trafo erection and commissioning

7/27/2019 trafo erection and commissioning

1/30

1. GENERAL

1. Introduction

The transformers are designed, manufactured and tested with care. With proper attention

during Installation and use, the user should receive from it the service, which he may reasonably

expect. The purpose of this Instruction Book is to explain how the transformer should be

installed, commissioned and maintained. The book is necessarily very general in character.

No transformer should have rated voltage applied to it until all the preliminary work and

checks described hereunder have been completed.

No high voltage tests should be made without making reference to the manufacture.

A transformer which has been commissioned and later withdrawn from service for any

considerable time should be re-checked as when first commissioned.

1.1 ARRANGEMENT FOR TRANSPORT

1.1.1 Transformers are thoroughly dried out before despatch. Oil, whether in the transformer

tank or in separate drums, is clean and dry when despatched.

1.1.2 Parts that are liable to be damaged in transit; are removed, packed and despatched in

separate leases. Radiators, bushings, explosion-vent/PRV dehydrating breathers, rollers and

buchholz relays. It always removed during despatch, wheather proof blanking plates being

provided where necessary. Transformers are despatched from the factory in either of the

following conditions:

a) With oil covering core and coils only

This is the usual arrangement wherever transport limitations permit. The tank is made oil

tight. Extra oil for filling up the tank, conservator and other fittings is sent in separate sealed

drums.

b) With no oil in the tank

The transformer is filled with nitrogen. The gas pressure and temperature at the time of

despatch from the factory are painted on the tank cover. Full quantity of oil is sent in separate

sealed drums.

1.2. INSPECTON

1.2.1 Before Despatch

The transformers are inspected and despatched from manufactirer works in first class

operating conditions, suitably packed for transport to destination.

1.2.2 At Site

1.2.2. External

A thorough external examination should always be made immediately on arrival at site.The various packages should be checked against the advice notes. Any discrepancies from the

advice notes should be immediately reported to the insurance underwriters, transporters under

intimation to supplier. In the event of outward damages, open delivery should be demanded

from transporters. Thereupon claims should be lodged with transporters and underwriters under

advice to supplier.

TRANSFORMER ERECTION & COMMISSIONING

267


2/30

1.2.2.2 Internal

An internal examination should be carried out to the maximum possible extent through

the inspection cover. The tightness of nuts and bolts should be particularly checked. Core and

coils should not be lifted unless there is reason to suspect internal damage.

1.2.3 Damage/Loss

1.2.3.1 Damage or loss of any component should be reported to supplier immediately. Photograph

or other useful evidence should be submitted where possible.

1.2.3.2 When the transformer is despatched filled with oil, a sample of oil should be taken from

the bottom of the tank and tested to I.S. 335. If the di-electric strength is below 40 kV BDV

when tested on a standard test gap as specified in I.S. 335, the matter should be reported to

supplier along with insulation resistance values of the various windings to earth.

1.2.3.3 Drums containing transformer oil which have been despatched separately should be

examined carefully for leaks or any signs of tampering. All drums are despatched filled up to

their capacity and any shortage should be reported. The drums should be stored in horizontal

(lying) position, with bung at 45 downwards.

1.2.4 Deterioration

1.2.4.1 The presence of rust or condensation anywhere within the tank indicates that moisture

is present. The transformer should not be energised until it has been dried out by the method

given in clause 2.5 of this booklet.

1.2.4.2 When transformers are despatched gas filled, the pressure and temperature, when

received at site, should be checked against the despatch pressure and temperature painted on

the tank cover. Pressure difference indicates that the seal is ineffective. If the pressure is the

same as the atmosphere, breathing may have taken place or the pressure may have gone down

because of temperature variations. In such a case the temperature of the transformer should

be raised above that of the 5urrounding air and a second measurement taken. If the internal

and atmospheric pressure are still the same, breathing is taking place and drying out may benecessary. This should be referred to supplier giving the pressure measured and the temperature

at which the measurements were taken.

1.3 HANDLING

1.3.1 Apart from the main lifting points designed to take the total weight of the unit, transformers

often have subsidiary lifting points suitable only for particular components. Care must be taken

to distinguish between them.

1.3.2 When lifting a tank, whether empty or with core, coils and oil, the cover must always be

bolted in position with the full complement of bolts.

1.3.3 Transformers should be jacked up using the projections specially provided for that purpose.

Jacks should never be placed under any valves

1.3.4 To avoid undue mechanical stresses the transformers must be kept on reasonably level

foundation.

1.3.5 Regarding oil please refer clause 2.3 of this booklet.

268


3/30

1.4 STORAGE

1.4.1 After arrival at site, it is desirable to erect and commission the transformer with minimum

delay. In Case this is not possible the transformer should be erected at its permanent location

with conservator and breather filled und dry oil filled to the correct level. Whether in storage or

in use, the dehydrating breather must be fitted on all transformers which are oil filled. The

breather incorporates an oil sealing device which must be filled with oil to the level marked to

be effective. The condition of silicagel must be checked periodically, particularly during monsoons.

The floating arm of the oil gauge on the conservator which is locked during despatch should be

released. The conservator and associated pipe work must be checked for rust or any foreign

matter and then flushed with dry oil before fitting. Breather inspection should be carried out.

Wherever possible, it is most desirable to keep the transformer energised even at a low voltage

so that the .oil temperature is about 10/150C higher than the surrounding ambient temperature.

1.4.2 All other accessories should be stored in a covered dry place. It should be ensured that

there is no damage to the gasketted joints and that all the blanked joints are tight.

1.4.3 Indoor type. transformers must be protect9d from the whether at all times. Outdoor

transformers may be stored outside, but it is preferable to store all transformers in a warm room

in which the temperature variation is small.

1.4.4 During the temporary storage, the tank should be kept on beams to enable free flow of air

underneath, unless the construction of the tank and base itself permits such a flow of air.

1.4.5 The 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.

1.4.6 Heaters for marshalling kiosks, etc. should be kept energised. If, for this purpose, power

supply at the appropriate voltage to suit the heaters is not available, temporary heaters should

then be installed to suit the site voltage. The provision of heaters is important and failure to

provide them may result in extensive damage to the contents of the kiosk because of condensation.

1.4.7 All exposed paint work should be examined periodically. If any touching up is required. it

should be carried out as described under clause 2.22 of this booklet.

1.4.8 Parts despatched separately are generally packed in cases or crates, although certain

major items are sometimes despatched without packing. All items should be stored unreceived

in a dry and covered place provided there are no sign of damage or rough handling.

1.4.9 If oil received in drums is not likely to be used immediately the drums should be stored in

a covererd space where the temperature variation is minimum, If it is necessary to store the oil

outside, adequate protection must be provided at all times. Drums should not stand on end but

should be placed on their sides with the bung at 45 downwards.

269


4/30

Fig.2TypicalUnloadingArrangementoftheTransformer

270


5/30

2 INSTALLATION

2.1 PRECAUTIONS

2.1.1 Workmen having access to the interior of a transformer should empty their pockets of all

loose articles. Any spanners or other tools used should be securely tied with a tape so that

they can be recovered, if accidentally dropped.

2.1.2 Fibrous cleaning material should not be used. The presence of loose fibers in suspension

in transformer oil can reduce its insulating properties. If any cleaning or wiping is necessary,

this should be done with clean and dry oil, using soft-non fluffy cloth.

2.1.3 All components despatched separately should be cleaned inside and outside before being

fitted. Rusted internal surfaces should receive special attention.

2.1.4 Transformer oil is inflammable and, under certain circumstances in a confined space, may

become explosive. Naked lights and flames should never be used near a transformer.

2.1.5 Transformer oil and the cellulose-based solid insulations normally used in transformer

construction absorb moisture easily, particularly when cold. Condensation can be caused if the

interior of a tank is exposed to atmosphere. But by taking adequate precautions this can be

minimised.

2.1.6 Whether despatched with or without oil, a transformer is best protected, from damp

hazards by circulating warm, dry, deaerated oil through it until its temperature is 5C to 10C

above ambient. This should be done before allowing external access to the interior of the tank.

The warm oil should be circulated all the time the transformer is open to atmosphere.

2.1.7 Any air entering a transformer through leaky joints or with the oil may get lodged in

vulnerable places such as windings. To prevent this, the oil pump and all joints in the oil pipe

work should be air tight.

2.2 LOCATION AND SITE PREPARATION

2.2.1 No special foundation is necessary for the installation of a transformer except a level floor

strong enough to support the weight and prevent accumulation of water. Foundation incorporating

special oil drainage facilities during fire and emergency is strongly recommended for large

transformers.

2.2.2 Transformer should be placed on the foundation so that easy access is available all

around and diagram plates, thermometers, valves, oil guages, etc., can be easily reached or

read. Adequate electrical clearances are also to be provided from various five points of the

transformer to earthed parts.

2.2.3 Type ON transformers 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.

2.2.4 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 clause

7.3 of I.S. 1886-1967.

271


6/30

2.2.5 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.

2.2.6 Where rollers are fitted, suitable rails or tracks should be used and the wheels locked toprevent accidental movement of the transformer. Where walls are provided it should be ensure

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.

2.3. Oil

2.3.1 When. Transformer oil is despatched to site separately. it is usually in sealed steel drums.

At the time of filling the drums it is ensured that the oil is filtered clean and dry.

2.3.2 If the drums are stored outdoors, they should be on their sides with the bungs downwards

at 45 to vertical.

2.3.3 Specification

The oil to be used for filling or topping up the transformers should be as per IS : 335 .

3.2.4 Precautions

2.3.4.1 Oil is easily contaminated. It is very important when sampling the oil and filling the tank,

to keep the oil free from contamination.

2.3.4.2 All equipment used for handling the oil should be cleaned and flushed with clean

transformer oil before use (the oil used for washing must be discarded). Particular attention

should be paid to the cleanliness of bungs, valves and other points where dirt or moisture tends

to collect.

2.3.4.3 For sampling, glass containers with glass stoppers are to be preferred over the metal

type which are susceptible to contamination by dirt. etc. Cleanliness is essential as even small

amounts of dirt and water will affect the accuracy of test results. Wax should not be used for

sealing the oil sample bottles, However, the stopper can be covered by a pack of silicagel tied

in a piece of cloth.

2.3.4.5 Flexible steel hose is recommended for handling insulating oil. Some kinds of synthetic

rubber or PVC hoses are also suitable but only those known to be satisfactory should be used.

Ordinary rubber, hose should not be used for this purpose as oil dissolves the sulphur from the

rubber and is thereby contaminated. Hose used for handling oil should be clean and free from

loose rust or scales.

2.3.4.6 Transformers must always be disconnected from the electricity supply system before

the oil level in the tank is lowered. .

2.3.4.7 Oil must not be emptied near naked lights as the vapour released is inflammable.

2.3.4.8 Minute quantities of moisture (particularly in the presence of fibres or dust) lower the

di-electric strength of the oil. To reduce the risk of condensation and moisture entering the oil.

containers taken into a warm room should not be opened until the entire body has attained the

same temperature as the room.

272


7/30

2.3.5 Sampling and Testing

Oil takes up moisture readily and its condition should always be checked before use. Oil

of a muddy colour is certain to be wet. Water and water saturated oil are both heavier than dry

oil and sink to the bottom of any container. Samples should therefore, be taken from the

bottom. Sample should not be taken unless the oil has been allowed to settle for 24 hours if

from a drum and for several days, if from a large transformer.

2.3.6 Samples from tank

Dirt from the draw-off valves or plug should be removed. To ensure that valve is clean,

some quantity of oil should be allowed to flow into a separate container before collecting

sample for testing. Samples should be collected in clear glass receptacles so that any water

present will settle at the bottom of the receptacles and will be clearly visible. Only glass bottles

with stoppers should be used for sampling.

2.3.7 Samples from drum

The drum should first be allowed to stand with bung vertically upwards for atleast 24

hours. The area around the bung should be cleaned. A clean glass or brass tube long enough toreach within 12 mm of the lowermost pmt of the drum should be inserted, keeping the uppermost

end of the tube sealed with the thumb whilst doing so. The thumb should be removed, allowing

oil to enter the bottom of the tube. The tube is to be resealed and an oil sample withdrawn.

The first two samples should be discarded. Thereafter the samples should be released into a

suitable receptacle.

2.4 PROCESSING OF OIL

In case when transformer is supplied with gas, it is customary to supply oil in drums. It

is not possible to filter each and every drum and test it before filling in transformer. It is

therefore necessary to have a container with suitable fittings so that the oil can be processed

to the required quality.

The figure 5 shows a typical arrangement of oil storage tank along with the fittingsrequired.

This storage vessel can be transported on a truck and can be lifted empty through the

lifting holes provided on the base channel.

2.4.1 Oil Filling of Transformer Tank

Before filling oil in the tank, it should be tested to meet the following requirements:

BDV PPM

(i) For transformers upto 132 kv class 60 kv 15

(ii) For transformers above 132 kv class 70 kv 10

In case the oil does not meet the requirements, it should be processed and should only

be used when it meets the requirements. For transformers despatched gas filled, the filling of oil

inside the tank should be done under vacuum. Transformers of high voltage ratings have their

tanks designed to withstand partial or full vacuum. The safe vacuum as consistent with the

tank design should be used for filling the oil. When filling a transformer with oil, it is preferable

that the oil is pumped from the bottom of the tank and that a filter press or other reliable oil

drying and cleaning device is interposed between the pump and the tank. If the maximum

suitable vacuum is maintained in the interior whilst oil is being pumped in, the hazard of bubbles

lodging in inaccessible corners of winding is very much reduced. Any air that remains will

273


8/30

dissolve in the partially ceaerated oil when the vacuum is broken. It should also be ensured that

the tap changer board, condenser bushing, relief vent diaphragm, buchholz relay, conservator,

etc., are not subjected to full vacuum as they are not designed for the same.

2.5 DRYING OF TRANSFORMERS

2.5.1 The process of drying out a transformer is one requiring care and good judgement. If the

drying out process is improperly performed, great damage may result to the transformer insulation

through overheating, etc. A properly dried out and correctly installed transformer is one of the

most reliable electrical appliances.

2.5.2 In no case should a transformer be left unattended during any part of the dry out period.

Transformer should be carefully watched throughout the dry out process and all observations

carefully recorded.

2.5.3 Drying of core and coils using oven

2.5.3.1 Where a suitable oven is available the core and coifs can be effectively dried in it by

raising the temperature to a level not exceeding 80C. This applies to oil-soaked transformers

also. A large volume of air should pass through the oven to remove moisture and vapour. Aninsulation resistance check will indicate when the coils are dry. The speed at which air impinges

on the core and coils must not exceed 600 metres per minute. The oil, in which the dry core

and coils will subsequently be immersed, must be processed and must meet the requirements la

ill down in clause 2.3.1.

2.5.3.2 An alternative method can be used where transformer is to keep the core and coils in

its own tank in an oven. This method can be used where the tanks are designed for full

vacuum. The vacuum will be drawn from the top of the tank by connecting a suitable pump to

any valve fitted at the top of the tank. The temperature in the interior of the tank should not

exceed 75C. Care should be taken that the heating and application of vacuum should be

alternated in sequence. Dry nitrogen gas should be used whenever vacuum is required to be

broken.

2.5.4 Drying out a transformer using Streamline filter machine

2.5.4.1 The most practical method of drying out is by circulation of hot oil through a Streamline

filter machine incorporating oil heater and vacuum chamber (or other oil cleaning and moisture

removing device). Before using any filter machine it should be ensured that any other oil filled in

the filter machine is first drained and refilled with oil supplied along with the transformer. The oil

should be circulated through the filter machine several times and oil sample should be tested. If

test results are in order, the filter machine can then be used for circulating the oil through

transformer. The vacuum pump of the filter machine should have the capacity of creating

vacuum as high as possible, but not less than 710 mm of Mercury. Where possible a vacuum

pump can be connected to the top tank cover to keep the oil in tar1k under vacuum consistent

with tank suitability. This may speed up the drying out process. It is preferable to lag or blanketthe transformer tank to prevent loss of heat. To start with, oil is drawn from the bottom and let

into the transformer at the top. This will remove any settled moisture/impurities. After about 8

to 12 hours circulation in this manner, the cycle is reversed i.e., oil is drawn from the top and

fed at the bottom.

274


9/30

2.5.4.2 The oil temperature as measured by the oil temperature indicator should be of the order

of 75C it should be seen that the oil temperature at the filter machine in no case exceeds

85C. The circulation should be continued till the insulation resistance and oil samples taken

from various points test satisfactorily.

2.5.4.3 The IR values taken at regular intervals should be plotted against temperature readings.It will be observed that, in the beginning, IR values drop down as the temperature goes up. The

IR values will be low till moisture is present inside. Once the moisture is removed, the IR values

will start rising and then become steady.

CURVE SHOWING VARIATION OF INSULATION

RESISTANCE WITH TIME OF DRYING

Fig.3 Curve Showing Variation of Insluation resistance with time of drying

275


10/30

IR-TEMPERATURE CURVE

Fig.4 IR - Temperature Curve

2.5.4.4 If necessary equipment is available, the power factor reading can be taken, which gives

a reliable check on the condition of the solid insulation in a transformer. Capacitive power factorangle is different for each transformer design and the factory tested figure at a suitable

reference temperature can be made available to the user, on request at the order stage.

2.5.4.5 The transformer can also be heated up by short circuiting the low voltage wind inn andsupplying a reduced voltage at high voltage terminals. The supply voltage should be maintainedin such a way that the current in the windings does not exceed 70% of normal full load currentand the oil temperature, as mentioned above, is about 75C. In this case, temperature of oilshould be measured at the bottom of the tank also. Constant watch is to be kept to ensurethat the temperature limits are not exceeded. The temperature of the windings which can be

measured by the following formula should in no case exceed 90C.

R2 (235 + T1)-235

T2 = ----

R1Where T2 = final average temperature of the copper.

T1 = Initial temperature of the copper.

R2 = Final resistance of the windings.

R1 = Initial resistance of winding.

This method is more effective in drying the insulation but requires care und supervision

of an experienced engineer.

276


11/30

Fig.

5

277


12/30

2.5.4.6 IR Temperature relation

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

converted at different temperature. We have drawn a curve (fig. 4) 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 forconversions of IR values at different temperatures. A simple example is given as to how to

calculate the value at different temperatures. Suppose IR value at 70C is 300 M. Ohms and

we want to convert at 40C, the curve will give you a factor K for difference in temperature i.e.

(70 - 40) = 30C

K for 30C temperature difference = 4.2

So the value at 400C = 4.2 x300 = 1260 M. Ohms. Value thus converted fairly tally with

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

Difference 10 20 30 40 50in temp. ( C)

Factor K 1.65 2.6 4.2 6.6 10.5

2.6 GASKETS AND GASKET JOINTS

2.6.1 Transformer gaskets are of neoprene bonded cork and 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, or in U-shaped pieces from which complete gaskets can be built

up.

2.6.2 Only scared joints should be used i.e. a 40 mm scarf in 5 mm thick material is recommended.Joints should be located away from corners and bolt halos, and should be well bonded smooth

and free from local thickening, Neoprene solution is used as an adhesive for joints.

2.6.3 Gaskets are best stored in hermetically sealed containers in a cool place. They must be

protected from damp, oil and grease.

2.6.4 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 inmaking joints, and in handling the gasket, it is possible to break and re-make a joint several

times, using the same gasket.

278


13/30

279


14/30

2.6.5 For making leak proof and good gasket joints, it is necessary that uniform pressure is

achieved all over the gasket after mating rims/flanges have boon clamped with bolts/studs.

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

(i) Tighten lightly the bolts/studs diagonally in the sequence as shown in the figure given

below:

Fig. 7

(ii) Tighten again bolts /studs in the same sequence with full pressure till gasket thickness isreduced to 2/3 of original thickness.

(iii) Do not overtighten otherwise gasket will get crushed.

2.7 BUSHINGS

2.7.1 Bushings are usually removed for transport and the bushing holes blanked off. Before

fitting, bushing should be examined for damage or cracks which may have occurred during

transit. They should be cleaned with a dry non-fluffy cloth. Bushings are normally despatched

fully assembled and filled to the correct level with the appropriate filling medium. Where a fluid

filling is used, a check of the level must be made and the bushing filled up to the correct level,

if necessary. Any leakage should be checked and stopped before the bushing is fitted.

2.7.2 Bushings should be assembled with their adaptors. if supplied, using the proper gaskets

for the various joints. When clamping the bushings to the tank, care should be taken to ensure

that an even pressure is obtained on the flange of the porcelain and its gasket by tightening

the diagonally opposite nuts alternatively. The line connections to all types of bushings should

be flexible to ensure that there is no strain exerted on the bushings because of line movement

such as swing, expansion. contraction. etc. If arcing horns are fitted, the gaps should be

carefully checked and reset to the values shown on the outline drawing or give in the instruction

book supplied along with the transformer.

For condenser bushing, please refer separate leaflet.

2.8 CABLE BOX

2.8.1 The laying of cables, connecting up and filling the cable boxes are usually the responsibility

of the cable contractor and as such have not been dealt with here.2.8.2 The cable box should be filled to the correct level with suitable filling medium (I S : 7084

class III) before energizing. lf the transformer is delivered to site with the cable box detached,

this should be assembled on the transformer before filling with compound and allowed to stand

with full oil head behind it for 24 hours. This is to check against possible oil leaks into the

compound.

280


15/30

Fig. 9

281


16/30

2.8.3 The cables should be supported separately to ensure that no undue strain is exerted onthe porcelain terminals in the cable box. The filling holes should be made weatherproof.2.8.4 Hot compound should never be poured directly on the porcelain of insulator.

2.9 CONSERVATOR

2.9.1 As the temperature of the transformer oil increases or decreases, there is a correspondingrise or fall in the oil volume. To account for this. an expansion vessel (conservator) is connectedto the transformer tank.2.9.2 Conservator, where fitted. should be assembled with its pipe work. etc., making sure thatall gasket joints ,He oil-tight and the pipe work is clean and free from moisture. The mechanismof the float type oil gauge inside the conservator is locked to prevent damage during transit.After placing the conservator in position, it should be released by turning the locking bolt in thedirection indicated on the plate.2.9.3 While topping up oil in the transformer. it should be ensured that oil is filled to the levelindicated by the oil gauge on the conservator and commensurate with the oil temperature atthat lime.

2.10 SILICAGEL DEHYDRATING BREATHERSSilicagel breaths arc despatched as loose items to prevent silicagel getting wet by oil in

the breather cup. The plug sealing the top of the breather should be removed and connectedto the associated pipe work. The oil cap at the bottom of the breather should be removed filledto the mark on the inside wall with transformer oil and the cap replaced. The oil seal ensuresthat silicagel is prevented from absorbing moisture when the transformer is not breathing (referto separate leaflet on breathers).

2.11 EXPLOSION VENT2.11.1 A temporary cover is provided over the explosion vent flange on the tank cover. Thisshould be removed and the explosion vent fitted with suitable gaskets, care being taken toensure that the top diaphragm with its gaskets makes on air tight joint. The explosion vent issent blanked from works and the top blanking plate should not be removed till the oil level inside

the transformer comes above the tank cover.2.11.2 Where a transformer is provided with a buchholz device, a bottom paper diaphragmwhich acts as deflector plate, is fitted at the base of the explosion vent to ensure that anygases liberated in the transformer will pass through the buchholz relay and not collect in thevent. A hole in the side permits the oil to enter the vent and equalise the pressure on either

side of the deflector diaphragm.

Arrangement of Explosion Vent

Fig. 9

282


17/30

2.11,3 In transformers of large ratings the space above the oil in the vent is usually connected

by n pipe to the top conservator to equalise the pressure and vent it to tip. atmosphere

through the breather. For transformers supplied without equalising pipe an air-cock is fitted at

the top which must be opened when the tank is being filled with oil and closed again.

2.12 PRESSURE RELIEF VALVE

In some of the temperature where explosion vent is not provided, Pressure Relief valve

is provided. Pressure relief valves are usually removed for transport and the mounting holes on

the transformer tank over blanked off. Examine the valve for damages, if any, before mounting.

Check the continuity of contacts with Avometer. The flag operation should be checked manually

by lifting the switch operating rod.

The following checks should be observed before installation of Pressure Relief Valve on

the transformers:

(1) Use gasket supplied along with the valve.

(2) Before P. R. V. put to service each valve should be cleaned with compressed air.

The rubber gasket supplied with the valve should be cleaned and located properly in the

recess carefully. All the bolts should be tightened evenly and sequentially so that equal pressureis exerted on gasket and base. The details of the pressure relief valve are as per figure no. 8

2.13 BUCHHOLZ RELAY

2.13.1 The buchholz relay is connected in the oil pipe work between the transformer tank and

the conservator: It a valve is fitted to isolate the conservator, the buchholz relay is usually on

the transformer tank side of the valve. The buchholz relay should be mounted in the pipe work

only alter oil level inside the transformer is above the tank cover.

2.13.2 To ensure successful operation of the relay, the pipe work on either side of the relay and

the relay itself, are all set at the same angle (3 - 7 to the horizontal for double float relay). A

machined surface is provided on the housing of the relay to check the level. The arrow on the

housing should point towards the conservator. The gas release pipe, when provided, is connected

to the top petcock of the buehholz relay. In service, the top petcock should be kept open and

the gas release pipe should be full of oil. When the gas is to be collected through the gas

release pipe oil will flow out initially followed by the gas.

Conservator tank oil pipe work showing the

portion of buchholz protection

Fig. 10

283


18/30

2.13.3 Before putting the transformer into service. all air which may have collected in the relay

should be allowed to escape through the top petcock. The alarm and trip terminals are clearly

marked for function and polarity. When making the connection from these terminals in conduits.

care must be taken that condensate should not drain into the terminal box. For details refer

separate booklet on buchholz relay.

2.14 TEMPERATURE INDICATORS

2.14.1 Oil temperature and winding temperature indicators operate on the principle of liquid

expansion and provide local indication of top oil and winding hot spot temperature respectively

at the marshalling kiosk. The thermometer bulb is connected by capillary tubing to the local

indicator. The bulb is enclosed in a pocket which is normally fixed in the transformer cover in the

hottest oil region. The oil temperature and winding temperature indicators are provided with a

maximum pointer and mercury switches as required by the customer. The oil temperature

indicator is the same as the winding temperature indicator except for the CT circuit and heating

element on the pocket.

2.14.2 In the case of WTI of the Heated Bellows type, heating element is incorporated in theinstrument itself. thus eliminating the heating element in the pocket.

2.14.3 The steel capillary tube is protected by a copper tubing which is strong enough to

withstand normal handling. It should not, however be bent sharply or repeatedly and should be

supported by clips to prevent sagging. On no account should it be cut. The thermometer

pockets for OTI and WT/ should be filled with transformer oil,

2.14.4 Normally winding temperature indicator is provided on one phase of the LV winding.

Where required, WTI can also be provided on the HV winding and also on different phases.

2.14 5 Before installing. the accuracy of the instrument snou1d be checked by hot oil or water

both. The mercury switches are adjusted to make contact at the desired temperatures depending

upon site conditions i.e., ambient temperature, loading condition, etc.

214.6 The CT connection of the WTI is made to the thermometer pocket as per instructions

given on the WTI terminal board. For details. refer separate book let on temperature controller.

2.15 COOLING EQUIPMENT

2.15.1 Radiators

Individual radiator, pipe work and headers should be cleaned and flushed with clean dry

oil before fitting. These should be assembled as shown in the outline drawing. The radiators

should then be filled with...clean and dry transformer oil. Air should be release from top of the

header and also from each radiator.

The inter-connecting valves between the tank and radiators are opened only when dry

out is completed.2.15.2 The fans and pumps should be wired to the respective terminals in the marshalling kiosk

and the direction of rotation should be checked. For details of pumps refer separate booklet.

2.15.3 OFW Coolers: Please refer separate booklet on OFW coolers.

284


19/30

2.16 ON-LOAD TAP CHANGER

2.16.1 The tap changer, if despatched separately, is to be fitted onto the tank. The insulation

resistance value of each tap changer lead to earth should be measured and the cause of low

values, if any, investigated. The leads from the tap changer should then be connected to their

respective positions on the terminal board provided on the tank. The tightness of all connections

on the selector switch and terminal board should be ensured, Any protective taping on the

diverter switch bushing is to be removed.

2.16.2 It is advisable to fit the tap changer on the transformer even during storage and

connect it to the conservator and breather.

2.16.3 In some cases, it may be difficult to get good IR values if there is moisture in the tap

changer terminal board or on any of the parts of the selector switch and diverter switch. It

should, therefore, be ensured that the selector switch and diverter switch are filled with clean

dry processed oil. To remove moisture from the terminal board, it will be preferable to circulate

the oil in the selector switch tank and the main transformer tank simultaneously and at as high

a temperature as permissible. If some other special instructions are provided, the same should

be followed.

2.16.4 For internally mounted (type MR) tap changer please refer separate booklet.

2.17 OFF CIRCUIT TAP SWITCH

Before changing taps on transformers designed for off circuit tap changing, isolate the

transformer from supply on all windings. It is to be noted that tap change is complete only

when the tap switch handle completes one full revolution. To prevent damage due to inadvertent

operation, the tap switch handle should never be left halfway and/or unlocked.

2.18 CURRENT TRANSFORMERS

All CT secondary terminals should be short-circuited or loaded before energising the

transformer. This will prevent excessive voltages developing across the CT secondary whichcan damage the CT and be a hazard if touched.

2.19 EARTHING

2.19.1 The tank should be permanently and effectively earthed by means of connections to the

earthing terminals provided for the purpose at the bottom of the tank. (Refer Indian Electricity

Rules for details on earthing).

2.19.2 A good low-resistance earth is essential for adequate protection. The earthing connection

should be of sufficient cross-section to carry the l V line current for 30 seconds. Stabilising

winding, if supplied, for the suppression of harmonics, should have one point earthed to prevent

it from developing a high potential. Other accessories such as fan motors, oil pump motors,

etc., should be also earthed.

2.20 PAINT WORK

2.20.1 All metal parts are given three coats of high quality paint before despatch from the

works. The first, which is applied to the clean metal surface, is a primary coat, followed by an

intermediate coat and a final finishing coat. which is normally dark grey to shade no. 632 of 1.5.

5 or B.S. 381C unless specified differently by the purchaser. If the point work has been

damaged during or erection, touching-up should be carried au with tl1o least delay.

285


20/30

2.20.2 The following paints which are available from our approved suppliers should be used.

FIRST AND SECOND COATS OF PRIMER

THIRD AND FINISHING COATS

2.20.3 It the recommended paints are not available. a high Quality alkyd resin-based paint may

be used. The surfaces to be repainted should be clean and free from grease and rust. If rust

has begun to form; the surrounding area should also be cleaned, thoroughly as rust tends to

spread beneath paint work.

2.20.4 Failure to maintain the paint work in good condition will result in considerable deterioration

of the apparatus during storage or service.

2.21 TOPPING UP OF OIL IN TRANSFORMER

After completing mounting of all fittings on the transformer. the transformer should be

topped up with clean and dry oil meeting the requirements as given in clause 2,3.3 through thebottom filter valve with the help of a filter machine. Care should be taken to keep all air releaseplugs and valves open to allow escape of trapped air. The filling is to be continued till theappropriate level, indicated on the conservator corresponding to the oil temperature, is reached.The filling should then be stopped and air release plugs and valves should be closed.

Sl.No. Supplier Name Reference

1

2

3

4

5

6

Asian Paints

Addisons

British Paints

Alkali Chemicals

Garware Paints

Goodlass Nerolac

32 / F 3257

LS 2907

BPL Red Oxide Zinc chromatePrimer

32 781

RR 4460

205 /023

1

2

3

4

5

6

Shalimar

Goodlass Nerolac

Asian Paints

Jenson & Nicholson

Alkali Chemicals

British Paints

Superlac Synthetic Enemel

500 / 850

Apcolite Synthetic Enamel

Robbialac Superlac

Deluxe Synthetic Enamel

LUXOL 3 HIGLOSS Synthetic Enamel

286


21/30

2.22 COMPLETION OF ERECTION WORKS

Any other work such as wiring of various alarm/trip contacts from various accessories of

the transformer to marshalling kiosk. settings of relays, checking of protection scheme, etc.,

should be completed before the transformer is cleared for commissioning,

2.23 PARALLEL OPERATION

2.23.1 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.

2.23.2 The voltage 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 below indicates the more common permissible combinations.

2.23.3 Reference to the connection diagrams of two transformers should indicate the terminals

to be paralleled.

2.23.4 The reversal of two leads on either side of a three-phase transformer will reverse the

polarity. Changing them in sequence (e. g. from A. B. C. to B.C.A. or C.A.B.) will swing thevectors through 1200

2.23.5 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.

TRANSFORMER (B)

HV Delta Star Delta Star

LV Delta Star Delta Star

H.V./L.V.

Delta / Star Yes Yes No No

TRANSFORMER (A) Star / Delta Yes Yes No No

Delta / Star No No Yes Yes

Star / Delta No No No Yes

3. COMMISSIONING

3.1 After the transformer is installed in accordance with Section 2 undernoted

precommissioning tests and checks on instruments should be done before putting the transformerin service.

3.2 DRY OUT TEST

3.2.1 IR values of various windings to earth should be checked. While checking these values;

no external lines. lightning arrestors, etc., should be in circuit. Bushings should be throroughly

cleaned with a dry clean piece of cloth before taking I R values. The IR values obtained should

287


22/30

be comparable with those indicated in BHEI test report. In humid weather condensation on the

sheds can affect IR values.

3.2.2 After filling the oil in the transformer, it will again be re-conditioned as per cl. no 2.4 till

the following characteristics are obtained:

BDV PPM(i) For transformer upto 132 kV 50 kV 15

(ii) For transfollllP.r above 132 kV 60 kV 10

3.3 RESISTANCE MEASUREMENT OF WINDINGS

Winding resistance should be measured at all tap positions.

3.4 RATIO TEST

Ratio test between HV and LV at all taps should be carried out. For 3 winding transformers

ratio between other pairs of windings should also be taken.

3.5 PHASE RELATIONSHIP TEST

Test for verification of phase relationship and vector group should be conducted at site.

3.6 TAP CHANGER TEST

a) Manual operation.

b) Local electrical operation.

c) Remote electrical operation

d) Parallel operation.

e) The operations should be checked from one end of the range to the other to check for

limit switch operations also.

f) IR values of motor and control wiring should be checked with 500 V megger.

3.7 BUCHHOLZ RELAY TEST

Buchholz relay operation for alarm and trip should be checked by injecting air through

the test petcock.

3.8 LOW OIL LEVEL ALARM

The low oil level alarm of the oil gauge should be checked.


The contacts of OTI and WTI for alarm, trip and control (where provided) should be

checked and set at required temperature.

3.10 COOLING GEAR TEST

a) The IR values and settings for operation of oil pump and fan motors should be checked.b) Alarm trip contact settings of differential pressure gauge, oil and water flow indicators,

where provided, should be checked.

3.11 MARSHALLING KIOSK

The wiring from various accessories to marshalling kiosk should be checked.

288


23/30

3.12 PROTECTIVE RELAYS

Tripping of the associated breakers is to be proved b actual operation of the differential

relay, over current relay, earth fault relay and other protective relays as applicable.

3.13 MAGNETISING CURRENT

The magnetising current can be measured by feeding 400 V. 3 phase, 50 Hz supply from

HV side keeping LV side open circuited and the values in different phases could be compared.

3.14 ADDITIONAL CHECKS

Checks should be made to see if :

a) All oil valves are in correct position closed or opened as required.

b) All air pockets are cleared.

c) Thermometer pockets me filled with oil.

d) Oil is at correct level in the bushings, conservator diverter switch. tank, etc.

e) Earthing connections are made.

f) Colour of silicagel is blue.

g) Bushing arcing horn gap is set correctly.h) CT polarity is correct (when bushing mounted CTs are provided).

If all the above tests/checks are found satisfactory a setting time of atleast 24 hours

should be allowed for the oil and air released from all points at 6 hourly intervals, before

commissioning the transformer. Wherever possible, while commissioning, the voltage should be

built up slowly and brought to the full level in about 4 to G hours. If the breaker is tripped on

differential. buchholz or any other device. the cause must be investigated before re-energising

the transformer. Sometimes tripping may be because of magnetic inrush which depends upon

the switching at a particular moment in the cycle. After commissioning magnetising current

should be measured by connecting with a meter in metering CT circuit and the reading should

be compared with the test report value.

4. MAINTENANCE

4.1 If a transformer is to give long and trouble -free service, it should receive a reasonable

amount of attention and maintenance. Mi1intenance consists of regular inspection, testing and

reconditioning where necessary. Records should be kept giving details of any unusual occurrence

and also of any test results taken.

The principal object of maintenance is to maintain the insulation in good condition.

Moisture, dirt the excessive heat are the main causes of insulation deterioration and avoidance

of these will, in general, keep the insulation in good condition.

No work should be done on tiny transformer unless it is disconnected from all external circuits

and all windings have been solidly earthed.

Note: The general notes and information given under storage, handling and installation etc. are

also applicable to routine maintenance.

4.2 TRANSFORMER OIL

Oil is used as a coolent and di-electric in the transformer and keeping it in good condition

will assist in preventing deterioration of the insulation which is immersed in it.

The peremissible limits of various characteristics as per IEC : 422 are given as per

Appendix A1 and A2 at the end.

289


24/30

A few short notes on the subject are given below

a) The oil level should be checked at frequent intervals and any excessive leakage of oil

investigated. There may be a slight loss of oil by evaporation; this need not cause

concern if the tank is topped up at regular intervals.

b) All minor leaks or seepages should be repaired as quickly as possible.

4.3 CORE & WINDINGS

It is recommended that the core and windings be removed from the tank for visual

inspection as per time schedule given in clause 4.17. The windings should be examined to

ensure that no sludge has been deposited blocking the oil ducts Any loose nuts and bolts

should be tightened.

Before lifting the core and windings from the tank it is usually necessary to disconnect

the windings from the bushings or cable boxes inside the tank, to disconnect the off-circuit tap

switch handle or leads of the on-land tap changer and to remove any earthing strips between

the core and the tank. The core and windings must be removed with great care, under cover

and in a dry place.

4.4 TAP CHANGER

See separate instruction booklet.

4.5 BUSHINGS

Porcelain insulators and rain sheds should be cleaned at convenient intervals when the

transformer is out of service. A metal pan scrubber can be used effectively to remove grime.

During cleaning, the porcelain should be examined for cracks or other defects and defective

ones replaced. Oil level in oil filled bushings should be checked at frequent intervals. The cause

of any serious loss of oil should be investigated. For details See separate maintenance booklet

on Bushings.

4.6 CABLE BOXES

The sealing arrangement for fillings holes should be chucked each year. When screwed

plugs are sealed with a bituminous compound, the compound should be exnll1irwd for cracks. If

the compound has cracked, it should be replaced as the cracks may lead to an accumulation of

water around the plug. Gasket joints should be examined and tightened.

4.7 COOLING EQUIPMENT

Please refer separate booklet.

4.8 CONSERVATOR

Conservators are so arranged that the lower part acts as a sump in which any impuritiesentering the conservator will collect. A valve is fitted at the lowest point of the conservator for

dri1ining and sampling. When sampling, care must be taken to run off any contaminated oil

before taking a sample for testing. The inside of the conservator should be cleaned as per

clause 4.17. A removable end is provided on each conservator for this purpose.

4.9 OIL GAUGES

Oil gauge glasses should be kept clean so that the gauges can fulfil their purpose.

290


25/30

Broken glasses should be replaced immediately; The gauges are normally fitted with armour

plate glass which is unbreakable under normal service conditions. When the conservator is

stripped for cleaning, the mechanism of the float type oil gauge, if fitted, should be inspected

and cleaned.

4.10 DEHYDRATING BREATHER

Breathers when fitted should be examined to ascertain if the silicagel requires changing.

The frequency of inspection depends upon local climate and operating conditions. More frequent

inspections are needed when the climate is humid and when the transformer is subject to

fluctuating load. The crystals of silicagel in the breather are tinted to act as an indicator and

change from blue to pink on becoming saturated with moisture. When the majority of the

crystals have turned pink, the silicagel should be replaced or reactivated. To reactivate the

gel, the crystals should be heated in a shallow pan at a temperature of 1500 C to 200C until

the original blue colour is regained. This usually takes 2 to 3 hours. The oil level in the oil seal

should be maintained up to the level indicated in the cup. (For full particulars see instruction

booklet on breathers.

4.11 BUCHHOLZ PROTECTOR

Routine operation and mechanical inspection test should be carried out as per clause 4.17. The

operation is tested by injecting air into the protector through the air injecting pipe. After

inspection, any air which has accumulated in the gas chamber must be released through the

upper petcock leaving the chamber full of oil.

To carry out mechanical inspection, the oil level should first be brought below the level

of protector and the mechanism removed. The floats should be able to rise and fall freely, and

the mercury switches should be tightly clamped and he in good order. If a switch is defective

swing to the glass being cracked there will be clouding of the glass. In such cases the switch

must be replaced, During operation, if there is an alarm and gas is found to be collecting, the

transformer should be isolated and the gas should be tested and analysed to find out thenature of fault. Sometimes it may be noticed that the gas collected is only air. The reasons for

this may be that the oil is releasing any absorbed air due to change in temperature or due to

leakage on the suction side of the pump. The absorbed air is released normally when no vaccum

is applied during filling of oil and also in the initial stages only. The internal faults can be

identified to a great extent by a chemical analysis of gas.

Buchholz may also give alarm/trip duo to oil level falling below the buchholz level.

Also see separate leaflet of the buchholz relays.

4.12 EXPLOSION VENT

The diaphragm, which is fitted at the exposed end of the vent, should be inspected at

frequent intervals and replaced. if damaged. Failure to replace the diaphragm quickly move

allow the ingress of moisture which, will contaminate the oil. If the diaphragm has broken

because of a fault in the transformer, an inspection should be carried out to determine the

nature and cause of the fault.

4.13 GASKETS

Gaskets sometimes shrink during service. It is therefore, necessary to check the tightness

of all bolts fastening the gasket joints. The bolts should be tightened evenly around the joints

291


26/30

to avoid uneven pressure. Leaking gaskets should be replaced as soon as the circumstances

permits.

4.14 SMALL PIPE WORK

The pipe work should be inspected atleast once a year leaks may be due to slackunions, which should be tightened, or due to badly seated joints caused by a misalignment. In

the latter case, the pipes should be aligned and joints remade.


At each yearly maintenance inspection, the level of oil in the pockets holding thermometer

bulbs should be chucked and the oil replenished if required. The capillary tubing should be

fastened down again if it has become loose. Dial-glasses should be kept clear and, if broken,

replaced as soon as possible to prevent damage to the instrument. Temperature indicators if

found to be reading incorrectly, should be calibrated with standard thermometer immersed in

hot oil bath.

4.16 PAINT WORK

During storage and service the paint work should be inspected once a year and, where

necessary, painting or retouching carried out. If the metal surface is exposed and becomes

dirty, rusty or greasy because of delay in repairing the paint work, the surface must be

thoroughly cleaned before repainting to ensure a good bond between metal and paint. If paints

recommended by supplier are not available, any good quality alkyd resin-based paint may be

used.

4.17 MAINTENANCE SCHEDULE

The maintenance schedule given at Table-A is a good indication 01 the attention required

to be given to the transformer under average conditions.

292


27/30

S.No. Items to beinspected

Inspection notes Action required

HOURLYAmbient temperature -- --

Winding temperature

Oil temperature

Check that temperaturerise is reasonable.

Shutdown the transformerand investigate, if either ispersistently higher thannormal.

Load amps

Load voltage

Check against ratedfigures.

An improper tap position cancause excessive core loss.Voltage and tap position

should be corrected.

DAILY

Relief ventCheck the relief ventdiaphragm to be intact.

Investigate, replace if foundbroken.

Oil level intransformerconservator

Check oil level from oil

gauge.

Top up, if found low.

Oil level in tapchanger diverterswitch.

Check oil level from thegauge glass


Oil level in bushings.Check the oil level fromgauge glass.


Pipework andaccessories forleakages

Inspect the transformerfor leakages.

If leakages are observed,tighten evenly the gasketjoints. Replace 'O' ring orwasher suitably. Replace

gasket if needed.

Dehydrating breatherCheck colour of silicagel. Reactivate/replace with new

charge, if found pink.

Leakages of water intocooler

-- Investigate and rectify aftertaking shutdown.

MONTHLY

Bushings

Examine for dirtdeposition and tightnessof oil filling plugs,Examine for cracks inporcelain.

Clean and tighten plugs.Cracked porcelain should berectified/ replaced.

Transformer oil andtap changer oil

Check for di-electric

strength and moisturecontent.

Take suitable action to

restore quality of oil.

293

TABLE-A


28/30

S.No. Items to beinspected

Inspection notes

QUARTERLY

Cooler fan, bearingand controls

Check contacts, manu

controls and interlock

On load tap changer

Examine contacts, chestep by step mechanioperation, end positiolimit switches,emergency limit switcand brakes.

Dehydrating breather

Check oil level in the and ensure air passagare free.

Ventilators Check that air passagare free.

Bushings

Check the oil dielectric strength moisture content.

HALF YEARLY

Oil cooler Check for pressure te

Insulation resistanceof transformer

Compare the values wcommissioning value.

Tap changer filterwherever provided

Check the free flow ofin the filter.

Cooling fans

Check vibrations ofimpellers and shaft.

NOTES :

1) In addition to the above instructions, reference should also be made to IS : 1886 - Code of

practice for installation and maintenance of transformers.

2) The inspection schedule specified for longer periods automatically includes those specified

for shorter periods.

3) All maintenance test results and observations should be specifically recorded.

4) Where matter given in the inspection book differs from that given in IS or BS referred to,

294


29/30

the transformer user should follow the instruction book.

5) In case of anything abnormal occuring during service, advice from suppliers should be

obtained giving them complete particulars as to the nature and extent of occurence,

together with the nameplate particulars in order to assist identification of the transformer.

6) Di-electric strength of at specified in the instruction manual is breakdown values with 2.5

mm gap - as given in IS : 335/1972.

7) As-efforts are being constantly made to improve designs and service, the transformer

supplied may differ in minor details from data given herein.

8) Any additional information required by the customer will be gladly supplied.

DOS FOR POWER TRANSFORMER

1. Check and thoroughly investigate the transformer whenever any alarm or protection is

operated.

2. Check the diaphragm of the relief vent, if cracked or broken, replace it.

3. Attend the leakages on the bushing immediately.

4. Examine the bushing for dirt deposits and coats and clean them periodically.

5. Check the oil in transformer and OLTC for. di-electric strength and moisture content andtake suitable action for restoring the quality of oil as per table-A of Maintenance Schedule.

6. Check the oil level in oil Clip and ensure air passages are free in the breather. If oil is less,

make up tile all.

7. Check the oil for acidity and sludge as per table-A of Maintenance Schedule.

8. If inspection covers are opened or any gasket joint is tightened then tighten the bolts

evenly to ovoid 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 tile 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 adjust 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 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 table-A 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. Close the equalising valve between tank and OLTC during operation.

23. Connect gas cylinder with automatic regulator transformer is to be stored for long, in order

to maintain positive pressure.

24. Fill the oil in the transformer at the earliest opportunity at site and follow storage instructions.

295


30/30

DONTS FOR POWER TRANSFORMER

1. Do not energise without thorough investigation of the transformer, wt1enever any alarm or

protection has operated.

2. Do not re-energise the transformer, unless the buchholz gas is analysed.

3. Do not re-energise the transformer 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 overload the transformer.

8. Do not leave tertiary terminals unprotected outside the tank, connect them to tertiary

lightning arrestors protection scheme when connected to load.

9. Do not allow WTI OTI temperature to exceed 750C during dry out of transformer, and filter

machine temperature beyond 850C.

10. Do not parallel transformers which do not fulfill the condition given in clause 2.25

11. Do not use low capacity lifting jacks on transformer for jacking.12. Do not move the transformer with bushings mounted.

13. Do not overload the transformer other than the specified limits as per IS : 6600.

14. Do not change the settings of WTI and OTI alarm and trip frequently. The setting should

be done as per the site condition.

15. Do not leave red pointer behind the black pointer in OTI and WTI.

16. Do not meddle with the protection circuits.

17. Do not leave any connection loose.

18. Do not allow conservator oil level to fall below 1/4 level.

19. Do not allow oil lewd to fall in the bushings, they must immediately be topped up.

20. Do not leave marshalling box doors open, they must be locked.

21. Do not switch off the heater in marshalling box except in summer.

22. Do not allow dirt and deposits on bushings, they should be periodically cleaned.23. Do not allow unauthorised entry near the transformer.

24. Do not leave ladder unlocked, when the transformer is ON in service, in case it is

provided.

25. Do not change the sequence of valve opening for taking standby pump and motor into

circuit.

26. Do not switch on water pump unless oil pump is switched all.

27. Do not allow water pressure more than oil pressure in differential pressure gauge.

28. Do not mix the oil, unless it conforms fully to IS : 335.

29. Do not allow inferior oil to continue in transformer. The oil should be immediately processed

and to be used only when BDV conforms to IS : 335.

30. Do not continue with pink silicagel, it should immediately be changed or regenerated.

31. Do not leave secondary terminals of an unloaded CT open

32. Do not commission the transformer in case IR values are not satisfactory.

33. Do not continue to run the transformer with relief vent diaphragm broken or cracked.

34. Do not store transformer for long after reaching site. It must be erected and commissioned

at the earliest.

35. Do not keep the transformer gas filled at site for a longer period.

296

trafo erection and commissioning

Documents