Digital distance relay reliability enhancement using real-time filter

In the conventional three - zone stepped directional distance scheme, distance relay is used to provide the primary as well as remote backup protection. However, the mal-operation of this relay under stressed conditions such as power swing and switching action transient instability may affect the reliability of the whole protective scheme. The voltage and current phasors needed by the distance relay for determining the impedance may be measured with integrated Phasor Measurement Unit. However, this measurement accuracy may be affected by several power disturbances such as fast and slow dc offsets decaying due to sudden current changes, inter-harmonics, etc. To avoid these effects for improving the quality of measurements and hence the distance relay reliability, this work proposes a new real-time filtering method for removing the unwanted DC offset and hence improving SDFT algorithm. To validate the present method, the performance of developed distance relay is tested using the data generated by Simulink/MATLAB simulator. The obtained simulation results are satisfactory.