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GE L90 Relay Current Differential Testing

One of the main element or function in the GE L90 relay is current differential. Testing of this element might be tricky. This require formula to calculate the pickup current depend on what terminal is used whether terminal 2 or 3. In this page, will be discuss what formula to use in different situation. But first, check the basic requirements of relay testing.

Basic Requirement For Relay Testing

Requirements: See the requirements page

Isolation of the equipment or circuits:

  • Complete the hazard sheet
  • Phone control room and pick up the necessary switching steps.
  • Complete all the issued switching steps as per approve operating order.
  • Do prove-test-prove and apply earths or lock and tag that issued by the controller as per approved operating order.
  • Define permit area that issued by the controller as per approve operating order.
  • Pick up the permit (Work Authority)
  • Complete the sign on/off sheet.
  • Do the isolation for relay testing as per approved ITP (Instruction Test Procedure)
  • Test the relay

Relay testing:

Download the as found relay settings. This will be use to compare the relay actual relay settings once the testing completed because, some protection elements will be turn off during testing. So, we need to compare the relay settings from the as found to make sure there are no setting changes when we leave.

Current Differential Testing Using Quick CMC Module.


Calculate the current differential pickup using this formula, where P is the pickup setting and S1 is the slope 1 or restraint 1 setting in decimal format.

For example, pickup is 0.40 pu, restraint 1 is 30% which is 0.03 in decimal format and Inom is 1 amps

Connect the test set current output to relay Phase A and connect the binary input 1 to the tripping contact of the relay then inject current value below the calculated pickup let’s say 90% of the calculated pickup which is equal to 0.5622 amps. The relay should not trip in current differential. If the relay tripped, check the relay targets make sure there’re no other elements trip. If there’re, disable them then inject current again. If the relay didn’t trip, slowly increase the current injection until the relay trip and note the pickup value. The level is 3% of reading of the maximum circuit current. Repeat the process to Phase B and C or three-phase to test the pickup.

To test the trip time of the current differential, inject more than the calculated pickup on three-phase let’s say 110% of the calculated pickup which is equal to 0.6872 then note the trip time value registered in the binary input when the relay trip. The typical trip time will be 1.0 to 1.5 power cycles or 20ms to 30ms in 50Hz power cycles.

If the relay CT rating uses 5 amps, the Inom will be 5 and test the relay using the value.

If 3-Terminal where used in the circuit, the pickup will be different. To check the number of terminals setting of the relay, in the relay display or relay software, go to System Setup -> L90 Power System. In the 3-Terminal circuit, one of the relay will be the master and the two others are slave. The formula for 3-Terminal for master relay will be

And the formula for the slave relay will be


In the example above, the calculated pickup value for master relay will be

Inject current of 0.4527 amps on Phase A then slowly increase until the relay operates and note the pickup value. Repeat the process to Phase B and C or three-phase to test the pickup. Inject current of 0.6633 amps on three-phase then note the trip time value registered in the test set.

And the pickup value for the slave relay will be

For the Ground Differential, the formula for 2-Terminals will be the formula below where PG is 87 Current Diff Gnd Pickup and SG is 87 Current Diff Gnd Restraint

The 3-Terminals formula for the master relay will be 

The 3-Terminals formula for the slave relay will be

To test the ground differential, inject current on one phase for example Phase A below the calculated pickup then slowly increase until ground differential operate and note the value. To test ground differential trip time, inject current more than the calculated pickup and note the trip time value. The typical trip time will be 1.5 to 2.5 power cycles or 30ms to 50ms in 50Hz power cycles.


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