Harmonic restraint is also used to prevent the transformer differential relay from operating during overexcitation events.
Overexcitation occurs when the volts per hertz (V/Hz) level rises significantly, resulting in transformer
saturation.
Transformer core flux is proportional to voltage and inversely proportional to frequency.
Overexcitation events can occur at an industrial facility during major system disturbances when industrial load is isolated from the utility system and is supplied from onsite generation.
When a transformer is saturated, excitation current, which is normally very low, increases and
unbalances the differential, causing it to operate.
Protection is required for overexcitation events, but the differential relay operation occurs so quickly that power system voltage control devices, such as generator automatic voltage regulators (AVRs), are not given enough time to operate to correct the problem.
When a transformer is overexcited, a significant amount of fifth-harmonic current is generated.
This harmonic is used to restrain the differential from operating.
Most digital relay manufacturers block relay operation when the fifth harmonic exceeds a specific value, generally around 30%.