Transformer Differential Protection Relay (87T) Test Procedure

This article is about to guide the responsible persons (Electrical Engineer) in conducting Transformer Differential Protection relay (87T) commissioning and testing procedures.

Testing transformer differential protection relays (87T) is critical to ensure the reliable operation of transformers and to safeguard against potential faults or failures.

Test Equipment: Secondary Injection test kit.

Safety Precautions: The following Safety precautions shall be taken in consideration prior, during and after conducting the test measurements.

• Safety tagging shall be implemented.
• Isolate the Area by Safety Warning Tape.
• Keep a Safe Distance from the device being tested.
• Wear Appropriate Personal Protective Equipment(PPE) Prior to starting any testing activity.
• Implementation of Proper Grounding.

Transformer Differential Protection Relay Test Procedure

1. Check Measurements & Final settings applied.

Ideal Setup on Directionality verification during Checking of Relay final setting for POWER TRANSFORMER.

2. Pickup/Drop-off Differential Current & Harmonics.

5. Operating time test for Diff. Current
   
   • 1. Measure and Record the relay’s Pick up Diff Current and contact timing for L1 to L2(Phase R), L2 to L3(Phase Y), L3 to L1(Phase B), by gradually increasing the injected current until such time the pick-up Diff current is reached. Repeat step for different curves (NI, EI, LTI, DT,..etc) as per requirement or as indicated in the Test form.
     
   • 2. Measure and Record the relay’s Drop off Diff Current and contact timing for
     L1 to L2(Phase R), L2 to L3(Phase Y), L3 to L1(Phase B),by gradually decreasing the injected current until such time the drop-off Diff current is reached. Repeat step for different curves (NI, EI, LTI, DT,..etc) as per requirement or as indicated in the Test form.
   • 3. Calculate the Reset Ratio with the formula: Reset Ratio = Idrop-off/Ipick-up x100%
6. Check that the differential element blocking with communication failure.
7. All B/I and B/O /LED configuration to be checked as per approved scheme.

9. If applicable, Also Perform the Capacitive charging current test with the following procedures:

9.1 Activate the VTS and set value for line susceptance.
9.2 Applied voltage at the local relay without current and read from the relays the differential current and base current.
9.3 Repeat the above steps with change the values of the voltage and susceptance.

10. If applicable, Also Perform Current transformer supervision test with the following procedures:
    • 10.1 Simulate CTS state at local relay by inject –ve sequence current.
    • 10.2 Increase currents gradually till reach the setting.
    • 10.3 Check the block state of the relay and measure the block time.
    • 10.4 Check the restrain state of the relay.
    • 10.5 Check the alarms appear at the relay at local & remote ends.
    • 10.6 Check the value of the relay operates (Is1CTS).
    • 10.7 Simulate CTS, then decrease currents gradually till the relay reset CTS state
    • 10.8 Check the value of relay operates.
    • 10.9 Repeat the above steps for remote end.

11. Insert test plug to block the relay in order to prevent wrong operation.
12. Check inter-trip scheme by Closing local CB then send inter-trip, remote CB will trip. During this, measure send voltage in local substation and measure receive voltage in remote substation and repeat for remote substation.
13. Check operating time (loop time).