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