Earth fault distance computation with fundamental frequency signals based on measurements in substation supply bay

This report describes three novel methods for earth fault location in unearthed, compensated and low-resistance grounded networks. In Finland, the medium voltage, 20 kV, distribution networks are either unearthed or compensated. Therefore, the last mentioned algorithm is primarily intended for the export industry of the Finnish relay manufacturers. The developed algorithms are based on the use of fundamental frequency signals, and they need only one measurement unit per primary transformer at the substation. This is due to the fact that the fault current can be determined from the negative sequence component of the measured signal in the supply bay. The scope is restricted to radially operated systems. The earth fault distance location is based on the line impedance calculation in these algorithms. For fault location in unearthed and resistance grounded networks, three different measurements are needed, which are spaced in time: pre-fault, fault and post-fault. In a compensated network, however, only two measurements are needed during an earth fault. The soundness of the algorithms was tested using simulated data. According to the results, the performance of the methods is good enough for practical use, and most likely the accuracy is comparable to transient based methods. The algorithms were tested when the faulty feeder was both loaded and unloaded, with 10 – 30 Ω fault resistances. The error in distance computation was about 12 km, depending on the loading and the fault resistance value.