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 . 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. What is the role of Harmonic 2 setting in Earth fault protection setting? In case of using phase CT summation for residual current measurement, there is a general agreement that the second harmonic is the preferred harmonic for use in harmonic restraint in current protection relays, especially for. Harmonic restraining in differential protection is a technique used in transformer protection to prevent false tripping during inrush or over-fluxing conditions by detecting and blocking specific harmonic currents—mainly the 2nd and 5th harmonics —commonly present during non-fault events. However, harmonics can distort measurements, leading to: False Tripping: Relays may misinterpret harmonic-rich inrush currents as faults. Testing of. Abstract—The terms “harmonic restraint” and “harmonic blocking” are sometimes used interchangeably when talking about transformer differential protection.
