Design of a protection relay for use with a measuring CT

An overcurrent relay is designed to measure the current flowing into a short-circuit fault and after an appropriate time trip its circuit breaker. Current transformers may saturate and distort the current measured by the relay. This distortion may affect the operating time and cause the maloperation of a time graded protection scheme operating with inverse overcurrent relays. The paper describes the design and evaluation of a directional overcurrent relay suitable for the protection of a distribution feeder. The relay is designed to operate with signals derived from iron-core "measuring "type" current transformers (CTs). This type of CT will saturate during a fault and the secondary current signals used to energise the relay will be severely distorted. The relay will use the information contained in the distorted current signal to predict the magnitude and phase of the primary current signal. A superposition algorithm and cross-correlation processing techniques are used to evaluate the direction to the fault and the amplitude of the fault current. These techniques were developed using MATLAB and tested using transient data generated by the EMTDC simulator. The design features of the directional and overcurrent algorithms and the results obtained during their evaluation are described in the paper.