Out-of-step protection (ANSI 78)
In electric power transmission systems electrical stability must be maintained at all times. If system conditions arise that threaten the stability measures must be taken to avoid an escalation. One such system is the out-of-step protection. The function can be applied as out-of-step protection or can be integrated into more complex systems for supervision and load control, i.e. system integrity protection systems (SIPS).
The out-of-step function constantly evaluates the impedance trajectory of the positive sequence impedance. The response characteristic is defi ned by impedance zones in the R/X plane. Accumulators are incremented depending on the point at which the impedance trajectory enters or exits the associated impedance zone. Tripping or signaling occurs when the set accu- mulator limits are reached. The out-of-step protection provides up to four independent impedance zones which can be adjusted and tilted according to the requirements of stability in the