Simple fault path indication techniques for earth faults

Fault indication is the key element in any fault management system. In case of short circuits, fault indication is straightforward, but especially in ungrounded and compensated neutral networks, the single phase fault currents are typically so small that simple fault current threshold detection hardly gives any satisfactory results. The problem can be solved by combining neutral voltage measurement to the sum current measurement using directional relay characteristics. However, the requirement of both current and voltage measurements makes the solution cost prohibitive especially when retrofitting in the existing secondary substations. Hence, there is the need for a simple but reliable earth fault indicator which is based on current measurements solely. This paper takes the above mentioned challenge and compares a number of possible fault indication solutions which are using local current measurements at the secondary substations only. The methods presented use either the sum current measurement directly, or are based on the comparison of the measured phase currents using symmetrical components.