Design and evaluation of an overcurrent relay suitable for operation with measurement current transformers

An inverse time overcurrent relay measures the current flowing into a fault and after an appropriate time trips its circuit breaker. Conventional relays are supplied from protection current transformers (CTs) rated to cope with the maximum possible fault current and worst case DC offset. This implies a physically large CT and a low burden secondary load. If the CT is inadequate or the burden excessive, saturation may occur and the current measured by the relay will be distorted. Distortion may affect the operating time and result in mal-operation of a time graded protection scheme. The paper describes the design and evaluation of an overcurrent relay suitable for use with iron-core “measurement type” CTs. This type of CT saturates when a fault occurs and this causes severe distortion of the secondary current. The relay uses an algorithm based on cross-correlation to analyze the distorted current and predict the magnitude of the primary current. A prototype relay was evaluated on an analogue test bench configured so that deep saturation of the CTs occurred during a fault