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Unit 1-2-b

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VTU Notes for Testing and commissioning of Electrical Equipment Department of Electrical and Electronics Faculty Name: Mrs Veena Bhat Designation: Assistant Professor Subject: Testing and Commissioning of Electrical equipment Semester: VII

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Unit 1-2-b

  1. 1. Testing of Transformer Preliminary Tests: Following tests are carried out in the works at different stages, before the core and the coil assembly is placed in its tank . 1.Core insulation: After the core is assembled, 2kV test is done to ensure that the insulation between clamp plates, and core bolts to find core is adequate. 2.Core loss test: Some turns are wound over the core and it is energized at normal flux density. Core loss and magnetizing current are noted and compared with design values.
  2. 2. Testing of Transformer 3. Check of Ratio, Polarity, Vector relationship and winding resistance of Transformer Assembly . a. Conducted to check that all connections to the bushings, tap changers etc. have been made correctly during final assembly. b. Conducted to ensure the correctness of voltage ratio between different windings on each tapping. c. The tolerance for ratio is ±0.5% of the declared ratio or ±10% of the percentage impedance voltage. d. In order to get accurate ratio, a ratiometer is employed . e. It also indicates the polarity of transformer windings. f. With the turns ratio tester, the turns ratio is directly read on the tester for each tap and for each phase of the winding.
  3. 3. Testing of Transformer g. The turns ratio can also be tested by applying a single phase ac voltage (approximately 230V) on the HV side and measuring the voltage on the low voltage side at all tap positions. h. For a three phase transformer a vector relationship test is also to be done. i. The dc resistance of each winding is measured by Kelvin’s double bridge to check that there is no faulty joint. 4. Preliminary Load loss and Impedance voltage measurements to be done.
  4. 4. Final Tests The completely assembled transformer is tested in accordance with the International standards. Routine tests: 1. Measurement of winding resistance. 2. Measurement of voltage ratio and check of voltage vector relationship. 3. Measurement of impedance voltage (principal tapping), short circuit impedance and load loss. 4. Measurement of no load loss and current. 5. Measurement of insulation resistance. 6. Dielectric tests. : Separate source ac voltage and Induced over voltage. 7. Tests on-load tap changers.
  5. 5. Final Tests Type Tests In a batch only one transformer will undergo these tests. 1. The temperature rise test 2. Lightning impulse test 3. Air Pressure test 4. Permissible flux density and over fluxing test 5. Noise level test
  6. 6. Final Tests Special Tests: Based on agreement between the manufacture and Purchaser. 1. Dielectric special tests. 2. Measurement of zero-sequence impedance of three phase transformers.(The zero-sequence is needed for earth-fault protection and earth-fault current calculations.) 3. Short-circuit test. 4. Measurement of acoustic noise level. 5. Measurement of harmonics of the no load current. 6. Measurement of power taken by the fans and oil pumps. 7. Measurement of capacitances between windings to earth and between windings. 8. Measurement of transferred surge voltage on low voltage windings. 9. Measurement of insulation resistance to earth of the windings, or measurement of dissipation factor of the insulation system capacitances.
  7. 7. Final Tests Note: • Special Tests and type tests are to be performed in the presence of the purchaser or his representative. • Commissioning tests are performed at site before commissioning.
  8. 8. 1.Measurement of Winding Resistance Importance of test: 1. Check transformer windings and terminal connections. 2. Use as reference for future measurements. 3. Calculate the load loss values at reference (e.g. 750C) temperature. 4. For calculation of I2R losses in the winding, it is necessary to measure dc resistance of each winding. 5. Winding resistances are measured between all connection terminals of windings and at all tap positions. 6. The resistance of each winding, the terminals between which it is measured and the temperature of the windings shall be recorded.
  9. 9. 1.Measurement of Winding Resistance 7. The measuring current can be obtained either from a battery or from a constant(stable) current source 8. The measuring current value should be high enough to obtain a correct and precise measurement and small enough not to change the winding temperature. 9. The resistance measurement should be done after the direct current circulating in the winding has reached a steady state.
  10. 10. 1.Measurement of Winding Resistance 11. In some cases, it may take some minutes depending upon the winding inductance. 12. Temperature of the winding must be stable and for this reason, this test is usually carried out before load loss measurement. 13. The average oil temperature is determined as the mean of top and bottom oil temperatures and is taken as average winding temperature. 14. Normally winding resistance values 1 ohm or above is measured using Wheatstone bridge and winding resistance values less than 1 ohm is measured using micro-ohm meter or Kelvin Bridge.
  11. 11. 1.Measurement of Winding Resistance R2 : winding resistance at temperature t2 R1 : winding resistance at temperature t1
  12. 12. 1.Measurement of Winding Resistance Refer Manual :Transformer Winding Resistance Measurement.docx
  13. 13. 2. Measurement of voltage ratio & check of voltage group  These tests are conducted to check that all connections to the bushings, tap changers etc. have been made correctly during final assembly. By using ratiometer this test can be conducted. Ratio test will confirms the polarity of the windings. Test is conducted on every transformer for position of every tap. Measurement of turn ratio is based on, applying a phase voltage to one of the windings using a bridge (equipment) and measuring the ratio of the induced voltage at the bridge. The measurements are repeated in all phases and at all tap positions, sequentially.
  14. 14. 2. Measurement of voltage ratio and check of voltage group In general, the measuring voltage is 220 V A.C. 50 Hz. However, equipments which have other voltage levels can also be used. The theoretical no-load turn ratio of the transformer is adjusted on the equipment by an adjustable transformer, it is changed until a balance occurs on the % error indicator. The value read on this error indicator shows the deviation of the transformer from real turn ratio as %. Voltage ratio & vector group test:volt ratio test transformer.docx
  15. 15. 3.Measurement of Impedance Voltage & Load loss  Short-circuit voltage is an important criteria especially during parallel operations of the transformers .  The short-circuit loss is a data which is also used in the heat test.  Load loss = I2R losses in the winding + stray losses due to eddy currents in conductors, clamps and tank  Stray losses vary with frequency  Hence in this test, supply is given to the transformer at rated frequency .  The test is carried out by short circuiting, usually the LV winding and by supplying the impedance voltage to HV winding.  The measured power will also include small core-loss .  Since the supply voltage during the test is a small fraction of normal voltage, this loss can be ignored.
  16. 16. 3.Measurement of Impedance Voltage & Load loss  For a high impedance transformer ( Z > 15% ) the core loss may become appreciable and can be deducted after separately measuring it at impedance voltage  As per IS: 2026(Part I) -1977, the measurements can be made at any current between 25% to 100%, but preferably not less than 50% of the rated current  Load loss and impedance voltage can be corrected for rated MVA as below
  17. 17. 3.Measurement of Impedance Voltage & Load loss While measuring load loss and impedance at different tap positions, readings should be taken quickly.  and the interval between measurements at different taps should be adequate to avoid significant errors due to momentary temperature rise of windings.  The difference in temperature between the top oil and bottom oil should preferably be small to enable the average temperature to be determined accurately .  Three wattmeter method should be used instead of two wattmeter method to avoid large value of wattmeter multiplier constant . •The power factor during load loss test can be less than 0.1 and wattmeters suitable for such low power factors should be used.
  18. 18. 3.Measurement of Impedance Voltage & Load loss Impedance voltage at rated current (principal tapping): If the principal tapping corresponds with the mean tapping position or with one of the two middle tapping positions: 1)two-winding transformers ± 10 percent of the declared impedance voltage for that tapping 2) multi-winding transformers ± 10 percent of the declared impedance voltage for one specified pair of windings ± 15 percent of the declared impedance for a second specified pair of windings
  19. 19. 3.Measurement of Impedance Voltage & Load loss
  20. 20. 4.Measurement of no load loss & current Importance of the test To assess the efficiency of transformer Also as a check that high voltage tests have not caused any damage to the winding insulation For large transformers, no-load loss measurement is carried out before and after completion of dielectric tests Test is carried out at rated frequency feeding usually LV winding I2R will be negligible Power factor for medium power transformers - around 0.3 Power factor for large power transformers - around 0.1 Core losses = hysteresis losses + eddy current losses  Hysteresis losses dependent on average value of voltage. Eddy current losses dependent on rms value of voltage. In this test, two voltmeters are used •(1) bridge rectifier type to indicate average voltage •(2) dynamometer type to indicate rms voltage
  21. 21. 4.Measurement of no load loss & current Supply is set such that specified value is indicated on the average voltmeter. Hysteresis component of no load loss is measured correctly . No load losses can be measured from the L.V. side using an adjustable three phase voltage source with neutral. The voltage and frequency should be steady and at rated values and as near as possible to 50 Hz and it should be measured. This test can give a basic value near rated conditions if all precautions are taken. The L.V. side is energised at the rated tap at rated voltage and power is measured by three watt meters or 3 phase, 4 wire single wattmeter/energy meter
  22. 22. 4.Measurement of no load loss & current Eddy current component will be either lower or higher than the true value, depending upon the form factor of the supply voltage Ratio between two components is known for any particular quality of core steel Where, P = no-load loss for sinusoidal voltage Pm = the measured no-load loss P1 = ratio of hysteresis losses to total iron losses P2 = ratio of eddy current losses to total iron losses K = ( rms voltage / ( 1.11*average voltage) )2 For normal flux densities and frequencies of 50Hz and 60Hz, P1 and P2 are each 0.5 for grain oriented steel •P1 and P2 are 0.7 and 0.3 , respectively, for non-grain oriented steel
  23. 23. 4.Measurement of no load loss and current
  24. 24. 5.Measurement of Insulation Resistance  Measured between two parts separated by insulation. Resistance between conducting part and earth. Instrument used is Megger. The ‘Megger’ consists of a D.C power source (hand operated or electrically driven D.C generator or a battery source with electronic circuit ) and a measuring system. Microprocessor based insulation testers are also now available.
  25. 25. 5.Measurement of Insulation Resistance(IR)  The insulation test reveals the condition of the insulation inside the transformer. The insulation resistance values are affected by temperature, humidity and presence of dirt on insulators and bushings. oil temperature is also recorded IR at 15th second and 60th second are noted for determining Polarization Index
  26. 26. Factors influencing Insulation Resistance (IR) value 1. Surface condition of the terminal bushing 2. Quality of oil 3. Quality of winding insulation 4. Temperature of oil 5. Duration of application and value of test voltage
  27. 27. IR values are monitored in Transformers 1. Winding to ground  HV to LV and earth connected together  LV to HV and earth. 2. Winding to winding. Eg. HV to LV 3. All windings to ground .Eg. HV and LV to earth.  By comparing the results obtained in insulation resistance measurements with periodical measurements, the insulation conditions can be evaluated.  For comparison they have to be at the same temperature
  28. 28. IR values are monitored in Transformers
  29. 29. Steps for measuring the IR 1. Shut down the transformer and disconnect the jumpers and lightning arrestors. 2. Discharge the winding capacitance. 3. Thoroughly clean all bushings 4. Short circuit the windings. 5. Guard the terminals to eliminate surface leakage over terminal bushings. 6. Record the temperature. 7. Connect the test leads (avoid joints). 8. Apply the test voltage and note the reading. The IR. value at 60 seconds after application of the test voltage is referred to as the Insulation Resistance of the transformer at the test temperature.
  30. 30. Steps for measuring the IR IR values recorded over a period of time may be plotted as a curve to study the history of the insulation resistance A curve showing a downward trend indicates a loss of IR due to unfavourable conditions such as oil deterioration, excessive moisture in paper, deterioration/damage to terminal bushings etc.  A very sharp drop is a cause for concern and action shall be taken to ascertain the exact cause of insulation failure and for corrective steps.
  31. 31. Polarization Index  Tells true idea about insulation quality and extent of dryness. As the insulation draws capacitive currents in the beginning current is more & within 15 secs insulation resistance measured by megger. Repeat the same for 60 secs. Take the reading. IoR60 / IoR15 = polarization index Dependence on which temperature insulation resistance is measured. Polarization index should be greater than 1 Reason for low value of Insulation 1. Dust & dirt sticking on the surface of the insulation. 2. Absorption of moisture by insulation & insulating oil.
  32. 32. 6.Dielectric Tests 1) Separate source voltage withstand test: Purpose – to verify the power frequency withstand strength of the winding under test to earth and other windings line terminals of windings under test are connected together and appropriate test voltage is applied to them Other windings and tank are connected together to the earth
  33. 33. 6.Dielectric Tests
  34. 34. Rated withstand voltages for transformer windings with highest voltage for equipment Um up to and including 420kV
  35. 35. 6.Dielectric Tests  Windings with graded insulation, which have neutral intended for direct earthing, are tested at 38kV  Supply voltage should be nearly sinusoidal The peak value of the voltage is measured  Digital peak voltmeter associated with capacitive voltage divider is used  Peak value divided by √2 shall be equal to the test value  Duration of application of test voltage is 60s
  36. 36. Procedure  The test shall be commenced at a voltage not greater than one-third of the specified test value and shall be increased to this value as rapidly as is consistent with measurements.  At the end of the test, the voltage shall be reduced rapidly to less than one-third of the test value before switching off The full test voltage shall be applied for 60s The test shall be successful if no collapse of the test voltage occurs
  37. 37. Example: HV high voltage test : LV winding connected together and earthed. HV winding connected together and given 28 KV ( for 11KV transformer) for 1 minute. LV high Voltage test : HV winding connected together and earthed. LV winding connected together and given 3 KV for 1 minute. Equipment used : High Voltage tester ( 100KV & 3KV)
  38. 38. Example:
  39. 39. 6.Dielectric Tests (2) Induced over-voltage withstand test: Purpose : to verify the power frequency withstand strength along the winding under test, between its phases, and to earth and other windings Transformers with uniformly insulated windings, the test voltage is twice the corresponding rated voltage For test voltages up to 66kV, three-phase transformers are generally supplied direct from a three-phase source For higher values, each HV terminal in turn is raised to a specified test voltage, by applying single-phase voltage to each winding
  40. 40. 6.Dielectric Tests For higher values, each HV terminal in turn is raised to a specified test voltage, by applying single-phase voltage to each winding Supply frequency higher than normal is used to avoid core saturation When the frequency is chosen is in the range of 150 to 240Hz, capacitive currents may cause overloading of the generator set Reactor is used Frequency must be selected properly to avoid series resonance Test duration in seconds = (120*Rated frequency)/Test frequency
  41. 41. 6.Dielectric Tests
  42. 42. 6.Dielectric Tests This test checks the inter turn insulation For a 11KV/433V transformer,866 Volts are applied at the 433V winding with the help of a Generator for 1 minute. This induces 22KV on 11KV side. The frequency of the 866V supply is also increased to 100HZ. Equipment used : MOTOR GENERATOR SET
  43. 43. 6.Dielectric Tests
  44. 44. Type Tests (i) Temperature rise test Purpose: to confirm that under normal conditions, the temperature rise of the windings and the oil will not exceed the specified limit Temperature rises are measured above the temperature of the cooling air In water cooled transformers, the temperature rise is measured above the inlet water temperature
  45. 45. Temperature Rise Test
  46. 46. Temperature rise for top oil LV winding is short circuited Voltage is applied to HV winding such that power input = no load loss + load loss corrected to a reference temperature of 75oC Measurement by three wattmeter method Losses are maintained constant until the top oil rise has reached a steady state Hourly readings of the top oil temperatures are taken Thermometer placed in a pocket in the transformer top cover Temperature at inlet and outlet of the cooler bank is also taken hourly and mean oil temperature is determined Ambient temperature is measured
  47. 47. Duration of temperature rise test Method (a) : Test should not be regarded as complete until the temperature rise increment of top oil is less than 3oC in 1 hour
  48. 48. Duration of temperature rise test Method (b): It should be demonstrated that the top oil temperature rise does not vary more than 1oC per hour during four consecutive hourly readings. The last reading is taken for the determination of the final oil temperature rise.
  49. 49. Winding temperature rise When top oil temperature rise is established, the current is reduced its rated value It is maintained for 1 hour to allow the winding to attain its normal temperature At the end of the test, supply is switched off Cooling fans or water pumps should be stopped Oil pumps should remain running Short circuit connection is removed
  50. 50. Winding temperature rise Hot resistance of winding is measured by KDB Time elapse between switching off of power supply and taking the first reading Resistances are measured at intervals over a period of about 15min Graph of hot resistance versus time is plotted From the extrapolated value, winding resistance at the instant of shut-down can be measured
  51. 51. Winding temperature rise
  52. 52. Winding temperature rise
  53. 53. Impulse Testing Lightning – common cause of flashover on overhead transmission lines (1) Lightning stroke makes direct contact with a phase conductor and produces a voltage in excess of impulse voltage level (2) stroke makes contact with an earth wire or tower -> back flashover Terminal equipment experience lightning impulses in service Measure of effectiveness of insulation strength is dielectric strength
  54. 54. Impulse Testing Power frequency tests alone are not adequate to check dielectric strength Distribution of impulse voltage stress along the winding is non-linear Necessary to ensure adequate dielectric strength of transformers for impulse conditions Switching impulses occur during switching operations in the system Magnitude and waveform of impulses differ from case to case Maximum voltage can be about 3.5 times the service voltage Switching impulse tests are worthwhile for transformers with greatly reduced insulation level
  55. 55. Lightning Impulses System disturbances from lightning can be represented by three basic wave shapes (1) Full wave 1.2/50 wave (2) chopped wave (3) front of waves
  56. 56. Lightning Impulses
  57. 57. Lightning Impulses These three waves – different in duration and in rate of rise and decay of voltage  Full wave - relatively long duration, cause major oscillations to develop in the windings, produce stresses in turn-to-turn and section-to-section insulation, stresses across large portions of winding, between windings to ground Chopped wave – higher turn-to-turn and section-to- section stresses due to rapid change in voltage Front of wave – high turn-to-turn and section-to- section voltages
  58. 58. Switching Impulse  Front time – several hundred microseconds Periodically damped or oscillating one IEC-60060 - 250/2500 Test at negative polarity SI – do not cause non-uniform voltage distribution along the winding They affect the insulation to ground, between windings and phases and the external insulation of the bushings Open circuited LV windings are subjected to impulse voltage stresses Separate test on LV winding is not essential
  59. 59. Generation of Impulses & Test circuit 
  60. 60. Lightning Impulse Test Circuit Basic Impulse Generator circuit Voltage measuring circuit Chopping circuit, where applicable Standards - 1.2μs/50μs Tolerances - ±3% on peak value, ±30% on front time, ±20% on time to half value
  61. 61. Lightning Impulse Test Circuit
  62. 62. Lightning Impulse Test Circuit
  63. 63. Line terminal testing
  64. 64. Neutral terminal testing
  65. 65. Test with transferred voltages
  66. 66. Test with transferred voltages Testing of HV windings of large transformers, Difficult to obtain wave front due to large capacitance of transformer, inherent self-inductance of the generator and the leads connecting the generator and transformer On LV windings, difficult to obtain specified wave tail due to extremely low impedance of these windings  Effective impedance of transformer controls the wave tail duration Effective impedance can be varied by different terminal impedances at the non-impulsed winding terminals If these are left isolated, effective impedance will be maximum
  67. 67. Test with transferred voltages They cannot be left isolated because high voltages are induced in these windings Voltage on terminals that are not being tested is limited to 75% of BIL Non-impulsed terminals of windings are earthed directly or through resistors( max 400Ω)
  68. 68. Full wave test sequence It consists of application of a reduced full-wave and three full waves The impulse test-sequence is applied to each of the line terminals of the tested winding in succession. In the case of a three-phase transformer, the other line terminals of the winding shall be earthed directly or through a low impedance, such as a current measuring shunt. If the winding has a neutral terminal, the neutral shall be earthed directly or through a low impedance, such as a current measuring shunt. The tank shall be earthed. In the case of a separate-winding transformer, terminals of windings not under test are likewise earthed directly or through impedances so that under all circumstances the voltage appearing on them is limited to less than 75 percent of their rated withstand voltage.
  69. 69. Test with impulse chopped on the tail Greater stresses on windings due to fast rate of collapse of voltage Time to chopping between 2 and 6μs Triggered type of chopping provides better consistency Failure during chopped wave test are reflected on application of full waves, immediately after the chopped wave
  70. 70. Test with impulse chopped on the tail
  71. 71. Sequence of tests with impulse chopped on the tail 1.One reduced full impulse 2.One 100% full impulse 3.One reduced chopped impulse 4.Two 100% chopped impulses 5.Two 100% full impulses In principle, the detection of faults during a chopped impulse test depends essentially on a comparison of the oscillographic records of 100 percent and reduced chopped impulses.
  72. 72. Measurement and Recording of Impulses Three types of dividers – resistive divider, capacitive divider and compensated divider Last two types also serve as load capacitor Chopping can be obtained with a rod gap, triggered sphere gap or with a multiple chopping gap
  73. 73. Oscillographic recording Lightning Impulse Test Applied voltage wave and one other parameter are recorded Oscillograms taken at reduced and full test levels should be recorded to give equal amplitude ( by using attenuators)
  74. 74. Fault detection Insulation failure will change the impedance of the transformer Causes a variation in the impulse current and in the voltage Reduced full wave oscillograms are taken as reference They match with subsequent full-wave oscillograms when transformer is healthy For fault detection , the following can be measured and used separately (1) the neutral current (2) capacitively transferred current (3) the tank current Also, transferred voltages to non-tested windings can be used
  75. 75. Fault detection
  76. 76. Interpretation of Oscillograms Current Oscillograms Most sensitive means of failure detection Major changes in the current oscillogram indicate probable breakdown within the winding and earth Localized high frequency oscillations spread over 2 or 3μs are a possible indication of severe discharges or a possible breakdown in the insulation between turns or coils Current oscillograms are recorded on two sweeps
  77. 77. Interpretation of Oscillograms
  78. 78. Interpretation of Oscillograms
  79. 79. Transformer accessories, fitments and safety devices Accessories Proper functioning Provide protection under fault condition
  80. 80. (1) Oil level indicator: Oil level in the conservator is maintained above a pre-determined minimum level Magnetic oil level gauge which also incorporates a mercury switch
  81. 81. (2) Pressure relief valve: Fitted on the tank to act as an exit for gases formed of oil Major fault inside the transformer causes sudden vaporization of the oil Leads to rapid build up gaseous pressure Pressure to be relieved within few milliseconds Otherwise transformer tank gets ruptured Oil spilling Damage and fire hazard
  82. 82. (2) Pressure relief valve: Pressure relief valves are usually installed behind the pressure gauge on sealed tank units. They are used in conjunction with pressurized nitrogen systems and can be mounted in the gas bottle cabinet or on the tank wall. The bleeder valve is set to bleed-off any pressures that exceed a, pre-set level (usually around 8-10 psi). This valve is an integral part of the pressurized gas system, and its failure can result in a rupture of the tank.
  83. 83. (2) Pressure relief valve: The operation of these devices can be checked by manually increasing the tank pressure to the preset level It is important not to exceed the maximum tank pressure. If the valve does not bleed off the excess pressure, it should be replaced
  84. 84. (3) Buchholz relay: Gas operated relay Fitted in the pipe between tank and conservator Minor fault –Damage to core bolt insulation –Local overheating Slow generation of gas in the oil Alarm for incipient faults Decomposition of oil due to fault contains more than 70% of hydrogen gas Hydrogen gas rises upward and tries to go into conservator Gas gets collected in the upper portion of the relay
  85. 85. (3) Buchholz relay: Oil level in the relay drops Float in the relay tilts with reduced oil level Mercury switch attached to the float is closed Switch closes the alarm circuit Transformer is disconnected as early as possible Gas sample is taken for test Test indicates type of fault and insulation level Relay operation is slow (0.1 sec and average time is 0.2sec)
  86. 86. (3) Buchholz relay: In case of short circuits inside transformer, pressure in the tank increases Oil rushes towards conservator The baffle(plate) gets pressed by the rushing oil and closes another switch This in turn closes the trip circuit of circuit breaker Relay for earthquake zones
  87. 87. (4) Sudden pressure relay(rate of rise of pressure relay):  Relay responds to sudden rise of pressure due to internal arcing
  88. 88. (5) Conservator: Large cylinder connected by pipe to the transformer Oil is filled up to some level in the conservator Remaining portion is filled with air Expansion and contraction of transformer tank oil is accommodated by air cushion in the conservator Direct contact with air is avoided Conservators are of two types (1) air cushion connected with external atmosphere through breather -air entering conservator is dried by silica gel in the breather (2) air cushion contained rubber bag installed within the conservator
  89. 89. (5) Conservator:
  90. 90. (6) Breather: One end of the breather is connected to air cushion in conservator Other end is towards external air It is filled with dry silica gel, which is blue in color During expansion or contraction of oil, air is breathed in/out through breather Silica gel absorbs moisture and admits only clean/fresh air When silica gel absorbs moisture it becomes pink It is dried during periodic maintenance
  91. 91. (7) Oil Temperature Indicator: Thermocouple is placed in the pocket provided with the tank near hot oil Both average reading and hot spot temperature gauges can use a bulk-type detecting unit that is immersed in the oil either near the top of the oil level or near the windings at the spot that is expected to be the hot test. A capillary tube is connected to the bulb and brought out of the tank. The temperature indication is provided either by a, linear marking on the tube itself, or by a dial-type indicator.
  92. 92. (7) Oil Temperature Indicator: (1) The lowest setting usually actuates external cooling fans that will come on at a preset temperature level. The fans will shut off once the temperature has been reduced to the prescribed level. (2) The contacts can also be set to actuate remote alarms that will alert maintenance personnel of the condition of the transformer. These devices must be reset even though the temperature has returned to normal. (3) The highest and most critical contact setting on the temperature gauge is connected to a relay or a circuit breaker that will trip out and de-energize the transformer.
  93. 93. (8) Winding Temperature Indicator(Hot spot indicator) with alarm and tripping contacts: Thermocouples are placed in the tank near hot oil Winding Temperature Indicator : The Winding is the component with highest temperature within the transformer and, above all, the one subject to the fastest temperature increase as the load increases.  Thus to have total control of the temperature parameter within transformer, the temperature of the winding as well as top oil, must be measured
  94. 94. (8) Winding Temperature Indicator(Hot spot indicator) with alarm and tripping contacts:  An indirect system is used to measure winding temperature, since it is dangerous to place a sensor close to winding due to the high voltage. The indirect measurement is done by means of a Built-in Thermal Image.  Winding Temperature Indicator is equipped with a specially designed Heater which is placed around the operating bellows through which passes a current proportional to the current passing through the transformer winding subject to a given load.
  95. 95. (8) Winding Temperature Indicator(Hot spot indicator) with alarm and tripping contacts:  Winding Temperature is measured by connecting the CT Secondary of the Transformer through a shunt resistor inside the Winding Temperature Indicator to the Heater Coil around the operating Bellows  It is possible to adjust gradient by means of Shunt Resistor  In this way the value of the winding temperature indicated by the instrument will be equal to the one planned by the transformer manufacturer for a given transformer load.
  96. 96. (9) Marshalling kiosk (control cabinet): Placed on the side of the tank  Control cables, power cables between transformers, coolers, control room, auxiliary supply switch gear etc. are via control cabinet  Start , stop switches for fan motors, cooler pump motors etc.  Temperature indicators and temperature controllers
  97. 97. (9) Marshalling kiosk (control cabinet):
  98. 98. (10) Surge Arrestors: One per phase is used One terminal is connected to the lead of the transformer terminal Other terminal is connected to earth Lightning and switching surges are discharged to earth Silicon carbide and ZnO surge arrestors
  99. 99. Maintenance of Power Transformer Maintenance of transformer:Maintenance of Power Transformer.docx
  100. 100. Sudden short-circuit withstand test Special test Conducted at short circuit testing station Ability of a transformer to withstand external short- circuits is proved by conducting these tests Transformer must withstand mechanical and thermal stresses caused by repeated external short circuit currents Due to short circuit currents, the transformer winding is subjected to radial electromagnetic forces The radial forces produce hoop stress on outer winding and compressive stress on inner winding  axial forces tend to collapse the winding, fracture the end rings, bending of conductor between stresses
  101. 101. Sudden short-circuit withstand test
  102. 102. Sudden short-circuit withstand test
  103. 103. Sudden short-circuit withstand test
  104. 104. Sudden short-circuit withstand test Designed to withstand short circuit forces
  105. 105. Short circuit testing of Power Transformers Special test Large amounts of power are required As per IEC 60076-5, Power Transformers are categorized as: Category I - Up to 2500kVA Category II – From 2501kVA to 100MVA Category III – Above 100MVA Symmetrical short circuit current = measured short circuit impedance of transformer + the system impedance
  106. 106. Requirements prior to short circuit test Transformer should pass all routine tests as specified in IEC 60076-I Lighting Impulse test is not to be required to be conducted prior to short circuit test At the beginning of short-circuit test, the average winding temperature shall be preferably between 10oC to 40oC
  107. 107. Peak value of short-circuit current
  108. 108. Peak value of short-circuit current
  109. 109. category 1: single-phase transformers Number of tests shall be three, the duration of each test being 0.5 second with a tolerance of ± 10 percent . Unless otherwise specified each of the three tests on a single-phase transformer with tapping's is made in a different position of the tap-changer, that is, one test in the position corresponding to the highest voltage ratio, one test on the principal tapping and one test in the position corresponding to the lowest voltage ratio .
  110. 110. category 1: three-phase transformers  Total number of tests shall be nine, that is three tests on each limb, the duration of each test being 0.5 second with a tolerance of ± 10 percent Unless otherwise specified the tests on each limb of a transformer with tapping’s are made in different positions of the tap-changer, that is, three tests in the position corresponding to the highest voltage ratio on one of the outer limbs, three tests on the principal tapping on the middle limb and three tests in the position corresponding to the lowest voltage ratio on the other outer limb
  111. 111. Detection of Faults and the Evaluation of the Results of the Short-Circuit Test Transformers of category 1 — All the routine tests shall be repeated The dielectric routine tests shall be at 75 percent of the original test value unless a higher value has been agreed between the manufacturer and the purchaser
  112. 112. Detection of Faults and the Evaluation of the Results of the Short-Circuit Test The transformer is deemed to have passed the short- circuit tests if, 1.The routine tests have been successfully repeated . 2.The results of the short-circuit tests, measurements during short-circuit tests and out of tank inspection do not reveal any defects (displacements, deformations of windings, connections or supporting structures, or traces of discharge), 3. The short-circuit reactance measured after the tests differs from that measured in the original state by not more than specified percent ( 2%)
  113. 113. Detection of Faults and the Evaluation of the Results of the Short-Circuit Test If the three conditions for passing the short-circuit tests have been met, the transformer is restored to its original state and any further routine tests necessary to prove fitness for service are repeated before dispatch. If any of the three conditions have not been met, it may be necessary to dismantle the unit as far as is required to establish the cause of the change of the conditions.
  114. 114. Transformers of category 2 and 3 Total number of tests –nine, three tests on each limb, the duration of each test being 0.25 second with a tolerance of ± 10 percent The transformer is deemed to have passed the short- circuit tests if, 1.The routine tests have been successfully repeated.
  115. 115. Transformers of category 2 and 3 2.The results of the short-circuit tests, measurements during short-circuit tests and out of tank inspection do not reveal any defects (displacements, deformations of windings, connections or supporting structures, or traces of discharge), 3. The short-circuit reactance measured after the tests differs from that measured in the original state by not more than specified percent ( 2% for category II and 1% for category III)
  116. 116. Efficiency The efficiency of transformer is the ratio of its output to input. The efficiency changes with load (both magnitude and quality of the load). The losses in transformers are a) No-load losses: includes both hysteresis loss and eddy current loss. As the core flux in a transformer remains practically constant at all loads, the core loss is also called as constant loss. The input power of a transformer under no-load, measures the core loss. b) Load losses: This loss is mainly due to holmic resistance of the transformer winding. Copper loss also includes the stray loss occurring in the mechanical structure and winding conductor due to the stray fluxes. c) Full load copper loss is measured by the short circuit test. Loading conditions of transformer: •Decided by permissible temperature rise of windings and oil. •Permissible oil temperature is 650C and hot spot temperature of the winding is 800C at rated current. •Life expectancy reduces if the transformer is overloaded for longer duration.
  117. 117. Regulation % Regulation = (V2 at No Load - V2 at Full Load/ V2 at full load)x100 % regulation of a loaded transformer at any given power factor = (R cosΦ+X sinΦ)+( X cosΦ- R sinΦ)2/200 where R= %age resistive drop, X= %age reactive drop. cos Φ power factor Note: Both SC and OC test results are used to predict the efficiency and regulation of transformer at any desired load & power factor.
  118. 118. Loading Conditions of Transformer •The loading of transformer is decided by permissible temperature rise of windings and oil. •Permissible oil temperature is 650C and hot spot temperature of the winding is 800C at rated current. •As load on the transformer varies according to the load curve, loading becomes an important operating problem. •Life expectancy reduces if the transformer is overloaded for longer duration.
  119. 119. Tap Changers • Fast growth of electrical load and distribution network voltage variation is normal. •Maintain system voltage within the specified limit for the better health of electrical equipments. • The system voltage may be adjusted by changing the tapings on the power transformer. • The variation in voltage may be brought in either by step or step less control. • Variation is generally achieved by means of tapings on the power transformer, as of the smaller currents to be dealt with, are normally located on the higher-voltage winding. •Tap Changer for Distribution Transformers.flv
  120. 120. Tap Changers Off circuit tap changer: •The economic method of changing the turns ratio of a transformer is the use of off-circuit tap changer. •It is necessary to de-energize the transformer before changing the tap. •A mechanical lock is provided to prevent unauthorized operation and inadvertent operation. •The transformers are normally provided with off-circuit taps with ± 2.5 percent and ±5 percent on the side. •The station transformers are preferably provided with OLTC with ± 10% in steps of 1.25 percent on the side.
  121. 121. Tap Changers On Load Tap Changers (OLTC): •OLTC are employed to change turns ratio of transformer to regulate system voltage while the transformer is delivering normal load. •All forms of OLTC circuit possess impedance, which is introduced to avoid short circuiting of tapping section during tap changer operation. •The OLTC can in general, be classified as resistor or reactor type. •Motor drive unit is initiated by a push button or voltage control relay, tap selector changes tap. •The diverter switch diverts the current. •The tap changers function is without interruption in load current. •Function Principle of the OLTC - YouTube (360p).mp4
  122. 122. Causes for Troubles & Failure in power Transformer 1. Failure in Magnetic circuit: Due to failure of insulation bolts used for clamping the core & yoke, circulating current flows and core is heating 2. Vibrations in core: Loose core clamping bolts between core results vibration of core leads to weaken the core insulation. 3. Excessive core heating, high magnetizing currents & high in rush current : Temperature rise of winding. 4. Failure of core bolt insulation: Moisture absorption leads to dielectric failure. 5. Over fluxing of power transformer, melting of core bolts, burning of core bolt insulation etc: Generator transformer fails.
  123. 123. Causes for Troubles & Failure in power Transformer and Preventive Action
  124. 124. Causes for Troubles & Failure in power Transformer and Preventive Action
  125. 125. Causes for Troubles & Failure in power Transformer and Preventive Action
  126. 126. Noise in the Transformer 1. Magnetostriction: Core laminations starts vibrating at harmonic frequency. (humming noise) 2. Mechanical vibrations developed by the laminations. 3. Mechanical vibrations of the tank: leakage flux pass through the tank walls produce vibration. 4. Noise by fans used for forced air cooling. 5. Noise in cooling system :oil pumps.
  127. 127. Maintenance of Transformer 1. Regular/routine inspection & minor works.  No need of opening the transformer cover.  Visual inspection • Checking oil level • Cooling system • Control circuit • Oil leakages.  BVD of oil sample and sample gas from Buchholz relay.  Cleaning the porcelain surface, tighten the bolts.  Replacement of silica gel.  Repair of external fitments and oil filtration. Note: operating personnel in power station and substation.
  128. 128. Maintenance of Transformer 2. Medium repairs (testing & reconditioning). • Dismantling of the transformer taken for inspection/repair of the core coil. Assembly. • Example: repair of conservator tank, tap changer, cooling system Note: Defects located in routine inspection & could not be corrected are taken for medium maintenance work
  129. 129. Maintenance of Transformer 3. Major repairs work • For serious internal fault / 8 to 10 years of prolonged service. • Complete dismantling of transformer for replacement of winding & insulation, tightening of core laminations. • Work is carried out by trained personnel. • Well planned. • Well equipped repair workshop.

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