20 MVA POWER TRANSFORMER TESTING METHODS | BUS BAR TESTING

1. INSULATION RESISTANCE TEST

Measure the Insulation resistance of the core against the core clamping structure. Open the link between the core and core clamping structure terminals. Connect the Megger terminals between the points of measurement.
Apply 2.5kV DC and record the value of insulation resistance after 60 seconds. After the measurement reconnects the link between the core and the core clamping structure.

1. HV winding with respect to LV connected to earth
2. LV winding with respect to HV connected to earth
3. HV & LV windings with respect to earth
4. HV, LV windings.

2. VECTOR GROUP TEST:

1. Connect 3-phase 50Hz 415V source to the HV winding terminals 1U, 1V, 1W.
2. Keep the LV winding open-circuited. 
3. Short terminals 1U and 2U.
4. Measure the voltages across 1U-1V, 1V-1W, 1W-1U, 2u-2v, 2v-2w, 2w-2u, 1V-2v, 1V-2w,1W-2v & 1W-2w

Pass Criteria:

In order to establish that the vector group of the transformer is Dyn11, the following conditions must be satisfied:

1. 1W-2w < 1W-2v
2. 1V-2w =1v-2v
3. 1U-2W = 1U-2v


3. RATIO TEST:

1. Position the OLTC on tap 1.
2. Apply three-phase 415V to HV winding using a 3-phase voltage source. Record the voltages on each phase of the HV and LV windings.
3. Calculate the voltage ratios for each phase and the average of the three ratios. Calculate the % difference with respect to the nominal ratio corresponding to the tap position.
4. Repeat the above for all the tap positions of the OLTC.

5. EXCITATION CURRENT TEST:

1. Connect the supply to the HV side, keeping the LV winding open circuited
2. Record the applied voltage and the excitation current drawn by the transformer.

6. MAGNETIC BALANCE TEST:

1. Keep the tap position in normal tap(NT).
2. Apply 415V AC across U – V phase of the HV winding using a 3-phase voltage source.
3. Record the phase-to-phase voltages across other combinations of HV windings. i.e. between
V – W and U – W.
4. Repeat the above procedure for the other two-phase combinations i.e. VW and WU.

7. WINDING RESISTANCE TEST:

 Measurement of resistance of HV winding.

1. Connect the Current source between terminals 1U and 1V to measure the resistance of U phase winding.  Keep the tap position at 1
2. Apply a current of 5 Amps. Record the value of voltage in the Voltmeter connected across the terminals. Repeat the measurements for all the tap positions.
3. Repeat the measurement in a similar fashion for 1V-1W and 1W-1U windings.
4. Calculate the resistance values at each tap position and their mean value corrected to 75°C.

 Measurement of resistance of LV winding.:

  1. Connect the Current source between terminals 2u and 2N to measure the resistance of U phase winding. 
    2. Apply a current of 5 Amps. Record the value of voltage in the Voltmeter connected across the terminals 2u and 2N.
    3. Repeat the measurements for V-phase and W-phase windings.
    4. Calculate the resistance values of U phase; V phase and W phase windings and their mean value corrected to 75°C.

NOTE:

R75  =  Rt x [ (235 + 75) / (235 + t°) ]

Rt     =  Resistance Measured at t°C.

8. TESTING OF OPERATION OF COOLER FANS:

1. Check the power supply source.
2. Check the operation of cooler fans. Verify that the fans are running and their direction of rotation is correct. Next change the position of the switch to auto position. Turn the needle on the WTI dial to 55°C and verify that the fans start running due to the closure of the WTI contact. Repeat the test for OTI operation.
3. Check for the control circuit as per the scheme.

9. FUNCTIONAL TEST ON BUCHHOLZ RELAY:

By operating the micro switch manually

10. FUNCTIONAL CHECKS OF PROTECTIVE RELAYS:

  1. Check the alarm and trip contact of the following devices.
  • Oil thermometer
  • Winding temp. indicator
  • Gas relay for transformer

2.    Max and min of Oil level gauge for transformer

3.    Trip contact of Protective relay for OLTC & pressure relief device

Bus Bar Testing

The following test to be conducted on the bus bar.

1. Contact Resistance test
2. Insulation Resistance check  before H.V test
3. H.V test
4. Insulation Resistance check  after  H.V test

1. Contact Resistance test:

Contact Resistance is measured by injecting the DC current through the busbar. With the feedback of voltage drop, the resistance value will have arrived in the micro ohmmeter.


2. Insulation Resistance checks before H.V test:

Insulation values were measured between phase to earth and between phases with the help of 5 KV megger for 60 seconds.

3. HV for  BUS BAR:

The high voltage test is conducted to find the healthiness of the dielectric material supporting the bus bar. This is achieved by applying 24 KV AC between phase – Earth (remaining phases are shorted and earthed) for 1 minute. With the help of an HV test kit.

The leakage current in mili amps will be measured and recorded for analyzing purposes the insulation should withstand this test for the specified period.

H.V Testing for Bus bar

4. Insulation Resistance checks after HV test:

Insulation value measured between phase to earth and between phases with the help of 5 KV megger for 60 seconds.After the HV test finds out any detonation of insulation material after the HV test voltage stress. And to be compared with the IR values taken before the  HV test.

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