Transformer Stability Test Procedure

OBJECTIVE:

The Purpose of Differential Stability test is to determine the Healthiness of the Differential CT circuits which is protecting the in Zone equipment from actual fault conditions and through fault conditions.

Principle of Protection scheme and Testing:

This scheme is applied to protect the Equipment from the in zone faults and to avoid spurious operation of scheme due to out side zone faults.  This Differential Testing mainly carried out to ensure the correctness of CT selection and setting parameters during fault sensing. The Differential relay monitors the primary & secondary side CT currents magnitude and vectors and operates for the in zone fault conditions, restrains for the through fault conditions.

VERIFICATION OF DIFFERENTIAL SCHEME Stability:

To verify the Differential CT circuits the following test / methods are adopted,

1.    First of all the current generated during the Differential stability test is calculated using the % z voltage and source to feed the current will have to be chosen.

2.    Apply the 440 VAC supply from HV side of the Transformer in such way that Primary CT’s are included in the circuit.

3.    Short the 3 phase of the Transformer secondary side with sufficient current carrying cable after the secondary CT’s. So by step 2 & 3 Differential CT’s will see the load / fault currents.

4.    Differential Relay has to be set as per the required calculated setting values before starting the stability test. (ie: CT Ratio correction factor, Vector Correction factor)

5.    Now the 440 VAC supply can be applied to verify the stability condition. Make sure the safety that no temporary earthing / discharging of cables if it’s a live network.

6.    Now the secondary currents of Primary & secondary CT’s, Bias currents, Relay currents, Restrain currents, Operate currents can be noted down from the Relay measuring menu. At the stability condition the Primary & secondary currents magnitude and vector should match. No current should pass through the operating coil of the relay (Refer Stability Test Report & Drawings).

VERIFICATION OF DIFFERENTIAL SCHEME Sensitivity:

To verify the Differential CT circuits sensitivity the following test / methods are adopted,

1.    After verifying the Stability conditions, switch off the 3 phase supply. Primary “R” phase CT polarity should be reversed. Switch ON the 3 phase supply and verify the Differential relay operation “R” phase element. The magnitudes of Primary & secondary CT currents, relay currents, Restraining coil currents shall be noted down. Now at operating coil we can measure the operating current.

2.    Switch off the 3 phase supply and return back the Primary “R” phase CT connections to normal. Now reverse the “Y” phase CT connections. Switch ON the 3 phase supply and verify the Differential relay operation “Y” phase element. The magnitudes of Primary & secondary CT currents, relay currents, Restraining coil currents shall be noted down. Now at operating coil we can measure the operating current.

3.    Switch off the 3 phase supply and return back the Primary “Y” phase CT connections to normal. Now reverse the “B” phase CT connections. Switch ON the 3 phase supply and verify the Differential relay operation “B” phase element. The magnitudes of Primary & secondary CT currents, relay currents, Restraining coil currents shall be noted down. Now at operating coil we can measure the operating current.

4.    Normalize all the CT connections at primary side. Repeat the same above procedure for all the 3 phases at secondary side CT’s and ensure the Differential relay operation of each corresponding phase elements.

After completing the sensitivity test by reversing both side CT’s, all CT connections to be normalized and again Stability of the CT circuits to be confirmed before concluding the test. After the stability test it is advised not to touch or disturb the CT circuits of both the sides to ensure proper operation of the Differential scheme.

METHOD OF STATEMENT FOR TRANSFORMER REF STABILITY TEST

OBJECTIVE:

The Purpose of REF Differential Stability test is to determine the Healthiness of the REF Differential CT circuits which is protecting the in Zone equipment from actual fault conditions and through fault conditions.


Principle of Protection:

This scheme is applied to protect the Equipment from the in Zone Earth faults. REF Protection provides an extremely fast, sensitive and stable method of detecting winding earth faults. REF must remain stable under switching and through fault conditions. This is achieved with by including stabilizing resistor in series with REF current measuring input.


VERIFICATION OF REF DIFFERENTIAL SCHEME Stability:


To verify the REF Differential CT circuits the following test / methods are adopted,

1.    Using the Primary Injection Test set this scheme can be tested.
2.    Inject the current between “R” phase of secondary side CT and Neutral CT. Bypass the Transformer (Refer to the drawing).
3.    Verify the CT secondary currents of the Phase CT and Neutral CT’s as per the ratio and primary current injected.
4.    Verify NIL current at REF operating coil and as well in REF measurement menu.
   
VERIFICATION OF DIFFERENTIAL SCHEME Sensitivity:

To verify the Differential CT circuits sensitivity the following test / methods are adopted,

NOTE: Before starting the sensitivity test bypass or remove the REF relay as a safety measure. Also short the METROSIL and Stabilizing resistors to avoid damage.

1.    Now stop the primary injection current, reverse the “R” phase CT polarity. Inject the Primary current again and verify the operation of REF element. Note down the current at REF operating coil.

2.    At the same time note down the Voltage developed across the METROSIL and Stabilizing resistor. And verify there should be NO voltage across REF relay terminals.

3.    Bring back “R” phase CT connections to normal. Now reverse the Neutral CT connections, repeat the step 1 & 2.

4.    Follow the same above steps 1, 2 & 3 for the “Y” and “B” phase to ensure the sensitivity of REF scheme

After completing the sensitivity test by reversing the CT’s, all 3 phase CT connections to be normalized and again REF Stability of the CT circuits to be confirmed before concluding the test. After the stability test it is advised not to touch or disturb the CT circuits of both the sides to ensure proper operation of the Differential scheme.

EARTH ELECTRODE RESISTANCE MEASUREMENT TESTING METHODS

Objective:

After Installation, checks must be made on an earthing system to verify if there is any significant change in the resistance over a period of time or under different soil moisture conditions. Such checks will indicate if the earth electrode resistance to earth has been exceeded by changing soil conditions or aging of the system.

General Test Procedure:

It is preferable that the earth electrode to be tested is first isolated from the circuit it is protecting, so that only the earth is measured and not the complete system. When this is done, the circuits and equipment must be de-energized. If however this is not possible, the earth electrode should be duplicated so that when it is disconnected for test purposes, the other one provides the necessary circuit protection. The safety precautions essential when working near high tension systems where any unintentional Live earths may be encountered between the site earth and remote earths established for test purposes. A Live earth is one that carries current from the mains supply or could do so under fault conditions.

1.    Connect the Wires in the Potential, Current spikes after firmly sitting them in the ground at the distance of 5 & 10Mtrs from the earth electrode and connect it to the Earth Megger.

2.    Connect the Earth electrode under test to the Earth megger.

3.    Start the testing by pressing the earth megger on (3 terminal method).

4.    Check for any adverse conditions are indicated, the cause should be rectified before the test continues. If the conditions for the test are satisfactory, the display will stabilize, and the reading given may be accepted.

5.    The earth value should be less than 2 Ohms for package s/s, 11KV Ring Main s/s and Transformers.  

6.    If the High resistance indicated, the cause should be rectified. This may be caused by an open circuit, poor test lead connections, or excessive earth resistance of the soil near the electrode under test. Moistening the soil around the Potential spike, re-siting the spike in a new position or using more than one spike may solve the problem.


Measurement of earth resistance by 3 pole method









OVER CURRENT AND EARTH FAULT RELAY TESTING METHODS

DESIGNATION: 50/51N

Remove packing & Transportation protection like stoppers / Support  / rubber bands.


Pick up test :

Set TMS to 0.1 and plug setting to the desired value. Inject the current above than the plug setting value and note down the disc starting & closing currents. Starting current should be in the range of  102 % -105 % and closing current should be in the range of 105% to 120%.

If any error this should be adjusted with the spring provided at bottom of the relay disc the relay.

The same procedure is followed for earth fault pick up test also. 

PHASE OVER CURRENT TIMING TEST:

           With desired current plug setting TMS 0.1 the tripping time should be Measured for 2 Is value and 5 Is value. For normal inverse curve 2 times of fault current the tripping time should be 10 seconds at TMS 1.0 For 5 times of fault current the tripping time should be 4.4 seconds at TMS 1.0.
    
INSTATANEOUS ELEMENT CHECKING:

Set the desired instantaneous value. Inject the current and measure the pick up value of instantaneous element. Apply 2 times of pick up value and note done the reading. This should be in the range of 50 milli-seconds.

AUXILIARY TRANSFORMER TESTING METHODS

1. Insulation Resistance Test.
2. Ratio Test.
3. Vector Group Test.
4. Winding Resistance Test.
5. Magnetizing Current Test.
6. Magnetic Balance Test.



1.Insulation Resistance Test:

To check the insulation level of transformer winding & oil this test is conducted
Apply the 5 KV on the primary to earth for 15 seconds & 60 seconds values to find polarization index value
Apply 2.5KV between primary to secondary and note down the 15 seconds & 60 second value.
Apply 1 KV between secondary to earth and note down the 15 seconds & 60 seconds value.

2.Ratio Test:

To find the transformation error this test is carried out. Apply 3-phase Voltage on the primary side and measure the secondary side voltage in TAP 3 & TAP 5.Calculate the ratio error by actual and measured value.

3.Vector Group Test:

To verify the vector group as specified by the manufacturer this test is carried out.
For example: Dyn11 Transformer
To verify, superimpose the 1U and 2U terminals and measured the voltages as per below equation and justify


Equation:
1.1U 1V = 1U 2n + 1V 2n
2.1V 2W=1V 2V
3.1W 2W < 1W 2V.

4.Magnetic Balance Test:

To verify the correct identification of cores and phases and to verify mutual flux inductance, this test is carried out.
Apply the voltage on one winding and measure the voltage sharing on the other windings. By theory the adjacent placed winding should share more flux than the other. That is if we apply on 1U –1V winding, 1V-1W should share  more voltage than 1W-1U. Note done and verify the magnetic balance as below


APPLIED VOLTAGE (V)
Measured  voltage(V)
1U-1V
1V-1W
1W-1U
1U1V
1V1W
1W1U
415
__
__
__


__
415
__

__

__
__
415


__



 5.Magnetizing Current Test:

To Asses the no load losses of the transformer this test is carried out. Measure the magnetizing current of the each winding / phase by tong tester or multimeter and verify with factory report.

6. Winding Resistance Test:

To verify the resistance value as specified by the manufacturer and to asses any damage on winding this test is carried out. Apply the DC current of 10Amps through the winding with the feedback of voltage drop the kit will calculate winding resistance value. Compare and verify with factory test report.

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 reconnect 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 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 the 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 transformer is Dyn11, 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 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 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 that the power supply source.
2. Check operation of cooler fans. Verify that the fans are running and their direction of rotation is correct. Next change the position of switch to auto position. Turn the needle on the WTI dial to 55°C and verify that the fans start running due to closure of WTI contact. Repeat the test for OTI operation.
3.Check for control circuit as per 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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