Does the second harmonic restraint have an expiration timer for 50/51 settings in the SEPAM protection relays?
Does the SEPAM have a timer for the second harmonic restraint?
SEPAM Series 20
SEPAM Series 40
SEPAM Series 60
SEPAM Series 80
The relay will only see high levels of second harmonic restraint when a non-symmetrical waveform with an a-periodic component occurs in the power system. Due to the physics of transformer energization, such events will generally be of a short duration (e.g., around 250 milliseconds). One point to keep in mind for SEPAM is that the second harmonic restraint threshold is fixed at 17%. Once the second harmonic levels in the waveform drop below that, the protection function is no longer restrained.
Another point to remember is that if the fault current somehow continues to increase and rises above 50% of "Isc min" after the initial closing of the breaker, then the second harmonic restraint will immediately be deactivated.