The application of distribution system current transformers for high frequency transient based protection

Wound current transformers are used in virtually all protection schemes associated with a power system plant. The potential for using them as the primary transducer for monitoring high frequency currents as well as the power system frequency components provides the basis for extensive research into the use of transient based protection using current signals. This investigation has examined the response of a representative sample of 50 Hz distribution system current transformers to signals in the range of 1 kHz to 100 kHz. This will provide an insight into whether or not they can be used reliably to detect signals within this frequency band. The performances of a variety of current transformers of different types are examined. These included standard distribution system CTs, class X CTs and earth fault passage indicator CTs. The ages of these CTs varied from new units to those that had been withdrawn after thirty or more years in service. The results have shown that the distribution system current transformers have a good accuracy in the range of frequencies from 1 kHz to 100 kHz. It is therefore concluded that such current transformers can be used as an appropriate interface for transient based protection using current signals.