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.
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.
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