Sensitive relay for power auto-transformer protection based on fifth harmonic criteria

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 problems which cause a false operation for the relay. A typical ATP-EMTP model for 500/220/11 kV Autotransformer is presented in this paper to evaluate the suggested algorithm. The algorithm is based on applying the percentage differential protection, the fifth harmonic component of the primary current and the root mean square value(RMS) of the primary current. The suggested technique satisfies high degree of security and dependability.