CURRENT TRANSFORMER TESTING METHODS |20 MVA POWER TRANSFORMER TESTING METHODS | Current Transformer Testing at Substations

The following test is to be conducted for a current transformer.

1. Insulation Resistance Test.
2. Winding Resistance Test.
3. Polarity Test.
4. Knee point Test.
5. Ratio Test.

1) Insulation Resistance Test:

To know the insulation value of CT windings. We will be applying primarily to Earth 5kv for 60 seconds, primary to secondary 2.5 kV for 60 seconds & secondary to Earth 1000v for 60 seconds.

2) Winding Resistance Test:

To verify the winding resistance as specified by the manufacturer and to check for any damage in the CT winding resistance arrived by injecting DC current through the winding and measuring the voltage drop compare the reading with a factory test report.

3) Polarity Test :  

This test is necessary to see the relative polarity of the primary and secondary terminals when terminals are not marked or to establish the correctness of the marking is already marked. This test is carried out with help of a 9v (or) 12v DC battery. Apply the DC source between primary Terminals P1 (+) P2 (-) and measure the secondary side using a Null deflection analog ammeter and check the correct deflection of the meter & compare with the polarity mark.

4) knee point voltage:

The saturation point of the CT core is measured by applying voltage in the secondary by keeping the primary open at the point where the exciting current increase by 50% for a 10% increase in secondary voltage is referred to as the saturation region.

5) Ratio Test:

To verify the ratio error of the CT this test is performed with help of a primary injection kit. Inject the current through P1- P2 (primary of CT Terminals) and measure the corresponding secondary side output current and calculate the ratio error from actual and measured values.

20 MVA POWER TRANSFORMER TESTING METHODS

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 the 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

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