ABSTRACT
By far the greater part of all the publications on the subject of selective digital protection are concerned with the determination of the distance between the relay location and faults on HV overhead lines. Conditioning the signals by passing them through digital bandpass filters prior to processing is essential for protection devices which evaluate the relationship between voltage and current, e.g. distance or phase-angle relays. The neutral current is also measured in addition to the phase currents mainly for ground fault protection purposes in ungrounded systems and systems with Petersen coils. Digital filtering is unavoidable where a fault current has to be measured and evaluated by an overcurrent device immediately a fault occurs. One of the simplest and most frequently applied method of determining the apparent component of a current variable in analogue systems is to measure its value at the zero-crossing of the associated voltage.
