THE EFFECT OF DC OFFSET ON CURRENT-OPERATED RELAYS

When a fault occurs on a power system, one or more phases will experie nce dc offset, This dc component, which will decay dependent on the L/ R time constant of the system, call produce saturation in the current transformers, as well as the input current transformers of tile protec tive relays sensing the fault. In addition, when the fault current is interrupted, the resulting de ail fan maintain the current above the r elay's pickup setting for a time dependent on the current transformer secondary circuit L/R time constant, This paper discusses this phenome non and how it can affect the operation of two types of current-operat ed relays.