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Divergent Engineering Monitors the Earth Fault of Motor Feeders via Protection Relays

Fault statistics suggest that Earth faults are the predominant fault type for motors. Without proper fault protection devices, such as Protection Relays, the consequences can be severe. Earth faults on motor feeders can cause site wide black outs affecting production. The team at Divergent Engineering was recently engaged to investigate some 11kV motors that had been rewound and were running hotter than normal. Our team implemented the IDMT curve in each relay for monitoring and tripping.

What is a Protection Relay?

Protection Relays monitor for parameters, such as electric currents and voltages, and respond for abnormalities such as electric faults. The selection of the appropriate relay depends on motor feeder specifications, such as voltage level and expected fault current. These relays offer features like current and voltage monitoring, as well as trip knowledge, ensuring comprehensive protection.

How do we implement an IDMT Curve in a Protection Relay?

IDMT (inverse definite minimum time) curves are used in protection devices to offer time and current based selectivity. In an IDMT curve, the relay's operating time is inversely proportional to the magnitude of the fault current. For a low amount of fault current, the relay will take a longer time to operate, while for high fault current, the relay trips (opens the circuit) faster.

To implement the IDMT curve, you must first determine the IDMT curve that best suits your application. Common IDMT curves include standard curves such as IEEE and IEC, each with different levels of inverse-ness. Configuring the protection relay to implement the IDMT curve involves setting the curve's slope, time multiplier and pickup current.

Challenges that Divergent Engineering Needed to Overcome

Whilst investigating the 11kV motors, Divergent engineering had to overcome the following challenges:

  • Motor protection relays were configured to monitor for earth’s faults by ‘residual’ configuration (the sum of the phase current enter the earth fault current input of the protection relay)

  • The protection relay installed (GE Multilin 369) only had one stage definite time earth fault element

  • During the starting of the motor, the phase currents are not balanced. This unbalance was incorrectly detected as an earth fault current by the relay due to ‘residual’ configuration.

  • A core balance CT is used for motor earth fault detection, which is less susceptible to the unbalanced starting currents. However, the client did not prefer installing core balance CT on the existing motor feeders.

  • To prevent the nuisance tripping on earth fault during motor startup, in the existing configuration, the time delay of the motor earth fault protection had to be set to 0.5 s, which encroached into the upstream protection device.

  • Upstream protection device setting could not be changed to achieve grading.

Our Solutions

To overcome the challenges listed above Divergent Engineering had to apply the following solutions:

  • Implement IDMT protection using two separate protection relays (SEL 751) that were decommissioned from the same site to offer a more cost-effective solution and meet ‘green’ targets in conjunction to the IDMT based earth fault protection based on the residual currents on six 11 kV motors.

  • With IDMT protection, the relay can be made immune (by selecting appropriate characteristic type) to residual currents because of unbalanced starting phase currents still offering quicker fault clearing at higher currents and maintaining 100% grading with the upstream protection device. The protection sensitivity is also increased.

  • Limiting the earth fault trip time to a maximum to offer maximum possible protection without nuisance tripping with the available installation configuration.

  • Configuring the relay logic such that each relay will trip only the respective motor feeder that experiences the earth fault, and the heathy motors remain unaffected.

  • The indication is configured to reflect the motor that is tripped, enabling operators to identify the faulty motor.



Regular testing and maintenance of the protection relay is crucial. The protection relay needs to be periodically tested to ensure it is functioning correctly. Calibration and functional testing should be part of the maintenance routine.

Proper protection relay configuration enhances the safety and reliability of your electrical system. It helps prevent damage to equipment, reduces downtime, and, most importantly, ensures the safety of personnel working with or around the equipment.

Contact Divergent Engineering to explore our capabilities with IEC 61850 GOOSE messaging as well as other ways we can help you with protection relays and more!


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