functions independently, check the 

air breather, nitrogen gas sealing device etc. If it functions 

combinedly with the over current relay etc. it is suspected that a 

serious internal defect is existent. In such a case take all tentative 

measures and contact BHEL immediately. 

3. Pressure When the internal pressure of the transformer rises above the set 

relief device value, the pressure relief device functions. It also functions, though 

occasionally by the choking of the air breather. The self- excited 

pressure relief plate is subject to brittleness caused by secular 

change. Comprehensive judgment based on the inspection of other 

protective relays is the key to determining whether the function of 

this device is an erroneous operation or a normal operation caused 

by internal defect. 

4. Differential This relay detects the internal defect by comparing the input current 

relay 

and the output current of the transformer. If it functions combinedly 

with other protective devices, it is indicative, in almost all cases, 

of an internal fault. Note that this relay sometimes operates due to 

inrush current when the transformer is energized, and this is of 

course not an internal fault. 

5. Dial thermo This thermometer gives alarm when the temperature rises beyond 

meter alarm the preset value. Insufficient cooling effect of cooling equipment 

contact due to contamination of cooling fins, Stoppage of oil pumps, fans 

and so forth is one of the reasons that operates this device. 

6. Overcurrent These are intended for detecting faults in the electric system 

relay, ground connected with the transformer. A fault in the transformer results 

fault relay in the function of these relays, which is always accompanied by 

the actuation of protective devices of the transformer. Detection 

of the fault in the electric system should therefore be made on the 

basis of a comprehensive judgment of these functions. 

7. Voltmeter If a sudden change in voltage or current is noted when no 

ammeter abnormalities are detected in the electric system, or an abnormal 

unbalance of voltage or current is noted between phases, investigate 

the cause. 

8. Human Note that human senses also play an important role in fault 

senses 

detection. Following are the points to be checked by your senses. 

1) Discolouration or odour of insulation oil (blackening or stink). If 

pieces of insulating materials or copper particles are found in 

the oil, it is suspected that an internal defect has developed. 

2) Abnormal vibration or sound. 


1. Measurement 

of insulation 

resistance 


of winding 

resistance 


of voltage ratio 


of exciting 

current 

5. Checking of 

insulation oil 


of dielectric 

loss factor, 

tan 

7. Analysis of gas 

accumulated in 

Buchholz relay 

8. Analysis of 

dissolved gas 

in oil 

Table 4.4: Check and refer standards 

No. Check Point Description Judging standard 

Grounding fault can be detected by 

this measurement, though to some 

extent. The measurement must be 

proceeded after disconnecting the 

transformer from lines and cleaning 

of bushing surfaces. 

This is a primary means to be 

employed for detecting the fault of 

windings. The resistance should 

preferably be measured independently 

by the D.C. drop of potential method. 

This can be conducted by applying a 

voltage of about 200V which can be 

readily obtained at job-site. Accurate 

measurement at job site entails some 

difficulties due to source voltage 

variation and errors of instruments. 

In case of 3 phase transformers, it is 

essential to measure the ratio phase 

by phase and check the variation of 

the ratio between phases. 

This measurement is very useful to 

detect fault in windings and/or cores. 

Local short- circuit in windings or 

cores often causes an increase in the 

exciting current. The test can be made 

by applying a low voltage (200-400 

volts) to a winding. 

Measure dielectric strength and check 

for carbon sludge, offensive odour 

and discoloration. 

When gas is accumulated in the 

Buchholz relay,its composition must 

be clarified by the gas chromatograph 

analyzer to check for the possible 

internal defect. 

Existence of internal defect can be 

judged, though to a limited degree, 

by sampling and analyzing oil by a 

gas chromatograph. (Refer 4.3.2). 

Judgment to be 

made by comparison 

with values obtained 

by periodical 

inspection. 

Judgment to be made 

by comparison with 

the factory test report. 



the factory test report. 

1 Comparison of 

the measured 

values with 

previously 

obtained ones. 

2 Significant 

difference in 

value among 

phases. 


with reference to 

Annexure 4.1 & 4.2. 



values obtained by 

periodical inspection. 

Contact BHEL 

Contact BHEL 


Table 4.5 Method for detecting oil or nitrogen leaks 

Leak Measures Description 

Oil leak Clean the suspected leak The following are susceptible 

part with thinner or gasoline to leakage. 

apply chalk powder to it. (1) Gasketed joints. 

The leak part will present (2) Welded parts. 

itself as a soiled spot. 

Nitrogen leak Step up Nitrogen gas pressure The following are susceptible 

(in case of to 0.17 kg/cm 2 to leakage 

Nitrogen sealed (1) Accessories for pressure 

system) relief 

(2) Flanges 

(3) Welded parts 

4.3.6 GAS ANALYSIS ON TRANSFORMER OIL 

Incipient faults in oil filled transformer are usually the result of electrical 

or thermal excess stress of either the transformer oil or insulating materials. 

It is known that such excessive stresses produce a mixture of gases 

characteristic of which give an indication of the type of faults, and materials 

associated with the faults. 

It is recommended that analysis of dissolved gases in transformer oil by 

gas chromatographic equipment is made, the time of commissioning and then 

after an interval of one year for transformers of 145 kV class and above. 

4.3.7 ANALYSIS METHOD 

4.3.7.1 SAMPLING OF OIL FROM TRANSFORMERS 

Oil in transformers can be sampled through drain or sampling valve near 

bottom of the tank. Special care shall be taken not to introduce air, foreign 

matter, or dirty oil into sampling container. For this purpose, first 0.5-1.0 litre 

of oil from the transformer shall be over-flown through the oil container. Shape 

of the sampling container may be that of shown in Fig. 4.2 and sampling 

method shall be in accordance with Fig. 4.3. See also IS:9497 Sampled oil 

shall not be exposed to air before analysis. 

4.3.7.2 GAS ANALYSIS 

Gases to be analysed and criteria for the gases found in transformer oil 

are tabulated in table 4.6. 

4.3.8 ASSESING THE TEST RESULTS 

Test results on gas contents in oil by some typical faults in transformer 

active part models are shown in table 4.7. 

Table 4.6 and table 4.7 may by referred to evaluate transformer condition. 

To analyse DGA test results, flow chart given at Appendix A of IS:10593 

should be followed. Standard values may be taken from CBIP Technical report 

no. 62 of April 88 titled "Guide for testing of Transformer by Sampling and 

analysis of free and dissolved gases" Ratio technique of IS:10593 should be 

used to know the nature of fault. IS:9434 and IS:10593 have been adopted 

from IEC pub 567 and IEC pub 599. 


Table 4.6 Gases to be analysed and criteria 

1. Gases to be analysed normally O 2 , N 2 , H 2 , Co, Co 2 , CH 4 

2. Gases to estimate abnormality H 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 

3. Gases to estimate deterioration Co, Co 2 , CH 4 

Table 4.7 Gas content in oil by faults 

Sl. Type of faults Decomposable gases in 

transformer oil 

1. Overheat of oil CH 4 , C 2 H 4 , H 2 , (C 2 H 6 , C 2 H 2 ,C 3 H 6 C 3 H 8 ) 

2. Arcing in oil H 2 , C 2 H 2 , (CH 4 , C 2 H 4 ) 

3. Overheat of solid insulating materials CO, CO 2 , (H 2 , C 2 H 4 ) 

4. Overheat of oil and paper combination CH 4 , C 2 H 4 , CO, CO 2 , H 2 

5. Arcing of oil and paper combination H 2 , C 2 H 2 , CO, CO 2 , (C 2 H 4 ) 

( ) shows gas contents which appear rarely. 

4.4 PARALLEL OPERATION 

(i) If it is desired to parallel a transformer which is not identical in 

design with BHEL transformer, it is preferable to refer the matter to 

BHEL for advice. 

(ii) The voltage ratings/ratios and impedance values at all taps should 

match for the two transformers that are to be paralleled. Only certain 

combinations of vector groups are capable of being paralleled. The 

table 4.8 below indicates the more common permissible combinations. 

Table 4.8 

TRANSFORMER (B) 

H.V. Delta Star Delta Star 

L.V. Star Delta Delta Star 

H.V./L.V. 

Delta/Star Yes Yes No No 

TRANSFORMER (A) 

Star/Delta Yes Yes No No 

Delta/Delta No No Yes Yes 

Star/Star No No Yes Yes 


(iii) Reference to the connection diagrams of two transformers should 

indicate the terminals to be paralleled. 

(iv) The reversal of two leads on either side of a three-phase transformer 

will reverse the polarity changing them in sequence (e.g. form UVW 

to VWU or WUV) will swing the vectors through 120. 

(v) Phase sequence and polarity can be checked by energising both 

transformers on the primary side before paralleling and measuring 

the open-circuit voltage appearing across each pair of terminals which 

will ultimately be paralleled. 

(vi) IS:2026 and IS:10561 may also be referred. 


SI Items to be 

inspected 

1. Ambient 

temperature 

2. Winding 

temperature 

3. Oil temperature 

4. Load amps/Load 

voltage 

1. Oil level in 

transformer 

conservator 

2. Oil level in tap 

changer diverter 

switch. 

3. Oil level in 

bushings. 

4. Pipework and 

accessories for 

leakages 

1. Buchholz Relay 

2. Dehydrating 

breather 

3. Leakages of 

water into 

cooler. 

1. Bushings 

2. Transformer oil 

and tap changer 

oil 

3. Cooler fan, 

bearing and 

controls, 

pumps. 

ANNEXURE 4.1 

MAINTENANCE SCHEDULE 

Inspection notes 

HOURLY 

- 

Check that temperature 

rise is reasonable. 

Check against rated figures 

DAILY 

Check oil level from oil 

gauge 

Check oil level from the 

gauge glass. 

Check the oil level from 

gauge glass/gauge. 

Inspect the transformer for 

leakages. 

MONTHLY 

Check oil level 

Check colour of silicagel. 

- 

QUARTERLY 

Examine for dirt deposition 

and tightness of oil filling 

plugs. Examine for cracks 

in porcelain.Clean and 

tighten plugs. 

Check for di-electric 

strength and moisture 

content. 

Check contacts, manual 

control and interlocks. 

Action required 

- 

Shutdown the transformer and 

investigate, if either is persistently 

higher than normal. 

An improper tap position can 

cause excessive core loss. Voltage 

and tap position should be 

corrected. 

Top up, if found low. 



If leakages are observed tighten 

evenly the gasket joints. Replace 

‘O’ ring or washer suitably. 

Replace gasket if needed. 

Release collected gas/air 

Reactivate/replace with new 

charge, if found pink Investigate 

and rectify after taking shutdown. 

Cracked porcelain should be 

rectified/replaced. 

Take suitable action to restore 

quality of oil. 

Lubricate the fan bearings. 

Replace worn out contacts and 

other parts. Clean/ adjust controls 

and interlocks. 


4. On load tap 

changer 

5. Dehydrating 

breather 


2. Cable box 

3. Alarm and 

protection 

circuits. 

4. OTI & WTI 

pockets 

5. Earth strip 

6. Packing gaskets 

of fans 

7. Fan motors 

8. Tank and 

accessories 

9. Gasket joints 

10. Divert servicing 

11. Diverter switch 

of on-load tap 

changer 

Examine contacts, check 

step by step mechanism 

operation, end position 

limit switches and brakes. 

Also check oil in driving 

gear mechanism. 

Check oil level in the cup 

and ensure air passages 

are free. 

YEARLY 

Check for characteristics in 

line with IS:1866 

Check for proper scaling of 

compound filling holes. 

Check for cracks in the 

compound (if used). 

Check all protections and 

alarm circuits by actual 

external initiation. Check 

operation of relays and 

their sensitivity. 

Check oil in the WTI and 

OTI pockets. 

Check earthing resistance 

Inspect the condition of 

packing gaskets. 

Check IR value of motor 

winding after rainy season. 

Noise & vibration of fans. 

Check painting and surface 

finish. Mechanical 

inspection of all 

accessories. 

Check the tightness of 

bolts 

One year after 

commissioning 

Check the contacts for 

burning or pitting marks. 

Replace all worn out and burnt 

contacts. Set limit switches in 

position. Clean/ replace brake shoe 

lining. Lubricate all bearing and 

coupling points suitably. (Refer 

Separate leaflet.) 

Top up, if found low. Clean air 

passages. 

Filter/replace as required to restore 

the quality. 

Replace sealing washer, if found 

damaged. 

Replace burnt out fuses Adjust the 

setting and check wiring circuits. 

Adjust relay setting, if needed. 

Replace damaged relays. 

Replenish, if required 

Take suitable action, if resistance 

is noted high. 

Replace if worn out or resilience 

lost. 

Dry out, if found low, check 

balancing of fans. 

Touch up/repaint, if required 

Replace any component found 

damaged. 

Tighten evenly all loose bolts, 

nuts, locking etc. 

Draw out diverter, clean, tighten 

contacts. 

Recondition/replace if required. 


12. Diverter switch 

oil 

13. Tank cover and 

conservator 

14. Bushing top 

connectors. 

1. Oil conservator 


3. Buchholz relay 

1. Transformer and 

tap changer 

Assembly 

Filter oil irrespective of 

strength. 

Dirt, Dust, Surface 

Check contact joints 

TWO YEARLY 

General Inspection 

including checking the 

operation of gauges on it. 

Check heathiness of air 

cell. 

Examine values as per 

IS:1866 

Mechanical inspection of 

buchholz 

7-10 YEARLY 

Overall internal inspection 

including lifting of core and 

coil assembly 

Replenish with new oil,if oil is too 

much contaminated. 

Clean touch-up painting. 

Retighten. 

Clean, if required. Ensure proper 

functioning of indicator/Replace if 

punctured. 

Filter oil if required. 

Set floats, if required. 

Tighten all clamping arrangement, 

loose cleatings etc. Tighten all 

nuts and bolts and check locking 

arrangements and fasteners. 

Wash core and coil with dry 

transformer oil. 

NOTES: 

1. In addition to the above instructions, reference should be made to 

IS:10028 “Code of practice for selection installation and maintenance 

of transformer’’ 

2. Reference of IS:1866 "Code of practice for Maintenance and 

Supervision of Mineral insulating oil in equipments'' should be made 

for maintenance of oil while transformer is in service. Table 1&2 of 

IS:1866 are given at annexure 4.2 & 4.3 for ready reference. 

3. The inspection schedule specified for longer periods automatically 

includes those specified for shorter period. 

4. All maintenance test results and observations should be specifically 

recorded. 

5. Where matter given in the inspection book differs from that given in 

standards referred to, the transformer user should follow the instruction 

book. 

6. In case of anything abnormal occuring during service, advice from 

BHEL should be obtained giving them complete particulars as to the 

nature and extent of occurrence, together with the name plate 

particulars in order to assist identification of the transformer. 

7. As efforts are being constantly made to improve designs and service, 

the transformer supplied may differ in minor details from data given 

herein. 


ANNEXURE 4.2 

APPLICATION AND INTERPRETATION OF TESTS ON OIL IN TRANSFORMERS AND 

REACTORS (INCLUDING SELECTOR TANKS OF ON-LOAD TAP CHANGERS 

(CLAUSE 5.1 (a), 5.2, 6.0 and 7.1) 

Sl.No CHARACTERISTIC EQUIPMENT TEST METHOD TEST VENUE SUGGESTED PERMISSIBLE ACTION IF 

VOLTAGE (REF TO IEC/ISO F=FIELD INITIAL LIMIT OUTSIDE 

OR APPENDIX) 

L=LABORAT- 

ARY 

(1) (2) (3) (4) (5) (6) (7) (8) 

(i) Electric strength 145 kV and IS:6792-1972* F/L After filling 50 kV Recondition 

(breakdown above (average of 6 or re-filling (Min) or alternatively 

voltage-kV) brealdowns prior to If more econmic 

72 kV and on one cell filling energizing, 40 kV or other tests 

less than with 2.5 mm gap then after (Min) dictate, replace 

145 kV spacing) three months 

and after one year 

oil. 

Below 72.5 kV 30 kV 

(Min) 

(ii) water content 145 kV and IS:335-1983 L After filling 25 PPM Recondition oil 

(ppm) above or re-filling (Max) or alternatively 

prior to If more econmic 

Below 145 kV energizing, 35 PPM or other tests 

3 months dictate, replace 

and after one year oil. 

(iii) Specific resistance All voltages IS:6103-1971 L After filling 0.1X1012 Recondition if 

(resistivity or re-filling ohm-cm the value of the 

ohm-cm) at prior to (Min) at DDF permits, 

900C (see Note 9) energizing, 900C reclaim or replace 

3 months 

and after 2 years 

if not. 

(iv) Dielectric dissipation 145 kV and IS:6262-1971 L After filling 0.2 (Max) Reclaim or 

factor (tan δ) above or re-filling replace oil (limit 

at 900C Below 145 kV prior to 1.0 (Max) to be adopted 

energizing, then depends on the 

after 2 years type of equipment 

and instructions 

from the 

manufacturer). 

(v) Neutralisation All voltage IS:1448 (P:2) F do 0.5 mg Reclaim or replace 

value (total 1967 or KOH/g oil 

acidity (see 

Note 10) 

L (Max) 

(vi) Sediment and/ All voltage Appendix A L do No sediment Recondition oil 

or precipitable or precipltable if sediment is 

sludge sludge should detected 

be detectable 

alternatively 

if more economic 

or other test 

dictate replace 

oil. Reclaim or 

replace oil if 

precipitable 

sludge is 

detected. 

(vii) Flash point All voltage IS:1448 (P:21) L do Decrease in Reclaim or 

(see Note 11) 1970 the flash point replace oil after 

15oC (Max) of 

the inital value 

minimum value 

125 

knowing causes. 

oC (viii) Interfacial All voltage IS:6104-1971 L do 0.018 N./m Reclaim or 

tension 

at 27 

(Min) replace oil. 

o PERIODICITY SATISFACTORY PERMISSIBILE 

OF TESTS FOR USE 

LIMIT SEE ALSO 

C 

(ix) Dissolved gas 145 kV and IS:9434-1979 L After filling IS:10593 Reclaim or 

analysis above or re-filling 

prior to 

energizing, 

3 months and 

after one year. 

1983 replace oil. 


Note 1 The voltages indicated in col 3 are highest system voltages. 

Note 2 The full application of these recommendations to small transformers 

upto 1 MVA or below 36 kV will be technically necessary only in 

cases where high reliability is required. It will normally be uneconomic. 

In most cases, it could be limited to the simpler field tests and the 

periodicity of tests will be determined with relation to the reliability 

required. Inspection of oil in pole- mounted transformers is assumed 

to be uneconomic. 

Note 3 Characteristics are normally well above the permissible limits given 

in col 7 at the time of test of transformer in the manufacturer’s 

works. However, no separate limits have been fixed for this stage. 

Note 4 Higher limits may be required for electric strength and lower limits 

for water content prior to testing in the factory and prior to energizing 

the transformer. Special instructions given by the manufacturer may 

be followed. 

Note 5 Suggested initial periodicity of tests have been given in col. 6. 

Subsequent intervals may vary depending on previous test results 

(progress of ageing) and on changed service conditions. 

Note 6 Instructions of equipment manufacturers should be taken into account 

in adoption of the recommendation of this table. 

Note 7 For application and interpretation of test in diverter tanks of on-load 

tap changers, guidance shall be taken from the manufacturer. 

Note 8 Alternatively an indication of the presence and effect of water content 

can be obtained from a resistivity comparison at 27 o C and 90 o C (see 

6.4.1.2). 

Note 9 Water content and acidity influence the values of the specific 

resistance. The limit is indicative only and when value falls below 

this limit the cause should be ascertained by other tests. 

Note 10 Perform test more frequently when value exceeds 0.3 mg KOH/g. 

For transformers subject to test (ii), method(s) it will be necessary 

to test more frequently to establish when neutralization value (total 

acidity) exceeds 0.1 mg KOH/g. 

Note 11 This test may also be required when an unusual odour is noted or 

when an internal fault has occured. 


ANNEXURE 4.3 

GUIDELINES FOR THE CHARACTERISTICS OF OIL TO BE OBTAINED BEFORE 

ENERGIZING NEW TRANSFORMERS USING NEW INSULATING OILS ACCORDING 

TO IS:335-1983 

SL. CHARACTERISTIC EQUIPMENT TEST METHOD PERMISSIBLE 

NO. VOLTAGE (REF TO IS:OR LIMIT 

APPENDIX) 

SATISFACTORY 

FOR USE 

(1) (2) (3) (4) (5) 

(1) Electric strength 

(breakdown 

voltage kV) 

(ii) Specific 

resistance 

(resistivity ohmcm 

at 90 o C) 

(iii) Dielectric 

dissipation 

factor (tanδ) at 

90 o C Max. 

(iv) Water content, 

ppm Max. 

(v) Interfacial 

tension at 27 o C 

(Min) N/m 

(vi) Dissolved gas 

content 

Below 72.5 kV 

72.5 kV and less 

than 145 kV 

145 kV and 

above 

All voltages 

All voltages 

Below 72.5 kV 

72.5 kV and less 

than 72.5 kV 

145 kV and 

above 

All voltages 

145 kV and 

above 

IS:6792-1972 

IS:6103-1971 

IS:6262-1971 

IS:335-1983 

IS:6104-1971 

(Under 

Consideration) 

40 kV (rms) min 

50 kv (rms) min 

60 kV (rms) min 

1X10 12 ohm-cm 

(min) 

0.05 

25 ppm 

20 ppm 

15 ppm 

0.030 




4.5 DO’S AND DON’TS 

4.5.1 DON’TS FOR POWER TRANSFORMER/REACTOR 

1. Do not energise without thorough investigation of the transformer/reactor, whenever 

any alarm of protection has operated. 

2. Do not re-energise the transformer/reactor, unless the Buchholz gas is analysed. 

3. Do not re-energise the transformer/reactor without conducting all pre-commissioning 

checks. The results must be comparable with results at works. 

4. Do not handle the off-circuit tap switch when the transformer is energised. 

5. Do not energise the transformer, unless the off-circuit tap switch handle is in 

locked position. 

6. Do not leave-off circuit tap switch handle unlocked. 

7. Do not leave tertiary terminals unprotected outside the tank, connect them to 

tertiary lightning arrestors protection scheme, when connected to load. 

8. Do not allow WTI/OTI temperature to exceed 55oC during dryout of transformer, 

and filter machine temperature beyond 60oC. 9. Do not parallel transformers which do not fulfil the condition given in clause 4.4. 

10. Do not use low capacity lifting jacks on transformer/reactor for jacking. 

11. Do not move the transformer/reactor with bushings mounted. 

12. Do not overload the transformer other than the specified limits as per IS:6600. 

13. Do not change the settings of WTI and OTI alarm and trip frequently. The setting 

should be done as per the site condition. 

14. Do not leave red pointer behind the black pointer in OTI and WTI. 

15. Do not leave any connection loose. 

16. Do not meddle with the protection circuits. 

17. Do not allow conservator oil level to fall below 1/4 level. 

18. Do not allow oil level to fall in the bushings, they must immediately to be topped 

up. 

19. Do not leave marshalling box doors open, they must be locked. 

20. Do not switch off the heater in marshalling box except to be periodically cleaned. 

21. Do not allow dirt and deposits on bushings, they should be periodically cleaned. 

22. Do not allow unauthorised entry near the transformer/reactor. 

23. Do not leave ladder unlocked, when the transformer/reactor is ‘ON’ in service, in 

case it is provided. 

24. Do not change the sequence of valve opening for taking standby pump and motor 

into circuit. 

25. Do not switch on water pump unless oil pump is switched on. 

26. Do not allow water pressure more than oil pressure in differential pressure gauge. 

27. Do not mix the oil, unless it conforms fully to IS:335. 

28. Do not allow inferior oil to continue in transformer/reactor. The oil should be 

immediately processed and to be used only when BDV/ppm conforms to IS:1866. 

29. Do not continue with pink silicagel, this should immediately be changed or 

regenerated. 

30. Do not leave secondary terminal of an unloaded CT open. 

31. Do not store transformer/reactor for long after reaching site. It must be erected 

and commissioned at the earliest. 

32. Do not keep the transformer/reactor gas filled at site for a longer period. 


4.5.2 DO’S FOR POWER TRANSFORMER/REACTOR 

1. Check and thoroughly investigate the transformer/reactor whenever any 

alarm or protection operated. 

2. Check air cell in conservator. 

3. Attend the leakages on the bushing immediately. 

4. Examine the bushings for dirt deposits and coats, and clean them 

periodically. 

5. Check the oil in transformer and OLTC for di-electric strength and moisture 

content and take suitable action for restoring the quality. 

6. Check the oil level in oil cup and ensure air passages are free in the 

breather. If oil is less, make up the oil. 

7. Check the oil for acidity and sludge as per IS:1866. 

8. If inspection covers are opened or any gasket joint is to be tightened, 

then tighten the bolts evenly to avoid uneven pressure. 

9. Check and clean the relay and alarm contacts. Check also their operation, 

and accuracy and if required change the setting. 

10. Check the protection circuits periodically. 

11. Check the pointers of all gauges for their free movement. 

12. Clean the oil conservator thoroughly before erecting. 

13. Check the buchholz relay and readjust the floats, switches etc. 

14. Inspect the painting and if necessary retouching should be done. 

15. Check the OTI and WTI pockets and replenish the oil, if required. 

16. Remove the air through vent plug of the diverter switch before you energise 

the transformer. 

17. Check the oil level in the diverter switch and if found less, top up with 

fresh oil conforming to IS:335. 

18. Check the gear box oil level, if less top up with specified oil. 

19. Examine and replace the burnt or worn out contacts as per Annexure 4.1 

of Maintenance Schedule. 

20. Check all bearings and operating mechanism and lubricate them as per 

schedule. 

21. Open the equalising valve between tank and OLTC, wherever provided 

at the time of filling the oil in the tank. 

22. Connect gas cylinder with automatic regulator if transformer is to be 

stored for long, in order to maintain positive pressure. 

23. Fill the oil in the transformer/reactor at the earliest opportunity at site 

and follow storage instructions. 

24. Check the door seals of marshalling Box. Change the rubber lining if 

required. 

25. Equalise the diverter compartment of the OLTC by connecting equalising 

pipe between flange joints provided on the tap changer head. 


4.6 DISPOSAL 

The items to be disposed off are mineral oil, silicagel, gasket and other 

insulating material. For disposal of these items, following procedure is 

recommended. 

4.6.1 INSULATING OIL : 

The transformer oil is mineral Hydro Carbon (Petroleum) oil. No special 

risks are involved in the handling and use of transformer oil. Howerver, attention 

is drawn to the need for personnel hygiene i.e washing of skin & clothing, 

which has come in contact with oil by personnel dealing with these products. 

Hands in such cases should be washed carefully before eating and drinking 

and contaminated clothing should be changed. Also inhalation of fumes or 

vapours should be avoided. 

When used oil has to be disposed off, certain precautions are necessary 

to avoid risk of environmental pollution such as large spillage and leakage 

from the containers, which may otherwise result into destruction of greenary, 

water birds, fishes. 

Normally, non contaminated oil free of PCB (Poly Chlorinated Biphenyls) 

can be destructed by burning or sent to local authorities for asphalt production. 

However contaminated oil containing PCB shall be filled in drums and sent to 

local authorities for disposal. 

4.6.2 SILICAGEL : 

When silicagel can not be regenerated and to be disposed off, it can be 

dumped in a pit and covered with earth. 

4.6.3 GASKET AND OTHER INSULATING MATERIAL : 

Crok Gaskets & Nitrile Rubber Gaskets can be disposed off by burning 

in a place separately marked and prepared for the purpose. In case these can 

not be burnt, these can be dumped in a pit and covered with earth.

instruction manual - BHEL Bhopal

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