Multi-function digital relay for power Autotransformer protection

This paper introduces a multi-function digital relay to provide a complete protection for the three phase power autotransformers against different types of abnormalities. The conventional methods of differential protection have many drawbacks such as Current Transformer (CT) mismatch, vector group and CT saturation problems which cause a false operation for the relay. A typical ATP-EMTP model for 500/220 KV Autotransformer is presented in this paper to evaluate the suggested algorithm. The algorithm is based on calculating the negative sequence differential current, the phase difference between the primary and secondary currents and the fundamental frequency for the primary and secondary currents. The suggested technique satisfies high degree of reliability, security and dependability.