MAINTAINENCE OF POWER TRANSFORMER & TESTINGS
INDEX
S.NO
PARTICULARS
PAGE NO.
1.
ABSTRACT
3
2.
INTRODUCTION
4-5
3.
POWER TRANSFORMER IN SUB STATION
6 – 16
4.
NAME PLATE DETAILS
17 - 19
5.
PROTECTION OF POWER TRANSFORMER
19 - 21
6.
COMMISSIONING OF POWER TRANSFORMER
21 - 24
7.
MAINTAINENCE OF POWER TRANSFORMER
25 - 28
8.
TESTING OF POWER TRANSFORMER AND CASE STUDY
29 - 32
9.
CONCLUSION
33
ABSTRACT
This project majorly deals with the performance, commissioning and testing of a POWER TRANSFORMER . It gives a brief idea of periodical maintenance of the power transformer in a 220/132/33-11 kv Renigunta substation. A practical power transformer is given here with all parts and its protective devices. The typical test results are also provided. Transformer is a vital link in a power system which has made possible the power generated at level of voltages (6600 to 22000 volts) to be stepped up to extra high voltages for transmission over long distances and then transformed to low voltages for utilization at proper load centers. With this tool in hands it has become to possible harness the energy sources at far off places from the load centers and connect the same through long extra high voltage transmission lines working on high efficiencies. At that, it may be said to be the simplest equipment with no motive parts. At every step, whether the voltage level should have to be increased or decreased we need a transformer. If there is any fault or damage in the transformer then there will be heavy loss to the distribution system. So, periodical maintenance and testing is very important for a power transformer.
INTRODUCTION:
The Transformer is a device that which transfers electrical energy from one electrical circuit to another electrical circuit without change in frequency. It is electrically isolated and magnetically coupled. Actually, the transformer is an electromagnetic energy conversion device, since the energy received by the primary is first converted to magnetic energy and it is then reconverted in to useful electrical energy. Power transformer is one which step-up or step-down the voltage level based on requirement without change in the frequency. Large-scale generation of electric power usually three phases at generated voltages of 11kv or higher. Transmission is generally at higher voltages of 132,220,400 and 750kv for which three phase transformers are necessary to step-up the generated voltage to that of the transmission line. Next at load centers the transmission voltage are reduced to distribution voltages of 66,33 and 11kv. Further at most of the consumers, the distribution voltages are still reduced to utilization voltages of 440 and 220V. Years ago it was a common practice to use suitably inter connected three single phase transformer instead of a single threephase transformer. PRINCIPLE: A Transformer is a static device by means of which electric power in one circuit is transformed into electric power of another circuit without change in the frequency. It can raise or lower the voltage level in a circuit with a corresponding decrease or increase in current. The transformer mainly works on the principle of electromagnetic induction between two circuits linked by a common magnetic flux. It consists of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance. The two coils posses high mutual inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up in the core, most of which is linked with other coil in which it produces mutually-induced emf based on Faraday’s laws of electromagnetic induction.
If the second coil circuit is closed, a current flows in it and so electrical energy is transferred from one coil to another. The first coil, in which electrical energy is fed from a.c supply mains, is called primary winding and the other from which energy is drawn out is called secondary winding. POWER TRANSFORMER IN A SUBSTATION : A Power transformer of capacity 30/50 MVA in 132/33 KV substation is shown in the figure below and its parts are described.
PARTS OF POWER TRANSFORMER: CORE: Core is made up of silicon steel laminations. The material used for core is special alloy steel of high resistance and low hysteresis losses is used in laminated form. The silicon rises the permeability at low flux densities,reduces hysteresis loss and eddy current loss. 5% silicon steel is difficult to punch and shear. But addition of nickel makes the silicon steel suitable for cold rolling and shearing. There are two kinds of transformer steel . High resistance steel having 4.5% silicon and nominal thickness 0.35mm.
The cold rolled grain oriented sheet or strip having 0.33mm nominal thickness for which the magnetic properties in the direction of rolling are superior. So, CRGO steel laminations are used in modern power transformer. In building the core high pressure is used to minimize the air gap between adjacent plates to avoid losses and also to minimize noise during operation.
TRANSFORMER TANK: Transformer tanks are made up of thin sheet steel. The tank provides a rigid support to the fittings and accessories. The function of it is to provide a protective cover to the core, windings, and other parts including transformer oil.
WINDINGS: Windings are arranged in concentric formation with lowest voltage winding next to the core. Over LV winding , HV tapping and HV main windings are placed. Some times tapping winding is placed after HV main winding depending upon requirement. Various types of winding used for making coils are as follows: Low voltage: helical/disc High voltage: partially inter leaved disc/layer/winding. Tapping winding: Inter wound spiral/helical coil. Paper covered insulated copper strips are continously transposed cables are used for making winding. Windings are compressed before assembly and remain in compressed position by means of clamping bolts. BUSHINGS: Bushing compress a central conductor surrounded by graded insulation. A bushing is necessary when a conductor is taken out through metallic or a well. Oil
filled bushings is used for 33kv applications. For making the bushings compact Synthetic resin bonded condenser bushings (SRBB) impregnated paper condenser bushing is used(OIP condenser bushing) The condenser bushing consists of central conductor surrounded by alternator layer of insulating paper and thin coil. The capacitor formed by alternator layer of thin foil and paper insulation results in uniform dielectric stress distribution between conductors surface and earthed flange.
The bushing core is coated with suitable resin. The assembly is enclosed in hollow porcelain and is provided with support flange and top hood. The porcelain is filled with dielectric oil. COOLING EQUIPMENT: The cooling systems are necessary for dissipating the heat generated in the transformer due to load loss .The various types of cooling system are mentioned in table .The radar fans are arranged for MVA at Renigunta substation.The arrangement makes use of air blast fans. COOLING SYSTEM FOR POWER TRANSFORMER: 1. O.N.A.N( Oil natural air natural): This is widely used for transformer upto 30MVA. Transformers are fitted with panel type radiators for oil natural circulation.
2. O.N.A.F( Oil natural air forced): This is widely used for transformers between 30 & 60 MVA. The panel type radiators are provided with cooling fans. Fans are switched on during heavy loads. 3. O.F.A.F( Oil forced air forced): This is method is used for transformers above 60MVA. Oil circulation through coolers. The coolers have cooling fans to exchange heat from oil to air. 4. O.F.W.F( Oil forced water forced): The heat is exchanged from oil to cooling water. Both oil and cooling water are circulated through radiator. 5. A.N( Air natural); The ambient air used for cooling in this method, is useful for oil-less transformer upto 1.5MVA. RADIATORS AND FANS: Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling. These are long, flat shaped of layers independent to each other through which transformer oil circulates. By using fans Oil-Natural Air-Forced cooling is provided such that rated power will increase as compared to oil-natural Air-Natural type of cooling. Fans can be either automatically or manually powered.
CONSERVATOR: The transformer is filled with dielectric oil.In case of oil filled transformer conservator is provided .It is installed at a slightly higher level than that of tank .During higher loads the oil in the tank expands and the level of oil in the conservator rises. The air cushion in the conservator permits expansion and contraction of the oil in the tank without contact with moist external air. The cushion in the conservator is connected to the external atmosphere via a silica ensuring dryness of incoming air during the breathing. Normally the conservator capacity is to be 10-20% of oil volume in the tank.
BREATHER: A breather is provided to prevent contamination of transformer oil in the conservator by the moisture present in the air entering the conservator. The outside air is drawn into the conservator through breather, every time the transformer cools down. The breather is packed with blue colored silica gel, which has the property of absorbing all water vapour contained in the air passing through it, there by making the air quite dry. The charge should be replaced if it becomes pink. The old silica gel can be reconditioned and used again by heating up in an Owen to 150-200 c to drive out all absorbed water.
PRESSURE VALVE: When oil expands, sometimes pressure inside the tank may increase rapidly so that tank may explode. By opening the valve, pressure inside the tank can get into normal operating point.
ON-LOAD TAP CHANGER: The taps help in changing the turns ratio so that transformer output voltage and hence the consumer’s terminal voltage can be controlled without any supply interruptions. This tap changer is used for daily or short period voltage alternations. During the operation of an on-load tap changer; 1. The circuit should not be opened otherwise dangerous sparking will occur. 2. No part of the tapped winding should get short-circuit.
TRANSFORMER OIL: The following characteristics are required for an oil to be used as transformer oil: 1. Mineral oil grade B should be used. 2. It should have a high di-electric strength i.e, not less than 30KV in the transformer tank. 3. It should contain negligible moisture content. 4. It should be perfectly clear and pale in color. Green color indicates presence of copper soaps. Cloudiness indicates presence of moisture or impurities like sludge or dust. 5. Acidity content should be very low, as it will cause precipitation of sludge and corrosion of metal surfaces. 6. It should be chemically stable, i.e. it should be impervious to the action of oxygen in the air even at high temperatures. PRESSURE RELIEF DEVICE: This PRD is designed to be used on power transformers.when pressure in the tank rises over predetermined safe limit ,this valve operates and performs following functions functions. 1) Allows the pressure to drop by instantanuosly opening aport of about 150mm dia. 2) Gives visual indication by raising a flag. 3) Operates a microswitch .This switch has 1 NO and 1NC contacts(four terminals ).Hence switch can be effectively used in control switch.
NAME PLATE DETAILS OF 132/33KV POWER TRANSFORMER TYPE POWER TRANSFORMER
TYPE OF COOLING
-
ONAN
ONAF
RATED POWER
- KVA
HV
37500
50000
KVA
LV
37500
50000
RATED VOLTAGE AT NO LOAD
- KV KV
HV LV
132 33
RATED LINE CURRENT
-
A A
HV LV
164.02 656.08
218.69 874.77
%IMPEDANCE AS PER GTP
-
12.5+/- 10%
%IMPEDANCE AT 50 MVA (BASE MEASURED) NO LOAD LOSS 20 KW
12.39
LOAD LOSS
-
145.5 KW
AUXILLIARY LOSS
-
1.5 KW
INSULATION LEVEL
-
HV LV HVN LVN
AC/LI AC/LI AC/LI AC/LI
230/550 70/170 38/95 70/170
YEAR OF MANUFACTURE
-
2009
TRANSFORMER SERIAL NUMBER FREQUENCY
-
900041B01 50 HZ
NUMBER OF PHASES
-
3
VECTOR GROUP
-
YNynO
SYMMETRICAL SHORT CIRCUIT CURRENT
-
8A
SYMMETRICAL SHORT CIRCUIT DURATION
-
1 SEC
MAXIMUM TEMPERATURE RISE OF OIL
-
50 o C
MAXIMUM TEMPERATURE RISE OF WINDING - 55 o C
REFERENCE AMBIENT TEMPERATURE
-
50 o C
TOTAL MASS
-
KG
86500
CORE & COIL MASS
-
KG
44600
TRANSPORT MASS (WITH OIL)
-
KG
77800
QUANTITY OF OIL
-
LTS 19100
PROTECTION OF POWER TRANSFORMER: Transformers are static devices, totally enclosed and generated oil immersed. Therefore, chances of faults occurring on them are very rare. However, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This necessity to provide adequate automatic protection for transformers against possible faults. Small distribution transformers are usually connected to the supply system through series fuses instead of circuit breakers. Consequently, no automatic protective relay equipment is required. However, the probability of fault on power transformers is undoubted more and hence automatic protection is absolutely necessary. TRANSFORMER PROTECTION: Different type of relays 1) Buchholz relay 2) Differential relays 3) O/F and E/F relays
4) Restricted earth fault relays 5) Over fluxing relays SAFTEY DEVICES TO POWER TRANSFORMER: Oil level indicator or Fluid level gauge: Low oil level is a harmful condition because internal insulation clearance, creepage etc; between loads, bushings and tanks are exposed to air when oiil drop below specified level. If the cooling tubes are partially cool or nearly at ambient temperature, it is an indication that the oil is not circulating in the cooling tubes and oil level has dropped below and level indicator gives an alarm,it may be a false alarm and level indicator needs checking. It position may be improper . The level indicator has a float and an arm .The float is suspended in the oil.When oil level drops down, the float hits the arm there by closing the alarm contacts. Pressure relief valve: Pressure relief valve is designed to use apower transformer .When the pressurein the tank rises above pre-determined limit this valve operates pressure in the transformer increases due to ? High over load peaks ? Prolonged over loads ? Arcing faults with in oil
The pressure relief valve is spring loaded and has a seal seat when pressure inside the tank increases above acertain valve ,the force on movable sub assembly exceeds the spring force and the valve operates .The alarm contacts are closed. After release of pressure, the valve may be manually reset.
Thermal over heating protection of power transformer: Thermocouples are the resistance measuring device. These are connected to abridge circuit .When temperature increases above safe level ,an alarm is sounded. If measures are not taken the circuit breaker is tripped after a certain temperature Some typical settings for oil temperature are as follows: Switch on fans : 60 degrees Alarm Trip : 95 degrees : 120 degrees
COMMISSIONING OF POWER TRANSFORMER: After the erection of PTR, oil filling the following pre-commissioning tests & checks are carried out on PTR to ensure the healthiness of PTR before commissioning. PRE COMMISSIONING TESTS: There are two types of pre-commissioning tests they are: 1. ELECTRICAL TESTS 2. INSULATION TESTS ELECTRICAL TESTS: TURNS RATIO TEST: The ratio should be checked on all taps and between all the windings and the result should tally with factory test reports as well as the rating and diagram plate details. Ratio shall be checked by applying a single phase 230-300 V supply on the high voltage side and measuring the voltage on the low voltage side at all tap positions.
Polarity and interface connections are checked while measuring the ratio. This can be checked by the voltmeter method. The primary and secondary windings are connected together at one point. A low voltage three phase supply is then applied to the terminals. Voltage measurements are then taken between various pairs of terminals. OPEN CIRCUIT OR MAGNETISING CURRENT TEST: A three phase low voltage supply 400V is given to the H.V winding of the three phase transformer and simultaneous current readings of the three phases are taken using low range A.C ammeters. Readings should be recorded for future reference. SHORT CIRCUIT TEST: Here LV winding of the three phase transformer is short-circuited. Here in this test we will find the % (percentage) impedance. And this % impedance must be around 2-5%. MAGNETIC BALANCE TEST: This test is conducted on HV side of the power transformer. Apply single phase 230V supply to each phase of a star connected winding, and measure voltage induced in other phases. When center phase is applied the other phase voltage should be 30 to 70 %. When extreme phase is applied, center phase voltage to be 50 to 95%. DC WINDING RESISTANCE TEST: Kelvin bridge meter should be used for the measurement of resistance at all windings. Tapped winding resistance shall be measured at all tap positions. Absolute steady reading should be obtained for minimum 1 minute. The tapped winding resistance trend shall be gradually increasing / decreasing.
VECTOR GROUP TEST: This test is mainly conducted to know the transformer connection, whether it is star-star or star-delta and so on. And also by this test we can know the phase lag between the primary and secondary connections. RN=Rn+Nn By=Yb If the conducted test would satisfy the following conditions, then the vector group specified by the company during manufacture was correct. INSULATION TESTS: The different types of insulation tests are described below: BREAK DOWN VOLTAGE TEST: The dielectric strength or break down voltage is the minimum voltage which when applied to an insulating material will result in destruction of its insulating properties. Dielectric strength is expressed in volts or kilo volts per unit thickness of the insulating material. Break down potential is that the value of voltage which when applied across 1 cm or 1 mm thickness dielectric medium or insulating material. The test equipment consists of a oil sampling container with 2 horn gap electrodes with the provision of adjusting the gap between electrodes and a high voltage transformer with a variac can vary the voltage across the electrodes from 0 to 100 KV. The moment the insulation placed between the electrodes breakdown accompanied a spark between electrodes the supply is tripped with the help a relay inside test set and the reading noted will be break down strength of the insulating material.
INSULATION RESISTANCE TEST: The insulation resistance between windings and between windings and earth should be measured with 2500/1000 volts megger and the values should be compared to the test report values. If there is much variation, the same should be intimated to the manufacturer. The dielectric absorption factor = IR at 60 sec/ IR at 10 sec should be > 1.3 It is preferable to have a motor operated megger and the readings taken after 1 minute from starting. Before measuring the insulation resistance, it should be made sure that the bushings are cleaned thoroughly with clean cotton cloth. They should also give reading of infinity before connecting up. Where, Polarizing index(P.I) = Insulation resistance at 10min Insulation resistance at 1 min
Dielectric absorption factor: Insulation resistance at 60sec POLARIZATION INDEX Less than 1 1.0-1.1 1.1-1.25 1.25-2.0 Above 2.0 INSULATION CONDITION Wet Poor Fair Good Dry
TAN DELTA TEST: When AC voltage is applied to perfect insulation there is no power loss and the charging current leads the applied voltage by 90 but in actual, the leakage current does not lead applied voltage by 90 and hence there is a definite amount of dissipation of energy which is dissipated in the form of heat. The phase angle is always less than 90. The complementary angle 90-theta is called dielectric loss angle. Dissipation factor/loss factor(tan delta) is defined as the ratio of resistive component (Ir) of current to that of the capacitive current(Ic) flowing in an insulating material. The more the resistive component of the current, the more will be the dissipation factor indicating the deterioration of the insulation. Power factor is the ratio of resistive current to that of total current. For very low value of resistive currents, the Ic will be almost equal to the total current and hence values of dissipation factor and power factor are same insulation factor or dissipation factor and capacitance measurement of bushings provide an indication of the quantity of the insulation of the bushing. A) Commissioning Check-list :
1. Air releasing from all points in the right sequence. Standing time and air release after standing time. 2. Silica gel color. 3. Condition of breather (cracks, holes etc.) and oil in the oil cup. 4. Buchholz relay shall be in service position and locked. 5. All inter connecting valves should be open. 6. Valves opened to atmosphere shall be closed and blanked. 7. Select WTI CT terminals as per manual. 8. All CT secondaries should be shorted either through respective instruments or directly. 9. OTI alarm/trip settings should be 85/95 0C . 10.WTI alarm/trip settings shall be 100/110 0C.
11.Earthing of components (thermo junction box, driving mechanism) Earthing of tank cover to tank. 12. Neutral earthing resistance should be within permissible limits. 13. Alarm indication shall be checked by closing alarm contacts of OTI, WTI, and Buchholz relay & pressure relief device. 14. Setting of over flux relay shall be 1.1 15. Overfluxing relays shall be connected to the untapped winding. 16. Setting and operation of differential protection shall be verified. 17. Over-current relay current setting shall be 120%. Check relay opera 18. Are separate annunciations available for different alarms? 19. Are the surroundings clean and degree of pollution with in tolerable levels? CHECKS DURING OPERATION AFTER COMMISSIONING: 1) 2) 3) 4) 5) 6) Transformer humming. Any abnormal noise from bushing. Any other abnormality observed. Observe for 12 hours for any abnormality. Any oil leaks from any point. Any other observation.
MAINTAINENCE SCHEDULE OF POWER TRANSFORMER: If the transformer is to give trouble free service, it should receive the required amount of attention during operation and a well planned periodic and preventive maintenance during service. Daily or periodic inspective will help in detecting abnormal conditions of a transformer and its parts before they cause any more serious troubles. A regular
program of periodic and preventive maintenance will help to ensure healthiness of transformer during service. This section describes inspection and maintenance methods to keep transformers in good condition. As for construction, functions and handling of accessories, the corresponding catalogue/maintenance manuals should be referred for more detailed information.
SL.NO Item of maintenance 1.
Periodicity
Checking the colour of silica jel Daily In the breather and also oil level of the oil seal. If silica jel colour changes from blue to pink by 50% the siica jel is to be reconditioned or replaced Observation of oil levels in (a)main conservator tank (b)OLTC conservator (c)bushing and examinefor oilleaks if any from the transformer Visual check for over heating if any at terminal connections(red hot ) and observation for any unusual internal noises Checking for noise,vibrations or any abnormality in cooling fans and oil pumps of power transformers stand by pumps and fans are also to be run condition to observed. Observation of oil and winding temperatures and recording . Visual check for explosion vent diaphragm for any cracks. Daily
2.
3.
Daily in each shift
4.
Daily
5. 6.
Hourly Daily
7. 8. 9.
10.
11. 12. 13. 14. 15. 16. 17. 18. 19.
20.
Checking for any water leakage in to cooler in case of force cooling system. Physical examination of diaphragm for any cracks . Cleaning of bushings, inspect for any cracks or chippings of the porcelain and checking of tightness of plants and jumpers. Measurement of I.R values of transformer with 2.5kv megger upto 33kv rating and 5kv megger above 33kv rating. Recording of the values specifying the temperature which measurements are taken. Cleaning of silica jel breather Checking temperature alarm by shorting contacts by operating the knob. Testing of main tank oil for BDV and moisture content. Testing OLTC oil for BDV and moisture content. Testing of buchholz surge relays and low oil level trips for correct operation. Checking auto-start of cooling fans and pumps. Checking of buchholz relay for any gas collection and testing the gas collector. Checking of buchholz relay by air injection ensuring actuation alarm and trips. Nothing the oil level in the inspection glass of buchholz relay and arresting of oil leakages if any. Checking of all connection of transformer for tightness such as bushings, tank earth
Daily Monthly Monthly
Monthly
Monthly Monthly Quarterly Quarterly Quarterly Quarterly Quarterly or during fault Half yearly or during shut down Monthly
Quarterly
21.
22.
23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
34.
connection. Lubricating/greasing all moving parts OLTC Quarterly or as given mechanism. the manufactures manual Checking of control circuitry, inter locks oil pumps and cooling fans for auto start and stop operations at correct temperature and also for manual operation. Testing of motors, pumps and calibration Half yearly pressure gauge Pressure testing of coil coolers Half yearly Testing of oil for dissolve gas analysis for Half yearly 100MVA transformers Testing of for dissolve gas analysis of EHV Once a year transformer up to 100 kva. Testing of oil in main tank acidity, tan delta Once in a year inter face tension specific resistivity Bushing testing for tan delta Once in a year Calibration of oil and winding temperature Repeats indicator Measurement of magnetizing current at normal Once in a year tap Measurement of dc winding resistance Once in year Turn ratio test at all taps Once in a year Inspection of OLTC mechanism and contacts it Once in a year or diverter switch number of operation as recommended by manufacture completed whichever is earlier Overhauls of tap changer and mechanism Once in a year
35. 36. 37. 38. 39. 40.
Replacement of oil in OLTC Calibration of thermometers(temperature indicators )and tap position indicator Remain old oil in thermometer pockets, clean the pockets and filling with new oil Checking in the air cell (for 100MVA and above transformers) Bushings partially discharge test and capacitance(EHV transformers) Filtration of oil /replacement of oil
Once in a year Yearly
Yearly Once in 5 years Whenever the IR values of transformer is below the permissible limits.
41.
General overhaul consisting: 1. Inspection of core and winding 2. Through washing of windings 3. Core tightening 4. Check up of the core bolt insulation 5. Replacement of gaskets 6. Overhaul OLTC
CASE STUDY: A 132/11 kv or 10/16 MVA PTR at 220 KV Renigunta substation is taken for carrying out the Pre commissioning Tests. NAME PLATE DETAILS : RATING VOLTAGES AT NO LOAD 10/16 MVA HV 132KV LV 11KV
CURRENT
-
HV 43.74/69.98 A LV 524.88/839.8 A
FREQUENCY PHASES
-
50 Hz HV 3 LV 3
TYPE OF COOLING IMPEDANCE VOLTAGE VECTOR GROUP REF. CORE AND WINDINGS WEIGHT OF OIL TOTAL WEIGHT OIL YEAR OF MANUFACTURE
-
ONAN/ONAF 10.181% YNyn0 19000 KG 10000 KG 40000 KG 11110 Litres 1982 50 0C
MAX. TEMPERATURE RISE IN OIL TEST RESULTS: TURNS RATIO TEST:
Tap no 5 10
Practical ratio Rn Yn Bn Theoretical ratio (HV/LV) (HV/LV) (HV/LV) 234/19=12.06 232/19.2=12.08 232/19.2=12.08 132/11=12 232/20.6=11.25 232/20.5=11.3 231/20.4=11.25 123.75/11=11.25
THEORITICAL CALCULATIONS: At tap no.5 HV=132 KV and LV=11 KV (according to name plate details) So, HV/LV=132/11=12 At tap no.10 HV=127.5 KV and LV=11KV (according to name plate details) So, HV/LV=127.5/11=11.25 Inference: Hence from the turns ratio test it is proved that the given theoretical ratio is approximately equal to the measured ratio. Open circuit test or magnetizing current test: Tap no (V) 5 Inference: From this magnetizing current test Short circuit test: Tap no RN (A) 5 2.06 HV side YN (A) 2.16 BN (A) 2.06 rn (A) 25.3 LV side yn (A) 28.5 bn (A) 25.7 236 Rn (mA) 0.69 (V) 234 Yn (mA) 0.50 (V) 233 Bn (mA) 0.69
Vph=232 and VL=415 V Therefore, % impedance = (Z/Zbase) *100 Z=(Vsc/Isc)=(232/2.1)=110.47 ohms Zbase=Vrated/Irated =76210/69.98=1089.02 ohms
%impedance= (Z/Zbase) *100 =110.47/1089.02 *100 =0.1014*100 =10.14% Inference: From this test it is proved that the % impedance i.e obtained is equal to the theoretical value. Magnetic balance test: HV side Tap number Rn 234 5 Inference: From this test it is proved that the flux is uniformly distributed in all the windings i.e the core is magnetically balanced. Breakdown voltage test: BVD (average of six samples): 65 Kv BVD (one minute with stand): 58 Kv This test is carried out under the temperature of 500 C Inference: As the BDV value is 65 Kv which is greater than the minimum value of 50 Kv. So the BDV oil provided is good for insulation. 12 25 Yn 220 236 220 Bn 21 108 235
Insulation resistance test: IR10 (M-ohm) HV-EARTH HV-LV LV-EARTH 30 25 22 IR60 (M-ohm) 30 25 22 IR600 (M-ohm) 38 34 25 1 1 1 1.26 1.36 1.13 DA PI
Polarization index = IR600/IR60 = 38/30 = 1.26 Dielectric absorption factor = IR60/IR10 = 30/30 = 1 Inference: As the value of polarization index is 1.25. So the insulation provided is fairly good. VECTOR GROUP TEST: RN =Rn + nN --- (I) RN = 238, Rn = 17, nN = 215 238 = 17 + 215 By=Yb ---- (II) By = 392 , Yb = 390 Inference: Since the above two conditions are satisfied the vector group specified by the manufacturer is correct.
Evaluation of test results: From the above test results it is evident that the power transformer insulation is healthy and sound as all the results are within the permissible limits. CONCLUSION: Power transformer is very important in the power transmission system and it must be closely monitored regarding its condition while in service. To take remedial measure at appropriate stage to avoid any failure and to enhance the reliable service of power transformer. Hence the above tests which are detailed in this project would essential, if evaluated/ assessed, indicate the condition of the insulation which plays a major role for the Service of power transformer and suggest the steps to be taken avoid the failure.
doc_304849402.docx
INDEX
S.NO
PARTICULARS
PAGE NO.
1.
ABSTRACT
3
2.
INTRODUCTION
4-5
3.
POWER TRANSFORMER IN SUB STATION
6 – 16
4.
NAME PLATE DETAILS
17 - 19
5.
PROTECTION OF POWER TRANSFORMER
19 - 21
6.
COMMISSIONING OF POWER TRANSFORMER
21 - 24
7.
MAINTAINENCE OF POWER TRANSFORMER
25 - 28
8.
TESTING OF POWER TRANSFORMER AND CASE STUDY
29 - 32
9.
CONCLUSION
33
ABSTRACT
This project majorly deals with the performance, commissioning and testing of a POWER TRANSFORMER . It gives a brief idea of periodical maintenance of the power transformer in a 220/132/33-11 kv Renigunta substation. A practical power transformer is given here with all parts and its protective devices. The typical test results are also provided. Transformer is a vital link in a power system which has made possible the power generated at level of voltages (6600 to 22000 volts) to be stepped up to extra high voltages for transmission over long distances and then transformed to low voltages for utilization at proper load centers. With this tool in hands it has become to possible harness the energy sources at far off places from the load centers and connect the same through long extra high voltage transmission lines working on high efficiencies. At that, it may be said to be the simplest equipment with no motive parts. At every step, whether the voltage level should have to be increased or decreased we need a transformer. If there is any fault or damage in the transformer then there will be heavy loss to the distribution system. So, periodical maintenance and testing is very important for a power transformer.
INTRODUCTION:
The Transformer is a device that which transfers electrical energy from one electrical circuit to another electrical circuit without change in frequency. It is electrically isolated and magnetically coupled. Actually, the transformer is an electromagnetic energy conversion device, since the energy received by the primary is first converted to magnetic energy and it is then reconverted in to useful electrical energy. Power transformer is one which step-up or step-down the voltage level based on requirement without change in the frequency. Large-scale generation of electric power usually three phases at generated voltages of 11kv or higher. Transmission is generally at higher voltages of 132,220,400 and 750kv for which three phase transformers are necessary to step-up the generated voltage to that of the transmission line. Next at load centers the transmission voltage are reduced to distribution voltages of 66,33 and 11kv. Further at most of the consumers, the distribution voltages are still reduced to utilization voltages of 440 and 220V. Years ago it was a common practice to use suitably inter connected three single phase transformer instead of a single threephase transformer. PRINCIPLE: A Transformer is a static device by means of which electric power in one circuit is transformed into electric power of another circuit without change in the frequency. It can raise or lower the voltage level in a circuit with a corresponding decrease or increase in current. The transformer mainly works on the principle of electromagnetic induction between two circuits linked by a common magnetic flux. It consists of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance. The two coils posses high mutual inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up in the core, most of which is linked with other coil in which it produces mutually-induced emf based on Faraday’s laws of electromagnetic induction.
If the second coil circuit is closed, a current flows in it and so electrical energy is transferred from one coil to another. The first coil, in which electrical energy is fed from a.c supply mains, is called primary winding and the other from which energy is drawn out is called secondary winding. POWER TRANSFORMER IN A SUBSTATION : A Power transformer of capacity 30/50 MVA in 132/33 KV substation is shown in the figure below and its parts are described.
PARTS OF POWER TRANSFORMER: CORE: Core is made up of silicon steel laminations. The material used for core is special alloy steel of high resistance and low hysteresis losses is used in laminated form. The silicon rises the permeability at low flux densities,reduces hysteresis loss and eddy current loss. 5% silicon steel is difficult to punch and shear. But addition of nickel makes the silicon steel suitable for cold rolling and shearing. There are two kinds of transformer steel . High resistance steel having 4.5% silicon and nominal thickness 0.35mm.
The cold rolled grain oriented sheet or strip having 0.33mm nominal thickness for which the magnetic properties in the direction of rolling are superior. So, CRGO steel laminations are used in modern power transformer. In building the core high pressure is used to minimize the air gap between adjacent plates to avoid losses and also to minimize noise during operation.
TRANSFORMER TANK: Transformer tanks are made up of thin sheet steel. The tank provides a rigid support to the fittings and accessories. The function of it is to provide a protective cover to the core, windings, and other parts including transformer oil.
WINDINGS: Windings are arranged in concentric formation with lowest voltage winding next to the core. Over LV winding , HV tapping and HV main windings are placed. Some times tapping winding is placed after HV main winding depending upon requirement. Various types of winding used for making coils are as follows: Low voltage: helical/disc High voltage: partially inter leaved disc/layer/winding. Tapping winding: Inter wound spiral/helical coil. Paper covered insulated copper strips are continously transposed cables are used for making winding. Windings are compressed before assembly and remain in compressed position by means of clamping bolts. BUSHINGS: Bushing compress a central conductor surrounded by graded insulation. A bushing is necessary when a conductor is taken out through metallic or a well. Oil
filled bushings is used for 33kv applications. For making the bushings compact Synthetic resin bonded condenser bushings (SRBB) impregnated paper condenser bushing is used(OIP condenser bushing) The condenser bushing consists of central conductor surrounded by alternator layer of insulating paper and thin coil. The capacitor formed by alternator layer of thin foil and paper insulation results in uniform dielectric stress distribution between conductors surface and earthed flange.
The bushing core is coated with suitable resin. The assembly is enclosed in hollow porcelain and is provided with support flange and top hood. The porcelain is filled with dielectric oil. COOLING EQUIPMENT: The cooling systems are necessary for dissipating the heat generated in the transformer due to load loss .The various types of cooling system are mentioned in table .The radar fans are arranged for MVA at Renigunta substation.The arrangement makes use of air blast fans. COOLING SYSTEM FOR POWER TRANSFORMER: 1. O.N.A.N( Oil natural air natural): This is widely used for transformer upto 30MVA. Transformers are fitted with panel type radiators for oil natural circulation.
2. O.N.A.F( Oil natural air forced): This is widely used for transformers between 30 & 60 MVA. The panel type radiators are provided with cooling fans. Fans are switched on during heavy loads. 3. O.F.A.F( Oil forced air forced): This is method is used for transformers above 60MVA. Oil circulation through coolers. The coolers have cooling fans to exchange heat from oil to air. 4. O.F.W.F( Oil forced water forced): The heat is exchanged from oil to cooling water. Both oil and cooling water are circulated through radiator. 5. A.N( Air natural); The ambient air used for cooling in this method, is useful for oil-less transformer upto 1.5MVA. RADIATORS AND FANS: Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling. These are long, flat shaped of layers independent to each other through which transformer oil circulates. By using fans Oil-Natural Air-Forced cooling is provided such that rated power will increase as compared to oil-natural Air-Natural type of cooling. Fans can be either automatically or manually powered.
CONSERVATOR: The transformer is filled with dielectric oil.In case of oil filled transformer conservator is provided .It is installed at a slightly higher level than that of tank .During higher loads the oil in the tank expands and the level of oil in the conservator rises. The air cushion in the conservator permits expansion and contraction of the oil in the tank without contact with moist external air. The cushion in the conservator is connected to the external atmosphere via a silica ensuring dryness of incoming air during the breathing. Normally the conservator capacity is to be 10-20% of oil volume in the tank.
BREATHER: A breather is provided to prevent contamination of transformer oil in the conservator by the moisture present in the air entering the conservator. The outside air is drawn into the conservator through breather, every time the transformer cools down. The breather is packed with blue colored silica gel, which has the property of absorbing all water vapour contained in the air passing through it, there by making the air quite dry. The charge should be replaced if it becomes pink. The old silica gel can be reconditioned and used again by heating up in an Owen to 150-200 c to drive out all absorbed water.
PRESSURE VALVE: When oil expands, sometimes pressure inside the tank may increase rapidly so that tank may explode. By opening the valve, pressure inside the tank can get into normal operating point.
ON-LOAD TAP CHANGER: The taps help in changing the turns ratio so that transformer output voltage and hence the consumer’s terminal voltage can be controlled without any supply interruptions. This tap changer is used for daily or short period voltage alternations. During the operation of an on-load tap changer; 1. The circuit should not be opened otherwise dangerous sparking will occur. 2. No part of the tapped winding should get short-circuit.
TRANSFORMER OIL: The following characteristics are required for an oil to be used as transformer oil: 1. Mineral oil grade B should be used. 2. It should have a high di-electric strength i.e, not less than 30KV in the transformer tank. 3. It should contain negligible moisture content. 4. It should be perfectly clear and pale in color. Green color indicates presence of copper soaps. Cloudiness indicates presence of moisture or impurities like sludge or dust. 5. Acidity content should be very low, as it will cause precipitation of sludge and corrosion of metal surfaces. 6. It should be chemically stable, i.e. it should be impervious to the action of oxygen in the air even at high temperatures. PRESSURE RELIEF DEVICE: This PRD is designed to be used on power transformers.when pressure in the tank rises over predetermined safe limit ,this valve operates and performs following functions functions. 1) Allows the pressure to drop by instantanuosly opening aport of about 150mm dia. 2) Gives visual indication by raising a flag. 3) Operates a microswitch .This switch has 1 NO and 1NC contacts(four terminals ).Hence switch can be effectively used in control switch.
NAME PLATE DETAILS OF 132/33KV POWER TRANSFORMER TYPE POWER TRANSFORMER
TYPE OF COOLING
-
ONAN
ONAF
RATED POWER
- KVA
HV
37500
50000
KVA
LV
37500
50000
RATED VOLTAGE AT NO LOAD
- KV KV
HV LV
132 33
RATED LINE CURRENT
-
A A
HV LV
164.02 656.08
218.69 874.77
%IMPEDANCE AS PER GTP
-
12.5+/- 10%
%IMPEDANCE AT 50 MVA (BASE MEASURED) NO LOAD LOSS 20 KW
12.39
LOAD LOSS
-
145.5 KW
AUXILLIARY LOSS
-
1.5 KW
INSULATION LEVEL
-
HV LV HVN LVN
AC/LI AC/LI AC/LI AC/LI
230/550 70/170 38/95 70/170
YEAR OF MANUFACTURE
-
2009
TRANSFORMER SERIAL NUMBER FREQUENCY
-
900041B01 50 HZ
NUMBER OF PHASES
-
3
VECTOR GROUP
-
YNynO
SYMMETRICAL SHORT CIRCUIT CURRENT
-
8A
SYMMETRICAL SHORT CIRCUIT DURATION
-
1 SEC
MAXIMUM TEMPERATURE RISE OF OIL
-
50 o C
MAXIMUM TEMPERATURE RISE OF WINDING - 55 o C
REFERENCE AMBIENT TEMPERATURE
-
50 o C
TOTAL MASS
-
KG
86500
CORE & COIL MASS
-
KG
44600
TRANSPORT MASS (WITH OIL)
-
KG
77800
QUANTITY OF OIL
-
LTS 19100
PROTECTION OF POWER TRANSFORMER: Transformers are static devices, totally enclosed and generated oil immersed. Therefore, chances of faults occurring on them are very rare. However, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This necessity to provide adequate automatic protection for transformers against possible faults. Small distribution transformers are usually connected to the supply system through series fuses instead of circuit breakers. Consequently, no automatic protective relay equipment is required. However, the probability of fault on power transformers is undoubted more and hence automatic protection is absolutely necessary. TRANSFORMER PROTECTION: Different type of relays 1) Buchholz relay 2) Differential relays 3) O/F and E/F relays
4) Restricted earth fault relays 5) Over fluxing relays SAFTEY DEVICES TO POWER TRANSFORMER: Oil level indicator or Fluid level gauge: Low oil level is a harmful condition because internal insulation clearance, creepage etc; between loads, bushings and tanks are exposed to air when oiil drop below specified level. If the cooling tubes are partially cool or nearly at ambient temperature, it is an indication that the oil is not circulating in the cooling tubes and oil level has dropped below and level indicator gives an alarm,it may be a false alarm and level indicator needs checking. It position may be improper . The level indicator has a float and an arm .The float is suspended in the oil.When oil level drops down, the float hits the arm there by closing the alarm contacts. Pressure relief valve: Pressure relief valve is designed to use apower transformer .When the pressurein the tank rises above pre-determined limit this valve operates pressure in the transformer increases due to ? High over load peaks ? Prolonged over loads ? Arcing faults with in oil
The pressure relief valve is spring loaded and has a seal seat when pressure inside the tank increases above acertain valve ,the force on movable sub assembly exceeds the spring force and the valve operates .The alarm contacts are closed. After release of pressure, the valve may be manually reset.
Thermal over heating protection of power transformer: Thermocouples are the resistance measuring device. These are connected to abridge circuit .When temperature increases above safe level ,an alarm is sounded. If measures are not taken the circuit breaker is tripped after a certain temperature Some typical settings for oil temperature are as follows: Switch on fans : 60 degrees Alarm Trip : 95 degrees : 120 degrees
COMMISSIONING OF POWER TRANSFORMER: After the erection of PTR, oil filling the following pre-commissioning tests & checks are carried out on PTR to ensure the healthiness of PTR before commissioning. PRE COMMISSIONING TESTS: There are two types of pre-commissioning tests they are: 1. ELECTRICAL TESTS 2. INSULATION TESTS ELECTRICAL TESTS: TURNS RATIO TEST: The ratio should be checked on all taps and between all the windings and the result should tally with factory test reports as well as the rating and diagram plate details. Ratio shall be checked by applying a single phase 230-300 V supply on the high voltage side and measuring the voltage on the low voltage side at all tap positions.
Polarity and interface connections are checked while measuring the ratio. This can be checked by the voltmeter method. The primary and secondary windings are connected together at one point. A low voltage three phase supply is then applied to the terminals. Voltage measurements are then taken between various pairs of terminals. OPEN CIRCUIT OR MAGNETISING CURRENT TEST: A three phase low voltage supply 400V is given to the H.V winding of the three phase transformer and simultaneous current readings of the three phases are taken using low range A.C ammeters. Readings should be recorded for future reference. SHORT CIRCUIT TEST: Here LV winding of the three phase transformer is short-circuited. Here in this test we will find the % (percentage) impedance. And this % impedance must be around 2-5%. MAGNETIC BALANCE TEST: This test is conducted on HV side of the power transformer. Apply single phase 230V supply to each phase of a star connected winding, and measure voltage induced in other phases. When center phase is applied the other phase voltage should be 30 to 70 %. When extreme phase is applied, center phase voltage to be 50 to 95%. DC WINDING RESISTANCE TEST: Kelvin bridge meter should be used for the measurement of resistance at all windings. Tapped winding resistance shall be measured at all tap positions. Absolute steady reading should be obtained for minimum 1 minute. The tapped winding resistance trend shall be gradually increasing / decreasing.
VECTOR GROUP TEST: This test is mainly conducted to know the transformer connection, whether it is star-star or star-delta and so on. And also by this test we can know the phase lag between the primary and secondary connections. RN=Rn+Nn By=Yb If the conducted test would satisfy the following conditions, then the vector group specified by the company during manufacture was correct. INSULATION TESTS: The different types of insulation tests are described below: BREAK DOWN VOLTAGE TEST: The dielectric strength or break down voltage is the minimum voltage which when applied to an insulating material will result in destruction of its insulating properties. Dielectric strength is expressed in volts or kilo volts per unit thickness of the insulating material. Break down potential is that the value of voltage which when applied across 1 cm or 1 mm thickness dielectric medium or insulating material. The test equipment consists of a oil sampling container with 2 horn gap electrodes with the provision of adjusting the gap between electrodes and a high voltage transformer with a variac can vary the voltage across the electrodes from 0 to 100 KV. The moment the insulation placed between the electrodes breakdown accompanied a spark between electrodes the supply is tripped with the help a relay inside test set and the reading noted will be break down strength of the insulating material.
INSULATION RESISTANCE TEST: The insulation resistance between windings and between windings and earth should be measured with 2500/1000 volts megger and the values should be compared to the test report values. If there is much variation, the same should be intimated to the manufacturer. The dielectric absorption factor = IR at 60 sec/ IR at 10 sec should be > 1.3 It is preferable to have a motor operated megger and the readings taken after 1 minute from starting. Before measuring the insulation resistance, it should be made sure that the bushings are cleaned thoroughly with clean cotton cloth. They should also give reading of infinity before connecting up. Where, Polarizing index(P.I) = Insulation resistance at 10min Insulation resistance at 1 min
Dielectric absorption factor: Insulation resistance at 60sec POLARIZATION INDEX Less than 1 1.0-1.1 1.1-1.25 1.25-2.0 Above 2.0 INSULATION CONDITION Wet Poor Fair Good Dry
TAN DELTA TEST: When AC voltage is applied to perfect insulation there is no power loss and the charging current leads the applied voltage by 90 but in actual, the leakage current does not lead applied voltage by 90 and hence there is a definite amount of dissipation of energy which is dissipated in the form of heat. The phase angle is always less than 90. The complementary angle 90-theta is called dielectric loss angle. Dissipation factor/loss factor(tan delta) is defined as the ratio of resistive component (Ir) of current to that of the capacitive current(Ic) flowing in an insulating material. The more the resistive component of the current, the more will be the dissipation factor indicating the deterioration of the insulation. Power factor is the ratio of resistive current to that of total current. For very low value of resistive currents, the Ic will be almost equal to the total current and hence values of dissipation factor and power factor are same insulation factor or dissipation factor and capacitance measurement of bushings provide an indication of the quantity of the insulation of the bushing. A) Commissioning Check-list :
1. Air releasing from all points in the right sequence. Standing time and air release after standing time. 2. Silica gel color. 3. Condition of breather (cracks, holes etc.) and oil in the oil cup. 4. Buchholz relay shall be in service position and locked. 5. All inter connecting valves should be open. 6. Valves opened to atmosphere shall be closed and blanked. 7. Select WTI CT terminals as per manual. 8. All CT secondaries should be shorted either through respective instruments or directly. 9. OTI alarm/trip settings should be 85/95 0C . 10.WTI alarm/trip settings shall be 100/110 0C.
11.Earthing of components (thermo junction box, driving mechanism) Earthing of tank cover to tank. 12. Neutral earthing resistance should be within permissible limits. 13. Alarm indication shall be checked by closing alarm contacts of OTI, WTI, and Buchholz relay & pressure relief device. 14. Setting of over flux relay shall be 1.1 15. Overfluxing relays shall be connected to the untapped winding. 16. Setting and operation of differential protection shall be verified. 17. Over-current relay current setting shall be 120%. Check relay opera 18. Are separate annunciations available for different alarms? 19. Are the surroundings clean and degree of pollution with in tolerable levels? CHECKS DURING OPERATION AFTER COMMISSIONING: 1) 2) 3) 4) 5) 6) Transformer humming. Any abnormal noise from bushing. Any other abnormality observed. Observe for 12 hours for any abnormality. Any oil leaks from any point. Any other observation.
MAINTAINENCE SCHEDULE OF POWER TRANSFORMER: If the transformer is to give trouble free service, it should receive the required amount of attention during operation and a well planned periodic and preventive maintenance during service. Daily or periodic inspective will help in detecting abnormal conditions of a transformer and its parts before they cause any more serious troubles. A regular
program of periodic and preventive maintenance will help to ensure healthiness of transformer during service. This section describes inspection and maintenance methods to keep transformers in good condition. As for construction, functions and handling of accessories, the corresponding catalogue/maintenance manuals should be referred for more detailed information.
SL.NO Item of maintenance 1.
Periodicity
Checking the colour of silica jel Daily In the breather and also oil level of the oil seal. If silica jel colour changes from blue to pink by 50% the siica jel is to be reconditioned or replaced Observation of oil levels in (a)main conservator tank (b)OLTC conservator (c)bushing and examinefor oilleaks if any from the transformer Visual check for over heating if any at terminal connections(red hot ) and observation for any unusual internal noises Checking for noise,vibrations or any abnormality in cooling fans and oil pumps of power transformers stand by pumps and fans are also to be run condition to observed. Observation of oil and winding temperatures and recording . Visual check for explosion vent diaphragm for any cracks. Daily
2.
3.
Daily in each shift
4.
Daily
5. 6.
Hourly Daily
7. 8. 9.
10.
11. 12. 13. 14. 15. 16. 17. 18. 19.
20.
Checking for any water leakage in to cooler in case of force cooling system. Physical examination of diaphragm for any cracks . Cleaning of bushings, inspect for any cracks or chippings of the porcelain and checking of tightness of plants and jumpers. Measurement of I.R values of transformer with 2.5kv megger upto 33kv rating and 5kv megger above 33kv rating. Recording of the values specifying the temperature which measurements are taken. Cleaning of silica jel breather Checking temperature alarm by shorting contacts by operating the knob. Testing of main tank oil for BDV and moisture content. Testing OLTC oil for BDV and moisture content. Testing of buchholz surge relays and low oil level trips for correct operation. Checking auto-start of cooling fans and pumps. Checking of buchholz relay for any gas collection and testing the gas collector. Checking of buchholz relay by air injection ensuring actuation alarm and trips. Nothing the oil level in the inspection glass of buchholz relay and arresting of oil leakages if any. Checking of all connection of transformer for tightness such as bushings, tank earth
Daily Monthly Monthly
Monthly
Monthly Monthly Quarterly Quarterly Quarterly Quarterly Quarterly or during fault Half yearly or during shut down Monthly
Quarterly
21.
22.
23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
34.
connection. Lubricating/greasing all moving parts OLTC Quarterly or as given mechanism. the manufactures manual Checking of control circuitry, inter locks oil pumps and cooling fans for auto start and stop operations at correct temperature and also for manual operation. Testing of motors, pumps and calibration Half yearly pressure gauge Pressure testing of coil coolers Half yearly Testing of oil for dissolve gas analysis for Half yearly 100MVA transformers Testing of for dissolve gas analysis of EHV Once a year transformer up to 100 kva. Testing of oil in main tank acidity, tan delta Once in a year inter face tension specific resistivity Bushing testing for tan delta Once in a year Calibration of oil and winding temperature Repeats indicator Measurement of magnetizing current at normal Once in a year tap Measurement of dc winding resistance Once in year Turn ratio test at all taps Once in a year Inspection of OLTC mechanism and contacts it Once in a year or diverter switch number of operation as recommended by manufacture completed whichever is earlier Overhauls of tap changer and mechanism Once in a year
35. 36. 37. 38. 39. 40.
Replacement of oil in OLTC Calibration of thermometers(temperature indicators )and tap position indicator Remain old oil in thermometer pockets, clean the pockets and filling with new oil Checking in the air cell (for 100MVA and above transformers) Bushings partially discharge test and capacitance(EHV transformers) Filtration of oil /replacement of oil
Once in a year Yearly
Yearly Once in 5 years Whenever the IR values of transformer is below the permissible limits.
41.
General overhaul consisting: 1. Inspection of core and winding 2. Through washing of windings 3. Core tightening 4. Check up of the core bolt insulation 5. Replacement of gaskets 6. Overhaul OLTC
CASE STUDY: A 132/11 kv or 10/16 MVA PTR at 220 KV Renigunta substation is taken for carrying out the Pre commissioning Tests. NAME PLATE DETAILS : RATING VOLTAGES AT NO LOAD 10/16 MVA HV 132KV LV 11KV
CURRENT
-
HV 43.74/69.98 A LV 524.88/839.8 A
FREQUENCY PHASES
-
50 Hz HV 3 LV 3
TYPE OF COOLING IMPEDANCE VOLTAGE VECTOR GROUP REF. CORE AND WINDINGS WEIGHT OF OIL TOTAL WEIGHT OIL YEAR OF MANUFACTURE
-
ONAN/ONAF 10.181% YNyn0 19000 KG 10000 KG 40000 KG 11110 Litres 1982 50 0C
MAX. TEMPERATURE RISE IN OIL TEST RESULTS: TURNS RATIO TEST:
Tap no 5 10
Practical ratio Rn Yn Bn Theoretical ratio (HV/LV) (HV/LV) (HV/LV) 234/19=12.06 232/19.2=12.08 232/19.2=12.08 132/11=12 232/20.6=11.25 232/20.5=11.3 231/20.4=11.25 123.75/11=11.25
THEORITICAL CALCULATIONS: At tap no.5 HV=132 KV and LV=11 KV (according to name plate details) So, HV/LV=132/11=12 At tap no.10 HV=127.5 KV and LV=11KV (according to name plate details) So, HV/LV=127.5/11=11.25 Inference: Hence from the turns ratio test it is proved that the given theoretical ratio is approximately equal to the measured ratio. Open circuit test or magnetizing current test: Tap no (V) 5 Inference: From this magnetizing current test Short circuit test: Tap no RN (A) 5 2.06 HV side YN (A) 2.16 BN (A) 2.06 rn (A) 25.3 LV side yn (A) 28.5 bn (A) 25.7 236 Rn (mA) 0.69 (V) 234 Yn (mA) 0.50 (V) 233 Bn (mA) 0.69
Vph=232 and VL=415 V Therefore, % impedance = (Z/Zbase) *100 Z=(Vsc/Isc)=(232/2.1)=110.47 ohms Zbase=Vrated/Irated =76210/69.98=1089.02 ohms
%impedance= (Z/Zbase) *100 =110.47/1089.02 *100 =0.1014*100 =10.14% Inference: From this test it is proved that the % impedance i.e obtained is equal to the theoretical value. Magnetic balance test: HV side Tap number Rn 234 5 Inference: From this test it is proved that the flux is uniformly distributed in all the windings i.e the core is magnetically balanced. Breakdown voltage test: BVD (average of six samples): 65 Kv BVD (one minute with stand): 58 Kv This test is carried out under the temperature of 500 C Inference: As the BDV value is 65 Kv which is greater than the minimum value of 50 Kv. So the BDV oil provided is good for insulation. 12 25 Yn 220 236 220 Bn 21 108 235
Insulation resistance test: IR10 (M-ohm) HV-EARTH HV-LV LV-EARTH 30 25 22 IR60 (M-ohm) 30 25 22 IR600 (M-ohm) 38 34 25 1 1 1 1.26 1.36 1.13 DA PI
Polarization index = IR600/IR60 = 38/30 = 1.26 Dielectric absorption factor = IR60/IR10 = 30/30 = 1 Inference: As the value of polarization index is 1.25. So the insulation provided is fairly good. VECTOR GROUP TEST: RN =Rn + nN --- (I) RN = 238, Rn = 17, nN = 215 238 = 17 + 215 By=Yb ---- (II) By = 392 , Yb = 390 Inference: Since the above two conditions are satisfied the vector group specified by the manufacturer is correct.
Evaluation of test results: From the above test results it is evident that the power transformer insulation is healthy and sound as all the results are within the permissible limits. CONCLUSION: Power transformer is very important in the power transmission system and it must be closely monitored regarding its condition while in service. To take remedial measure at appropriate stage to avoid any failure and to enhance the reliable service of power transformer. Hence the above tests which are detailed in this project would essential, if evaluated/ assessed, indicate the condition of the insulation which plays a major role for the Service of power transformer and suggest the steps to be taken avoid the failure.
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