Testing Of Transformers

PRE COMMISSIONING TESTS ON EQUIPMENT AT 33/11 KV SUB STATIONS TESTS ON TRANSFORMERS

1. IR Values This is measured to measure the Insulation Resistance of the whole transformer. a) For 33/11 KV Power Transformer 2500 V megger is to be need. b) Power Transformer neutral Earthing is to be disconnected. c) Line terminal of the megger is to be connected to one of the HV Bushings of Power transformer and Earth terminal of megger is to be connected to Power transformer Body Earth Point. IR Values are to be read on the megger by meggering the Power transformer i) The above Value is to be noted as HV to Body or Earth. ii) Then IR Value between LV terminal and body of Power transformer is to be measured & noted. ii) IR Values between HV & LV terminals are to be measured & noted. iv) The temperature of transformer oil at which the IR Values are measured is also to be noted. v) Particulars of megger is also to be noted. The maximum value shall be 60MΩ at 600C temperature for transformer which are in service. For new transformer the value obtained shall be tallied with manufactures test results. Example: - HV – E ---- 5000 Mega Ohm OR INFINITE LV – E ---- 5000 Mega Ohm OR INFINITE HV – LV ---- 5000 Mega Ohm OR INFINITE

2. Continuity test :Checking of continuity of HV windings and LV windings with megger or multimeter. To know the continuity of windings of the transformer. Example: - On HV side – RY-YB-BR- Zero Megaohm or continuity On LV side – ry-yb-br- Zero Megaohm or continuity

3. D.C. Resistance or Winding Resistance The Resistance of HV winding LV winding between their terminals are to be measured with precision milliohm meter/ micro ohm meter OR Wheatstone Bridge or DC resistance meter. To know the resistance of windings. Example :- For 100 KVA 11/0.4 KV trf. HV side in ohms RY= 120 to 160 YB= 120 to 160 BR=120 to 160 LV side in milliohms ry = 125 to 180 yb = 125 to 180 br =125 to 180 rn = 60 to 90 yn= 60 to 90 bn =60 to 90 Similarly for 5 MVA power trf. HV side in ohms RY= 1 to 2 YB=1 to 2 LV side in milliohms ry = 30 to 100 yb =30 to 100 rn = 15 to 50 yn= 15 to 50 -1-

BR=1 to 2 br =30 to 100 bn =15 to 50

The values shall be compared with original test results which varies with the transformer ratings.

4. Turns Ratio Test With turns Ratio meter, turns Ratio between HV & LV windings at various taps to be measured & recorded. At normal tap for 33/11 KV Delta/Star transformer the turns turns ratio is 5.196. At other taps values will be as per the percentage raise or lower at the respective tap positions. In case of Delta/Star transformers of 33/11 KV rating the ratio measure between RY-rn, YB-yn and BR-bn. Being Delta/Star transformers the voltage ratio between HV winding and LV winding in each phase limb at normal tap is 33 KV OR 33 √3 = 5.196 11 KV / √3 11 At higher taps (i-e high voltage steps) less number of turns are in circuit than normal. Hence ratio values increase by a value equal to. 5.196 + {5.196 x (no. of steps above normal) x (% rise per each tap)} 100 Similarly for lower taps than normal the ratio is equal to 5.196 - {5.196 x (no. of steps above normal) x (% rise per each tap)} 100

5.Voltage Ratio Test :When “Turns Ratio meter” is not available, Voltage Ratio Test is done at Various tap position by applying 3 phase LT (415V) supply on 33 KV side of Power transformer. At Various taps applied voltage and Resultant voltages LV side between various Phases and phases& neutral measured with precision voltmeter & noted. Example :- For any 33/11 KV power trf. Tap position VRY VYB VBR Vry Vyb Vbr Vrn Vyn Vbn 420 413 419 131 130 131 75 74 75 1 2(normal) 418 412 418 132 131 133 76 75 75 . . This test is done to check both the transformer voltage ratio and tap changer.

6. Short Circuit Test The four terminals on LV side of Power transformer are shorted with 50 sq. mm. copper cable. Three phase LT supply is applied on HV side of power transformer at normal tap and currents measured in all the phases on HV side and phases & neutral on LV side values noted. In transformer factory the above test is done to measure full load transformer loss. In this case the the four terminals on LV side of Power transformer are shorted with 50 sq. mm. copper cable. Three phase LT supply is applied on HV side of power transformer at normal tap with rated current on HV side. Measure the loss in all the three phases with the help of 3 watt meter method. Total no load loss or iron loss of the trf = W1 + W2 W3 The loss varies from trf to trf. -2-

Example :- For any 33/11 KV power trf. Short circuit current = Rated primary current of the trf X applied voltage X100 Rated primary voltage of the trf percentage Imp of trf The Resultant HV & LV currents are as follows:HV side current in all Phases = Rated HV current of PTR X LT Voltage applied on 33 KV side of PTR X100 Rated primary voltage i-e 33000 X Percentage Impedance Volts LV side phase Current in all phases = Rated LV current of PTR XLT Voltage applied on 33 KV side of PTR X 100 Rated primary voltage i-e 33000 X Percentage Impedance Volts LV side neutral current shall be zero or less than 1.0 amp. For example in a 1.6 MVA trf. the results are:VRY VYB VBR IR IY IB Ir Iy Ib In 417

406

412

5.83

5.75

5.50

17.61

17.05

17.50

0.45

Short circuit current = 28 x 417 x100 = 5.75 A similarly for LT side On HT side 33000 x 6.25

7. Open Circuit Test Three phase LT Voltage of 415 V applied on HV side of Power transformer keeping LT open. Measure the loss in all the three phases with the help of 3 watt meter method. Total no load loss or iron loss of the trf = W1 + W2 +W3 The loss varies from trf to trf. Tap position VRY VYB VBR Vry Vyb Vbr Vrn Vyn Vbn 2(normal) 418 412 418 132 131 133 76 75 75 For a new 63 KVA trf. no load loss <=160watt For a new 63 KVA trf. full load loss <=1235watt

8. Magnetic Balance Test 415V, Two phase supply is to be applied to any two phases terminals on HV side of Power transformer and voltages in other two phase combination are to be measured with LT open. Sum of the Resultant two values shall be equal to the voltage applied. RY YB BR a) V (415 V) V1 V2 b V1 V (415 V) V2 ) c) V1 V2 V (415 V) Example :- For any 33/11 KV power trf. Given VRY VYB VBR Vry (RY) 407 311 99 138 (YB) 189 390 198 111 (BR) 106 302 404 39

Result V= V1+ V2 V= V1+ V2 V= V1+ V2 Vyb 41 112 134

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Vbr 94 02 96

Vrn 76 35 19

Vyn 59 74 56

Vbn 17 37 76

9. Vector Group Test Connect HV side ‘R’ Phase terminal to LV side ‘r Phase terminal of Power transformer. Apply 3Phase 415 V supply to HV side of Power transformer. Measure Voltages between various terminals as follows. R,r n

y

b B

Y

Voltages between Rn; Yn; RY Voltages between Yb; Yy; Voltages between Bb; By; VRY VRn VYn VYb 414 78 334 300

VRY = V Rn + VYn VYb = VYy VY b > VBb VYy 302

VBb 293

For YY0 vector group ry=yb=br and Yy=Bb

10. BDV Test of Oil Oil samples from top oil & bottom oil of Power transformer main tank as well as OLTC tank are to be collected and tested. Across 2.5 mm gap test Kit, The oil shall stand for 60 KV For/ minute. And 4 mm gap for distribution trf. oil. Power trf. oil is called as EHV GRADE POWER TRF. OIL OR ASCAREL OIL. Note :- To improve the BDV value the oil is filtered and to remove moisture from winding it is kept in heat chamber.

11. Crickel Test of Oil This test is performed to test the presence of moisture or water content in the trf oil. In this method take a one side closed steel pipe then heat the closed side of the pipe. Put the closed side of the pipe inside the oil and hear the sound from the other open side. You will hear hissing sound if there is no moisture inside the oil and you will hear boiling side if there is moisture.

12. Magnetization Current Three phase LT Voltage of 415 V applied on HV side of Power transformer keeping LT open .Measure the currents of HT side in three phases with milli ammeter. 1 to 2 percent of rated full load Applied current of PTR X Voltage 33000 Example: - let for 1.6 MVA trf. full load current is 28/84 A. VRY VYB VBR Vry Vyb Vbr Vrn 401 385 391 128 126 131 75 The value shall be =

Magnetising current

IR = 0.0033 A or IY = 0.0033 A or IB = 0.0033 A or

3.39 A 3.39 A 3.39 A

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Vyn 72

Vbn 73

TESTS ON TRANSFORMERS AT FACTORY In transformer factory some more tests are conducted to know the healthy condition of a transformer they are as follows:1) High voltage test or source voltage test:This test is done to check the insulation level between HV to LV, HV to Earth, LV to earth .This test is done through single phase high voltage testing set. (a)For distribution trf. HV winding testing:11/0.4 KV distribution trf. for 11 KV side 28 KV voltage is applied on HT side for 1 minute by shorting all HV windings and then connect it to the LV winding of testing set lead. Again short all LV windings with transformer tank then connect it to the LV winding of testing set lead. For a repair transformer 21 KV is applied on HV side i-e 75% of 28 KV. (b)For a new trf. LV winding testing:Similarly same procedure is applied but 2.5 KV is applied on LV side for mew trf. and 1.87 KV for repair trf. i-e 75% of 2.5 KV. (c)For power trf. HV winding testing:33/ 11 KV power trf. for 33 KV side 68 KV voltage is applied on HT side to one phase PT and shorting all three phases for 1 minute by shorting all LV windings and then connect it to earth. For a repair transformer 21 KV is applied on HV side i-e 75% of 51 KV. (d)For power trf. LV winding testing:28 KV is applied on LV side for new trf. and KV for repair trf. i-e 75% of 21 KV keeping HT open. 2) Double voltage and double frequency test

or induced over

voltage test:This test is done to check the insulation strength between inter turn of coils or windings. In this test double the rate of voltage and double rate of frequency is applied to the secondary side of the trf. for one minute keeping HV open. For a 11/0.4 KV distribution trf. 866 volt and 100 HZ is applied on LV side of the trf. for 1 minute keeping HV open.. For a 33/11 KV power trf. 22 KV and 100 HZ is applied on LV side of all the three phases of the trf. through a auxiliary transformer for 1 minute keeping HV open.. Auxiliary trf. is a 11/0.4 KV trf. to whom 800 volt is applied on LT side so that 22 KV will come on HT side that 22 KV is applied to the LT side of power trf. and 100 HZ through a synchronous motor.

12. Phase sequence test :This test is done to check the proper phase sequence of trf. n with the help of phase sequence indicator.

i-e R-Y-B and r-y-b-

PRE COMMISSIONING CHECKS ON POWER TRANSFORMERS -5-

Check that 1) Top & Bottom valves of radiators are in open position 2) All drain valves are in closed position with dummy plates in position duly fitted with bolts & nuts 3) All filler valves in closed position with dummy plates in position duly fitted with bolts & nuts 4) No oil leakages from radiators, valves, dummies, top cover, inspection covers, oil level gauges etc. 5) Silica gel breather is in position with silica gel filled & oil in oil cup 6) The air path of silica gel breather is free with out any seals to holes of the oil cup 7) Equalizing valve of the interconnecting pipe of vent pipe to conservator tank is in open position 8) Top oil filling point of conservator tank is in closed position with cover duly bolted. 9) The valves on either side of Bucholtz relay are in open position. 10) Thermometer pockets on main tank top cover are having oil inside & Thermometer sensing bulbs are in position duly nuts fixed properly. 11) The double earthing of neutral of Power transformer is done as per standards. 12) The double earthing of body of Power transformer is done as per standards. 13) The oil levels in conservator tank, OLTC Conservator tank. 14) The operation of main Bucholtz relay & OLTC Bucholtz relay ( Trips &alarms to be checked) 15) Operation of Transformer alarms. 16) The operation of PRV to trip the breakers or operation of alarms 17) The vent pipe diaphragm is intact. 18) The lock pieces are provided, duly welded to base channels, for the support wheels of transformers (If wheels are provided) 19) Tightness of all bolts, nuts, clamps, jumpers etc. 20) Testing of earth resistance by earth megger Generally earth resistance of a s/s is measured with the help of earth megger or earth tester. A earth megger is like a insulation megger. It has five point terminals i-e C1,P1,G,P2,C2 and three earth spikes with wires. First put the first spike inside the ground then connect the spike wire to the point C1 making C1 and P1 short in the megger. Again put another spike inside the ground at a distance of 12.5 mt from the first spike in a straight line and connect the wire to P2 similarly put another spike at a distance of 12.5 mt from the second spike in a straight line and connect it to point C2 in the megger. The move the handle of the megger to measure the earth resistance it should be in-between 1 to 3. (<1 is good) 21)Before charging the “PTR” air is to be released from air releasing points of main tank cover of PTR, OLTC top cover, Bucholtz relays, radiators, bushings, (All releasing bolts, screws, valves shall be kept in position fully tightened after releasing air from respective points).

Tests on Breakers A)

I) Status of Breaker “Open” Measure IR values between “IN” & “Out” terminals Each limb of breaker shall be more than 3000 MΩ II) Status of Breaker “Close” Measure resistant of each limb by connecting “IN” & “Out” terminals to a precision Micro ohm meter it should be zero.

The values shall tally with manufacturer’s test report. -6-

III) Status of Breaker “Close” Measure IR values between phase & Body ground terminal of breaker for all limbs the values shall be more than 3000 MΩ B) Opening time & closing time tests to be done on all limbs. The values shall tally with test results of the Manufacturer. C) Test mechanical ON and OFF of VCB and indication of VCB and breaker. D) Similarly electrical ON and OFF of VCB and indication of VCB and breaker on both remote and local. E) As follows: a) IR values of current Transformers are to be measured. b) Polarity check on current transformers is to be made. c) Primary injection test on current transformer is to be done to check operation of relays functioning of meters and CT accuracy. Let CT ratio is 400-200-100/1-1 A then test the ct in different current ratings in all the three adopted ratios like 400/1 and 200/1 or 100/1 A by injecting currents like 25,50,100,150,200,250,300,400 etc. testing is done at different PSM. d) Secondary Injection test on relays is to be done. e) DC tests on breakers panel are to be done. f) Calibration of meters is to be done. g) D.C. interlock are to be tested. h) Check that all Jumpers, clamps etc. are in to CT i) Clean all the bushings. j) Ensure no leakages from the CTs. k) check proper earthing of CT’s, Panel and breaker with flat separate earth. l) before wiring study carefully the Ckt. Diagram of VCB. Tests on Potential Transformers A) IR value are to be checked with megger. B) Ratio test is to be done. C) Polarity check is to be done. D) Check that there are no oil leakages E) Clamps & Jumpers properly tightened. Tests on CURRENT Transformers A) IR value are to be checked with megger. B) Ratio test is to be done through primary injection CT injection. C) Polarity check is to be done. D) Check that there are no oil leakages E) Clamps & Jumpers properly tightened Tests on protective relays A) Current injection of Relay through secondary injection kit. B) tripping time test through secondary injection kit..

Tests on Lightening Arrestors -7-

a) IR value are to be checked. b) Jumper connection to be checked Other tests in a Sub Station a) b) c) d)

Battery charger & Battery to be checked for proper operation Earth Resistance is to be measured & noted. Meggering of Bus Bars is to be done. Check the entire earthing system in the Sub Station is as per standards.

FUSE RATING CALCULATION OF DISTRIBUTION AND POWER TRF. For 100 KVA 11/0.4 KVA distribution trf. H.V. side line current =

Rating of trf. in KVA √3 X Voltage on HV side in KV = 100/1.73X11 =5.24 A

L.V. side line current =

Rating of trf. in KVA √3 X Voltage on LV side in KV = 100/1.73X0.4 =133.34 A

Or H.V. side line current =KVA/20 =100/20=5 A L.V. side line current =KVA + 1/3 rd of KVA= 100 + 33.33=133.33 A For 1000 KVA 33/11 KV power trf. H.V. side line current =

Rating of trf. in KVA √3 X Voltage on HV side in KV L.V. side line current = Rating of trf. in KVA √3 X Voltage on LV side in KV L.V. side line current =KVA/20 =1000/20=50A H.V. side line current = L.V. side line current /3=17 A

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