Transformer Manual

INSTRUCTIONS FOR

INSTALLATION, COMMISSIONING, OPERATION AND MAINTENANCE OF

TRANSFORMERS AND REACTORS

A n I S O 9 0 0 1 : 2 0 0 0 C o m p a n y

Transformer Manual

I N D E XSr. No. PARTICULARS PAGE NO.

1.0 LIST OF STANDARDS RELATED TO OUR PRODUCTS1.1 LIST OF INDIAN STANDARDS REFERRED 11.2 LIST OF IEC STANDARDS REFERRED 22.0 TRANSPORTATION, DESPATCH AND STORAGE2.1 TRANSPORTATION 32.2 RECEIPT OF TRANSFORMER AT SITE 32.3 STORAGE 53.0 INSTALLATION3.1 LOCATION AND SITE PREPARATION 63.2 ASSEMBLING THE TRANSFORMER 63.3 GASKETS 73.4 BUSHINGS 73.5 TAP CHANGERS 113.6 COOLING OF TRANSFORMERS 143.7 RADIATORS 143.8 COOLING FANS 153.9 OIL PUMP 153.10 OIL FLOW INDICATOR 163.11 HEAT EXCHANGER 173.12 DIFFERENTIAL PRESSURE GAUGE 173.13 CONSERVATOR TANK 183.14 MAGNETIC OIL LEVEL GAUGE 183.15 COMMISSIONING OF AIR CELL 193.16 GAS AND OIL OPERATED (BUCHHOLZ) RELAYS 203.17 SILICA GEL BREATHER 223.18 PRESSURE RELIEF VALVE (PRV) 233.19 DOUBLE DIAPHRAGM EXPLOSION VENT 243.20 MARSHALLING BOX 253.21 OIL TEMPERATURE INDICATOR 253.22 WINDING TEMPERATURE INDICATOR 253.23 CURRENT TRANSFORMER 283.24 CONTROL CABLING 283.25 ASSEMBLY OF VALVES 283.26 ASSEMBLY OF PIPE WORK 283.27 OIL FILLING 283.28 EARTHING 313.29 COMPLETION OF ERECTION WORK 313.30 TOUCH-UP PAINTING 314.0 COMMISSIONING4.1 GENERAL CHECK POINTS 324.2 DO’S & DON’TS FOR POWER TRANSFORMER 334.3 INSULATION RESISTANCE TEST (MEGGER) 344.4 VOLTAGE RATIO TEST 344.5 MAGNETIZATION CURRENT 354.6 VECTOR GROUP 354.7 MAGNETIC BALANCE 364.8 WINDING RESISTANCE 364.9 OLTC CONTINUITY CHECK 364.10 OIL CHARACTERISTICS 374.11 TESTING OF ON LOAD TAP CHANGER 374.12 OFF CIRCUIT TAP CHANGER 374.13 TEST ON CT 384.14 TESTING OF COOLING CIRCUIT 384.15 TERTIARY WINDING PROTECTION 384.16 PROTECTION AND ALARMS 384.17 DETAILS OF CIRCUIT BREAKER 394.18 ENERGIZING 394.19 RECORD AFTER CHARGING 395.0 MAINTENANCE AND OPERATION5.1 GENERAL 405.2 ROUTINE MAINTENANCE INSPECTION OF OLTC 405.3 OIL FILTRATION AND MOISTURE REMOVAL 425.4 TEMPERATURE SUPERVISION 435.5 IR TEMPERATURE RELATION 435.6 SUPERVISION AND CONTROL OF OIL 445.7 TROUBLE SHOOTING 445.8 GAS ANALYSIS ON TRANSFORMER OIL 445.9 MAINTENANCE SCHEDULE 48

• TARIL Transformers are designed, manufactured and tested with care. With proper attention duringinstallation and use, the user should receive from it the maximum expected performance.

• This maintenance procedure gives a good understanding about handling and maintenance of thetransformer.

• It is recommended to go through all information contained in this before installing, operating andmaintenance of the transformer.

• These instructions have been prepared to provide information on assembly, installation, commissioningand regular maintenance of the transformers and shall form part of the Instruction Manual.

• In operating the Transformer, 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. may please be referred.

• In case there is doubt about any portion or wants more details. Please contact us in the followingaddress.

• If any problem occurs during operation, please inform T&R at following address by specifying thefollowing.

1. SL. NO.

2. YEAR OF MANUFACTURE

3. VOLTAGE

4. PROBLEM FACED

TRANSFORMERS & RECTIFIERS (INDIA) LTD.

Survey No. 344 to 350, Sarkhej Bavla Highway, N.H. 8-A, Village: Changodar,Ta. Sanand, Dist. Ahmedabad - 382210. Gijarat, INDIA

Phone: +91-2717-661661 Fax: +91-2717-661716 Email: info@transformerindia.comwww.transformerindia.com

P R E F A C E

Transformer Manual

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1. LIST OF STANDARDS RELATED TO OUR PRODUCTS

1.1 LIST OF INDIAN STANDARDS REFERREDSr No IS/EC NO AND YEAR SPECIFICATIONS

POWER TRANSFORMERS

A1. SPECIFICATIONS

1 IS 1885 (Part 38) : 1977 Electrical Vocabulary Part 38 Transformers

2 IS 2026 (Part 1) : 1977 Power Transformers: General

3 IS 2026 (Part 2) : 1977 Power Transformers: Temperature Rise

4 IS 2026 (Part 3) : 1981 Power Transformers: Insulation level & Dielectric Tests

5 IS 2026 (Part 4) : 1977 Power Transformers: terminal Marking, Tappings & Connections

6 IS 2026 (Part 5) : 1994 Transformer/Reactor bushings minimum external clearance in air- Specification

7 IS 6600 : 1972 Guide for loading of oil immersed Transformers

8 IS 10561 : 1983 Application guide for Power Transformer

9 IS 335 : 1993 Insulating Oil

10 IS 10028 (Part 1) : 1985 Code of practice for selection installation & maintenance of Transformers

11 IS 10028 (Part 2) : 1981 Installation

12 IS 10028 (Part 3) : 1981 Maintenance

13 IS 1180 Outdoor Type three-phase distribution transformers up to & Including 100 KVA in

11 KV.

14 IS 1180 (Part 1) : 1989 Non Sealed Type

15 IS 1180 (Part 2) : 1989 Sealed Type

A2. MATERIALS, FITTINGS & ACCESSORIES OF POWER TRANSFORMERS.

1 IS 1576 : 1992 Solid Pressboard for electrical purpose

2 IS 2312 : 1976 Propeller type AC ventilating fans

3 IS 3024 : 1965 Electrical steel sheets (oriented)

4 IS 3231 : 1986 1987 Electrical relays for Power Transformers

5 IS 3402 : 1992 Silica gel.

6 IS 3588 : 1987 Electrical Axial Flow fans

7 IS 3637 : 1966 Gas Operated relays

8 IS 3639 : 1966 Fittings & Accessories for Power Transformers

9 IS 4253 (Part 1) : 1980 Specification for cork composition sheets

10 IS 4253 (Part 2) : 1980 Cork & Rubber

11 IS 6088 : 1988 Oil to water heat exchangers for Transformers

12 IS 7404 (Part 2) : 1991 Paper covered Copper conductors. Rectangular conductor

13 IS 8468 : 1977 On-load tap changers

14 IS 9147 : 1979 Cable sealing boxes for oil immersed transformers

15 IS 9700 : 1991 Activated Alumina

16 IS 8478 : 1977 Application guide for On Load Tap Changers

17 IS 5561 : 1970 Electric Power Conductors

18 IS 12943 : 1990 Brass glands for PVC Cables

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1.2 LIST OF IEC STANDARDS REFERREDSR NO IEC NO EDITION YEAR DESCRIPTION

1 60-1 SECOND 1989-11 HIGH-VOLTAGE TEST TECHNIQUES (GENERAL DEFINITIONS AND TEST REQUIREMENT)

2 60-2 SECOND 1994-11 HIGH VOLTAGE TEST TECHNIQUES (MEASURING SYSTEMS)

3 76-1 SECOND 1993-03 POWER TRANSFORMERS (GENERAL)

4 76-2 SECOND 1993-04 POWER TRANSFORMERS (TEMPERATURE RISE)

5 76-3 SECOND 2000-03 POWER TRANS. INSULATION LEVELS ,EXTERNAL CLEARANCE & DIELECTRIC TESTS

6 76-3-1 FIRST 1987 POWER TRANSFORMERS (INSULATION LEVELS AND DIELECTRIC TESTS EXTERNALCLEARANCES IN AIR)

7 60076-5 SECOND 2000-07 POWER TRANSFORMER – ABILITY TO WITHSTAND SHORT CIRCUIT

8 60076-5 THIRD 2006-02 POWER TRANSFORMER – ABILITY TO WITHSTAND SHORT CIRCUIT

9 60076-7 FIRST 2005-12 LOADING GUIDE FOR OIL-IMMERSED POWER TRANSFORMERS

10 60076-8 FIRST 1997-10 POWER TRANSFORMERS-APPLICATION GUIDE

11 60076-10 FIRST 2001-05 POWER TRANSFORMER-DETERMINATION OF SOUND LEVELS

12 60076-10-1 FIRST 2005-10 DETERMINATION OF SOUND LEVELS-APPLICATION GUIDE

13 60076-11 FIRST 2004-05 POWER TRANSFORMERS-DRY TYPE TRANSFORMERS

14 TS 60076-14 FIRST 2004-11 DESIGN & APPLICATION OF LIQUID-IMMERSED POWER TRANSFORMERS USINGHIGH-TEMPERATURE INSULATION MATERIAL

15 60137 FIFTH 2003-08 INSULATED BUSHING FOR ALTERNATING VOLTAGES ABOVE 1000V

16 289 SECOND 1988 BOBINES D’INDUCTANCE (REACTORS)

17 296 SECOND‘ 1982 SPECIFICATION FOR UNUSED MINERAL INSULATING OILS FOR TRANSFORMERS

18 317-0-2 2.1 2000-01 SPECIFICATION FOR PARTICULAR TYPES OF WINDING WIRES

19 396 SECOND 1991-02 TEST METHOD FOR INDUCTION CHANNEL FURNACES

20 554-1 FIRST 1977 SPECIFICATION FOR CELLULOSE PAPER FOR ELECTRICAL PURPOSES

21 554-3-3 FIRST 1980-01 SPEC.. FOR CELLULOSE PAPER FOR ELECTRICAL PURPOSES

22 641-1 FIRST 1979 SPEC. FOR PRESSBOARD & PRESS PAPER FOR ELECTRICAL PURPOSE

23 641-3-1 FIRST 1992-04 SPEC. FOR PRESSBOARD & PRESS PAPER FOR ELECTRICAL PURPOSE

24 60038 6.2 2002-07 IEC STANDARD VOLTAGES

25 60044-1 2000-07 CURRENT TRANSFORMER

26 60214-1 FIRST 2003-02 TAP CHANGERS-PERFORMANCE REQUIREMENTS & TEST METHODS

27 60214-2 FIRST 2004-10 TAP CHANGERS-APPLICATION GUIDE

28 60270 THIRD 2000-12 HIGH VOLTAGE TEST TECHNIQUES - PARTIAL DISCHARGE MEASUREMENT

29 60317-27 THIRD 1998-01 SPEC. FOR PARTICULAR TYPES OF WINDING WIRES

30 60404-8-7 SECOND 2008 MAGNETIC MATERIAL

31 60554-2 SECOND 2001-11 CELLULOSE PAPERS FOR ELECTRICAL PURPOSES

32 60599 SECOND 1999-03 MINERAL OIL IMPREGNATE ELECTRICAL EQUIPMENT IN SERVICE GUIDE TOTHE INTERPRETATION OF DISSOLVED AND REC GASES ANALYSIS

33 62271-209 FIRST 2007-08 HIGH VOLTAGE SWITCHGEAR AND CONTROL GEAR - CABLE CONNECTION FORGAS INSULATED METAL ENCLOSED SWITCHGEAR FOR RATED VOLTAGE ABOVE 52 KV

Transformer Manual

2. TRANSPORTATION, DESPATCH AND STORAGE

2.1 TRANSPORTATION“Transformers,” depending upon the restriction imposed by transport weight and/or other considerations, have to betransported either filled with Oil or Nitrogen as per the description given below.

WITH OILThe Transformer is filled with oil such that sufficient tank volume is left clear for expansion to limit the excess internalair pressure to 0.35 kg/cm2. Fittings dismantled before transport is packed in packing cases in line with packing list.

All openings resulting from removal of fittings for shipment are sealed with suitable blanking plates during transport.

The balance oil quantity required for the complete filling of the transformer is supplied separately.

A transformer dispatched according to this method can be stored at site or elsewhere for one year provided, that allinsulating materials are covered with oil and silica gel breather is mounted and quality of oil is maintained as per IS: 1866.

WITHOUT OIL (FILLED WITH NITROGEN N2)Large oil filled transformers are not normally oil filled during shipping due to weight limitations. To protect the activeparts against moisture the transformer tank is filled with dry Nitrogen at a low internal pressure 0.2kg/cm2 at TARILworks before dispatch.

All openings resulting from the removal of fittings for shipment are sealed with suitable gasketted blanking platesduring transport.

The gas pressure & temperature at the time of dispatch from factory are painted on the tank cover. Full quantity of oilis sent in separate sealed drums.

For control of gas pressure and maintaining the pressure during transport and possible storage before assembling, thetransformer is equipped with two back up N2 Cylinders and is kept in a steel frame provided on tank side.

N2 regulator reduces cylinder high pressure of 120 to 140 kg/cm2 to required low pressure of 0.2kg/cm2. One gaugeprovided on the first stage indicates the cylinder pressure, whereas other gauge provided on second stage indicatesthe tank pressure.

The gas consumption during transport and possible storage is difficult to estimate as it depends on ambient temperaturevariation, possible leakages as well as on the duration of transport and storage. However, two back up cylinders areprovided to meet the gas requirement. Normally one cylinder is kept open and the other shut. When the pressure offirst cylinder falls to 0.5 kg/cm2 this should be considered as empty and immediately its valve should be closed and thevalve of second cylinder should be opened.

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. If for some reasonoil filling is not possible, then nitrogen must be continuously maintained at a positive pressure.

INSTRUCTIONS FOR FILLING DRY PURE NITROGEN GAS WITH BACK UP CYLINDERS (AT WORKS)1. Lower the oil level to the minimum necessary to dismantle the items (such as bushing and turrets) which must be

removed for shipping.

2. Pull 500mm. mercury vacuum after blanking off all openings.

3. Break the vacuum by admitting dry nitrogen through a convenient valve at the top of the tank and drain the oilcompletely.

4. Continue to supply nitrogen until it maintains a steady pressure of 0.14 ± 0.02 kg/cm2 above the atmosphere.

5. Pressure of nitrogen gas shall be maintained at 0.14 ± 0.02 kg/cm2 at same reference temperature. Pressurewould be monitored by taking three readings within 24 hours to ensure that there is no leakage of gas

6. Shut off the gas supply valve and fit dry nitrogen back up cylinders through nitrogen regulator valve.

2.2 RECEIPT OF TRANSFORMER AT SITETo ensure that a Transformer will function satisfactorily it is important that handling, lifting, storing and assembling arecarried out with great care and cleanliness by experienced personnel who are thorough with various working operations.

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This section gives instructions how handling, lifting, storing and assembling should be carried out. For large Transformersit is recommended that the work is done by TARIL or is under supervision by experts from TARIL.

INSPECTIONIn connection with receiving and unloading at site, and at the final storing place before assembling, the transformersshall be inspected carefully for external visible damages as dents, paint damages etc.

After the arrival of the material at receiving points, the customer should, in case of possible damage/loss of anycomponent, make the necessary claims with the contractor’s representatives under intimation to supplier so that suchclaims can be registered with the transport agents. Before unloading, the condition of packing and of the visible partsshould be checked and possible traces of leaks verified (condenser bushings). If necessary, appropriate statements andclaims should be made.

Drums containing oil which have been dispatched separately should be examined carefully for leaks or any sign oftampering. All drums are dispatched filled up to their capacity and any shortage should be reported. As a principle wenever supply partly filled drums.

In order to protect the active part against moisture, the transformer tank is filled with nitrogen during transport at anover pressure of 0.2kg/cm2 approximately at room temperature.

Check immediately the gas pressure at the arrival. A positive pressure indicates that the tank and the transformercomponents 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 pressure of 2 PSIwithout loss of time.

DAMAGE/LOSSDamage or loss of any component should be reported to Supplier immediately. Photographs or other useful evidentshould be submitted wherever possible.

When the transformer is dispatched filled with oil, level in transformer main tank should be checked & if the level isbelow the indicated level in main tank the same should be reported to the supplier immediately & also the sameshould be mentioned in LR copy. When the transformer is dispatched filled with oil, a sample of oil should be takenfrom bottom of the tank and tested to IS: 1866. If the dielectric strength is below 50KV, when tested on standard IS:1866, the matter should be reported to supplier along with insulation resistance values of the various windings toearth.

Drums containing transformer oil which have been dispatched separately should be examined carefully for leaks orany signs of tempering. All drums are dispatched filled up to their capacity and any shortage should be reported to usimmediately.

UNLOADINGThe transformer should be unloaded by means of crane or suitable device of sufficient capacity (Please refer ratingplate for weight detail).

For lifting purpose lifting lugs are provided on the top cover & on the sides of tank. Lifting lugs on the sided of tank arelifting the complete transformer, whereas the lugs provided on the tank cover should be used only for lifting the cover.Lifting lugs for transformer lifting are painted red. Check at lifting of complete transformer that the lifting wires/ropesare not in contact with bushing or other components on the cover.

Four jacking pads are also provided for lifting the transformer with the help of jacks.

Bidirectional skids are also provided on the bottom of the transformer, in the form of channels having towing holes onboth the sided & also holes for foundation of transformer.

Transformer should be jacked up using the projections specially provided for that purpose. Jack should never be placedunder any valves.

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

ASSEMBLY OF WHEELSWhenever wheels are supplied with transformer, movement of transformer at site is carried out by mounting thesewheels.

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Transformer Manual

Mounting of wheels under transformer is to be done as per roller arrangement drawing. In power transformer wheelsare designed such a way that conservator side wheels are of slight higher in height fin the view of keeping transformershortly inclined to release trapped air from main tank and to avoid accumulating rain water on the tank top cover.While fixing the rollers, the flange should come on the inner side of the rails.

Transformer placement can be with or without rollers as per applicable GA/foundation drawing. When the transformeris in the final position, the wheels shall be locked to prevent accidental movement of the transformer.

2.3 STORAGE1. After arrival at site, it is desirable to erect and commission the transformer with minimum delay. In case this is

not possible the transformer shall be erected at its permanent location with conservator and breather fitted anddry oil filled to the correct level. Whether storage or in use, the dehydrating breather must be fitted on all thetransformers which are oil filled. The breather incorporates an oil sealing device which must be filled with oil, tothe level marked, to be effective. The condition of silica gel must be checked periodically, particularly duringmonsoons.

2. Dismantled equipment and components are packed to be protected against normal handling and transportstresses.

3. All other accessories should be stored in a covered dry place. It should be ensured that there is no damage to thegasketted joints and that all the blanked joints are tight.

4. Goods stored outdoors must not be placed directly on the ground, and should be covered carefully with tarpaulinor similar material.

5. The tap changer, if provided, should be operated at 6 monthly intervals. Two of three runs from one end of therange to the other and back are sufficient.

6. Heaters on Marshalling kiosks, etc. should be kept energized. If for this purpose, power supply at the appropriatevoltage to suit the heater is not available, temporary heaters should then be installed to suit the site voltage. Theprovision of heaters is important and failure to provide them may result in extensive damage to the contents ofthe kiosk because of condensation.

7. If oil received in drums is not likely to be used immediately the drums should be stored in a covered space wherethe temperature variation is minimum. If it is necessary to store the oil outside, adequate protection must beprovided at all times. Oil drums should be stored in horizontal (lying) position with both the bungs also inhorizontal position.

INDOOR STORAGEThe following items shall be stored indoor.

• All the Bushings.

• Insulating Materials & Paints.

• M.Box R.T.C.C. & Motor Drive of OLTC.

• Turrets with C.T. mounting.

• Components like B. Relay, MOO, PRy, Pressure gauges & Flow Indicators, Breather.

• Cooling fans & Pumps.

OUT DOOR STORAGE• Oil Drums: To be kept Horizontal and over two wooden planks. Care is taken that the caps shall be at

45 deg. from Vertical

• Radiators & Conservator.

• Frames, Pipes & Pipe supports.

• Rollers, Cable Box.

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3 INSTALLATION

3.1 LOCATION AND SITE PREPARATION1. Transformer 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 tobe provided from various live points of the transformer to earthed parts.

2. ONAN type transformers depend entirely upon the surrounding air for carrying away the heat generated due tolosses. For indoor installation, therefore, the room must be well ventilated so that the heated air can escapereadily and be replaced by cool air. Air inlets and outlets should be of sufficient size and number to passadequate air to cool the transformer. The inlets should be as near the floor as possible and outlets as high as thebuilding will allow. Where necessary, exhaust fans can be installed for the purpose.

3. The transformers should always be separated from one another and from all walls and partitions to permit freecirculation of air.

Where walls are provided, it should be ensured that the transformer gets good ventilation as mentioned abovefor indoor transformers. Provision should be made for the emergency drainage of the oil from the transformers(e.g. in case of fire in neighboring apparatus or bushing or the transformer tank), by surrounding the transformerplinth with sump filled with small pebbles.

3.2 ASSEMBLING THE TRANSFORMERBy means of the Part list and the Transformer GA Drawing, the assembling of a fully completed transformer iscarried out according to the following Instructions. The following precautions are to be taken:

1. Fire-fighting equipment shall be available near the oil-treatment equipment as well as at work on and adjacentto the transformer.

2. Transformer oil is inflammable and under certain circumstances in a confined space may become explosive.Naked lights & flame should never be used near a transformer.

3. Check that there is no overpressure in the transformer when blanking plates or connection lids are to be opened.

4. Workmen having access to the interior of a transformer should empty their pockets of all loose articles. Anyspanners or other tools used should be securely tied with a tape so that they can be recovered, if accidentallydropped.

5. All loose objects, tools, screws, nuts etc., shall be removed from the transformer cover before opening theconnection and blanking lids.

6. All loose objects (tools, pencils, spectacles etc.,) shall be removed from the boiler-suit pockets etc. Beforestarting the work through man holes.

7. Tools to be used inside the transformer/reactor -e.g. for tightening of screw-joints- shall be fastened to the wristor another fixed point by means of cotton tape or string.

8. Tools with loose sleeves and tools with catches must not be used at work inside the transformer.

9. Fibrous cleaning material should not be used as it can deteriorate oil when mixed with it. The presence of loosefibers in suspension in transformer oil can reduce its insulating properties. The presence of loose fibers in suspensionin transformer oil can reduce its insulating properties. If any cleaning or wiping is necessary this should be donewith clean and dry oil, using soft non-fluffy cloth.

10. Check healthiness of all the accessories and components dispatched separately with respect to GA Drawing andpacking list.

11. All components dispatched separately should be cleaned inside and outside before being fitted.

12. A transformer is best protected from damp hazard by circulating warm, dry oil through it until temperature is 5°C to 10° C above ambient. This should be done before allowing external access to the interior of the tank. Thewarm oil should be circulated as long as transformer is open to atmosphere,

13. Oil pump & all joints in the oil pipe work should be airtight to avoid entrance of air through leakage joints.

14. The active part (core and winding) should be exposed to the surrounding air as short time as possible. Opentherefore only one blanking plate or connection lid at a time for remounting of bushings, valves etc.

15. Check Conditions of Leads, Connections, Tap changer, General conditions of insulation etc.

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Transformer Manual

16. Check that the bushing leads set without being too close to ground or other points of different potential.

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

18. Before entering into a N2 filled transformer make sure that sufficient Oxygen is available inside.

3.3 GASKETSThe sealing system normally used against oil and gas in transformers and belonging components has rubber bondedcork gaskets at joints.

Whenever the blanking plates are removed to fix detached parts such as bushing turrets, etc., a new gasket shall beused while fixing the same. A set of new unused gaskets of correct size and thickness is supplied with every transformerfor this purpose.

Gaskets are best stored in such a way, that they must be protected from damp, oil and grease

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 itsoriginal thickness.

Joints should not be subjected to pressure until tightening is complete. If care is taken in making joints, and in handlingthe gasket, it is possible to break and remake a joint several times, using the same gasket.

Bolts/studs should be tightened lightly and diagonally in the sequence. Do not over tighten, otherwise gasket will getcrushed.

3.4 BUSHINGSAfter completing the internal connections, windings leads are brought out through suitable bushing.

Normally three types of bushings are used.

PLAIN PORCELAIN TYPEAssembly and dismantling of this type of bushing is possible without disturbing active part, tank cover and tank.

PLAIN OIL FILLED TYPEBushings supplied are oil communicating type. Oil communicating type bushing must be filled with transformer oil inoperation and this is achieved by keeping conservator above bushing level so that bushing remain always filled with oil.

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CONDENSER TYPE

DESIGN AND CONSTRUCTION

The active part of the bushing consists of an oil impregnated paper core built up around a centre pipe/rod withAluminum foils at pre-designed locations for best possible internal & external dielectric strengths. The active part isbuilt under heat and pressure on microprocessor controlled broadband winding machine.

The active part is dried under heat and vacuum and impregnated with insulating oil. The impregnated core is assembledinside porcelains Aluminum flange, and conservator with oil resistant synthetic rubber seals and are held together bya spring pack which also takes care of expansion & contraction of the centre pipe/rod. The annular space between thecore and the porcelain is filled with the same oil. A nitrogen gas cushion is provided in conservator to take care ofexpansion and contraction of oil. An oil level gauge (prismatic / magnetic) is provided on the conservator. At themounting flange an air vent plug is provided for air release of the transformer turret or for connecting to a Buchholzrelay.

The outermost condenser layer of core is taken out electrically through the test tap and grounded through a screwed- on cap.

Arcing horns are provided (If ordered) and are kept inside the packing case itself. The upper arcing horn fixingarrangement is shown in the lower arcing horn support should be fixed to any of the mounting bolts.

CAUTION: THE TEST TAP MUST ALWAYS BE EARTHED BY THE SCREWED ON CAP AND SHOULD NEVER BEREMOVED DURING OPERATION.

PACKING AND TRANSPORTThe bushings are packed horizontally in sturdy wooden cases with oil level indicator facing down to avoid Nitrogen gasto come in contact with active part. Immediately on receipt, inspect the bushing thoroughly for any damage afteropening top cover of the case. It is recommended to use the same packing case for storage/further transport.CAUTION OIL LEVEL INDICATOR SHOULD ALWAYS FACE DOWN DURING STORAGE / TRANSPORT

HANDLING AND STORAGEBushing can be lifted from packing case It is recommended to use two point lifting arrangement. The bushing can bestored in the same packing case with oil level indicator facing down or can be kept vertical on stands with flangefirmly fixed stand.

Generally condenser type bushings are used for 72.5 KV and above. Design of this bushing is such that their mountingis independent of oil level in conservator, whenever this bushing are mounted on bushing pockets or raised truncatedportions. Air vent pipes are provided for carrying away air or gases from these pockets to Bucholz relay during service.

The bushing should be examined for damage at oil end and as well as the porcelain before fixing which may haveoccurred during transit.

The bushing shall be lifted by using the lifting eyes and soft ropes. Steel wire ropes or slings shall not be used.

The turrets are often individually adapted; check therefore that they are remounted in correct places, which appearfrom the GA drawing and, part list.

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Transformer Manual

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1 Oil Impregnated Core2 Central Metal Tube3 Fixing Flange4 Porcelain - Upper5 Porcelain - Lower6 Gaskets7 Springs8 Expansion Bowl9 Cable Bolt

10 Air Releasing Screw11 Test Tap12 Top Terminal13 Oil Filling Plug14 Oil Sight Glass15 Base Plate /

Stress Shield16 Upper Arcing Horn17 Lower Arcing Horn18 Nitrogen Filling Plug

Bushings should be cleaned by dry & non fluffy cloth.Bushing insulator and metal parts are dispatched separately.Refer to G.A. Drawing for the position of bushing on transformer.Ensure that the oil level is below the tank cover. (Please check prismatic oil level gauge on main tank or Check it byunscrewing air release plug provided on the tank cover. No oil should come out).Remove blanking plates from tank cover. Check the gasket is OK. Replace it if required.Put the lock washer over the connecting stud. Screw the connecting stud to the connecting plug.

BUSHING ARCING HORN GAP SETTING

KV RATING BIL RECOMMENDED VALUES

66kV 325 380 mm

132Kv 650 / 550 800 / 635 mm

220kV 1050 / 900 1400 / 1250 mm

400kV 1425 2250 mm

NOTE: IN CASE LIGHTNING ARRESTERS ARE FIXED CLOSE TO TRANSFORMER ARCING HORNS ARE NOT REQUIRED.

ASSEMBLY OF PLAIN PORCELAIN & PLAIN OIL FILLED TYPE OF BUSHING USED FOR VOLTAGERATING UP TO 36KV.1. Fix the stem on the top of the connecting stud and then lock them with lock nuts.

2. Insert the clamping ring over the bottom of the insulator before kept in the position. Place the insulator on thecover property, insert threading stem assembly and insulating tube through it. Ensure that stem is properlyengaged inside the groove of the bushing.

3. Insert seal ring, cap and fix them in position by hexagonal nut.

4. Clamp the insulator by means of clamping ring, clamp, washer and hexagonal nut. See that the insulator istightened evenly and carefully at all places. Turn the cap so that mounting locations for spark gap horns whichare fitted later are in the position.

5. Fix the hexagonal nut, plain washer and spring washer.

ASSEMBLY OF CONDENSER TYPE OF BUSHING USED FORVOLTAGE RATING 72.5KV & ABOVE1. The line lead of HV winding if coiled inside the transformer is

drawn through the bushing using a string when the bushing islowered into position.

2. The thimble brazed at the end of line lead is fixed in position atthe top of the bushing brass tube. Fix the stem on top of the endcap and lock them with lock nut. Fix the washer over the end cap

3. The lower end of the bushing shall be inspected from inspectionwindow for proper sealing.

4. The line connection should be tight and should not strain theterminal.

Fix the upper and lower spark gap horn and adjust gap setting as perrequired.

Release the trapped air inside the bushing by unscrewing hexagonalnut and depressing the stem slightly or unscrewing the vent screw onthe top side of the metal part till a little quantity of oil comes out.

Before mounting on transformer, clean the bushing thoroughly and check for any damage. The lower part and insideof the centre pipe (in case of draw load/rod type bushing) should be further cleaned with transformer oil.

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BUSHINGS WITH DRAW LEAD/DRAW ROD CONNECTIONSRemove the terminal and pass a pilot wire through the centre pipe and lift the bushing upright of any desiredinclination as shown in fig.. Gently rock it back and forth to release any gas trapped inside insulation before lifting up.After tying the transformer lead/rod to the pilot wire, bring down the bushing while pulling up the wire. Fix the flangeto the transformer after checking the direction of oil level indicator, tightening the mounting bolts in a fraction of a turnat a time, working progressively in one direction until the bolts are uniformly tight. The thimble/rod is located bymeans of the pin and the terminal is screwed down fully with the gasket. The centre pipe can be deaerated byloosening the terminal. In case of 245 & 420 kv bushings, an additional cover is to be fixed after the terminal istightened, for compressing the gasket. Bushings as per terminations as per CEA requirements.

STEM TYPE BUSHINGSIn stem type bushings, the central conductor itself acts as .the current carrying part and the transformer lead is to beconnected at the bottom terminal through the inspection window of the transformer. The busbar/conductor can beconnected directly to the top terminal with the help of a proper terminal connector.

PRE-COMMISSIONING CHECKS1. Check capacitance/tan delta of the bushing preferably at 600 V and less than 10 KV, between High voltage

terminal and test tap (after removing the cap) and compare with factory test results. Any abnormality shouldimmediately be reported to TARIL. After the test, put back the cap.

CAP SHOULD NOT BE LEFT OPEN UNDER ANY CIRCUMSTANCES.

2. Check oil level of the bushing. In vertical condition, oil level should be at the middle of the indicator/gauge.

3. Check that the HV terminal is firmly tightened with the gasket to avoid passage of moisture to transformer.

4. Ensure that the transformer is energized minimum 36 Hrs. after mounting the bushing.

ROUTINE CHECKS AND MAINTENANCEAs the bushings are hermetically sealed, these are practically maintenance free. How ever, a periodic check of oillevel and cleaning of the porcelain insulator will suffice. It is recommended to check capacitance and tan delta andcompare with pre commissioning test results every year. Tan delta value of 0.01 or more, and increase in capacitanceby 10% or more, should be viewed seriously and should be immediately informed to TRAIL.

3.4.8 DEMOUNTING AND PACKINGDemounting of the bushing from transformer should be done in the reverse fashion as illustrated under “INSTALLATION”.Repacking of the bushing is to he done in sturdy wooden cases and in the same way it is received.

3.5 TAP CHANGERS

OFF CIRCUIT TAP CHANGERThe off circuit tap changer is an integral part of transformer & its operation is to be carried out from outside. The actualposition of tap changer is confirmed when the ratio tests are done.

That tap change is complete only when the tap switch handle hole matches with the hole provided on indicatingplate.

The transformer must be de-energized from both primary and secondary sides before handling Off Circuit Tap Changer.In no case should the tap switch handle be left half way and unlocked to prevent damage due to inadvertentoperation.

Off circuit tap changer should be operated only after de-energizing the transformer.

ON LOAD TAP CHANGER:On load tap changer (OLTC) function in the same as off circuit tap changing switch. However, in case of OLTC thetransformer need not be electrically isolated from HV & LV side. The tap changer can be operated on load. The tapchanger is an integral part of the transformer and is suitable for power flow in the direction indicated on the OLTCname plate. It is used for transformer where the system voltage fluctuations are frequent and over wide range. It haslocal manual, local electrical and remote electrical controls and the panel located in the control room has the necessary

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switches for raising or lowering the taps and is provided with a remote tap changer control cubical (R.T.C.C). Automaticvoltages regulation can be provided as an optional fitting.

Where transformers have to operate in parallel, special circuits are employed such as Master - Follower and will begiven with each scheme by the supplier in agreement with user.

For constructional details, refer manuals of manufacturer.

It is advisable to fit the tap changer on the transformer even during storage & connect it to the conservator andbreather.It should be ensured that the selector switch & diverter switch are filled with clean dry processed oil to keep selectorswitch & diverter switch moisture & dust free.

OIL SURGE RELAY

A Protective oil operated relay is supplied and the dispatched loose. This relay is sensitive to both low oil level and oilsurge conditions. The relay must be fitted in a pipe connection from the tap changer conservator pad to the conservatorand its contacts must be connected to the inter tripping system so as to isolate both sides of the transformer. The pipework rising to the conservator should be arranged at an angle of 5° above the horizontal to ensure the effectiveoperation of the protective relay. The connection of the relay contacts in the trip circuit such that the transformer isfully isolated will prevent damage to the tap changer the in case of low oil level.

Extensive damage will also be prevented in remote cases of incipient faults in the tap changer.

If the relay is painted at site care should be taken to see that the vent and drain holes in the terminal box are notclogged. A separate indication to show operation of tap changer oil surge relay should be available to avoid unnecessaryinspection of tap changer in case only the transformer gas and oil surge relay operates.

OILThe oil in the tap changer should conform to IS 335 and thus be thoroughly dried out by filtration. The electricalwithstand level of Oil should be minimum 50kV when tested as per IS 335 - and IS: 5792-1972. The oil should be freefrom solid contaminations as well.

HEATERHeater is provided in the drive mechanism in order to eliminate condensation troubles due to changes in ambienttemperature. The heater is controlled by an ON-OFF Switch and should be switched on when there is a possibility ofmoisture condensation such as during monsoons.

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COMMISSIONING OF THE TAP CHANGERIt is absolutely - important that the Drive Mechanism and the Tap changer Head show the same tap number, otherwise the electrical and mechanical limits which are contained only in the Drive Mechanism become inoperative forthe main Tap changer.

Fit the protective Relay, to a suitable outlet using pipe Connection

Fill the Oil Vessel and Protective Relay completely with oil.

COMMISSIONING CHECKS

1. The associated transformer should be de-energized, isolated and earthed during these checks to prevent anyunintended damage to life equipment. Switch off auxiliary supply to tap changer.

2. Inspect the outside of the tap changer and inside of the drive mechanism to ensure that nothing is damaged orremoved and that the mechanism chamber is clean. Touch up paint and lubricate if required.

3. Ensure that the tap changer is filled with oil to the level required in the conservator. Test oil.

4. Check that the breather charge is blue.

5. Check tripping operation of surge relay.

6. Operate the tap changer manually to extreme position, and check the operation of the limit switches and crankhandle interlock visually. Bring the tap changer manually to a position in the middle of the tapping range.

7. Switch on mechanism auxiliary supply.

8. Stall motor by holding coupling and ensure that the motor protection relay trips.

9. Check that the damper is locked.

10. Check that the mechanism travels in the correct direction when operated electrically.

11. Check that the friction device is pressing tightly on the coupling drum.

12. Check that during electrical operation the pointer is approximately in the middle of the white sector when themotor comes to rest after tap change.

13. Check that the heaters are working.

14. Check counter operation.

15. Carry out approximately 100 electrical operations through the full range.

16. Air Release as follows:

• From the Head Cover: Note that there is provision of a small air cushion on top of oil in the Head andtherefore you may see an air bubble under the numbering disk glass window.

• From Suction pipe, where this is provided

• From Transformer Tank Space under head, through plug.

17. Before commissioning the Tap changer must be taken at least once over the entire range manually using thecrank handle. During this operation the following checks should be carried out.

• The number on the Tap changer Head and the Drive Mechanism should tally at each tap.

• At the end positions, the electrical limits should be operative.

• If the Drive Mechanism is driven by crank handle beyond the end position, then the mechanical limit shouldoperate, thereby decoupling the drive to the tap changer.

18. Only after carrying out the above checks, should power be switched on to the Motor Drive.

19. See also separate Operating Instruction for Motor Drive.

20. Check operation of all electrical control both local and remote as well as paralleling where provided.

21. Operate the tap changer one complete cycle with the transformer energized. It is not essential that the transformerbe loaded for this test. If the tap changer oil surge relay trips during this test do not operate tap changer further.Isolate the whole equipment electrically and examine the tap changer completely, including the selector switch.Rectify any faults found and recommission.

22. Record the counter reading at the end of commissioning checks.

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PUTTING INTO OPERATIONBefore connecting the motor drive to the mains, check whether voltage, current and power of the supply shouldcoincide with the required values. When checking the voltage for the motor circuit take care that the RYB phasesequence is clockwise.

As the gearing and the ball bearings of the driving motor are sufficiently supplied with grease a regular maintenanceis not necessary. We recommend, however, Check that all wiping faces, gear teeth, cam faces etc. are lubricated withspecial molybdenum disulphide lubricant. No oil should be used for lubrication. Ensure that drive mechanism chamberis clean and free from dust.

3.6 COOLING OF TRANSFORMERS

TYPE OF COOLING1. OIL NATURAL AIR NATURAL COOLED TRANSFORMER (ONAN)

2. OIL NATURAL AIR FORCED COOLED TRANSFORMER (ONAF)

3. OIL FORCED AIR FORCED COOLED TRANSFORMER (OFAF)

4. OIL FORCED WATER COOLED TRANSFORMER (OFW)

COOLING EQUIPMENTS USED IN TRANSFORMER AS PER THEIR COOLING SYSTEM

TYPE OF COOLING ONAN ONAF OFAF OFW

COOLING EQUIPMENTS USED

RADIATORS √ √ √

FAN √ √

OIL PUMP √ √

OIL FLOW INDICATOR √ √

HEAT EXCHANGERS √

DIFFERENTIAL PRESSURE GAUGE √

The cooling equipments & associated pipe work & fittings are to be thoroughly cleaned thoroughly before assembly.Valves which are not dismantled like shut-off valves for radiators, coolers and possible headers shall be provided withblanking 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.

The pressure gauge, differential pressure gauges, etc. should be fitted in position.

Check that each radiators and possible headers are assembled in the correct positions according to GA drawing.

The shut-off valves against the transformer tank shall be closed until the oil-filling is started.

The cooler and associated pipe work is then filled with clean dry oil keeping all the cooler circuit open. Air is releasedfrom all the pipe work during filling.

3.7 RADIATORSTransformer is provided with detachable pressed sheet radiators with isolating valve. Due to transport limitation andconsidering possible transit damages, the radiator valve are closed at the time of dispatch and radiators are detachedand sent separately keeping the valve in position on tank flange. The valve blanking plate is to be removed only whenthe radiator is ready for mounting on flanges. On removal of blanking plate, oil will seep out. After filling the oil inradiators and venting air from them, radiator valves at top and bottom must be kept in OPEN position and sealed.

In large transformers the radiators are sometimes separately mounted. In such cases there will be a header each at top& the bottom, which are supported on frames. Flanges are provided on these headers for fixing the radiators. Radiatorsvalves are fitted to the headers and dispatched. The end frames are to be erected first. The frames should bepositioned correctly with respect to the transformer. The distance between centre lines of transformer and coolershould be strictly as per GA drawing otherwise the connecting pipe work will not match. After erecting the end frames

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the top and bottom headers are mounted. The headers will have to be properly leveled so that the connecting pipework can be easily fixed. Radiators should be mounted in last.

If the conservator is to be provided on the cooler bank, the same may be mounted on it and all fittings for the sameattached.

The interconnecting pipe work may be done taking care to connect correct pieces at the correct location. Usuallyexpansion joints are provided in the pipeline connecting the transformer tank to cooler. Special care should be takento see that these are installed correctly.

MOUNTING OF RADIATORSEnsure that the transformer and radiators are not damaged in transit.Special care should be taken while removing radiators from crates subsequent handling operations and while mountingon tank. Any damage while handling may result in cracks and oil may leak when fitted on tank and filled.

1. Lift the radiator vertically. Bring the radiator nearer to the valve. Remove the blanking plates from valve. Slidethe radiator on the bolt from pipe flange the radiator valve and tighten using plain washer, spring washer andnuts. Tighten nuts in sequence applying uniform pressure on the gasket to make a proper leak proof fitting.Assembly all the radiators in the same manner.

2. Clamping arrangement at outer ends of the radiators is to be fitted to minimize the vibrations of radiators.Required number of bracing straps in standard lengths are sent separately with hole arrangement.

3. Make arrangements to fill the transformer oil through the main conservator. It is recommended to use hotfiltered oil directly from the filter.

4. Fill only one radiator at a time. Open the bottom valve. Slowly unscrew the air release plug on the top of theradiators until the air starts escaping. The oil from the main tank will now flow in the radiator. Start putting freshoil in the conservator.

5. After filling open valve at top.

6. Fill the remaining radiators in the same manner.

3.8 COOLING FANSCooling fans are provided whenever transformer is designed for dual rating. This are mounted on brackets which arefixed on tank wall or ground. The fans are controlled from Fan Cooling Control and some times from remote tapchanger control (RTCC) panel and can be operated manually and on auto mode. When fans are on auto mode theyare switched on using one of the mercury switches of the WTI mounted FCC. The setting of the switch should be suchthat no hunting takes place. For this the temperature differential should be minimum 5º C to 7º C between switchingon and off the fans.

3.9 OIL PUMPThe pump and driving motor are assembled as a single unit or a common shaft. The transformer oil circulates throughthe windings thus eliminating cooling fan, shaft seal and avoiding the danger of oil leakage or entry of air. Thecomplete unit is weather proof and can be arranged for horizontal or vertical mounting. Lubrication of the bearing andcooling of the motor windings is achieved with the circulated transformer oil. This results in a very efficient rate ofcooling of the motor windings with minimum recirculation losses and maximum utilization of cooling of the motorwindings with minimum utilization of available motor power.

In Case of forced oil cooled transformer, oil pumps are provided for circulating the oil. The pumps are dispatchedseparately after blanking both suction and delivery sides. The pump should be connected at proper position as per theGA drawing. New gaskets provided with the accessories should be used at the joints and the bolts should be tightened.In some pumps an air release plug is provided on the body. This plug should be checked for tightness.

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3.10 OIL FLOW INDICATOR

FUNCTIONFlow indicators are mainly designed as a safety device looking to the need of an electrical signal on failure of forcedcirculation of liquid in pipe line. They perform following functions:

1. Indicate the rate of full flow in proper direction in specified pipe.

2. Operate one or two mercury switches when rate of flow drops near to 70% of specified full flow.These switches can be used to initiate precautionary systems of safety devices.

3. To introduce these Flow Indicators in a pipe line a “T-Mounting” is necessary.

CONSTRUCTION & WORKINGA suspended vane is used as sensor. Its surface is kept at right angle to direction of flow. When liquid starts flowingthrough pipe, the vane gets deflected along with vane shaft. This deflection is used to indicate flow inside the pipeand to operate mercury switches as follows:

A pair of permanent magnets is used as glandless coupling. The driving magnet is connected with vane; therefore itrotates with vane and remains in liquid. The follower magnet, separated by a nonmagnetic wall, is kept outside in airand acquires position corresponding to driving magnet. The follower magnet carries a pointer and a cam. The pointeris set to read the specified rate of full flow and the cam is set to operate one or two mercury switches when flow dropsnear to 70% of full flow. A return spring is used which acts in the opposite direction of flow. Hence any steady positionof pointer is the result of equilibrium of force due to impinging liquid on vane spring tension & weight of vane. Thepointer does not remain steady for unsteady flows. The size and shape of vane and tension on return spring is adjustedto suit specified full flow. The vane occupies very small area out of full cross section of pipe at NO-FLOW position. Atfull flow it becomes almost parallel to flow. Hence there is practically no head-loss across Flow Indicator. The flowindicators are distinguished in two series 4021 & 4022. Basic difference between two series is that in Series 4021 onlyone mercury switch is provided. Whereas in Series 4022 two mercury switches are provided. Proportionately, numberof terminals, size of switch-box and size of terminal box differ.

DIAL MARKINGUsually PUMP ON-PUMP OFF or FULL FLOW-NO FLOW type of limit markings are printed. Intermediate markings suchas ALARM, TRIP are also printed if required. Besides, specifications such as value of full flow, liquid, pipe size anddirection of flow are also printed. The background of dial is white. Direction indicating arrow is in red color and allother markings in black color.

SWITCHESOne or two mercury switches are provided. Their contact position can be set on NO or NC at full flow. If required,switch can be changed from NO to NC or vice versa at site.

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In case of single switch, it is set to operate near 70% when flow is fallen. In case of two switches, both are mountedon same plate. Hence both operate at same rate of flow. But in order to satisfy design requirements, one switch canlie set to operate near 80% and the other near 70% or one near 70% and the other near 60%. It is not possible to seta switch to operate at lower value than 60%. Even when both switches are set to operate at same rate of flow, someswitching differential is observed due to use of mercury as switching element. We can modify setting suitably ifswitching requirements are clearly specified.

WIRINGLeads from mercury switch/switches are brought into a terminal box at the bottom of indicator. Terminals are accessiblefor wiring after removing cover. A hole with conduit threads is provided for fixing cable gland to box. Terminals aremarked. Wiring should be done as per respective wiring diagrams.

NOTES:1. Flow indicators are not interchangeable direction of flow or rate of flow.

2. Locking pin in the terminal box should be removed before taking into service.

3.11 HEAT EXCHANGERIn case of forced water-cooled transformers the oil to water shell tube heat exchangers are dispatched separately andproperly blanked. On receipt at site, it shall be checked whether blanking is all right. The bracket for mounting theheat exchanger may be attached to the transformer first. The heat exchanger should be mounted on the support asper GA drawing. The oil pump, OFI & the connecting pipe may be fixed after this, in the correct position. In the watercircuit necessary pies may be fitted. A water flow mater is placed on the outlet pipe to indicate that there a positivewater flow. It is to be made sure that there is no restriction in the water outlet pipe as any obstruction in this pipe willincrease the pressure in the water circuit and may result in the water pressure exceeding the oil pressure and creatingleakage of water into oil circuit, which is detrimental to the transformer. Water discharge shall be of free flow type.

The heat exchanger oil circuit is sealed from the water circuit with special seals. Pressure Gauges & TemperatureGauges should be mounted in water & oil pipe line inlet & outlet the heat exchanger to monitor the pressure &temperature, of inlet & outlet, water & oil. Temperature of outlet oil should be lower that the inlet while oppositephenomena should be in water circuit.

3.12 DIFFERENTIAL PRESSURE GAUGEDifferential Pressure Gauge is mounted to ensure that the water circuitpressure should not increase the oil circuit pressure.

Differential Pressure Gauge Consisting of two bourdon tubes, one foroil and one for water line and the pointer of the instrument give directreading of differential pressure. Each bourdon tube can take maximumpressure of 1.3 times of maximum pressure shown on the dial.

Oil side tube should be connected to Oil pipe line at outlet of HeatExchanger and water side tube should be connected to Water pipe lineat inlet of Heat Exchanger.

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3.13 CONSERVATOR TANKThe conservator, which may be with or without air cell 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 -functionssatisfactorily. Before erection of conservator make sure that all gasketted joints are oil tight and pipe work is cleanand free of moisture.

The breather is connected to the oil conservator, and it is very important that joints and couplings in the pipe betweenbreather and conservator are air tight.

3.14 MAGNETIC OIL LEVEL GAUGEThis model of indicator is manufacturedconsidering Transformer application. It can alsobe used as Content Gauge on other tanks wherelevel of liquid inside the tank is required to beindicated continuously on a dial.This is direct oil level indicator devices providedon conservator. The low oil contacts provided onthe magnetic oil level indicator can be used forautomatic alarm when the oil level in theconservator falls to a low level. This protectionprevents the damages of the transformer fromdamages attributed to loss of oil due to anyreason.

During dispatch, the indicator & the arm andfloat are detached to prevent damage to thebevel gear and mercury switch. The mercuryswitch operating arm and bevel gear are alsolocked in position.

CONSTRUCTION, WORKING & FEATURESA float is used as sensor of liquid level inside the conservator (tank). Swing of float due to change in liquid level isutilized to indicate level on a calibrated dial and to operate a switch for external alarm unit.

Use of magnetic coupling in the indicator achieves complete sealing off the liquid inside the conservator from surroundingatmosphere. This results in avoiding any leakage of costly oil and avoiding contamination of insulating oil due toseepage of surrounding air in the conservator.

The glass mercury switch is nylon encapsulated to avoid breakage and spilling of mercury.

Switch is accessible for servicing while indicator is mounted on the conservator without any necessity of draining the oil.

The detail of the subassembly/parts are as below.

1. Gear Assembly

2. Magnetic Couple

3. Float with arm

4. Cam assembly with Mercury Switch

5. Dial with pointer

The Schematic diagram of the full assembly is

shown here.

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EXAMPLES OF APPLICATION OF MOG

The float is hinged and swings up or down when oil level rise or falls. This rise or fall rotates the bevel gear and thusthe pinion of the gear assembly. The pinion in turn rotates the driving magnet inside the conservator. The followermagnet positioned outside carries a pointer and a cam. The pointer reads oil level and the cam set to operate themercury switch at a predetermined low level. MGOs are with different specifications are not interchangeable.

3.15 COMMISSIONING OF AIR CELL

1. Before assembly, ensure that there is no leakage in the air cell. Keep the air cell fixed inside the conservator.Inflate air cell by dry air or Nitrogen to a pressure of 0.08 kg/sqcm max. through the opening for breatherconnection. There should be one valve and a pressure gauge fitted in between this valve and air cell close thevalve after required pressure is achieved. Keep in this condition for 24Hrs ensure no leakage in air cell with thehelp of pressure gauge.

2. Assemble the air cell conservator on the transformer.

3. Connect the conservator to transformer tank through Bucholz relay pipe line.

4. Close the valve between Bucholz relay & conservator.

5. Fill the oil in the transformer up to upper tank flange level under vacuum.

6. Keep the air cell inflated to a pressure of 0.08 kg/sqcm by Nitrogen / dry air through the opening for breather.

7. Open the valve between Bucholz relay & conservator.

8. Start further filling through the transformer lower filter valve slowly. Continue filling till the oil appears through‘C1’. Close vent holes ‘C1’ and when oil appears through vent holes ‘C2’ close it. During oil filling, the pressuregauge should be kept 0.08 kg/sqcm.

9. Start further filling through the transformer lower filter valve slowly. Continue filling till the oil appears through‘C1’. Close vent holes ‘C1’ and when oil appears through vent holes ‘C2’ close it. During oil filling, the pressuregauge should be kept 0.08 kg/sqcm.

10. Now remove the pressure supply connection of conservator & fix the breather with its pipeline. Keep furtherfilling of oil and stop at the value selected for the filling temperature.

11. Before applying pressure or vacuum close the valve between Bucholz relay & conservator perfectly.

12. While air cell in service never open the vent holes ‘C1’ & ‘C2’ to check oil level. This will cause sudden fall in oillevel. If oil level is to be checked first inflate the separator to 0.08kg/sqcm pressure and then open the vent holes.

3.16 GAS AND OIL OPERATED (BUCHHOLZ) RELAYSBuchholz relay is very sensitive, gas and oil operated instrumentwhich detects low oil level, formation of gas or developmentof sudden pressure inside the oil filled transformer. It has tobe connected to protection circuits to give any early audiblealarm in case of low oil level/gas collection and to disconnectthe transformer from supply in case of severe fault inside thetransformer. When mounted on the pipe work, the correctdirection is maintained with the help of arrow provided. Theangle of inclination is also to be checked and should bebetween 3 to 7’.

CONTENTSProtective devices In the ideal case must be sensitive to all faults, simple in operation, robust for service and economicallyfeasible. The relay operates on the fact that almost every type of electric fault in oil filed transformer gives rise to gas.This gas is collected in the body of the relay and is used in some way or other to cause the alarm or the tripping circuitto operate. The Relay is capable of indicate incipient fault thereby preventing further spreading of the fault andextensive damage and thus saving expensive and protracted repairs.

Double element relays are being used in detecting minor or major faults in transformer. The alarm element willoperate, after a specified volume of gas has collected to give an alarm indication. Examples of incipient faults are.

1. Broken - down core bolt insulation,

2. Shorted laminations

3. Bad contacts

4. Overheating of part of windings.

The alarm element will also operate in the event of oil leakage and if air gets into the oil system.

The trip element will be operated by an oil surge in the event of more serious faults such as

1. Earth faults

2. Winding short circuits

3. Puncture of bushings

4. Short circuit between phases

The trip element will also be operated if a rapid loss of oil occurs. Single element relays can be used to detect eitherincipient or major faults in oil filled potential transformers, reactors, capacitors etc. A special single element relay isavailable for the protection of on load tap-change equipment.

CONSTRUCTIONThe Buchholz relay essentially consists of enclosure i.e. an oil tight container and an inserted assembly comprising oftwo floated elements fitted with switches. The enclosure is provided with inlet and outlet for the liquid. To allowconnection of pipes, the enclosure openings can be designed with pipe thread or flange. The front inspection glass isprovided with a graduation to allow reading the accumulated volume of gas and observe color of gas for fault analysis.For checking the function of the inserted assembly a Test Key is provided. There is also a visual indication on the scale

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of service and Test/Locking position, in the form of movement of red indicator on the reading scale. This allows thetesting of the alarm and Trip circuits before installation and even when the transformer is in service.Just above the inspection glass an arrow shows the specified direction of flow of the insulation liquid to the conservator.Enclosure and its inserted Assembly cover are made out of Cast iron/weather resistant light alloy.

FUNCTIONAL TEST1. Testing the relay function with the Test Key.

A test system is provided in the Buchholz relay that allows the functional test of the upper and lower switchingsystem. To test the relay function loosen the nut on the Test Key and rotate the key with a screwdriver in theAnticlockwise direction the SLOT on the Test Key points towards the T/L position Both the alarm (upper switchingsystem) and Trip (lower switching system) will show continuity.

On bringing the SLOT on the Test Key to S (Service) position by rotating the key clockwise the Alarm and Tripcircuits will not show continuity. The circuits will be actuated to ‘ON’ position only when there will be a fault inthe transformer. Repeat the functional test each time a relay is started or maintenance completed.

2. Testing the relay function with draining the oil. Close the valves both the side of the relay. Put continuity testeracross the alarm & trip contacts of the relay terminal box. Open the air vent lug on the top of the relay. Drain theoil from relay from drain plug at the bottom of the relay. As oil start draining alarm & trip contact will close.

INSTALLATIONMount the Buchholz relay as close as possible to the tank in the pipeline between transformer and conservator.Keep pipe bends as wide as possible. Avoid close bends.

Make sure pipe ascends to the conservator at angle between one degree to nine degrees.

Make pipe bends as wide as possible between transformer and conservator.

See that the relay enclosure is not subjected to stress. If necessary, use expansion compensators.

Ensure that the slot on the Test Key remains in the T/L (Test/Locking) position during storage or loose transportation ofthe relay.

Ensure that the slot on the Test key remains in the ‘S (Service) position and the Test Key Bolt is tightened just beforecommissioning of the relay.

MAINTENANCENo servicing is needed during operation. On routine inspections of the protection equipment test the function of theBuchholz relay as described earlier and check the alarm and trip devices connected to them.

BASIC CHARACTERISTICSThe Gas and Oil relay provides protection against a number of internal faults but it is also able to indicate in severalcases the tyke of fault. This is possible because the gas collected in relay can, from its color, odor and composition,indicate where the fault may be and what its nature is. By examining the gases collected it is possible to identify thenature of the fault.

DIAGNOSISI. If the gas is colorless and odorless or with only a faint odor of oil, the gas is air trapped in the oil or the insulation.

2. If the gas is Greyish White with sharp and penetrating odor and be non-inflammable it is due to overheated orfaulty insulation.

3. If the gas is Yellowish in color and inflammable it may be due to surface leakage on material like wood.

4. If the gas is dark Grey and inflammable it may be due to a flashover in oil or due to excessive overheating of theoil caused by a fault in the winding or the core.

On the operation of the alarm if investigation of the collected gas does not indicate a serious fault it is possibleto leave the transformer In service till it Is convenient to carry out a thorough inspection. This occurrence ispossible on a newly commissioned transformer due to air trapped in the oil, or the

insulation. On repeated and frequent alarm signals the transformer should be taken out of service for thoroughcheck up.

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FUNCTIONThe function of a double element relay will be described here. During normal operation of a transformer the Buchholzrelay is completely oil filled.

In the event some fault in the interior of the transformer tank gas bubbles are produced and accumulate in theBuchholz relay on the way to the conservator. In consequence, the oil level in the relay enclosure drops which in turnlowers the upper bucket.

This causes the mercury switch/Reed switch to operate an alarm signal.

The lower bucket does not change its position, because when the gas reaches the upper inside wall of the pipe it canescape into the conservator. Hence, minor fault in the transformer tank will not trigger the lower switching assemblyand will not trip the transformer.

In case the liquid continues to drop due to loss of oil, the lower bucket also goes down in consequence lower switchingsystem operates if the level of oil goes below the bottom level of the pipe connected the relay.

Alternately in the event the liquid flow exceeds a specific value the lower bucket is forced down, thus triggering thelower switching system to operate.

As the liquid flow rate decreases or the level of the liquid raises the bucket returns to its original position.

The single element relay has only one operating element and it responds to either gas collection or oil surges. Thesingle element OIL SURGE RELAY has been specifically designed for use with on load tap change equipment and itwill bypass normal amounts of gas which are generated by tap change operations and will only responds to oil surgesand loss of oil.

OPERATING CHARACTERISTICSNormally Relays of the double element type are made in three sizes GOR1, GOR2 & GOR3 according to the diameter25, 50 and 80 mm respectively of the bore of the oil pipe connecting the transformer to the conservator tank.

All relays are normally open types.

When transformers are required for use in areas subject to earthquakes or for traction application, we can supplyspecial relays which use magnetically operated Reed Switches instead of mercury switches.

3.17 SILICA GEL BREATHER

OPERATIONDry silica gel crystals which are dark bluein color have a very good capacity toabsorb moisture. When the air fromoutside is breathed in the conservator,it passes through the crystals and themoisture in the air is absorbed. Thus theair that reaches the conservator is dry.Dust particles are partly trapped in theoil seal and partly trapped by the crystalsof the silica gel. Silica gel crystals changetheir color from dark blue to pinkdepending upon absorption of certainquantity of moisture.

Silica gel crystals can be reactivated a numbers of times and therefore, they can be used over a long period. Theyshould be observed for the color of the silica gel on weekly basis. Change oil in oil seal once the color changes fromlight yellow to dark brown.

BREATHING PROCESS OF THE TRANSFORMER:When Transformer is loaded or unloaded, the oil temperature inside the transformer tank rises or falls. Accordingly theair Volume inside the tank changes, by either sucking in or pushing out the air.The phenomenon is called Breathing of the transformer. The air which is being sucked in contains either foreign

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Transformer Manual

impurities and/or Humidity which changes dielectric strength of transformer oil. Hence, it is necessary that, the airentering into the transformer is free from moisture & foreign impurities.

OPERATION & WORKINGThe Breather- is connected to an outlet pipe of the Conservator Vessel and the air which Is being sucked by transformeris made to pass through the Silica Gel Breather to de humidify the air and to remove foreign impurities.The Silica Gel which is filled in the Breather Is hard blue Crystals, which has considerable absorption power formoisture. When, it gets saturated with moisture, it changes it color to pinkish white. For proper dehumidification of air,its is absolutely necessary that this charge of Silica Gel is reconditioned from pinkish white to deep blue by heating it.

The air, which is passed through Gel is first made to pass through the oil compartment of the Breather. This oil removesall foreign Impurities from air which enters the Gel compartment.

Hence, Oil sealed type Silica Gel Breather will keep the oil properties constant, thereby ensuring proper working andhence longer life of the transformer.

INSTALLATIONThe Breather is connected to transformer by either threaded or flanged joint (after removing the Seal) as per detailsmentioned below. Before putting the Breather Into service, remove the Oil Cup and fill it up with fresh transformer oilup to the line marked on Cup. Remove the seals on the air holes of the Cup and fit the Cup to the Breather TheBreather is how ready for installation.

MAINTENANCEAs the color of the Silica Gel changes to pinkish whit after use, it needs recharging.

Take out the Silica Gel and heat it, until color of Gel changes to deep blue. The Gel content or the size of the Breatheris determined by the Volume contents of the air passing through the Breather, the Oil Transformer and the atmosphericconditions of installation. Hence, the Transformer Manufacturer or the ultimate user should decide the size of Breatherwould shut the transformer.

3.18 PRESSURE RELIEF VALVE (PRV)Pressure Relief Valve should be mounted as per GA drawing. Check operation of alarm/trip contacts. The purpose ofthe pressure relief valve is to prevent mechanical damage to the transformer tank by releasing any excess pressureover the set limit. This valve is basically a spring loaded device which closes automatically when the pressure insidethe tank drops to the set limit after venting excess pressure over the set limit.

APPLICATIONThis PRV is designed to be used on power Transformer. When pressure in the tank rise above, predetermined safe limitthis valve operates and allows the pressure to drop by instantaneously opening a port of about 150 mm diameter &give valve & operation by raising a flag and also operates a switch. This switch has 1 NO and 1 NC contacts (As perrequirement). Hence switch can be effectively used in control circuit.

SPECIFICATIONSNormally PRVs are used of two sizes T-3 & T-6 each are having variety of three pressure ratings of 0.42 kg/sqcm (6 PSI),0.49 kg/sqcm (7 PSI) and 0.56 kg/sqcm (8 PSI).

CONSTRUCTION & WORKINGThe Pressure Relief Valve consists of a pressure die cast Aluminum flange (1) with Nitrile gasket (2) for mounting ontransformer. The stainless steel diaphragm (3) is loaded with two reverse wound calibrated springs (4) and seals the 75/150mm port against top and side gasket (5 & 6). The deep drawn cover (7) retains and is held in place by six screws (8).

The cover and the operating disc have specially designed retainers to prevent dislocation of springs during repeatedoperations.

The switch assembly and houses one/two NO and NC contact as per requirement. It also has a visual indicator (10).

When pressure in the tank rises above the safe limit, the operating disc moves slightly upwards from top gasket. This

23

exposes the transformer pressure to a greater of side gasket, resulting sudden increase of force. The disk liftsinstantaneously and vents gases, vapor and liquid till the pressure falls to allowable value.

The PRVs are recommended to be used on power transformers. They are much more effective, durable and suitablefor repeated operations than the conventionally used explosion vent.

MOUNTINGThe PRV should be preferably mounted in the horizontal position, top side up. However, it can be mounted on its side,in vertical plane also.

INSTALLATIONThis PRV has ‘an integral flange with four/six holes for mounting. Check that no parts are damaged in transit. Cleansurface of mounting pad on tank and place the flange with gasket. Each valve should be cleaned form inside (tankside). All particles should be removed from tank side. All the bolts should be tightened evenly so that equal pressureis exerted on gasket aid base. Ensure that the gasket is placed in the groove provided in the flange. Check theoperation of switch by manually lifting the operation rod. After checking, the switch should be manually reset byplacing the roller type knob back to its original position.

MAINTENANCEThe PRV has a rugged construction and does not require any maintenance. The operating pressure is factory presetand cannot be changed at site. The operation of switch may be periodically tested by manually lifting the operatingrod and should be reset before putting the instrument in service.

3.19 DOUBLE DIAPHRAGM EXPLOSION VENTThe purpose of the explosion vent is to prevent damage to transformer tank by releasing any excess pressure that maybe generated inside the tank.

CONSTRUCTIONExplosion vent consists of a bent pipe with aluminum diaphragms at both ends. A protective wire mesh is fitted on theopening of the transformer to prevent the piece of ruptured diaphragm from entering the tank.

Near the lower end of the vent, there is small oil level indicator. When lower diaphragm ruptures, transformer oil risesin the vent pipe and is visible through the indicator indicating the failure of the lower diaphragm. In case the pressuredeveloped is not reduced to safe value after the bursting of lower diaphragm, upper diaphragm also gives waythrowing the gas and oil outside and thus protecting the transformer tank from mechanical damage.

Sometimes conservator and explosion vent are connected with an equalizer pipe with or without a valve. The pipeequalizes the pressure between conservator and explosion vent.

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Transformer Manual

It should be fitted after vacuum application. Care being taken to ensure that the top diaphragm with its gasketsmakes an air tight joint.

3.20 MARSHALLING BOXThe transformer is protected by certain protective devices mounted on the transformer at different locations. Thiscircuit usually has contacts or terminals which are connected to the protection scheme. The connection to protectivescheme gives various types of alarm indications and also disconnects the transformer from incoming and outgoingsupply in case of severe condition. In order to facilitate connections of all such devices to the protective scheme, theleads of all protective device contacts are wired to a weatherproof kiosk. This kiosk also houses indicators for oiltemperature, winding temperature, cooling circuit and heater.

CONSTRUCTIONHinged lockable front door is provided with a glass window from which the temperature readings on the indicators canbe observed. The capillary from each indicator take off from the indicators, come out from the side of the marshallingbox. The entire kiosk has a rain shed provided on top to prevent ingress of water due to heavy rains falling directly overthe marshalling kiosk.

3.21 OIL TEMPERATURE INDICATOROil temperature indicator consist of direct reading dial gauge and serves for monitoring the oil temperature of thetransformer. The dial gauge of the transformer can be fixed away from the point of temperature measurement on thetop of the transformer because the active gas bulb element is connected to the mechanism by a long capillary tube.The length of capillary does not influence the accuracy of measurement.

OPERATIONThe liquid inside the bulb in contact with the hot oil in the thermometer pocket expands and the pressure of the vaporin side the system increases. This increase in pressure results in movement of the bellow. A link and lever mechanismamplified this movement to the disc carrying pointer and switches which indicates the temperature on the dial of thethermometer.

3.22 WINDING TEMPERATURE INDICATORFor oil immersed transformers operating under varying of the winding temperature is of great importance. The use ofwinding temperature indicator permits the winding temperature to be constantly monitored and harmful overloadscan be avoided.

OPERATIONThe indicator is similar to OTI. The winding temperature indicator is calibrated to indicates the hottest spot of thewinding.

The switches are adjusted to make contact at desired temperature depending upon the site conditions, i.e., ambienttemperature, loading conditions, etc.

The capillary tube is protected adequately to withstand all normal handling. It should not, however, be bent sharply orrepeatedly and should be supported by clips to prevent sagging.

The thermometer pocket should be filled with transformer oil.

For OFAF & OFW cooling OTI & WTI are provided with four contacts, used for Alarm, Trip, Fan Starting & PumpStarting.

The instrument indicates the temperature rise of the transformer winding over, the top oil temperature for anyspecified load current.

The maximum Input current to the heater coil temperature not exceed 2.0 Amps. The maximum permissible secondarycurrent from the Current Transformer at rated load is 2.0 Amps.

25

OPERATING MECHANISMThe system is self contained and does not depend on any outside power source for its operation. It consists of a liquidfilled bellows capillary tube of required length and sensing element or bulb. This system is thermally balanced by acompensating bellow and a second capillary which terminates at the head of the bulb. The two bellows are linkedtogether via a lever In such a manner that temperature changes on the capillary line and head are automaticallycompensated for. Temperature changes on the bulb motivates the operating bellows only which is linked to the singlerotating switch platform and indicating pointer.

The power available from this type of system is adequate to operate up to four switches and a potentiometer withoutany visible pointer detect. This robustness also ensures that the instrument does not suffer from normal shock conditionssuch as may be experienced in handling and transport.

DIALA 4 1/2 long segment dial is provided with bold graduations. The standard range is 0-150°C with 5 per graduation.

MAXIMUM POINTERA maximum indicating pointer red in color is fitted to the instrument window aid operated by a peg driven by theindicating pointer. It can be reset with a standard screwdriver blade, after removing the knurled weatherproof knob.

As switch testing knob is fitted on the bottom of the lid to check the switch without removing the lid.

Instrument should be mounted only in vertical position: Inclined mounting will upset zero setting of the mercuryswitches.

The transport lock should be removed after opening the cover before putting the instrument to use. The capillary linecan be supplied up to 15 meters in length. It is protected by flexible stainless steel armor. Ordinary care should betaken while running the capillary and sharp bends should be avoided, particularly where it joins the instruments caseand the bulb. The capillary should he supported by suitable clips at 12to 18 Inches and necessary length left so that thebulb may be freely installed and removed.

26

MODEL 1002

MODEL 1001

MODEL 1003

MODEL 1030

Transformer Manual

27

STANDARD SENSING BULB FOR RANGE 0-150° C

STANDARD WIRING DIAGRAM FOR FOUR CONTACT SWITCH

3.23 CURRENT TRANSFORMERCurrent transformer for winding temperature indicator is provided on winding of the transformer at either the terminalsor on the bus bar of terminal gear. The secondary winding of the current transformers are connected to a terminal box.Current transformer terminals from terminal box are wired to winding temperature indicator mounted in marshalling box.

The connections between terminals are made by links which can also be arranged to short circuit and earth thecurrent transformer when disconnected. For detail of CTs refer Rating and Diagram plate drawing.

Current transformers are used in transformer for measuring and also for protection purpose. CTs are provided as percustomer’s requirement and specification. All current transformer secondary terminals should be short circuited orloaded before commissioning the transformer. This will prevent excessive voltage developing across CT terminalswhich can damage the CT and be hazard if touched. CTs should not be dismantled & if required extra care should betaken in handling it.

CLASS OF ACCURACY USED FOR DIFFERENT PURPOSE OF CTS

PURPOSE CLASS OF ACCURACY

MEASUREMENT 0.2, 0.5

REF & DIFFERENTIAL PROTECTION PS

WTI 5

BACK UP PROTECTION 5P10, 5P20

3.24 CONTROL CABLINGReassemble the control cables according to the drawing of wiring system and connect the cable ends to terminalblocks in instruments, terminal boxes, junction boxes and control cabinets according to valid connection diagram.

3.25 ASSEMBLY OF VALVESValves which may affect the loading gauge, or will be subjected to damages, is dismounted before delivery. Remountthe valves -the positioning appears from the GA drawing and part list. The gasketting surfaces shall be cleaned welland new gaskets fitted. Check that all valves are closed.

3.26 ASSEMBLY OF PIPE WORKPipes with flanges for connection of conservator, radiators, as well as pipes for equalizing of turrets etc. are mostlydelivered completely ready for assembling according to GA & part list.

In certain cases -e.g. at a separately assembled oil conservator -certain fitting and welding of pipes and flanges on siteis however required.

3.27 OIL FILLING

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29

PRECAUTIONS FOR N2 FILLED TRANSFORMERN2 gas cylinder to be disconnected and the N2 gas valves on the tank to be closed.

Ensure that all the valves are in closed position and all openings blanked off.

Remove N2 gas from the tank by means of a vacuum pump connected to the top filter valve.

Oil is easily contaminated. When sampling the oil and filling the tank, it is very important to keep the oil free fromcontamination.

All equipments used in handling the oil must be cleaned & should be washed with clean transformer oil before use.The oil used for washing must be discarded.

Particular attention shall be paid to the cleanliness of valves & other points where the dirt or moisture tends to collect.

Hose used for handling oil should be clean and free from loose rust or scale.

Oil must not be emptied near naked light heater/fire, as vapor released is inflammable.

Minute quantities of moisture (particularly in presence of fibers or dust) lower the dielectric strength of the oil.Therefore, care should be taken that oil should not come in contact with moisture.

Not to mix oil from different supplier, if required it should meet the requirements of IS: 1866.

TABLE 1 - SPECIFICATION OF TRANSFORMER OIL AFTER FILLING IN TRANSFORMER TANK BEFORECOMMISSIONING AS PER IS: 1866.

PROPERTY HIGH VOLTAGE EQUIPMENT

<72.5kV 72.5 - 170kV >170kV

Appearance Clear free from sediments andSuspended matters

Density at 29.5°C (g/cm) Max 0.89 0.89 0.89

Viscosity at 27°C (cst) Max. 27 27 27

Flashpoint (°C)Min. 140 140 140

Pourpoint (°C)Max. -6 -6 -6

Neutralization value mgKOHIg/gm of oil - Max. 0.03 0.03 003

water content (ppm) Max. 20 15 10

Interfacial tension (Mn/M) Min 35 35 35

Dielectric dissipation factor at 90°C and 0.015 0.015 0.01040 to6O Hz, Max.

Resistively at 90°C 6 6 6

Break down voltage (BDV) (kV rms) Min. 40 50 60

Oxidation stability (un inhibited oil)a) Neutralisation value (mgKOHIg) Max. 0.4 0.1 0.4 0.1 0.4 0.1b) Sludge (% by mass) Max.

Ocidation stability (inhibited oil) Same values as before filling.

OIL SAMPLINGOil takes up moisture readily and its condition should always be checked before use. Oil of a muddy color is certain tobe wet. Water and water-saturated oil are both heavier that dry oil and sink to the bottom of any container. Samplesshall, therefore, be taken from bottom.

While taking sample from Transformer Tank care should be taken that dirt from the draw-off valve or plug should beremoved. To ensure that the valve is clean, some quantity of oil should be allowed to flow into a separate containerbefore collecting sample for sampling. Sample should be collected either in glass bottle or in stainless steel bottle.

For taking sample from oil drum, the drum should first be allowed to stand with the bung vertically upwards for at least

24 hours. The area around the bung should be cleaned. A clean tube long enough to reach to within 10 mm of thedrum should be inserted, keeping the uppermost end of the tube sealed with the thumb whilst doing so. Remove thethumb, thereby allowing oil to enter the bottom of the tube.

The sample should have following information

1. Date of sampling.

2. MVA & Voltage Class of Transformer.

3. WO & Sr. No. of Transformer

4. Whether the sample from Main Tanker or OLTC.

5. Sample Taken from Top or Bottom.

6. Before filtration or After filtration

7. Before charging or After Charging.

OIL FILLINGBefore filling oil in the tank, it should be tested to meet the requirements as per IS:1866. In case oil does not meet therequirement, it should be processed and shall only be used when meets the requirements.

For transformers dispatched gas filled, remove N2 gas from the tank by means of a vacuum pump and take fullvacuum and check & rectify if any leakage is there & keep the transformer under vacuum for 12:00 HRS. The fillingof oil inside the tank should be done under vacuum & fill the transformer main tank up to top cover. Do not open anyblank flange for erection without filling oil up to top cover. Transformer of high voltage rating (66KV and above) havetheir tanks designed to withstand full vacuum. Below 66KV class transformer, full or partial vacuum should not beapplied without consulting manufacturer.

When filling a transformer with oil it is preferable that the oil be pumped into the bottom of the tank through filtrationmachine.

It should also be ensured that the tap changer board, relief vent diaphragm, conservator, etc. are not subjected to fullvacuum as these may not designed for the same.

In case the transformer is provided with OLTC of in-tank type, while evacuating the main transformer tank, thediverter switch compartment must also be evacuated simultaneously so that no undue pressure is allowed on the TapChanger Chamber. While releasing vacuum, the Tap Changer vacuum should also be released simultaneously. Forthis, one equalizer pipe should be connected between main tank and tap changer.

DRYING OF TRANSFORMER USING FILTER MACHINEBefore the drying out is started all fittings coolers and associated accessories shall be fitted. Drying out process oftransformer is requiring care and good judgment. If the drying out process is carelessly or improperly performed, greatdamage may result to the transformer insulation through overheating etc.

Transformer should not be left unattended during any part of drying out period. Transformer should be carefullywatched throughout the dry out process and also observations shall be carefully recorded.

The most practical method of drying out is by circulation of hot oil through a high vacuum filter machine incorporatingoil heater and vacuum chamber. The vacuum pump of the filter machine should have capacity of creating vacuum ashigh as possible but not less than 710 mm of mercury. Where possible, a vacuum pump can be connected to the tanktop cover to keep the oil in tank under vacuum consistent with tank suitability. This may speed up the drying outprocess. Oil should be filled in tank under vacuum.

Coolers and Tap Changers are filled with clean dry oil. Oil samples are taken out from them and tested. Furthercirculation of oil is carried out till the oil results are satisfactory.

The oil temperature as measured by the oil temperature indicator should be of the order of 60’C. In no case temperatureof oil inside the filter machine should exceeds 70’C. The circulation is continued till insulation resistance and oilsamples test are satisfactory.

Plot IR values taken at regular intervals against temperature readings. It will be observed that in the beginning IRvalues drop down as the temperature goes up. The IR values will be low till moisture is coming out of the insulationand start rising before steadying.

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Transformer Manual

3.28 EARTHINGTank should be permanently and effectively earthed by means of connection to the earthing terminals provided for thepurpose at the bottom of the tank.

A good low-resistance earth is essential for adequate protection. The earthing connection should be of sufficient cross-section to carry the LV line current for 30 seconds. Other accessories such as fan motors, oil pump motors, windingneutrals etc. should also be earthed.

Earthing terminals are provided on tank for earthing of tank mentioned in General Arrangement Drawing. Ensure thatpaint on the earthing terminal is removed before connecting the earthing strips.Earth resistance should be measured and it should be within acceptable limit.

3.29 COMPLETION OF ERECTION WORKFinal topping up is now done up to a level in conservator commensurate with filled oil temperature. Other such aswiring of various alarm/trip contacts, fan motors, pumps motors and other apparatus, earthing of neutral and tank isalso to be completed. The interposing valves between the radiators and the tank are opened. The transformer and itsequipment are cleaned carefully from dirt, oil, lubricating grease etc. If the paint work has been damaged duringtransit or erection, touching-up should be carried out with the least delay. The surface to be repainted should becleaned and free from rust and grease. If rust has begun to form, the surrounding area should also be cleanedthoroughly as rust tends to spread beneath paint work. Failure to maintain the paint work in good condition will resultin considerable deterioration of the apparatus during storage or service. After completion of erection work it should bechecked thoroughly that there should be not leakage. If any leakage found, it should be immediately arrested.

3.30 TOUCH-UP PAINTINGA basic principle at touch-up painting should be to restore a damaged paint coat on a surface to the same quality andfinish as of the surrounding surface. The touch-up paint should be limited to a surface as small as possible. If the paintwork has been damaged during transit or erection, touching-up should be carried out with the least delay. The surfaceto be repainted should be cleaned and free from rust and grease. If rust has begun to form, the surrounding areashould also be cleaned thoroughly as rust tends to spread beneath paint work. Failure to maintain the paint work ingood condition will result in considerable deterioration of the apparatus during storage or service.

Both damaged and surrounding surface should be cleaned so that all grease, dust and other impurities will beremoved.

Damages on the primer paint and grinding down to the steel surface should be painted with two coats anticorrosiveprimer paint.

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4 COMMISSIONINGAfter completion of proper erection, transformer is to be kept ready for commissioning. Before putting transformer inservice, following check points and Testing should be done to ensure the healthiness of the transformer.

4.1 GENERAL CHECK POINTS1. After erection & oil filling, transformer should be checked for any oil leakages from each and every part. If found

necessary tighten, replacement of gasket and “o” ring should be done.

2. Valves between Conservator to main tank should be fully open. All radiator valves are also fully opened andlocked in proper position.

3. Buchholz relay is to be mounted in position inclined angle of 5 to 7 degree. Ensure arrow mark of relay towardsconservator.

4. Air release from all air release plug / screws on Main tank cover, radiators, bushings, buchholz relay, OLTC andBushing turret to make ensure that no air remains inside the transformer tank.

5. Check the level of oil in main tank and OLTC conservator. Check the MOG operation by depressing the float.

6. Ensure that, all the thermometer pockets are filled with transformer oil.

7. Minimum clearance between live parts to earth should be checked as per drawing.

8. Check the various protective devices (i.e. Buchholz relay, PRV, OSR, OTI, WTI and MOG) should give properalarm and trip indication.

9. Check the Buchholz relay practically by injecting the air from test cock or by draining the oil from buchholz relayby closing both side valves. Ensure that, buchholz relay floats are not locked and the relay is set at the desiredposition.

10. Check the color of silica gel in breather is dark blue. Also the oil in oil seal to be filled up to the level indicator.Check the air passage of breather is not blocked.

11. Check weather transformer is earthed properly by two independent earth connections. Paint between earthingterminal and earthing strip should cleaned properly for better connectivity. Also OLTC, Marshalling box, cablebox should also earthed properly. Also check for earthing for neutral bushings.

12. Check the incoming voltage and set the OLTC on the corresponding tap position. In case of off circuit tapchanger brought to desired position and locked with bolt in plate hole. AVR can also set as per required voltagevariation, if applicable.

13. Check terminal clamp on bushings and cable and termination on bus bars.

14. Check functions of the breakers with transformer actual tripping operation.

15. Check oil level in OIP bushing from glass or small MOG (Mounted on bushing). It should be in required level.

16. Check Explosion vent assembly mounting.

17. Check OLTC Explosion diaphragm in healthy condition.

18. Check Transformer painting. Apply touch paint at required places.

19. Check Name plate provided properly.

20. Check Roller wheel stoppers provided.

21. Check Valve between OLTC conservator and OLTC surge relay open.

22. Check BCTs for protection and metering connected to respective circuits properly.

23. Check Bushing thimbles and terminal cops tightness.

24. Check Anchor bolts for roller frames and cooler frame provided.

Following details should be collected from name plate of transformer for reference before testing.

Transformer Sr. No: Work order No:

Name of Manufacture: Customer Name:

MVA/KVA Rating: Voltage Ratio:

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Transformer Manual

4.2 DOS AND DON’TS FOR POWER TRANSFORMER

DON’TS FOR POWER TRANSFORMER1. Do not energies without thorough investigation of the transformer whenever any alarm of protection has operated.

2. Do not reenergize the transformer unless the Buchholz relay gas is analyzed.

3. Do not reenergize the transformer without conducting all pre-commissioning checks. The results must becomparable with results at works.

4. Do not operate the off-circuit tap switch when the transformer is energized.

5. Do not energize 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 arrestor protectionscheme, when connected to load.

8. Do not allow WTI/OTI temperature to exceed 65°C during drying out of transformer and filter machine temperaturebeyond 70’C.

9. Do not parallel transformer which do not fulfill the Paralleling condition.

10. Do not use low capacity lifting jacks on transformer for jacking.

11. Do not move the transformer with bushings mounted (above 33 KV’ class).

12. Do not overload the transformer other than the specific 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 sitecondition.

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 protection circuits.

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

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 unauthorized entry near the transformer.

23. Do not leave ladder unlocked, when the transformer 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 continue with pink silica gel, this should immediately be changed or regenerated.

29. Do not store Transformer for long after reaching site it must be erected and commissioned at the earliest.

30. Do not leave secondary terminal of CT open.

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

32. Do not top up oil from conservator with air cell bag inside.

DO’S FOR POWER TRANSFORMER1. Check and thoroughly investigate the transformer whenever any alarm or protection operated.

2. Check air cell conservator (optional).

3. Attend the leakages on the bushing immediately.

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

5. Check the oil in transformer and OLTC for dielectric strength & moisture content and take suitable action forrestoring the quality.

6. Check the oil level in oil cup and ensure air passages are free in the breather.

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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 avoiduneven pressure.

9. Check & clean the relay and alarm contacts. Check also their operation and accuracy and if required, changethe setting.

10. Check the protection circuit periodically.

11. Check the pointer of all gauges for their free movement.

12. Clean the oil conservator thoroughly before erecting.

13. Check the bucholz relay.

14. Inspect the painting and if necessary retouching should be done.

15. Check the OTI & WTI pockets and replenish the oil, if required.

16. Examine and replace the burnt or worn out contacts.

17. Check all bearings and operating mechanism and lubricate them as per schedule.

18. Open the equalizing valve between tank and OLTC, wherever provided at the time of filling the oil in the tank.

19. Connect gas cylinder with automatic regulator if transformer is to be stored for long in order to maintain positivepressure.

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

21. Check the door seals of marshalling Box.

22. Equalize the diverter compartment of the OLTC by connecting equalizing pipe between flange joints provided onthe tap changer head.

23. Follow the oil filling instruction for topping up of oil for transformer with air cell type of oil, preservation system.

4.3 INSULATION RESISTANCE TEST (MEGGER)Purpose of IR test is to check over all condition of insulation of transformer. Insulation resistance is a relative measureof the integrity of the insulation structure. The test equipment used is a DC insulation tester (Megger). The IR testresults are co-related with transformer insulation quality by parameters like Absorption ratio, polarization index (PI),Step voltage test etc.Temperature, humidity, dust or pollution on the surface of insulation and changed atmosphere inthe vicinity influences.

Range of Insulation Resistance Tester

Make

Temp

Insulation Resistance Value

Temperature = Deg C

SI. No. Configuration IR value in Mega Ohms

15 sec 60 sec P.I.= 60sec/15sec

1 HV - LV

2 HV - E

3 LV - E

4.4 VOLTAGE RATIO TESTRatio test should be conducted in every transformer to check the design ratio. Ratio at all taps is to be checked. Startthe tap changer from lower tap and then go on increasing tap. Apply 3 Phase 3 Wire 400 Volts A.C to HV. Keep HVneutral isolate (if in Star connection). Measure voltage between phase to phase and phase to neutral on LV side.

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35

TAP APPLIED VOLTAGE VOLTAGE MEASURED ON L.V SIDE CAL.NO. ON HV SIDE RATIO

U-V V-W W-U u-v v-w w-u u-n v-n w-n

1

2

3

.

.

n

4.5 MAGNETIZATION CURRENTThe exciting current test at low voltage is very useful in location problem such as defects in magnetic core balance, inmagnetic core structure, shifting of winding, failures in to turn-to-turn insulation, or problem in tap changers. Theacceptance criteria for the results of exciting current measurement should be based on the comparison with theprevious site test results or factory test results. The general pattern is two similar high reading on the outer phases andone lower reading on the center phase, in case of three phase transformers.

Apply 3 phase, 3 wire 415 volt from HV side & keep LV and neutral isolated.

Voltage Applied Current Measured in mA

1U-1V Phase Volts 1U Phase m Amps

1V-1W Phase Volts 1V Phase m Amps

1W-1U Phase Volts 1W Phase m Amps

For LV winding of more then 1.1 KV class, Apply 3 phase, 3 wire 415 volt from LV side & keep HV and neutral isolated.Measure the excitation current in all 3 LV.

Voltage Applied Current Measured

2U-2V Phase Volts 2U Phase m Amps

2V-2W Phase Volts 2V Phase m Amps

2W-2U Phase Volts 2W Phase m Amps

Note: Magnetising current on LV side is applicable for power transformer only. This shall be not applicable for furnaceduty, rectifier duty and distribution transformer.

4.6 VECTOR GROUPThe purpose of this test is to check correctness of HV &LV winding connections in a transformer. Measurement of thistest, connect the primary and secondary of one phase together and then measure the voltage between variousterminals by applying three phase supply.

Short 1U to 2U. Apply 415 volts, 3 phase, 3 wire supply to 1U, 1V, 1W to HV winding and measure suitable manner.

VECTOR GROUP DIAGRAM OF DIAGRAM AS REMARKSCONNECTION CONNECTED FOR TEST

4.7 MAGNETIC BALANCEPurpose of this test is to check healthiness windings, core assembly condition and flux distribution in the transformer.Single phase supply (preferable 230 volts AC) is applied to each winding and the induced voltages are measuredacross other two phases. When the supply is fed to outer phase of a transformer, the voltage induced in the centerphase shall be 50 to 90% of the applied voltage. However, when the center phase is excited then the voltage inducedin the outer phases shall be 30 to 70% of the applied voltage.

Apply 230 volt AC on primary/Secondary winding and measure the voltage.

UV/UN VW/VN WU/WN

(SUPPLY) MEASUREMENT MEASUREMENT

MEASUREMENT (SUPPLY) MEASUREMENT

MEASUREMENT MEASUREMENT (SUPPLY)

Note: Magnetic balance on LV side is applicable for power transformer only. This shall be not applicable for furnaceduty, rectifier duty transformer.

4.8 WINDING RESISTANCETransformer winding resistance are to be measured at site in order to check for abnormities due to loose connection,broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings. The results aregenerally measured with original data measured in factory. The test shall be conducted at all taps of the transformerwindings and the values are to be converted to 75º C. Winding resistance measured shall be done after measurementof magnetization current.

WINDING TAP NO. PHASE U PHASE V PHASE W TEMP.

HV 1

2

3

.

.

n

LV

WINDING RESISTANCE AT 75’C = (235 + 75) X Measured Wdg. Res.

(235 + Ambient Temp.)

4.9 OLTC CONTINUITY CHECK

CONTINUITY TEST BY OPEN CIRCUIT TESTApply 440 volts supply on HV connect volt meters on LV. Check for discontinuity of induced voltage while tap changeris operated throughout the range in both raise and lower directions.

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Transformer Manual

CONTINUITY TEST BY SHORT CIRCUIT TEST1. Short LV side.

2. Give 440V Supply on HV side.

3. Put ‘A’-meter across all the phases.

4. Tap changing keeping supply switched ON

5. Continuity between line and neutral or line to line should check with multimeter on all taps and phases to beconfirmed.

No abnormal variation/discontinuity should be observed during tap changing. This should be checked for raise andlower operations for all taps.

HV CURRENT AT APPLIED VOLTAGE = (RATED HV CURRENT/IMPEDANCE IN VOLTS)X

APPLIED VOLTAGE

IMPEDANCE IN VOLTS = (PERCENT IMPEDANCE X IIV VOLTS)/100

4.10 OIL CHARACTERISTICSTake necessary precaution, while withdrawing the oil sample. Each sample should be free of air bubbles and shouldnot be tested when it is hot. The sample should satisfy IS: 1866.

SL. OIL SAMPLE TIME & MOISTURE B.D.V IN RESISTIVITY TAN-NO. FROM CONDITION IN PPM KV AT 90º C 90º C

1 Tank (Top)

2 Tank (Bottom)

3 OLTC Diverter

4.11 TESTING OF ON LOAD TAP CHANGER

Sr. No. Description Observation Remarks

1 Visual inspection of OLTC

2 Hand operation in all taps

3 Complete wiring of the circuit

4 Upper/Lower Limit switch

5 Over running device.

6 Over load relay of driving motor

7 Local operation

8 Remote operation

9 Tap position indicator

10 Out of step relay

11 Tap changer stuck up relay

12 Function of A.V.R

4.12 OFF CIRCUIT TAP CHANGERTap switch handle should not be left halfway or unlock. Limit

Switch for Maximum/Minimum should be operated.

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4.13 TEST ON CT1. Ratio

2. Polarity

3. Magnetising current

4. IR value

4.14 TESTING OF COOLING CIRCUITPhase sequence of supply to fans and pumps to be checked for rotation in direction of arrow.

WTI Setting Set for OTI Setting Set for

Alarm 90º C Alarm 80º C

Trip 100º C Trip 90º C

Fan start 65º C

Pump start 70º C

NOTE: FOR OFW COOLING & UNIT COOLERS PUMP WILL BE ON WHEN TRANSFORMER IN OPERATION.

4.15 TERTIARY WINDING PROTECTION

TICK IN ONE OF THE FOLLOWING OPTION IT TRANSFORMER IS HAVING TERTIARY WINDING

TERTIARY OPEN AND CONNECTED TO LIGHTENING ARRESTER.

TERTIARY CLOSED INSIDE AND EARTHED INSIDE.

TERTIARY CLOSED INSIDE AND EARTHED OUTSIDE.

TERTIARY CLOSED OUTSIDE AND EARTHED OUTSIDE

TERTIARY UNLOADED / LOADED AND CONNECTED TO LIGHTNING ARRESTORS

4.16 PROTECTION AND ALARMSTest the tripping of various devices and relays by actual operation. Tripping of HV and LV breakers checked through allthe below devices.

Sl. No. Device Alarm Trip Remarks

1 Buchholz Relay

2 Winding Temp. Indicator

3 Oil Temp. Indicator

4 Pressure relief valve

5 Oil surge relay

6 Magnetic oil level gauge

7 Oil floe failure

8 Water flow failure

9 Oil pump trip

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Transformer Manual

Sl. No. Relays on HV & LV Current setting Time setting Make / Type

1 Differential relay

2 Over current relay

3 Earth fault relay

4 Over fluxing relay

5 R.E.F relay

6 Over voltage relay

7 Under voltage relay

4.17 DETAILS OF CIRCUIT BREAKER

1. H.V Circuit Breaker :

Make : Type :

MVA Rating : KV Class :

2. L.V Circuit Breaker :

Make : Type :

MVA Rating : KV Class :

4.18 ENERGISINGWhen all checks and tests are found satisfactory, Air should be vented from all air release plug/screws provided on thetransformer, radiators, bushings, buchholz relay, oil surge relay. After completely venting the air out, the transformershould be energised and maintained on no load for 24 hours. The initial Magnetising

current at the time of switching will be very high. This depends on the particular movement in the cycle. Hummingsound and chattering of arcing horn should also be monitored after charging of few minutes.

After 24 hours no load operation, the transformer should be de-energised and air venting should be carried out againto release all the trapped air. After this venting operation, the transformer should be re-energised and maintained onno load for 6 hours. After this period of 6 hours the transformer can be put on load gradually.

4.19 RECORD AFTER CHARGING1. No load current at relay terminal

CT Ratio -

U Phase : Amp

V Phase : Amp

W Phase : Amp

2. Temperature (At the time of charging)

O.T.I : º C

W.T.I : º C

Ambient : º C

3. Maximum temperature after 24 houres. º C

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5 MAINTENANCE AND OPERATION

5.1 GENERAL MAINTENANCEIn order to avoid faults and disturbance, it is important that a careful and regular supervision and control of thetransformer and its components is planned and carried out. A regular system of inspection and preventive maintenancewill give long life and trouble free service. The causes of breakdown of transformer are generally classified as:

1. Overload 4. Poor workmanship2. Incorrect installations 5. Accident3. Faulty design 6. Surges on transformer

DIRT/DUST:

The external transformer surface shall be inspected regularly and when required cleaned from dust, inserts, leaves andother airborne dirt. Dust collection on bushing, in control cubicle should be inspected.

RUST DAMAGES & PAINTING:

A rusting of various ferrous parts in transformer should be inspected regularly. A regular inspection of the externalsurface treatment of the Transformer should be carried out. Possible rust damages are removed and the surfacetreatment restored to original state by means of primer and finish paint.

POSSIBLE LEAKAGE:

After energizing of the transformer, a certain setting may appear in sealing joints. This should therefore be retightenedaccording to schedule.

5.2 ROUTINE MAINTENANCE INSPECTION OF OLTCIMPORTANT CAUTION:Dismantling of the Diverter Switch and Energy Storage Mechanism by untrained personnel is not recommended. Incase it is desired to have a detailed inspection, we would strictly recommend that the Tap changer Manufacturershould be contacted for assistance.

For maintenance of Motor Drive, Please see separate pamphlet on Motor Drive. All gear units and moving parts in theDrive Mechanism are lubricated for life before dispatch by us. No renewal of lubrication is needed.

All pins and nuts, on the Drive Shaft arrangement need periodic inspection and greasing to prevent corrosion.

The protective Relay does not require any maintenance. But it is advisable to periodically open the terminal box coverat the top to check for ingress of water inside the terminal. Chang the gasket if necessary.

SUCTION PIPEAs the Diverter Oil Vessel has only very small oil volume, no provision for circulation is made as standard.

However a Suction pipe may be provided optionally to remove oil from the bottom of the Oil Vessel.

In case where such a Suction Pope is provided it is absolutely imperative 1: that while filling the Diverter Oil Vessel, thevent of the suction Pipe should be opened.

Schedule of Routine Inspection Operating Current Number of Tap changes Time between Routine Inspection.

Up to l5OA 100000Over l50A 70000Up to 400 A 50000Up to 500 A 50000 or 3 years which ever is earlierUpto400A 100000Upto500A 100000Upto 800 A 70000U5to120OA 85000 above.

Figures given above are applicable to units at the star point. For three phase delta Tap changers and for Line End unitstake Number of operations at 60% of

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Transformer Manual

INSULATING OILThe switch tank is fitted with drain valve and filtration may be carried out via this valve and the tank top cover.The oil should be filtered after every 30,000 operation.

At intervals preferably during filtration the diverter switch unit should be washed down with clean’ oil and all traces ofcarbon removed.

The electrical withstand strength of oil must not be less that 30 KV at any time. Oil from selector switch should betested every six months.

DRIVE MECHANISM UNITThe mechanism has been designed on the basis of no maintenance being necessary over a long period. It is importantthat NO OIL be applied to any part of the mechanism since special lubrication arrangements are made as follows:Ball and roller bearings are pre-packed with special long life grease and protected by dust. The remaining surfaceswhich require lubrication that is cam faces gear teeth, rollers and wiping surfaces are lubricated with molybdenumdisulphide in a grease base. This lubricant is spread over the working faces and will not migrate. A small supply of thisspecial lubricant should, however, be kept in store to replace any that might be inadvertently wiped away duringinspection of cleaning.

A first service is strongly recommended after 4000 operations or 6 months from commissioning whichever is earlier.

Thereafter servicing should be done as per the Servicing Schedule.

Following should be carried out after every six months:

1. Touch up paint, as required, the drive mechanism.2. Check electrical and manual operations.3. Check operation of limit switches, crank handle interlock and oil surge relay tripping.4. Ensure silica gel breather is dry.5. Test oil if electrical withstand strength is below 30 KV, filter oil. The electrical withstand strength of filtered oil

should be not less than 50 KV.6. Clean air filters.7. Ensure that all gaskets On drive mechanism door are in good condition and the window glass intact.

SERVICING SCHEDULE1. De-energize, isolate and earth the transformer. Drain Oil from tap changer.2. Inspect the selector switch through the top inspection cover.3. Check condition of fixed and moving contacts. The tips of these will appear to be slightly roughened due to

arcing. This is normal. If however, the contacts are found to be worn out replace with new contacts.4. Check Pat in the rest position the resistor the fixed contacts.5. Check and ensure that all nuts, screws and other fasteners are secured properly and all connections are in good

condition.6. Be careful not to alter the vernier setting and clarifiers of the coupling between the drive mechanism and the

selector switch but the tightness of the nuts should be checked.7. Check visual and by continuity test that the transition resistors are in good condition.8. Wash down the switch with dry hot transformer oil. Do not use cotton waste. A piece of cloth should be used to

clean bottom of tank. Ensure that no foreign particles should be left inside the switch.9. Refill with clean dry hot oil. The oil must have an electrical withstand value of not less than 50KV.10. Examine silica-gel breather, and reactivate or replace as necessary.11. Clean drive mechanism chamber and tighten fasteners.12. Ensure that all gaskets on drive mechanism door are in place and in good condition and the window glass is

intact. Replace as found necessary.13. Lubricate all surfaces such as gear teeth, wiping surfaces etc. with special lubricant as required.14. If any scratches are found on painted surfaces, touch up with paint.15. Clean air filters.16. Check electrical operation.17. Check functioning of limit switches, crank handle interlock, motor protection relay and heaters.18. Ensure damper is locked in minimum position.19. Operate the switch by hand one cycle, and electrical through two cycles to check all functions. The switch should

show snappy action all the time.

41

20. Check operation of oil surge relay trip.21. PLEASE DO NOT:

1. Work with naked lights.2. Allow moisture to get into selector tank while servicing.3; Allow other solid contamination of oil.4. Alter disposition of connections to terminal board in any manner.5. Stress terminal board stems while trying to tighten connection.6. Leave any foreign matter in the selector switch tank.

5.3 OIL FILTRATION AND MOISTURE REMOVALTransformer oil absorbs moisture easily. Moisture observation is prevented by means of following alternatives.

1. Oil conservator with air cell (rubber bellow):The system consists of an oil conservator with a built-in air cell as a separating wall between the transformer oiland surrounding air.

2. Oil conservator with breather:The moisture absorption prevented by a breather connected to the conservator. The active drying agent (silicagel) should be exchanged or regenerated when 2/3 of the agent has been pink by absorbed moisture. Thebreather contains also an air-lock that prevents spontaneous air circulation. This lock should be filled with oil tolevel specified.

3. Oil level indicator for oil conservator:The transformer oil conservator is provided with an oil level indicator. At an oil temperature of 45º C, theconservator should be half filled. If the level exceeds the value “full”, the oil must be drained off. If the value is“low”, oil must be filled in. Normal oil level should be at 35º C mark.

4. Gas operated Relay:The use of gas operated relay as protection for oil immersed transformers is based on the fact that fault as flashover, short circuit and local overheating normally results in gas-generation. The gas bubbles gathering in the gasoperated relay affect a float-controlled contact which gives an alarm signal.

5. Venting:Certain transformers are provided with venting nipple on higher level components in order to remove possiblytrapped air in the oil system. This nipple should be checked after the installation of the transformer, or after aninspection that opening of the oil system.

6. Moisture:If the oil has been exposed to moisture air due to prolonged storage, moisture contents and dielectric withstandtests shall be carried out. If values are low, oil should be filtered.

7. Sludge and Acid in the Oil:Oil which has not been too much oxidized may be regenerated by circulation through the oil filtering machine.If sludge is being produced in the oil, the oil changes colour and becomes darker and turbid. The sludge can beremoved by means of filtering, but if the sludge formation has started, it is increased with time. The oil shouldtherefore be exchanged if the neutralization value exceeds 0.5 mg KOH/g. Old oil should not mix with new oilas the new oil will get infected and will age up quickly.

5.4 TEMPERATURE SUPERVISIONThe service life of a transformer is highly dependent on the temperature prevailing on the core and windings underoperation It is therefore essential to keep a watch on the oil and winding temperature continuously. The temperature shouldbe read regularly and the measured value registered. In automatic monitoring system, the oil and winding temperature arescanned and monitored, continuously. When temperature approaches limits, the alarm system will be operated.

1. Overload: The transformer can be overloaded during lower ambient temperature. However, the permissibleoverload must not be judged only with regard to the oil temperature. We must also consider that the temperaturedifference between oil and winding will increase with the load. Therefore winding temperature should besupervised during overloading.

If the temperature in a transformer shows a tendency of rising without a corresponding increase of a load, thismay be caused by a reduction of the cooling ability of the cooling equipment. The temperature should also bechecked in this case.

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Transformer Manual

2. Thermometer for measuring of Top-oil temperature: The thermometer consists of a cylindrical sensing body witha flange, a capillary tube, and a thermometer housing with dial and contact device. The measuring system isfilled with liquid, which changes its volume at the temperature variations, and affects spring bellows. The movementsof the bellows are transferred to the pointer and signal contacts of mercury switches. The contact can be setindependently of each other.

3. Winding Temperature Indicator: The thermometer system consist of a sensitive body with assembling details, acapillary tube and a thermometer housing, which consist a scale with an indicating pointer and a maximumtemperature pointer, heating and adjusting resistor, as well as contact-device for four micro switches.

4. Remote control of winding temperature: For remote control of winding temperature a resistance temperaturedevice has been used with a heating element built in to the same housing. The heating element is fed by thesecond current of a current transformer.

For setting of the heating element there is a parallel connected adjustment resistor installed in the control cabinet.

5. Radiators: The external cooling surface 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 suitable carried outby means of water flushing at high pressure. Normally, no measures are necessary for keeping the internalcooling surfaces clean as long as oil is in good condition. If, however, sludge formation has set in, the sludge maydeposit 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.

6. Bushings: Bushings porcelains shall be cleaned from dust and dirt regularly. In areas where the air containsimpurities as salt, cement dust, smoke or chemical substances, shorter intervals are required.

7. Connection: In order to avoid prohibited temperature rises in the electrical connections of the transformers, allscrew joints should be checked and retightened according to schedule.

8. Accessories: Separate leaflet should be followed for various fittings for, fans, pumps, Flow indicators, pressuregauge, Oil gauge, pressure reducing valve, OLTC, Off circuit tap changer, Coolers etc.

5.5 IR TEMPERATURE RELATION:It has been always been the question from the customer as to how the IR can be converted at different temperatures.A simple example is given as to how to calculate the IR value at different temperature. The factors are tabulated asbelow:

Difference 10º C 20º C 30º C 40º C 50º Cin Temp.

Factor K 1.65 2.6 4.2 6.6 10.5

Suppose IR value of 70º C is 300 M ohms and we want to convert at 40º C, the factor K will give you difference intemperature i.e. (70º C - 40º C) = 30º C.

K for 30º C temperature difference = 4.2

so the value at 40º C = 4.2 * 300 = 1260 M ohms.

Value thus converted fairly tally with actual value cooling of transformer.

5.6 SUPERVISION AND CONTROL OF OILThe oil in a transformer provides for an electrically insulating and a cooling medium. The service reliability of an oil-immersed transformer is therefore mainly, depending on the oil quality.

Schedule:-

The oil should be maintained as per the guidelines of IS: 1866. Periodicity of test and permissible limits for importantparameters of oil - electrical strength, water contents, receptivity, tan-delta, neutralization value, sediment and perceptiblesludge, flash point and interfacial tension are given in Annexure. Health of transformer can be effectively monitored byDissolved Gas Analysis (DGA Test). This technique helps in detection of incipient fault in transformer.

43

5.7 TROUBLE SHOOTINGGeneral measures for trouble shooting are described in this section.

1. Following information to be informed to T & R in case of trouble in transformer.• Specification of transformers (Capacity, Sr. No.)• Load current (A) and operating Voltage• Operating tap position• Condition at the time of trouble.

2. Transformer trouble and checks items, Troubles, their possible causes and item to be checked are given in Table.

3. Table for Detective devices and their functions.

4. Table for Check items and judging.

5. Detection method of oil or gas leaks.

5.8 GAS ANALYSIS ON TRANSFORMER OILINTRODUCTIONDissolved gas analysis is a powerful tool to detect incipient faults inside the transformers. Because of not so simplecalculations, operating staff always seek the help of experts to analyse the results DGA. On many occasions it isnoticed that data are lost due improper sampling. This paper gives a procedure for correct sampling and explainsmethods to analyse data with out an experts help. Wherever a detailed assessment is required recommendation ofIEC 7600599 may be followed

Advantages of regular gas analysis:

1. Advance warning of developing faults 3. Status checks of new or repaired transformers2. Determining the improper use of transformers 4. Monitoring transformers under over loading

FOLLOWING GASES ARE DETECTED IN DGA• OXYGEN O2 • ETHANE C2H6

• NITROGEN N2 • ETHYLENE C2H4• HYDROGEN H2 • ACETYLENE C2H2• CARBON MONOXIDE CO • PROPANE C3H8• CARBON DIOXIDE CO2 • PROPYLENE C3H6• METHANE CH4

SAMPLINGMetal cylinders as in figure are to be used collectoil samples from transformers. Proper samplingprocedure is required to get representativesample from transformers. Take sample from aposition where oil is in circulation. Sampling valveif provided at middle is best to get arepresentative sample. Stagnant oil from thesampling valve of the transformer may beremoved before sampling. To achieve this,remove 2 to 3 liters of oil to waste container beforeconnecting metal cylinder to sampling valve.

Attach bottom valve of container to the samplingvalve of the transformer through clear PVCtubing. PVC tube connected to top valve mayrun to a waste oil container. Sampling may bedone in following sequence.

1. Open Sampling valve of transformer (C)2. Open bottom valve of metal cylinder (A)3. Open top valve of metal cylinder (B)

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Transformer Manual

Allow 200ml of oil to flow through cylinder to waste container. While filling the cylinder sides may be tapped with ahard object to dislodge air bubbles trapped inside.

After Sampling valves may be shut in the reverse order.

1. Close top cylinder valve (B)2. Close bottom cylinder valve (A)3. Close transformer sampling valve (C)

After the filling shake the cylinder and feel for any signs of incomplete filling. If not completely filled, redo filling.

Close transformer valve. Remove PVC tubing. Apply sealing to the valves opening. Send sample to lab after properidentification.

ANALYSING RESULTSNo immediate action is required if values of the gases are with in the values given table no.1 except for newtransformers and for transformers with earlier test data are available. For new transformers and transformers withearlier data are available, rate of rise shall be limited to values given in table 2

Table -1

Maximum Values in ppm

Type of transformer H2 CO CO2 CH4 C2H6 C2H4 C2H2

Power transformer Withoutcommunicating OLTC 150 900 15000 110 90 280 50

Power transformer Withcommunicating OLTC 150 850 12000 130 70 250 270

Furnace transformers 200 800 6000 150 150 200 *

Distribution transformers 100 200 5000 50 50 50 5

*- Value of this is influenced by design and assembly of tap changer

Table - 2

Maximum rise

Gas H2 CO CO2 CH4 C2H6 C2H4 C2H2

milli liters per day <5 <50 <200 < 2 < 2 < 2 < 0.1

Equation to calculate rate of rise of gas(y1-y2) q

rate =

(d2-d1)

Wherey1 is the value of reference analysis

y2 is the value of last analysis

d1 date of reference sample y1

d2 date of last sample y2

q Quantity of oil in the transformer

ANALYSINGIf gas contents are more than table 1 or table 2 depending on the case, analyse as given below.

Divide quantity of H2, CH4, C2H6, C2H4 and C2H2 by maximum value of gas out of these gases. Mark position ofthese gases in X axis and ratio of particular gas with highest quantity in y axis, plot a graph. Compare this pattern withtypical patterns given below:

45

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Transformer Manual

47

TEMPERATURE OF FAULTTemperature of fault may be calculated by using following formulaTemperature in ºC = 322log (C2H4/C2H6) +525

INVOLVEMENT OF SOLID INSULATIONThe ratio of CO2 with CO is less than 3 is an indication of involvement of solid insulation in the fault.

5.9 MAINTENANCE SCHEDULE:The maintenance schedule given below is a good indication required to be given to the transformer under averagecondition.

48

SR. NO. ITEMS TO BE INSPECTED INSPECTION NOTES ACTION REQUIRED

HOURLY

1 Ambient temperature - -2 Winding temperature Check that temperature Shut down the transformer &3 Oil temperature rise is reasonable investigate if persistently higher

than normal.4 Load amps/Load voltage Check against rated figures An improper tap position can cause

excessive core loss. Voltage and tapposition should be corrected.

DAILY

1 Oil level in transformer conservator Check oil level from oil gauge Top up, if found low2 Oil level in tap changer diverter switch Check oil level from the gauge glass Top up, if found low3 Leakage of water into cooler - Shut down the transformer &

investigate.4 Relief vent diaphragm - Replace if cracked or broken5 Dehydrating breather Check colour of the active agent If silica gel is pink change by spare

charges. The old charge may bereactivated for further use

6 Pipe work and accessories for leakages Inspect the transformer for leakages If leakages are observed tightenevenly the gasket joints. Replace ‘O’ring or washer suitably. Replacegasket if needed

QUARTERLY

1 Bushing Examine for dirt deposition and Cracked porcelain should betightness of oil filling plugs. Examine rectified/replaced.for cracks in porcelain. Clean andtighten plugs.

2 Transformer oil and tap changer oil Check for die-electric strength Take suitable action to restoreand moisture content quality of oil

3 Cooler fan, bearing and controls, pumps Check contacts, manual control Lubricate the fan bearings. Replaceand interlocks worn out contacts and other parts.

Clean/adjust controls and interlocks4 On load tap changer Examine contacts, check step by Replace all worn out and burnt

step mechanism operation, end contacts. Set limit switches in position.position limit switches and brakes. Clean/replace brake shoe lining.Also check oil in driving gear Lubricate all bearing and couplingmechanism. points suitably.(Refer separate leaflet.)

5 Dehydrating breather Check oil level in oil cup & that air Make up oil if required.passage are free.

6 Ventilators Check that air passages are free.

HALF YEARLY

1 Test for pressure2 On load changer and driving gear. Check for all moving parts, contacts, Clean, adjust or replace as required.

brake shoes, motor etc.3 Non-conservator transformer. Check for moisture under cover. Improve ventilation, check oil.

Transformer Manual

49

SR. NO. ITEMS TO BE INSPECTED INSPECTION NOTES ACTION REQUIRED

YEARLY

1 Oil in transformer Check for acidity & sludge. Filter or replace.2 Insulation resistance. Compare with values at the time of Process if required.

commissioning3 Oil filled bushing. Test oil. Filter or replace.4 Gasketted joints. Tighten all bolts evenly to avoid

uneven pressure5 Cable box Check for sealing arrangements Replace gaskets, if leaking

for filling holes.Examine compound for cracks.

6 Surge diverter and gaps Examine for cracks and dirt deposits Clean or replace7 Relays, alarms, their circuits,etc. Examine relay and alarm contacts Clean the components or replace

and their operation, check fuse,etc. contacts and fuses if necessary.Relay accuracy etc. Change the setting if required.

8 Temperature indicator Pockets holding thermometers Oil to be replenished if required.should be checked

9 Dial type oil gauge Check pointer for freedom of Adjust if requiredmovement

10 Paint-work Should be inspected Any painting or retouching should bedone if necessary

11 Earth resistance Take suitable action if earthresistance is high.

12 Diverter switches for tap changer Check worn or burnt out contacts. Replace worn out parts.having non-arcing tap selection. filter oil irrespective of oil tests.

13 Tank and accessories. Check painting and surface finish. Touch up/repaint, if required.Mechanical inspection of all Replace any component found damage.accessories.

14 Fan motors Check IR value of motor winding Dry out, if found low,after raining season. check balancing of fans.Noise and vibration of fans.

15 Bushing top connection Check contact joints Retighten

2 YEARLY

1 Oil conservator. Internal inspection. Should be thoroughly cleaned.2 Bucholz Relay Mechanical inspection. Adjust floats, switches etc.3 Transformer oil. Examine values as per IS:1866 Filter oil if required.

3 YEARLY

1 Non-arcing selector tap of General inspection. Replace worn out parts, filter oil.on load tap changer.

5 YEARLY

1 Non-conservator transformer or Over all inspection including lifting Wash by hosing down with clean dryconservator transformers up to 3MVA of core and coils. oil. Tighten coil clamping arrangement

and other bolts, nuts if loose.

7 to 10 YEARLY

1 Transformer and tap changer assembly. Overall inspection including lifting Tighten all clamping arrangement,core and coils. loose coatings etc.

Tighten all nuts and bolts and checklocking arrangements and fasteners.Wash core and coil with drytransformer oil.

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