Harmonic restraint is used within transformer differential relays to provide both inrush and overexcitation restraints.
Inrush restraint is required when a transformer is energized.
The transient magnetizing current to energize the transformer can be as high as 8–12 times the transformer rating.
With a high current in the primary winding and no current in the secondary windings, a high differential current will result.
The transformer differential relay sees this unbalance as a trip condition.
The magnitude of inrush current depends on the residual magnetizing flux in the transformer core, the source impedance, and the point on the voltage wave when the circuit breaker contact closes.
When the circuit breaker closes, all three phase contacts close at approximately the same time.
The three-phase voltages, however, are displaced from each other by 120องศา.
Thus, two of the phasevoltages will be near maximum while one is near 0 V.
This imbalance in voltages results in inrush currents being asymmetrical in each of the three phases.
Inrush current is not entirely a 60-Hz sinusoidal current but is composed of a significant level of even harmonics, with the most dominant being the second harmonic. For over 50 years, relay designers have used second-harmonic restraint to
prevent false differential operation on transformer energizing.
Most digital relay designers also used the second harmonic for inrush restraint. Digital relays are also designed so that the second harmonics in all three phases are combined in some manner to restrain the differential.