A high-speed differential relay for generator protection

RECENT papers have shown that high-speed differential relaying is considerably complicated by the presence of the d-c transient in asymmetrical through fault currents. This is particularly true of generator differential relaying, since an asymmetrical fault near the terminals of a generator provides a more severe d-c transient than at any other point on the system. The effect of the d-c component is to produce a false differential current during a through fault because of dissimilar performance of the current transformers, even though they may nominally be duplicate. When this false differential current reaches tripping proportions with respect to the restraining current, even for as short a time as one cycle with a high-speed relay, the relay problem becomes one of properly discriminating between a false and a true differential current of tripping magnitude.