A DIGITAL TRANSFORMER DIFFERENTIAL AND RESTRICTED EARTH FAULT RELAY

Many kinds of relays are used to detect faults in power transformers. Two of these relays are the restricted earth fault and transformer differential relays. The concepts of harmonic restraint and variable percentage of differential characteristic have been widely used in the conventional transformer differential relays. In the past ten years several methods have been proposed for designing digital transformer differential relays. This project investigates the feasibility of designing a digital differential and restricted earth fault relay. The differential protection segment includes the harmonic restraint and variable percentage differential characteristic features. Non-recursive digital filters are used to determine the fundamental and harmonic frequency components of the transformer currents. The filters have been designed using the least error squares approach, the Fourier series method and the frequency sampling method. The frequency responses of the designed filters have also been investigated. A digital logic implementing the restricted earth fault and transformer differential relay has been developed. The relay has been tested using simulated internal faults and magnetizing inrush conditions. Some test results are also included