Transformer Manual An
INSTRUCTIONS FOR INSTALLATION, COMMISSIONING, OPERATION AND MAINTENANCE OF TRANSFORMERS AND REACTORS
I Sr. No. 1.0 1.1 1.2 2.0 2.1 2.2 2.3 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22 3.23 3.24 3.25 3.26 3.27 3.28 3.29 3.30 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9
PARTICULARS LIST OF STANDARDS RELATED TO OUR PRODUCTS LIST OF INDIAN STANDARDS REFERRED LIST OF IEC STANDARDS REFERRED TRANSPORTATION, DESPATCH AND STORAGE TRANSPORTATION RECEIPT OF TRANSFORMER AT SITE STORAGE INSTALLATION LOCATION AND SITE PREPARATION ASSEMBLING THE TRANSFORMER GASKETS BUSHINGS TAP CHANGERS COOLING OF TRANSFORMERS RADIATORS COOLING FANS OIL PUMP OIL FLOW INDICATOR HEAT EXCHANGER DIFFERENTIAL PRESSURE GAUGE CONSERVATOR TANK MAGNETIC OIL LEVEL GAUGE COMMISSIONING OF AIR CELL GAS AND OIL OPERATED (BUCHHOLZ) RELAYS SILICA GEL BREATHER PRESSURE RELIEF VALVE (PRV) DOUBLE DIAPHRAGM EXPLOSION VENT MARSHALLING BOX OIL TEMPERATURE INDICATOR WINDING TEMPERATURE INDICATOR CURRENT TRANSFORMER CONTROL CABLING ASSEMBLY OF VALVES ASSEMBLY OF PIPE WORK OIL FILLING EARTHING COMPLETION OF ERECTION WORK TOUCH-UP PAINTING COMMISSIONING GENERAL CHECK POINTS DO’S & DON’TS FOR POWER TRANSFORMER INSULATION RESISTANCE TEST (MEGGER) VOLTAGE RATIO TEST MAGNETIZATION CURRENT VECTOR GROUP MAGNETIC BALANCE WINDING RESISTANCE OLTC CONTINUITY CHECK OIL CHARACTERISTICS TESTING OF ON LOAD TAP CHANGER OFF CIRCUIT TAP CHANGER TEST ON CT TESTING OF COOLING CIRCUIT TERTIARY WINDING PROTECTION PROTECTION AND ALARMS DETAILS OF CIRCUIT BREAKER ENERGIZING RECORD AFTER CHARGING MAINTENANCE AND OPERATION GENERAL ROUTINE MAINTENANCE INSPECTION OF OLTC OIL FILTRATION AND MOISTURE REMOVAL TEMPERATURE SUPERVISION IR TEMPERATURE RELATION SUPERVISION AND CONTROL OF OIL TROUBLE SHOOTING GAS ANALYSIS ON TRANSFORMER OIL MAINTENANCE SCHEDULE
X PAGE NO. 1 2 3 3 5 6 6 7 7 11 14 14 15 15 16 17 17 18 18 19 20 22 23 24 25 25 25 28 28 28 28 28 31 31 31 32 33 34 34 35 35 36 36 36 37 37 37 38 38 38 38 39 39 39 40 40 42 43 43 44 44 44 48
P R E F A C E • TARIL Transformers are designed, manufactured and tested with care. With proper attention during installation and use, the user should receive from it the maximum expected performance. • This maintenance procedure gives a good understanding about handling and maintenance of the transformer. • It is recommended to go through all information contained in this before installing, operating and maintenance of the transformer. • These instructions have been prepared to provide information on assembly, installation, commissioning and 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 following address. • If any problem occurs during operation, please inform T&R at following address by specifying the following. 1.
YEAR OF MANUFACTURE
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: [email protected]
Transformer Manual 1. LIST OF STANDARDS RELATED TO OUR PRODUCTS 1.1 LIST OF INDIAN STANDARDS REFERRED Sr No IS/EC NO AND YEAR
SPECIFICATIONS POWER TRANSFORMERS A1. SPECIFICATIONS
IS 1885 (Part 38) : 1977
Electrical Vocabulary Part 38 Transformers
IS 2026 (Part 1) : 1977
Power Transformers: General
IS 2026 (Part 2) : 1977
Power Transformers: Temperature Rise
IS 2026 (Part 3) : 1981
Power Transformers: Insulation level & Dielectric Tests
IS 2026 (Part 4) : 1977
Power Transformers: terminal Marking, Tappings & Connections
IS 2026 (Part 5) : 1994
Transformer/Reactor bushings minimum external clearance in air- Specification
IS 6600 : 1972
Guide for loading of oil immersed Transformers
IS 10561 : 1983
Application guide for Power Transformer
IS 335 : 1993
IS 10028 (Part 1) : 1985
Code of practice for selection installation & maintenance of Transformers
IS 10028 (Part 2) : 1981
IS 10028 (Part 3) : 1981
Outdoor Type three-phase distribution transformers up to & Including 100 KVA in 11 KV.
IS 1180 (Part 1) : 1989
Non Sealed Type
IS 1180 (Part 2) : 1989
A2. MATERIALS, FITTINGS & ACCESSORIES OF POWER TRANSFORMERS. 1
IS 1576 : 1992
Solid Pressboard for electrical purpose
IS 2312 : 1976
Propeller type AC ventilating fans
IS 3024 : 1965
Electrical steel sheets (oriented)
IS 3231 : 1986 1987
Electrical relays for Power Transformers
IS 3402 : 1992
IS 3588 : 1987
Electrical Axial Flow fans
IS 3637 : 1966
Gas Operated relays
IS 3639 : 1966
Fittings & Accessories for Power Transformers
IS 4253 (Part 1) : 1980
Specification for cork composition sheets
IS 4253 (Part 2) : 1980
Cork & Rubber
IS 6088 : 1988
Oil to water heat exchangers for Transformers
IS 7404 (Part 2) : 1991
Paper covered Copper conductors. Rectangular conductor
IS 8468 : 1977
On-load tap changers
IS 9147 : 1979
Cable sealing boxes for oil immersed transformers
IS 9700 : 1991
IS 8478 : 1977
Application guide for On Load Tap Changers
IS 5561 : 1970
Electric Power Conductors
IS 12943 : 1990
Brass glands for PVC Cables 1
1.2 LIST OF IEC STANDARDS REFERRED SR NO IEC NO
HIGH-VOLTAGE TEST TECHNIQUES (GENERAL DEFINITIONS AND TEST REQUIREMENT)
HIGH VOLTAGE TEST TECHNIQUES (MEASURING SYSTEMS)
POWER TRANSFORMERS (GENERAL)
POWER TRANSFORMERS (TEMPERATURE RISE)
POWER TRANS. INSULATION LEVELS ,EXTERNAL CLEARANCE & DIELECTRIC TESTS
POWER TRANSFORMERS (INSULATION LEVELS AND DIELECTRIC TESTS EXTERNAL CLEARANCES IN AIR)
POWER TRANSFORMER – ABILITY TO WITHSTAND SHORT CIRCUIT
POWER TRANSFORMER – ABILITY TO WITHSTAND SHORT CIRCUIT
LOADING GUIDE FOR OIL-IMMERSED POWER TRANSFORMERS
POWER TRANSFORMERS-APPLICATION GUIDE
POWER TRANSFORMER-DETERMINATION OF SOUND LEVELS
DETERMINATION OF SOUND LEVELS-APPLICATION GUIDE
POWER TRANSFORMERS-DRY TYPE TRANSFORMERS
TS 60076-14 FIRST
DESIGN & APPLICATION OF LIQUID-IMMERSED POWER TRANSFORMERS USING HIGH-TEMPERATURE INSULATION MATERIAL
INSULATED BUSHING FOR ALTERNATING VOLTAGES ABOVE 1000V
BOBINES D’INDUCTANCE (REACTORS)
SPECIFICATION FOR UNUSED MINERAL INSULATING OILS FOR TRANSFORMERS
SPECIFICATION FOR PARTICULAR TYPES OF WINDING WIRES
TEST METHOD FOR INDUCTION CHANNEL FURNACES
SPECIFICATION FOR CELLULOSE PAPER FOR ELECTRICAL PURPOSES
SPEC.. FOR CELLULOSE PAPER FOR ELECTRICAL PURPOSES
SPEC. FOR PRESSBOARD & PRESS PAPER FOR ELECTRICAL PURPOSE
SPEC. FOR PRESSBOARD & PRESS PAPER FOR ELECTRICAL PURPOSE
IEC STANDARD VOLTAGES
TAP CHANGERS-PERFORMANCE REQUIREMENTS & TEST METHODS
TAP CHANGERS-APPLICATION GUIDE
HIGH VOLTAGE TEST TECHNIQUES - PARTIAL DISCHARGE MEASUREMENT
SPEC. FOR PARTICULAR TYPES OF WINDING WIRES
CELLULOSE PAPERS FOR ELECTRICAL PURPOSES
MINERAL OIL IMPREGNATE ELECTRICAL EQUIPMENT IN SERVICE GUIDE TO THE INTERPRETATION OF DISSOLVED AND REC GASES ANALYSIS
HIGH VOLTAGE SWITCHGEAR AND CONTROL GEAR - CABLE CONNECTION FOR GAS 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 be transported either filled with Oil or Nitrogen as per the description given below.
WITH OIL The Transformer is filled with oil such that sufficient tank volume is left clear for expansion to limit the excess internal air 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 all insulating 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 active parts against moisture the transformer tank is filled with dry Nitrogen at a low internal pressure 0.2kg/cm2 at TARIL works before dispatch. All openings resulting from the removal of fittings for shipment are sealed with suitable gasketted blanking plates during transport. The gas pressure & temperature at the time of dispatch from factory are painted on the tank cover. Full quantity of oil is sent in separate sealed drums. For control of gas pressure and maintaining the pressure during transport and possible storage before assembling, the transformer 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 gauge provided on the first stage indicates the cylinder pressure, whereas other gauge provided on second stage indicates the tank pressure. The gas consumption during transport and possible storage is difficult to estimate as it depends on ambient temperature variation, possible leakages as well as on the duration of transport and storage. However, two back up cylinders are provided to meet the gas requirement. Normally one cylinder is kept open and the other shut. When the pressure of first cylinder falls to 0.5 kg/cm2 this should be considered as empty and immediately its valve should be closed and the valve 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 reason oil 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.
Pull 500mm. mercury vacuum after blanking off all openings.
Break the vacuum by admitting dry nitrogen through a convenient valve at the top of the tank and drain the oil completely. Continue to supply nitrogen until it maintains a steady pressure of 0.14 ± 0.02 kg/cm2 above the atmosphere.
4. 5. 6.
Pressure of nitrogen gas shall be maintained at 0.14 ± 0.02 kg/cm2 at same reference temperature. Pressure would be monitored by taking three readings within 24 hours to ensure that there is no leakage of gas Shut off the gas supply valve and fit dry nitrogen back up cylinders through nitrogen regulator valve.
2.2 RECEIPT OF TRANSFORMER AT SITE To ensure that a Transformer will function satisfactorily it is important that handling, lifting, storing and assembling are carried out with great care and cleanliness by experienced personnel who are thorough with various working operations. 3
This section gives instructions how handling, lifting, storing and assembling should be carried out. For large Transformers it is recommended that the work is done by TARIL or is under supervision by experts from TARIL.
INSPECTION In connection with receiving and unloading at site, and at the final storing place before assembling, the transformers shall 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 any component, make the necessary claims with the contractor’s representatives under intimation to supplier so that such claims can be registered with the transport agents. Before unloading, the condition of packing and of the visible parts should be checked and possible traces of leaks verified (condenser bushings). If necessary, appropriate statements and claims should be made. Drums containing oil which have been dispatched separately should be examined carefully for leaks or any sign of tampering. All drums are dispatched filled up to their capacity and any shortage should be reported. As a principle we never supply partly filled drums. In order to protect the active part against moisture, the transformer tank is filled with nitrogen during transport at an over 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 transformer components respectively are tight, and that the active part including the insulation materials is dry. If there is no positive gas-pressure, transformer should be immediately filled with dry Nitrogen gas at pressure of 2 PSI without loss of time.
DAMAGE/LOSS Damage or loss of any component should be reported to Supplier immediately. Photographs or other useful evident should be submitted wherever possible. When the transformer is dispatched filled with oil, level in transformer main tank should be checked & if the level is below the indicated level in main tank the same should be reported to the supplier immediately & also the same should be mentioned in LR copy. When the transformer is dispatched filled with oil, a sample of oil should be taken from 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 to earth. Drums containing transformer oil which have been dispatched separately should be examined carefully for leaks or any signs of tempering. All drums are dispatched filled up to their capacity and any shortage should be reported to us immediately.
UNLOADING The transformer should be unloaded by means of crane or suitable device of sufficient capacity (Please refer rating plate 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 are lifting 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/ropes are 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 on both 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 placed under any valves. To avoid undue mechanical stresses the transformers must be kept on reasonably level foundation.
ASSEMBLY OF WHEELS Whenever wheels are supplied with transformer, movement of transformer at site is carried out by mounting these wheels.
Transformer Manual Mounting of wheels under transformer is to be done as per roller arrangement drawing. In power transformer wheels are designed such a way that conservator side wheels are of slight higher in height fin the view of keeping transformer shortly 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 transformer is in the final position, the wheels shall be locked to prevent accidental movement of the transformer.
2.3 STORAGE 1.
After arrival at site, it is desirable to erect and commission the transformer with minimum delay. In case this is not possible the transformer shall be erected at its permanent location with conservator and breather fitted and dry oil filled to the correct level. Whether storage or in use, the dehydrating breather must be fitted on all the transformers which are oil filled. The breather incorporates an oil sealing device which must be filled with oil, to the level marked, to be effective. The condition of silica gel must be checked periodically, particularly during monsoons.
Dismantled equipment and components are packed to be protected against normal handling and transport stresses.
All other accessories should be stored in a covered dry place. It should be ensured that there is no damage to the gasketted joints and that all the blanked joints are tight.
Goods stored outdoors must not be placed directly on the ground, and should be covered carefully with tarpaulin or similar material.
The tap changer, if provided, should be operated at 6 monthly intervals. Two of three runs from one end of the range to the other and back are sufficient.
Heaters on Marshalling kiosks, etc. should be kept energized. If for this purpose, power supply at the appropriate voltage to suit the heater is not available, temporary heaters should then be installed to suit the site voltage. The provision of heaters is important and failure to provide them may result in extensive damage to the contents of the kiosk because of condensation.
If oil received in drums is not likely to be used immediately the drums should be stored in a covered space where the temperature variation is minimum. If it is necessary to store the oil outside, adequate protection must be provided at all times. Oil drums should be stored in horizontal (lying) position with both the bungs also in horizontal position.
INDOOR STORAGE The 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.
3 INSTALLATION 3.1 LOCATION AND SITE PREPARATION 1.
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 to be provided from various live points of the transformer to earthed parts. ONAN type transformers depend entirely upon the surrounding air for carrying away the heat generated due to losses. For indoor installation, therefore, the room must be well ventilated so that the heated air can escape readily and be replaced by cool air. Air inlets and outlets should be of sufficient size and number to pass adequate air to cool the transformer. The inlets should be as near the floor as possible and outlets as high as the building will allow. Where necessary, exhaust fans can be installed for the purpose. The transformers should always be separated from one another and from all walls and partitions to permit free circulation of air. Where walls are provided, it should be ensured that the transformer gets good ventilation as mentioned above for indoor transformers. Provision should be made for the emergency drainage of the oil from the transformers (e.g. in case of fire in neighboring apparatus or bushing or the transformer tank), by surrounding the transformer plinth with sump filled with small pebbles.
3.2 ASSEMBLING THE TRANSFORMER
1. 2. 3. 4.
5. 6. 7. 8. 9.
10. 11. 12.
13. 14. 15.
By means of the Part list and the Transformer GA Drawing, the assembling of a fully completed transformer is carried out according to the following Instructions. The following precautions are to be taken: Fire-fighting equipment shall be available near the oil-treatment equipment as well as at work on and adjacent to the transformer. 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. Check that there is no overpressure in the transformer when blanking plates or connection lids are to be opened. Workmen having access to the interior of a transformer should empty their pockets of all loose articles. Any spanners or other tools used should be securely tied with a tape so that they can be recovered, if accidentally dropped. All loose objects, tools, screws, nuts etc., shall be removed from the transformer cover before opening the connection and blanking lids. All loose objects (tools, pencils, spectacles etc.,) shall be removed from the boiler-suit pockets etc. Before starting the work through man holes. Tools to be used inside the transformer/reactor -e.g. for tightening of screw-joints- shall be fastened to the wrist or another fixed point by means of cotton tape or string. Tools with loose sleeves and tools with catches must not be used at work inside the transformer. Fibrous cleaning material should not be used as it can deteriorate oil when mixed with it. The presence of loose fibers in suspension in transformer oil can reduce its insulating properties. The presence of loose fibers in suspension in transformer oil can reduce its insulating properties. If any cleaning or wiping is necessary this should be done with clean and dry oil, using soft non-fluffy cloth. Check healthiness of all the accessories and components dispatched separately with respect to GA Drawing and packing list. All components dispatched separately should be cleaned inside and outside before being fitted. 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. The warm oil should be circulated as long as transformer is open to atmosphere, Oil pump & all joints in the oil pipe work should be airtight to avoid entrance of air through leakage joints. The active part (core and winding) should be exposed to the surrounding air as short time as possible. Open therefore only one blanking plate or connection lid at a time for remounting of bushings, valves etc. Check Conditions of Leads, Connections, Tap changer, General conditions of insulation etc.
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 GASKETS The sealing system normally used against oil and gas in transformers and belonging components has rubber bonded cork gaskets at joints. Whenever the blanking plates are removed to fix detached parts such as bushing turrets, etc., a new gasket shall be used while fixing the same. A set of new unused gaskets of correct size and thickness is supplied with every transformer for 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 its original thickness. Joints should not be subjected to pressure until tightening is complete. If care is taken in making joints, and in handling the gasket, it is possible to break and remake a joint several times, using the same gasket. Bolts/studs should be tightened lightly and diagonally in the sequence. Do not over tighten, otherwise gasket will get crushed.
3.4 BUSHINGS After completing the internal connections, windings leads are brought out through suitable bushing. Normally three types of bushings are used.
PLAIN PORCELAIN TYPE Assembly and dismantling of this type of bushing is possible without disturbing active part, tank cover and tank.
PLAIN OIL FILLED TYPE Bushings supplied are oil communicating type. Oil communicating type bushing must be filled with transformer oil in operation and this is achieved by keeping conservator above bushing level so that bushing remain always filled with oil.
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 with Aluminum foils at pre-designed locations for best possible internal & external dielectric strengths. The active part is built 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 assembled inside porcelains Aluminum flange, and conservator with oil resistant synthetic rubber seals and are held together by a spring pack which also takes care of expansion & contraction of the centre pipe/rod. The annular space between the core and the porcelain is filled with the same oil. A nitrogen gas cushion is provided in conservator to take care of expansion and contraction of oil. An oil level gauge (prismatic / magnetic) is provided on the conservator. At the mounting flange an air vent plug is provided for air release of the transformer turret or for connecting to a Buchholz relay. 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 fixing arrangement 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 BE REMOVED DURING OPERATION.
PACKING AND TRANSPORT The bushings are packed horizontally in sturdy wooden cases with oil level indicator facing down to avoid Nitrogen gas to come in contact with active part. Immediately on receipt, inspect the bushing thoroughly for any damage after opening 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 STORAGE Bushing can be lifted from packing case It is recommended to use two point lifting arrangement. The bushing can be stored in the same packing case with oil level indicator facing down or can be kept vertical on stands with flange firmly fixed stand.
Generally condenser type bushings are used for 72.5 KV and above. Design of this bushing is such that their mounting is independent of oil level in conservator, whenever this bushing are mounted on bushing pockets or raised truncated portions. 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 have occurred 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 appear from the GA drawing and, part list.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Oil Impregnated Core Central Metal Tube Fixing Flange Porcelain - Upper Porcelain - Lower Gaskets Springs Expansion Bowl Cable Bolt Air Releasing Screw Test Tap Top Terminal Oil Filling Plug Oil Sight Glass Base Plate / Stress Shield 16 Upper Arcing Horn 17 Lower Arcing Horn 18 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 by unscrewing 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
650 / 550
800 / 635 mm
1050 / 900
1400 / 1250 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 VOLTAGE RATING UP TO 36KV. 1. 2.
Fix the stem on the top of the connecting stud and then lock them with lock nuts. Insert the clamping ring over the bottom of the insulator before kept in the position. Place the insulator on the cover property, insert threading stem assembly and insulating tube through it. Ensure that stem is properly engaged inside the groove of the bushing. Insert seal ring, cap and fix them in position by hexagonal nut. Clamp the insulator by means of clamping ring, clamp, washer and hexagonal nut. See that the insulator is tightened evenly and carefully at all places. Turn the cap so that mounting locations for spark gap horns which are fitted later are in the position. Fix the hexagonal nut, plain washer and spring washer.
ASSEMBLY OF CONDENSER TYPE OF BUSHING USED FOR VOLTAGE RATING 72.5KV & ABOVE 1.
The line lead of HV winding if coiled inside the transformer is drawn through the bushing using a string when the bushing is lowered into position. 2. The thimble brazed at the end of line lead is fixed in position at the top of the bushing brass tube. Fix the stem on top of the end cap and lock them with lock nut. Fix the washer over the end cap 3. The lower end of the bushing shall be inspected from inspection window for proper sealing. 4. The line connection should be tight and should not strain the terminal. Fix the upper and lower spark gap horn and adjust gap setting as per required. Release the trapped air inside the bushing by unscrewing hexagonal nut and depressing the stem slightly or unscrewing the vent screw on the 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 inside of the centre pipe (in case of draw load/rod type bushing) should be further cleaned with transformer oil.
Transformer Manual BUSHINGS WITH DRAW LEAD/DRAW ROD CONNECTIONS Remove the terminal and pass a pilot wire through the centre pipe and lift the bushing upright of any desired inclination 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 flange to the transformer after checking the direction of oil level indicator, tightening the mounting bolts in a fraction of a turn at a time, working progressively in one direction until the bolts are uniformly tight. The thimble/rod is located by means of the pin and the terminal is screwed down fully with the gasket. The centre pipe can be deaerated by loosening the terminal. In case of 245 & 420 kv bushings, an additional cover is to be fixed after the terminal is tightened, for compressing the gasket. Bushings as per terminations as per CEA requirements.
STEM TYPE BUSHINGS In stem type bushings, the central conductor itself acts as .the current carrying part and the transformer lead is to be connected at the bottom terminal through the inspection window of the transformer. The busbar/conductor can be connected directly to the top terminal with the help of a proper terminal connector.
PRE-COMMISSIONING CHECKS 1.
2. 3. 4.
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 should immediately be reported to TARIL. After the test, put back the cap. CAP SHOULD NOT BE LEFT OPEN UNDER ANY CIRCUMSTANCES. Check oil level of the bushing. In vertical condition, oil level should be at the middle of the indicator/gauge. Check that the HV terminal is firmly tightened with the gasket to avoid passage of moisture to transformer. Ensure that the transformer is energized minimum 36 Hrs. after mounting the bushing.
ROUTINE CHECKS AND MAINTENANCE As the bushings are hermetically sealed, these are practically maintenance free. How ever, a periodic check of oil level and cleaning of the porcelain insulator will suffice. It is recommended to check capacitance and tan delta and compare with pre commissioning test results every year. Tan delta value of 0.01 or more, and increase in capacitance by 10% or more, should be viewed seriously and should be immediately informed to TRAIL.
3.4.8 DEMOUNTING AND PACKING Demounting 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 CHANGER The off circuit tap changer is an integral part of transformer & its operation is to be carried out from outside. The actual position 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 indicating plate. 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 inadvertent operation. 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 the transformer need not be electrically isolated from HV & LV side. The tap changer can be operated on load. The tap changer is an integral part of the transformer and is suitable for power flow in the direction indicated on the OLTC name plate. It is used for transformer where the system voltage fluctuations are frequent and over wide range. It has local manual, local electrical and remote electrical controls and the panel located in the control room has the necessary 11
switches for raising or lowering the taps and is provided with a remote tap changer control cubical (R.T.C.C). Automatic voltages 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 be given 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 and breather. It should be ensured that the selector switch & diverter switch are filled with clean dry processed oil to keep selector switch & 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 oil surge conditions. The relay must be fitted in a pipe connection from the tap changer conservator pad to the conservator and its contacts must be connected to the inter tripping system so as to isolate both sides of the transformer. The pipe work rising to the conservator should be arranged at an angle of 5° above the horizontal to ensure the effective operation of the protective relay. The connection of the relay contacts in the trip circuit such that the transformer is fully 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 not clogged. A separate indication to show operation of tap changer oil surge relay should be available to avoid unnecessary inspection of tap changer in case only the transformer gas and oil surge relay operates.
OIL The oil in the tap changer should conform to IS 335 and thus be thoroughly dried out by filtration. The electrical withstand level of Oil should be minimum 50kV when tested as per IS 335 - and IS: 5792-1972. The oil should be free from solid contaminations as well.
HEATER Heater is provided in the drive mechanism in order to eliminate condensation troubles due to changes in ambient temperature. The heater is controlled by an ON-OFF Switch and should be switched on when there is a possibility of moisture condensation such as during monsoons.
Transformer Manual COMMISSIONING OF THE TAP CHANGER It is absolutely - important that the Drive Mechanism and the Tap changer Head show the same tap number, other wise the electrical and mechanical limits which are contained only in the Drive Mechanism become inoperative for the 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 any unintended 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 or removed 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 crank handle 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 the motor 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 and therefore 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 the crank 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 should operate, 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 transformer be 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.
PUTTING INTO OPERATION Before connecting the motor drive to the mains, check whether voltage, current and power of the supply should coincide with the required values. When checking the voltage for the motor circuit take care that the RYB phase sequence is clockwise. As the gearing and the ball bearings of the driving motor are sufficiently supplied with grease a regular maintenance is not necessary. We recommend, however, Check that all wiping faces, gear teeth, cam faces etc. are lubricated with special molybdenum disulphide lubricant. No oil should be used for lubrication. Ensure that drive mechanism chamber is clean and free from dust.
3.6 COOLING OF TRANSFORMERS TYPE OF COOLING 1. 2. 3. 4.
OIL NATURAL AIR NATURAL COOLED TRANSFORMER (ONAN) OIL NATURAL AIR FORCED COOLED TRANSFORMER (ONAF) OIL FORCED AIR FORCED COOLED TRANSFORMER (OFAF) OIL FORCED WATER COOLED TRANSFORMER (OFW)
COOLING EQUIPMENTS USED IN TRANSFORMER AS PER THEIR COOLING SYSTEM TYPE OF COOLING
COOLING EQUIPMENTS USED RADIATORS FAN OIL PUMP
OIL FLOW INDICATOR
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 with blanking plates during the transport. Remove the blanking plates when the assembling of the coolers is to be started. Check first that the valves are closed. 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 released from all the pipe work during filling.
3.7 RADIATORS Transformer is provided with detachable pressed sheet radiators with isolating valve. Due to transport limitation and considering possible transit damages, the radiator valve are closed at the time of dispatch and radiators are detached and sent separately keeping the valve in position on tank flange. The valve blanking plate is to be removed only when the radiator is ready for mounting on flanges. On removal of blanking plate, oil will seep out. After filling the oil in radiators 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. Radiators valves are fitted to the headers and dispatched. The end frames are to be erected first. The frames should be positioned correctly with respect to the transformer. The distance between centre lines of transformer and cooler should be strictly as per GA drawing otherwise the connecting pipe work will not match. After erecting the end frames 14
Transformer Manual the top and bottom headers are mounted. The headers will have to be properly leveled so that the connecting pipe work 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 same attached. The interconnecting pipe work may be done taking care to connect correct pieces at the correct location. Usually expansion joints are provided in the pipeline connecting the transformer tank to cooler. Special care should be taken to see that these are installed correctly.
MOUNTING OF RADIATORS Ensure 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 mounting on 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. Slide the radiator on the bolt from pipe flange the radiator valve and tighten using plain washer, spring washer and nuts. 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 hot filtered 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 the radiators until the air starts escaping. The oil from the main tank will now flow in the radiator. Start putting fresh oil in the conservator. 5. After filling open valve at top. 6. Fill the remaining radiators in the same manner.
3.8 COOLING FANS Cooling fans are provided whenever transformer is designed for dual rating. This are mounted on brackets which are fixed on tank wall or ground. The fans are controlled from Fan Cooling Control and some times from remote tap changer control (RTCC) panel and can be operated manually and on auto mode. When fans are on auto mode they are switched on using one of the mercury switches of the WTI mounted FCC. The setting of the switch should be such that no hunting takes place. For this the temperature differential should be minimum 5º C to 7º C between switching on and off the fans.
3.9 OIL PUMP The pump and driving motor are assembled as a single unit or a common shaft. The transformer oil circulates through the windings thus eliminating cooling fan, shaft seal and avoiding the danger of oil leakage or entry of air. The complete unit is weather proof and can be arranged for horizontal or vertical mounting. Lubrication of the bearing and cooling of the motor windings is achieved with the circulated transformer oil. This results in a very efficient rate of cooling of the motor windings with minimum recirculation losses and maximum utilization of cooling of the motor windings 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 dispatched separately after blanking both suction and delivery sides. The pump should be connected at proper position as per the GA 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.
3.10 OIL FLOW INDICATOR
FUNCTION Flow indicators are mainly designed as a safety device looking to the need of an electrical signal on failure of forced circulation 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 & WORKING A suspended vane is used as sensor. Its surface is kept at right angle to direction of flow. When liquid starts flowing through pipe, the vane gets deflected along with vane shaft. This deflection is used to indicate flow inside the pipe and to operate mercury switches as follows: A pair of permanent magnets is used as glandless coupling. The driving magnet is connected with vane; therefore it rotates with vane and remains in liquid. The follower magnet, separated by a nonmagnetic wall, is kept outside in air and acquires position corresponding to driving magnet. The follower magnet carries a pointer and a cam. The pointer is set to read the specified rate of full flow and the cam is set to operate one or two mercury switches when flow drops near to 70% of full flow. A return spring is used which acts in the opposite direction of flow. Hence any steady position of pointer is the result of equilibrium of force due to impinging liquid on vane spring tension & weight of vane. The pointer does not remain steady for unsteady flows. The size and shape of vane and tension on return spring is adjusted to suit specified full flow. The vane occupies very small area out of full cross section of pipe at NO-FLOW position. At full flow it becomes almost parallel to flow. Hence there is practically no head-loss across Flow Indicator. The flow indicators are distinguished in two series 4021 & 4022. Basic difference between two series is that in Series 4021 only one mercury switch is provided. Whereas in Series 4022 two mercury switches are provided. Proportionately, number of terminals, size of switch-box and size of terminal box differ.
DIAL MARKING Usually PUMP ON-PUMP OFF or FULL FLOW-NO FLOW type of limit markings are printed. Intermediate markings such as ALARM, TRIP are also printed if required. Besides, specifications such as value of full flow, liquid, pipe size and direction of flow are also printed. The background of dial is white. Direction indicating arrow is in red color and all other markings in black color.
SWITCHES One 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.
Transformer Manual In case of single switch, it is set to operate near 70% when flow is fallen. In case of two switches, both are mounted on same plate. Hence both operate at same rate of flow. But in order to satisfy design requirements, one switch can lie set to operate near 80% and the other near 70% or one near 70% and the other near 60%. It is not possible to set a switch to operate at lower value than 60%. Even when both switches are set to operate at same rate of flow, some switching differential is observed due to use of mercury as switching element. We can modify setting suitably if switching requirements are clearly specified.
WIRING Leads from mercury switch/switches are brought into a terminal box at the bottom of indicator. Terminals are accessible for wiring after removing cover. A hole with conduit threads is provided for fixing cable gland to box. Terminals are marked. 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 EXCHANGER In case of forced water-cooled transformers the oil to water shell tube heat exchangers are dispatched separately and properly blanked. On receipt at site, it shall be checked whether blanking is all right. The bracket for mounting the heat exchanger may be attached to the transformer first. The heat exchanger should be mounted on the support as per GA drawing. The oil pump, OFI & the connecting pipe may be fixed after this, in the correct position. In the water circuit necessary pies may be fitted. A water flow mater is placed on the outlet pipe to indicate that there a positive water flow. It is to be made sure that there is no restriction in the water outlet pipe as any obstruction in this pipe will increase the pressure in the water circuit and may result in the water pressure exceeding the oil pressure and creating leakage 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 & Temperature Gauges 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 opposite phenomena should be in water circuit.
3.12 DIFFERENTIAL PRESSURE GAUGE Differential Pressure Gauge is mounted to ensure that the water circuit pressure should not increase the oil circuit pressure. Differential Pressure Gauge Consisting of two bourdon tubes, one for oil and one for water line and the pointer of the instrument give direct reading of differential pressure. Each bourdon tube can take maximum pressure of 1.3 times of maximum pressure shown on the dial. Oil side tube should be connected to Oil pipe line at outlet of Heat Exchanger and water side tube should be connected to Water pipe line at inlet of Heat Exchanger.
3.13 CONSERVATOR TANK The 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 -functions satisfactorily. Before erection of conservator make sure that all gasketted joints are oil tight and pipe work is clean and free of moisture. The breather is connected to the oil conservator, and it is very important that joints and couplings in the pipe between breather and conservator are air tight.
3.14 MAGNETIC OIL LEVEL GAUGE This model of indicator is manufactured considering Transformer application. It can also be used as Content Gauge on other tanks where level of liquid inside the tank is required to be indicated continuously on a dial. This is direct oil level indicator devices provided on conservator. The low oil contacts provided on the magnetic oil level indicator can be used for automatic alarm when the oil level in the conservator falls to a low level. This protection prevents the damages of the transformer from damages attributed to loss of oil due to any reason. During dispatch, the indicator & the arm and float are detached to prevent damage to the bevel gear and mercury switch. The mercury switch operating arm and bevel gear are also locked in position.
CONSTRUCTION, WORKING & FEATURES A float is used as sensor of liquid level inside the conservator (tank). Swing of float due to change in liquid level is utilized 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 surrounding atmosphere. This results in avoiding any leakage of costly oil and avoiding contamination of insulating oil due to seepage 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.
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 thus the pinion of the gear assembly. The pinion in turn rotates the driving magnet inside the conservator. The follower magnet positioned outside carries a pointer and a cam. The pointer reads oil level and the cam set to operate the mercury switch at a predetermined low level. MGOs are with different specifications are not interchangeable.
3.15 COMMISSIONING OF AIR CELL
2. 3. 4. 5. 6. 7. 8.
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 breather connection. There should be one valve and a pressure gauge fitted in between this valve and air cell close the valve after required pressure is achieved. Keep in this condition for 24Hrs ensure no leakage in air cell with the help of pressure gauge. Assemble the air cell conservator on the transformer. Connect the conservator to transformer tank through Bucholz relay pipe line. Close the valve between Bucholz relay & conservator. Fill the oil in the transformer up to upper tank flange level under vacuum. Keep the air cell inflated to a pressure of 0.08 kg/sqcm by Nitrogen / dry air through the opening for breather. Open the valve between Bucholz relay & conservator. 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 pressure gauge should be kept 0.08 kg/sqcm.
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 pressure gauge should be kept 0.08 kg/sqcm.
10. Now remove the pressure supply connection of conservator & fix the breather with its pipeline. Keep further filling 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 oil level. 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) RELAYS Buchholz relay is very sensitive, gas and oil operated instrument which detects low oil level, formation of gas or development of sudden pressure inside the oil filled transformer. It has to be connected to protection circuits to give any early audible alarm in case of low oil level/gas collection and to disconnect the transformer from supply in case of severe fault inside the transformer. When mounted on the pipe work, the correct direction is maintained with the help of arrow provided. The angle of inclination is also to be checked and should be between 3 to 7’.
CONTENTS Protective devices In the ideal case must be sensitive to all faults, simple in operation, robust for service and economically feasible. 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 circuit to operate. The Relay is capable of indicate incipient fault thereby preventing further spreading of the fault and extensive 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 will operate, 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 either incipient or major faults in oil filled potential transformers, reactors, capacitors etc. A special single element relay is available for the protection of on load tap-change equipment.
CONSTRUCTION The Buchholz relay essentially consists of enclosure i.e. an oil tight container and an inserted assembly comprising of two floated elements fitted with switches. The enclosure is provided with inlet and outlet for the liquid. To allow connection of pipes, the enclosure openings can be designed with pipe thread or flange. The front inspection glass is provided 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
Transformer Manual of service and Test/Locking position, in the form of movement of red indicator on the reading scale. This allows the testing 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 TEST 1.
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 switching system. To test the relay function loosen the nut on the Test Key and rotate the key with a screwdriver in the Anticlockwise direction the SLOT on the Test Key points towards the T/L position Both the alarm (upper switching system) 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 Trip circuits will not show continuity. The circuits will be actuated to ‘ON’ position only when there will be a fault in the transformer. Repeat the functional test each time a relay is started or maintenance completed. Testing the relay function with draining the oil. Close the valves both the side of the relay. Put continuity tester across the alarm & trip contacts of the relay terminal box. Open the air vent lug on the top of the relay. Drain the oil from relay from drain plug at the bottom of the relay. As oil start draining alarm & trip contact will close.
INSTALLATION Mount 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 of the relay. Ensure that the slot on the Test key remains in the ‘S (Service) position and the Test Key Bolt is tightened just before commissioning of the relay.
MAINTENANCE No servicing is needed during operation. On routine inspections of the protection equipment test the function of the Buchholz relay as described earlier and check the alarm and trip devices connected to them.
BASIC CHARACTERISTICS The Gas and Oil relay provides protection against a number of internal faults but it is also able to indicate in several cases 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 the nature of the fault.
DIAGNOSIS I. 2. 3. 4.
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. If the gas is Greyish White with sharp and penetrating odor and be non-inflammable it is due to overheated or faulty insulation. If the gas is Yellowish in color and inflammable it may be due to surface leakage on material like wood. If the gas is dark Grey and inflammable it may be due to a flashover in oil or due to excessive overheating of the oil 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 possible to leave the transformer In service till it Is convenient to carry out a thorough inspection. This occurrence is possible 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 thorough check up.
FUNCTION The function of a double element relay will be described here. During normal operation of a transformer the Buchholz relay is completely oil filled. In the event some fault in the interior of the transformer tank gas bubbles are produced and accumulate in the Buchholz relay on the way to the conservator. In consequence, the oil level in the relay enclosure drops which in turn lowers 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 can escape into the conservator. Hence, minor fault in the transformer tank will not trigger the lower switching assembly and 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 switching system 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 the lower 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. The single element OIL SURGE RELAY has been specifically designed for use with on load tap change equipment and it will bypass normal amounts of gas which are generated by tap change operations and will only responds to oil surges and loss of oil.
OPERATING CHARACTERISTICS Normally Relays of the double element type are made in three sizes GOR1, GOR2 & GOR3 according to the diameter 25, 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 supply special relays which use magnetically operated Reed Switches instead of mercury switches.
3.17 SILICA GEL BREATHER OPERATION Dry silica gel crystals which are dark blue in color have a very good capacity to absorb moisture. When the air from outside is breathed in the conservator, it passes through the crystals and the moisture in the air is absorbed. Thus the air that reaches the conservator is dry. Dust particles are partly trapped in the oil seal and partly trapped by the crystals of the silica gel. Silica gel crystals change their color from dark blue to pink depending upon absorption of certain quantity of moisture. Silica gel crystals can be reactivated a numbers of times and therefore, they can be used over a long period. They should be observed for the color of the silica gel on weekly basis. Change oil in oil seal once the color changes from light 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 the air 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 22
Transformer Manual impurities and/or Humidity which changes dielectric strength of transformer oil. Hence, it is necessary that, the air entering into the transformer is free from moisture & foreign impurities.
OPERATION & WORKING The Breather- is connected to an outlet pipe of the Conservator Vessel and the air which Is being sucked by transformer is 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 for moisture. 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 removes all 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 and hence longer life of the transformer.
INSTALLATION The Breather is connected to transformer by either threaded or flanged joint (after removing the Seal) as per details mentioned below. Before putting the Breather Into service, remove the Oil Cup and fill it up with fresh transformer oil up to the line marked on Cup. Remove the seals on the air holes of the Cup and fit the Cup to the Breather The Breather is how ready for installation.
MAINTENANCE As 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 Breather is determined by the Volume contents of the air passing through the Breather, the Oil Transformer and the atmospheric conditions of installation. Hence, the Transformer Manufacturer or the ultimate user should decide the size of Breather would 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 of the pressure relief valve is to prevent mechanical damage to the transformer tank by releasing any excess pressure over the set limit. This valve is basically a spring loaded device which closes automatically when the pressure inside the tank drops to the set limit after venting excess pressure over the set limit.
APPLICATION This PRV is designed to be used on power Transformer. When pressure in the tank rise above, predetermined safe limit this 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 per requirement). Hence switch can be effectively used in control circuit.
SPECIFICATIONS Normally 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 & WORKING The Pressure Relief Valve consists of a pressure die cast Aluminum flange (1) with Nitrile gasket (2) for mounting on transformer. 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 repeated operations. 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
exposes the transformer pressure to a greater of side gasket, resulting sudden increase of force. The disk lifts instantaneously 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 suitable for repeated operations than the conventionally used explosion vent.
MOUNTING The PRV should be preferably mounted in the horizontal position, top side up. However, it can be mounted on its side, in vertical plane also.
INSTALLATION This PRV has ‘an integral flange with four/six holes for mounting. Check that no parts are damaged in transit. Clean surface of mounting pad on tank and place the flange with gasket. Each valve should be cleaned form inside (tank side). All particles should be removed from tank side. All the bolts should be tightened evenly so that equal pressure is exerted on gasket aid base. Ensure that the gasket is placed in the groove provided in the flange. Check the operation of switch by manually lifting the operation rod. After checking, the switch should be manually reset by placing the roller type knob back to its original position.
MAINTENANCE The PRV has a rugged construction and does not require any maintenance. The operating pressure is factory preset and cannot be changed at site. The operation of switch may be periodically tested by manually lifting the operating rod and should be reset before putting the instrument in service.
3.19 DOUBLE DIAPHRAGM EXPLOSION VENT The purpose of the explosion vent is to prevent damage to transformer tank by releasing any excess pressure that may be generated inside the tank.
CONSTRUCTION Explosion vent consists of a bent pipe with aluminum diaphragms at both ends. A protective wire mesh is fitted on the opening 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 rises in the vent pipe and is visible through the indicator indicating the failure of the lower diaphragm. In case the pressure developed is not reduced to safe value after the bursting of lower diaphragm, upper diaphragm also gives way throwing 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 pipe equalizes the pressure between conservator and explosion vent. 24
Transformer Manual It should be fitted after vacuum application. Care being taken to ensure that the top diaphragm with its gaskets makes an air tight joint.
3.20 MARSHALLING BOX The transformer is protected by certain protective devices mounted on the transformer at different locations. This circuit usually has contacts or terminals which are connected to the protection scheme. The connection to protective scheme gives various types of alarm indications and also disconnects the transformer from incoming and outgoing supply in case of severe condition. In order to facilitate connections of all such devices to the protective scheme, the leads of all protective device contacts are wired to a weatherproof kiosk. This kiosk also houses indicators for oil temperature, winding temperature, cooling circuit and heater.
CONSTRUCTION Hinged lockable front door is provided with a glass window from which the temperature readings on the indicators can be observed. The capillary from each indicator take off from the indicators, come out from the side of the marshalling box. The entire kiosk has a rain shed provided on top to prevent ingress of water due to heavy rains falling directly over the marshalling kiosk.
3.21 OIL TEMPERATURE INDICATOR Oil temperature indicator consist of direct reading dial gauge and serves for monitoring the oil temperature of the transformer. The dial gauge of the transformer can be fixed away from the point of temperature measurement on the top 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.
OPERATION The liquid inside the bulb in contact with the hot oil in the thermometer pocket expands and the pressure of the vapor in side the system increases. This increase in pressure results in movement of the bellow. A link and lever mechanism amplified this movement to the disc carrying pointer and switches which indicates the temperature on the dial of the thermometer.
3.22 WINDING TEMPERATURE INDICATOR For oil immersed transformers operating under varying of the winding temperature is of great importance. The use of winding temperature indicator permits the winding temperature to be constantly monitored and harmful overloads can be avoided.
OPERATION The indicator is similar to OTI. The winding temperature indicator is calibrated to indicates the hottest spot of the winding. The switches are adjusted to make contact at desired temperature depending upon the site conditions, i.e., ambient temperature, loading conditions, etc. The capillary tube is protected adequately to withstand all normal handling. It should not, however, be bent sharply or repeatedly 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 & Pump Starting. The instrument indicates the temperature rise of the transformer winding over, the top oil temperature for any specified load current. The maximum Input current to the heater coil temperature not exceed 2.0 Amps. The maximum permissible secondary current from the Current Transformer at rated load is 2.0 Amps.
OPERATING MECHANISM The system is self contained and does not depend on any outside power source for its operation. It consists of a liquid filled bellows capillary tube of required length and sensing element or bulb. This system is thermally balanced by a compensating bellow and a second capillary which terminates at the head of the bulb. The two bellows are linked together via a lever In such a manner that temperature changes on the capillary line and head are automatically compensated for. Temperature changes on the bulb motivates the operating bellows only which is linked to the single rotating switch platform and indicating pointer. The power available from this type of system is adequate to operate up to four switches and a potentiometer without any visible pointer detect. This robustness also ensures that the instrument does not suffer from normal shock conditions such as may be experienced in handling and transport.
DIAL A 4 1/2 long segment dial is provided with bold graduations. The standard range is 0-150°C with 5 per graduation.
MAXIMUM POINTER A maximum indicating pointer red in color is fitted to the instrument window aid operated by a peg driven by the indicating 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 mercury switches. The transport lock should be removed after opening the cover before putting the instrument to use. The capillary line can be supplied up to 15 meters in length. It is protected by flexible stainless steel armor. Ordinary care should be taken while running the capillary and sharp bends should be avoided, particularly where it joins the instruments case and the bulb. The capillary should he supported by suitable clips at 12to 18 Inches and necessary length left so that the bulb may be freely installed and removed.
STANDARD SENSING BULB FOR RANGE 0-150° C
STANDARD WIRING DIAGRAM FOR FOUR CONTACT SWITCH
3.23 CURRENT TRANSFORMER Current transformer for winding temperature indicator is provided on winding of the transformer at either the terminals or 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 the current 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 per customer’s requirement and specification. All current transformer secondary terminals should be short circuited or loaded before commissioning the transformer. This will prevent excessive voltage developing across CT terminals which can damage the CT and be hazard if touched. CTs should not be dismantled & if required extra care should be taken in handling it.
CLASS OF ACCURACY USED FOR DIFFERENT PURPOSE OF CTS PURPOSE
CLASS OF ACCURACY
REF & DIFFERENTIAL PROTECTION
BACK UP PROTECTION
3.24 CONTROL CABLING Reassemble the control cables according to the drawing of wiring system and connect the cable ends to terminal blocks in instruments, terminal boxes, junction boxes and control cabinets according to valid connection diagram.
3.25 ASSEMBLY OF VALVES Valves which may affect the loading gauge, or will be subjected to damages, is dismounted before delivery. Remount the valves -the positioning appears from the GA drawing and part list. The gasketting surfaces shall be cleaned well and new gaskets fitted. Check that all valves are closed.
3.26 ASSEMBLY OF PIPE WORK Pipes with flanges for connection of conservator, radiators, as well as pipes for equalizing of turrets etc. are mostly delivered 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 site is however required.
3.27 OIL FILLING
Transformer Manual PRECAUTIONS FOR N2 FILLED TRANSFORMER N2 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 from contamination. 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 BEFORE COMMISSIONING AS PER IS: 1866. PROPERTY
HIGH VOLTAGE EQUIPMENT 170kV
Clear free from sediments and Suspended matters
Density at 29.5°C (g/cm) Max
Viscosity at 27°C (cst) Max.
Neutralization value mgKOHIg/gm of oil - Max.
water content (ppm) Max.
Interfacial tension (Mn/M) Min
Dielectric dissipation factor at 90°C and 40 to6O Hz, Max.
Resistively at 90°C
Break down voltage (BDV) (kV rms) Min.
Oxidation stability (un inhibited oil) a) Neutralisation value (mgKOHIg) Max. b) Sludge (% by mass) Max.
Ocidation stability (inhibited oil)
Same values as before filling.
OIL SAMPLING Oil takes up moisture readily and its condition should always be checked before use. Oil of a muddy color is certain to be wet. Water and water-saturated oil are both heavier that dry oil and sink to the bottom of any container. Samples shall, 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 be removed. To ensure that the valve is clean, some quantity of oil should be allowed to flow into a separate container before 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 29
24 hours. The area around the bung should be cleaned. A clean tube long enough to reach to within 10 mm of the drum should be inserted, keeping the uppermost end of the tube sealed with the thumb whilst doing so. Remove the thumb, 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 FILLING Before filling oil in the tank, it should be tested to meet the requirements as per IS:1866. In case oil does not meet the requirement, 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 full vacuum and check & rectify if any leakage is there & keep the transformer under vacuum for 12:00 HRS. The filling of oil inside the tank should be done under vacuum & fill the transformer main tank up to top cover. Do not open any blank flange for erection without filling oil up to top cover. Transformer of high voltage rating (66KV and above) have their tanks designed to withstand full vacuum. Below 66KV class transformer, full or partial vacuum should not be applied without consulting manufacturer. When filling a transformer with oil it is preferable that the oil be pumped into the bottom of the tank through filtration machine. It should also be ensured that the tap changer board, relief vent diaphragm, conservator, etc. are not subjected to full vacuum 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, the diverter switch compartment must also be evacuated simultaneously so that no undue pressure is allowed on the Tap Changer Chamber. While releasing vacuum, the Tap Changer vacuum should also be released simultaneously. For this, one equalizer pipe should be connected between main tank and tap changer.
DRYING OF TRANSFORMER USING FILTER MACHINE Before the drying out is started all fittings coolers and associated accessories shall be fitted. Drying out process of transformer is requiring care and good judgment. If the drying out process is carelessly or improperly performed, great damage 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 carefully watched 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 incorporating oil heater and vacuum chamber. The vacuum pump of the filter machine should have capacity of creating vacuum as high as possible but not less than 710 mm of mercury. Where possible, a vacuum pump can be connected to the tank top cover to keep the oil in tank under vacuum consistent with tank suitability. This may speed up the drying out process. 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. Further circulation 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 temperature of oil inside the filter machine should exceeds 70’C. The circulation is continued till insulation resistance and oil samples test are satisfactory. Plot IR values taken at regular intervals against temperature readings. It will be observed that in the beginning IR values drop down as the temperature goes up. The IR values will be low till moisture is coming out of the insulation and start rising before steadying.
Transformer Manual 3.28 EARTHING Tank should be permanently and effectively earthed by means of connection to the earthing terminals provided for the purpose at the bottom of the tank. A good low-resistance earth is essential for adequate protection. The earthing connection should be of sufficient crosssection to carry the LV line current for 30 seconds. Other accessories such as fan motors, oil pump motors, winding neutrals etc. should also be earthed. Earthing terminals are provided on tank for earthing of tank mentioned in General Arrangement Drawing. Ensure that paint 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 WORK Final topping up is now done up to a level in conservator commensurate with filled oil temperature. Other such as wiring of various alarm/trip contacts, fan motors, pumps motors and other apparatus, earthing of neutral and tank is also to be completed. The interposing valves between the radiators and the tank are opened. The transformer and its equipment are cleaned carefully from dirt, oil, lubricating grease etc. If the paint work has been damaged during transit or erection, touching-up should be carried out with the least delay. The surface to be repainted should be cleaned and free from rust and grease. If rust has begun to form, the surrounding area should also be cleaned thoroughly as rust tends to spread beneath paint work. Failure to maintain the paint work in good condition will result in considerable deterioration of the apparatus during storage or service. After completion of erection work it should be checked thoroughly that there should be not leakage. If any leakage found, it should be immediately arrested.
3.30 TOUCH-UP PAINTING A basic principle at touch-up painting should be to restore a damaged paint coat on a surface to the same quality and finish as of the surrounding surface. The touch-up paint should be limited to a surface as small as possible. If the paint work has been damaged during transit or erection, touching-up should be carried out with the least delay. The surface to be repainted should be cleaned and free from rust and grease. If rust has begun to form, the surrounding area should also be cleaned thoroughly as rust tends to spread beneath paint work. Failure to maintain the paint work in good 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 be removed. Damages on the primer paint and grinding down to the steel surface should be painted with two coats anticorrosive primer paint.
4 COMMISSIONING After completion of proper erection, transformer is to be kept ready for commissioning. Before putting transformer in service, following check points and Testing should be done to ensure the healthiness of the transformer.
4.1 GENERAL CHECK POINTS 1. 2. 3. 4. 5. 6. 7. 8. 9.
13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
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. Valves between Conservator to main tank should be fully open. All radiator valves are also fully opened and locked in proper position. Buchholz relay is to be mounted in position inclined angle of 5 to 7 degree. Ensure arrow mark of relay towards conservator. Air release from all air release plug / screws on Main tank cover, radiators, bushings, buchholz relay, OLTC and Bushing turret to make ensure that no air remains inside the transformer tank. Check the level of oil in main tank and OLTC conservator. Check the MOG operation by depressing the float. Ensure that, all the thermometer pockets are filled with transformer oil. Minimum clearance between live parts to earth should be checked as per drawing. Check the various protective devices (i.e. Buchholz relay, PRV, OSR, OTI, WTI and MOG) should give proper alarm and trip indication. Check the Buchholz relay practically by injecting the air from test cock or by draining the oil from buchholz relay by closing both side valves. Ensure that, buchholz relay floats are not locked and the relay is set at the desired position. 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. Check weather transformer is earthed properly by two independent earth connections. Paint between earthing terminal and earthing strip should cleaned properly for better connectivity. Also OLTC, Marshalling box, cable box should also earthed properly. Also check for earthing for neutral bushings. Check the incoming voltage and set the OLTC on the corresponding tap position. In case of off circuit tap changer brought to desired position and locked with bolt in plate hole. AVR can also set as per required voltage variation, if applicable. Check terminal clamp on bushings and cable and termination on bus bars. Check functions of the breakers with transformer actual tripping operation. Check oil level in OIP bushing from glass or small MOG (Mounted on bushing). It should be in required level. Check Explosion vent assembly mounting. Check OLTC Explosion diaphragm in healthy condition. Check Transformer painting. Apply touch paint at required places. Check Name plate provided properly. Check Roller wheel stoppers provided. Check Valve between OLTC conservator and OLTC surge relay open. Check BCTs for protection and metering connected to respective circuits properly. Check Bushing thimbles and terminal cops tightness. 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:
Transformer Manual 4.2 DOS AND DON’TS FOR POWER TRANSFORMER DON’TS FOR POWER TRANSFORMER 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.
Do not energies without thorough investigation of the transformer whenever any alarm of protection has operated. Do not reenergize the transformer unless the Buchholz relay gas is analyzed. Do not reenergize the transformer without conducting all pre-commissioning checks. The results must be comparable with results at works. Do not operate the off-circuit tap switch when the transformer is energized. Do not energize the transformer, unless the off-circuit tap switch handle is in locked position. Do not leave-off circuit tap switch handle, unlocked. Do not leave tertiary terminals unprotected outside the tank; connect them to tertiary lightning arrestor protection scheme, when connected to load. Do not allow WTI/OTI temperature to exceed 65°C during drying out of transformer and filter machine temperature beyond 70’C. Do not parallel transformer which do not fulfill the Paralleling condition. Do not use low capacity lifting jacks on transformer for jacking. Do not move the transformer with bushings mounted (above 33 KV’ class). Do not overload the transformer other than the specific limits as per IS: 6600. Do not change the settings of WTI and OTI alarm and trip frequently. The setting should be done as per the site condition. Do not leave red pointer behind the black pointer in OTI and WTI. Do not leave any connection loose. Do not meddle with protection circuits. Do not allow conservator oil level to fall below 1/4 level. Do not leave marshalling box doors open, they must be locked. Do not switch off the heater in marshalling box except to be periodically cleaned. Do not allow dirt and deposits on bushings, they should be periodically cleaned. Do not allow unauthorized entry near the transformer. Do not leave ladder unlocked, when the transformer is ‘ON’ in service, in case it is provided. Do not change the sequence of valve opening for taking standby pump and motor into circuit. Do not switch on water pump unless oil pump is switched on. Do not allow water pressure more than oil pressure in differential pressure gauge. Do not mix the oil, unless it conforms fully to IS: 335. Do not continue with pink silica gel, this should immediately be changed or regenerated. Do not store Transformer for long after reaching site it must be erected and commissioned at the earliest. Do not leave secondary terminal of CT open. Do not keep the transformer gas filled at site for a longer period. Do not top up oil from conservator with air cell bag inside.
DO’S FOR POWER TRANSFORMER 1. 2. 3. 4. 5. 6.
Check and thoroughly investigate the transformer whenever any alarm or protection operated. Check air cell conservator (optional). Attend the leakages on the bushing immediately. Examine the bushing for dirt deposits on coats, and clean them periodically. Check the oil in transformer and OLTC for dielectric strength & moisture content and take suitable action for restoring the quality. Check the oil level in oil cup and ensure air passages are free in the breather. 33
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
Check the oil for acidity and sludge as per IS: 1866. If inspection covers are opened or any gasket joint is to be tightened, then tighten the bolts evenly to avoid uneven pressure. Check & clean the relay and alarm contacts. Check also their operation and accuracy and if required, change the setting. Check the protection circuit periodically. Check the pointer of all gauges for their free movement. Clean the oil conservator thoroughly before erecting. Check the bucholz relay. Inspect the painting and if necessary retouching should be done. Check the OTI & WTI pockets and replenish the oil, if required. Examine and replace the burnt or worn out contacts. Check all bearings and operating mechanism and lubricate them as per schedule. Open the equalizing valve between tank and OLTC, wherever provided at the time of filling the oil in the tank. Connect gas cylinder with automatic regulator if transformer is to be stored for long in order to maintain positive pressure. Fill the oil in the transformer at the earliest opportunity at site and follow storage instructions. Check the door seals of marshalling Box. Equalize the diverter compartment of the OLTC by connecting equalizing pipe between flange joints provided on the tap changer head. 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 measure of the integrity of the insulation structure. The test equipment used is a DC insulation tester (Megger). The IR test results 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 in the vicinity influences. Range of Insulation Resistance Tester Make Temp Insulation Resistance Value Temperature =
SI. No. Configuration
IR value in Mega Ohms 15 sec
HV - LV
HV - E
LV - E
4.4 VOLTAGE RATIO TEST Ratio test should be conducted in every transformer to check the design ratio. Ratio at all taps is to be checked. Start the tap changer from lower tap and then go on increasing tap. Apply 3 Phase 3 Wire 400 Volts A.C to HV. Keep HV neutral isolate (if in Star connection). Measure voltage between phase to phase and phase to neutral on LV side.
Transformer Manual TAP NO.
APPLIED VOLTAGE ON HV SIDE U-V
VOLTAGE MEASURED ON L.V SIDE u-v
1 2 3 . . n
4.5 MAGNETIZATION CURRENT The exciting current test at low voltage is very useful in location problem such as defects in magnetic core balance, in magnetic core structure, shifting of winding, failures in to turn-to-turn insulation, or problem in tap changers. The acceptance criteria for the results of exciting current measurement should be based on the comparison with the previous site test results or factory test results. The general pattern is two similar high reading on the outer phases and one 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
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
Note: Magnetising current on LV side is applicable for power transformer only. This shall be not applicable for furnace duty, rectifier duty and distribution transformer.
4.6 VECTOR GROUP The purpose of this test is to check correctness of HV &LV winding connections in a transformer. Measurement of this test, connect the primary and secondary of one phase together and then measure the voltage between various terminals 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 CONNECTION
DIAGRAM AS CONNECTED FOR TEST
4.7 MAGNETIC BALANCE Purpose 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 measured across other two phases. When the supply is fed to outer phase of a transformer, the voltage induced in the center phase shall be 50 to 90% of the applied voltage. However, when the center phase is excited then the voltage induced in 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
Note: Magnetic balance on LV side is applicable for power transformer only. This shall be not applicable for furnace duty, rectifier duty transformer.
4.8 WINDING RESISTANCE Transformer 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 are generally measured with original data measured in factory. The test shall be conducted at all taps of the transformer windings and the values are to be converted to 75º C. Winding resistance measured shall be done after measurement of magnetization current. WINDING
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 TEST Apply 440 volts supply on HV connect volt meters on LV. Check for discontinuity of induced voltage while tap changer is operated throughout the range in both raise and lower directions.
Transformer Manual CONTINUITY TEST BY SHORT CIRCUIT TEST 1. 2. 3. 4. 5.
Short LV side. Give 440V Supply on HV side. Put ‘A’-meter across all the phases. Tap changing keeping supply switched ON Continuity between line and neutral or line to line should check with multimeter on all taps and phases to be confirmed. No abnormal variation/discontinuity should be observed during tap changing. This should be checked for raise and lower 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 CHARACTERISTICS Take necessary precaution, while withdrawing the oil sample. Each sample should be free of air bubbles and should not be tested when it is hot. The sample should satisfy IS: 1866. SL. NO.
OIL SAMPLE FROM
TIME & MOISTURE CONDITION IN PPM
B.D.V IN KV
RESISTIVITY TANAT 90º C 90º C
4.11 TESTING OF ON LOAD TAP CHANGER Sr. No.
Visual inspection of OLTC
Hand operation in all taps
Complete wiring of the circuit
Upper/Lower Limit switch
Over running device.
Over load relay of driving motor
Tap position indicator
Out of step relay
Tap changer stuck up relay
Function of A.V.R
4.12 OFF CIRCUIT TAP CHANGER Tap switch handle should not be left halfway or unlock. Limit Switch for Maximum/Minimum should be operated.
4.13 TEST ON CT 1. 2. 3. 4.
Ratio Polarity Magnetising current IR value
4.14 TESTING OF COOLING CIRCUIT Phase sequence of supply to fans and pumps to be checked for rotation in direction of arrow. WTI Setting
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 ALARMS Test the tripping of various devices and relays by actual operation. Tripping of HV and LV breakers checked through all the below devices.
Winding Temp. Indicator
Oil Temp. Indicator
Pressure relief valve
Oil surge relay
Magnetic oil level gauge
Oil floe failure
Water flow failure
Oil pump trip
Transformer Manual Sl. No.
Relays on HV & LV
Over current relay
Earth fault relay
Over fluxing relay
Over voltage relay
Under voltage relay
Make / Type
4.17 DETAILS OF CIRCUIT BREAKER 1.
H.V Circuit Breaker : Make :
MVA Rating :
KV Class :
L.V Circuit Breaker : Make :
MVA Rating :
KV Class :
4.18 ENERGISING When all checks and tests are found satisfactory, Air should be vented from all air release plug/screws provided on the transformer, radiators, bushings, buchholz relay, oil surge relay. After completely venting the air out, the transformer should 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. Humming sound 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 again to release all the trapped air. After this venting operation, the transformer should be re-energised and maintained on no load for 6 hours. After this period of 6 hours the transformer can be put on load gradually.
4.19 RECORD AFTER CHARGING 1.
No load current at relay terminal CT Ratio U Phase : V Phase : W Phase :
Temperature (At the time of charging) O.T.I : W.T.I : Ambient :
Amp Amp Amp
ºC ºC ºC
Maximum temperature after 24 houres.
5 MAINTENANCE AND OPERATION 5.1 GENERAL MAINTENANCE In order to avoid faults and disturbance, it is important that a careful and regular supervision and control of the transformer and its components is planned and carried out. A regular system of inspection and preventive maintenance will give long life and trouble free service. The causes of breakdown of transformer are generally classified as: 1. Overload 4. Poor workmanship 2. Incorrect installations 5. Accident 3. Faulty design 6. Surges on transformer DIRT/DUST: The external transformer surface shall be inspected regularly and when required cleaned from dust, inserts, leaves and other 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 external surface treatment of the Transformer should be carried out. Possible rust damages are removed and the surface treatment 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 retightened according to schedule.
5.2 ROUTINE MAINTENANCE INSPECTION OF OLTC IMPORTANT CAUTION: Dismantling of the Diverter Switch and Energy Storage Mechanism by untrained personnel is not recommended. In case it is desired to have a detailed inspection, we would strictly recommend that the Tap changer Manufacturer should be contacted for assistance. For maintenance of Motor Drive, Please see separate pamphlet on Motor Drive. All gear units and moving parts in the Drive 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 cover at the top to check for ingress of water inside the terminal. Chang the gasket if necessary.
SUCTION PIPE As 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, the vent of the suction Pipe should be opened. Schedule of Routine Inspection Operating Current Number of Tap changes Time between Routine Inspection. Up to l5OA Over l50A Up to 400 A Up to 500 A Upto400A Upto500A Upto 800 A U5to120OA
100000 70000 50000 50000 or 3 years which ever is earlier 100000 100000 70000 85000 above.
Figures given above are applicable to units at the star point. For three phase delta Tap changers and for Line End units take Number of operations at 60% of 40
Transformer Manual INSULATING OIL The 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 of carbon removed. The electrical withstand strength of oil must not be less that 30 KV at any time. Oil from selector switch should be tested every six months.
DRIVE MECHANISM UNIT The mechanism has been designed on the basis of no maintenance being necessary over a long period. It is important that 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 surfaces which require lubrication that is cam faces gear teeth, rollers and wiping surfaces are lubricated with molybdenum disulphide in a grease base. This lubricant is spread over the working faces and will not migrate. A small supply of this special lubricant should, however, be kept in store to replace any that might be inadvertently wiped away during inspection 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 SCHEDULE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
De-energize, isolate and earth the transformer. Drain Oil from tap changer. Inspect the selector switch through the top inspection cover. 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. Check Pat in the rest position the resistor the fixed contacts. Check and ensure that all nuts, screws and other fasteners are secured properly and all connections are in good condition. 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. Check visual and by continuity test that the transition resistors are in good condition. 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. Refill with clean dry hot oil. The oil must have an electrical withstand value of not less than 50KV. Examine silica-gel breather, and reactivate or replace as necessary. Clean drive mechanism chamber and tighten fasteners. 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. Lubricate all surfaces such as gear teeth, wiping surfaces etc. with special lubricant as required. If any scratches are found on painted surfaces, touch up with paint. Clean air filters. Check electrical operation. Check functioning of limit switches, crank handle interlock, motor protection relay and heaters. Ensure damper is locked in minimum position. 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 REMOVAL Transformer 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 oil and surrounding air. 2. Oil conservator with breather: The moisture absorption prevented by a breather connected to the conservator. The active drying agent (silica gel) should be exchanged or regenerated when 2/3 of the agent has been pink by absorbed moisture. The breather contains also an air-lock that prevents spontaneous air circulation. This lock should be filled with oil to level 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, the conservator 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 flash over, short circuit and local overheating normally results in gas-generation. The gas bubbles gathering in the gas operated 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 possibly trapped air in the oil system. This nipple should be checked after the installation of the transformer, or after an inspection 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 withstand tests 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 be removed by means of filtering, but if the sludge formation has started, it is increased with time. The oil should therefore be exchanged if the neutralization value exceeds 0.5 mg KOH/g. Old oil should not mix with new oil as the new oil will get infected and will age up quickly.
5.4 TEMPERATURE SUPERVISION The service life of a transformer is highly dependent on the temperature prevailing on the core and windings under operation It is therefore essential to keep a watch on the oil and winding temperature continuously. The temperature should be read regularly and the measured value registered. In automatic monitoring system, the oil and winding temperature are scanned 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 permissible overload must not be judged only with regard to the oil temperature. We must also consider that the temperature difference between oil and winding will increase with the load. Therefore winding temperature should be supervised during overloading. If the temperature in a transformer shows a tendency of rising without a corresponding increase of a load, this may be caused by a reduction of the cooling ability of the cooling equipment. The temperature should also be checked in this case.
Transformer Manual 2.
6. 7. 8.
Thermometer for measuring of Top-oil temperature: The thermometer consists of a cylindrical sensing body with a flange, a capillary tube, and a thermometer housing with dial and contact device. The measuring system is filled with liquid, which changes its volume at the temperature variations, and affects spring bellows. The movements of the bellows are transferred to the pointer and signal contacts of mercury switches. The contact can be set independently of each other. Winding Temperature Indicator: The thermometer system consist of a sensitive body with assembling details, a capillary tube and a thermometer housing, which consist a scale with an indicating pointer and a maximum temperature pointer, heating and adjusting resistor, as well as contact-device for four micro switches. Remote control of winding temperature: For remote control of winding temperature a resistance temperature device has been used with a heating element built in to the same housing. The heating element is fed by the second current of a current transformer. For setting of the heating element there is a parallel connected adjustment resistor installed in the control cabinet. 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 out by means of water flushing at high pressure. Normally, no measures are necessary for keeping the internal cooling surfaces clean as long as oil is in good condition. If, however, sludge formation has set in, the sludge may deposit on horizontal surfaces in radiators. In such a case, the radiator should be flushed internally with clean oil. If the sludge does not loosen, we can firstly flush with petrol etc. and then with oil. Bushings: Bushings porcelains shall be cleaned from dust and dirt regularly. In areas where the air contains impurities as salt, cement dust, smoke or chemical substances, shorter intervals are required. Connection: In order to avoid prohibited temperature rises in the electrical connections of the transformers, all screw joints should be checked and retightened according to schedule. Accessories: Separate leaflet should be followed for various fittings for, fans, pumps, Flow indicators, pressure gauge, 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 as below: Difference in Temp.
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 in temperature 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 OIL The oil in a transformer provides for an electrically insulating and a cooling medium. The service reliability of an oilimmersed 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 important parameters of oil - electrical strength, water contents, receptivity, tan-delta, neutralization value, sediment and perceptible sludge, flash point and interfacial tension are given in Annexure. Health of transformer can be effectively monitored by Dissolved Gas Analysis (DGA Test). This technique helps in detection of incipient fault in transformer.
5.7 TROUBLE SHOOTING General 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 OIL INTRODUCTION Dissolved gas analysis is a powerful tool to detect incipient faults inside the transformers. Because of not so simple calculations, operating staff always seek the help of experts to analyse the results DGA. On many occasions it is noticed that data are lost due improper sampling. This paper gives a procedure for correct sampling and explains methods to analyse data with out an experts help. Wherever a detailed assessment is required recommendation of IEC 7600599 may be followed Advantages of regular gas analysis: 1. Advance warning of developing faults 3. Status checks of new or repaired transformers 2. Determining the improper use of transformers 4. Monitoring transformers under over loading
FOLLOWING GASES ARE DETECTED IN DGA • • • • • •
OXYGEN O2 • NITROGEN N2 • HYDROGEN H2 • CARBON MONOXIDE CO • CARBON DIOXIDE CO2 • METHANE CH4
ETHANE C2H6 ETHYLENE C2H4 ACETYLENE C2H2 PROPANE C3H8 PROPYLENE C3H6
SAMPLING Metal cylinders as in figure are to be used collect oil samples from transformers. Proper sampling procedure is required to get representative sample from transformers. Take sample from a position where oil is in circulation. Sampling valve if provided at middle is best to get a representative sample. Stagnant oil from the sampling valve of the transformer may be removed before sampling. To achieve this, remove 2 to 3 liters of oil to waste container before connecting metal cylinder to sampling valve. Attach bottom valve of container to the sampling valve of the transformer through clear PVC tubing. PVC tube connected to top valve may run to a waste oil container. Sampling may be done 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) 44
Transformer Manual Allow 200ml of oil to flow through cylinder to waste container. While filling the cylinder sides may be tapped with a hard object to dislodge air bubbles trapped inside. After Sampling valves may be shut in the reverse order. 1. 2. 3.
Close top cylinder valve (B) Close bottom cylinder valve (A) 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 proper identification.
ANALYSING RESULTS No immediate action is required if values of the gases are with in the values given table no.1 except for new transformers and for transformers with earlier test data are available. For new transformers and transformers with earlier data are available, rate of rise shall be limited to values given in table 2 Table -1 Maximum Values in ppm Type of transformer
Power transformer Without communicating OLTC
Power transformer With communicating OLTC
*- Value of this is influenced by design and assembly of tap changer Table - 2 Maximum rise Gas
milli liters per day