Remanent flux in current-transformer cores

The presence of remanent flux in current-transformer cores can affect the behaviour of associated protective schemes and metering equipment. At the instant of interruption of a system fault, the core flux density in current transformers carrying the fault current may be very high, perhaps even at saturation level, and remanent flux densities up to the maximum possible for the particular core material may be left. The paper considers the subsequent operation of such transformers, and it is shown that some residual flux may remain in a core indefinitely, since the passage of normal load current may not bring about complete demagnetisation. It is concluded that the large flux variations, which are normally present in the cores of metering transformers, will cause such large amounts of any residual flux present to be eliminated that the accuracy will only be affected to a negligible degree. In protective current transformers, however, where the normal flux variations are small, any residual-flux reduction will tend to be small, and protective schemes must be designed to allow for any effects which remanent flux may cause.