A current transformer monitors a current that passes through an electrical apparatus such as a power transformer, a generator, or a circuit breaker. To ensure that a current transformer is accurately monitoring the current, the metering or the relaying accuracy of the current transformer is verified. The metering accuracy determines the ratio error and the phase angle of the current transformer through a stringent certification process established by the Institute of Electrical and Electronics Engineers (IEEE) under IEEE C57.13 (1993). In comparison, the relaying accuracy relates to measuring the secondary excitation and resistance associated with the current transformer. Generally, current transformers intended for relaying applications do not undergo the certification process under IEEE C57.13 unless the current transformer is retrofitted for a metering application.
Before an accuracy test can be performed under IEEE C57.13 (1993), the current transformer is disconnected and removed from its position within the electrical apparatus. In a traditional accuracy test, a current, referred to as a primary current, circulates through the primary winding of the current transformer and a known standard transformer of the same current ratio. The ratio and phase displacement errors of the current transformer are then determined by comparison to the known standard. This process consumes a substantial amount of time and incurs a significant cost due to removing the current transformer from its installation on or within the electrical apparatus. Frequently, it is more cost-effective to replace rather than test the existing current transformer. It is therefore desirable to have a test method that overcomes the disadvantages associated with conventional methods for determining the accuracy of a current transformer.