The effect of DC offset on current operated relays

When a fault occurs on a power system, one or more phases will experience DC offset. This DC component, which will decay dependent on the L/R time constant of the power system, can produce saturation in current transformers, as well as the input current transformers of the protective relays sensing the fault. In addition, when the fault current is interrupted, the resulting DC tail can maintain the current above the relay's pickup setting for a time dependent on the CT secondary circuit L/R time constant. This paper discusses this phenomenon and how it can affect the operation of two types of current operated relays.