Digital Distance Relay mho Elements Using Bit-Slice Technology

Distance relays are a common method of protecting power transmission lines. This paper deals with a digital method for implementing mho elements of a distance relay. The method is based on calculation of an apparent impedance which is compared to a fixed circular characteristic. Digital mho elements have been designed and implemented using a bit-slice microprocessor and a fast array multiplier. Samples of voltage and current for a three-phase system are taken 16 times per cycle at a power line frequency of 60 Hz. The system contains a total of 12 mho elements for six phase-ground and six phase-phase loops.