Q and A: Protective relay testing

Electrical Tester – 27 July 2017

Differential protection is employed in many important parts of the power system such as generators, transformers, busbars, motors and others. It provides selective and sensitive operation for faults within the protected zone. To address the challenges associated with testing differential protection in the field, it is necessary to understand the basic concepts and how each component within the protection scheme operates. To help provide this understanding, here are answers the questions Megger’s technical support team is most frequently asked about differential protection.

Q: Why is it necessary to perform harmonic block tests on transformer differential relays?

A: Differential protection issues an instantaneous trip if the current recorded by CTs on the primary and secondary windings are not equal for each phase of the transformer. When a transformer is energized, the inrush current flowing through the primary is not reflected on the secondary winding, and this would cause the differential function to operate. To prevent this, the protection relays have filter circuits that measure the harmonic content in the waveform to differentiate inrush conditions from the internal faults. The presence of harmonics indicates an in-rush condition and hence the differential operation is blocked/restrained.

Typically, when checking the operation of transformer differential protection relays, 2nd and 5th harmonic block tests are performed. Correct functioning is verified by injecting a current waveform containing fundamental and harmonics into the relay. The relay blocks/restrains the differential operation if the 2nd and 5th harmonic values in the injected waveform are higher than the relay settings.

Megger facilitates this test by providing automated modules in the Relay Test Management Software (RTMS) used with its protection relay test systems. The “waveform selection” and “calculator” options allow the user to superimpose and inject up to four individual waveforms with different frequencies from each voltage and current channel. These facilities make it possible to test the harmonic blocking feature of transformer differential protection relays accurately and reliably.

Q: Are there any differences between generator and transformer differential protection?

A: The working principle is the same for all differential protection: under normal operating conditions, the vector sum of currents flowing through a node should be zero. That being said, the calculations associated with transformer differential protection are complicated, as the primary and secondary windings have different voltage and current magnitudes. Based on the transformer configuration, phase angle deviation between primary and secondary must also be considered. Proper care should be taken when calculating the relay compensation factors for manual testing.

The “differential screen” in Megger’s RTMS software is designed to simplify the testing of transformer differential relays. The single-phase pickup factors are calculated based on the vector group of the transformer. Users can also enter the values manually if the relay manufacturers have provided specific pickup factors.