Reliability Analysis Of Harmonic Based Single Phase To Ground Fault Protection In Non-solidly Earthed Network

Single phase to ground fault protection based on harmonic is one typical technique among various methods developed for earth fault protection in non-solidly earthed network. It is now applied abroad in some areas due to its advantages, as it has no special requirement in hardware and can be applied to resonant grounded system. Nevertheless the practical field performances of the technique are not satisfactory. This paper has an analysis in theory of those key factors directly affecting its reliability, in order to find out the main reasons for its low reliability and to provide reference to scholars or engineers who research on this technique. The reliability of harmonic based earth fault protection depends mainly on the magnitude of harmonic current at fault point, so its reliability can be judged by distribution characteristics from analysis of harmonic voltage previous to fault and harmonic current of fault point along feeders. This paper classifies those reliability-affecting factors into three main aspects: harmonic source from system side, from load side and from fault point. This paper first discusses the three factors respectively, and then takes all these factors into considerations. Provided harmonic source only exists in system side. Harmonic voltage and input impedance of fault point both become smaller as distance from fault point to bus gets longer, so the magnitude of faulty harmonic current changes little with fault point positions. Harmonic current depends mainly on the magnitude of harmonic source. The maxim magnitude of harmonic current at fault point is much smaller than that of capacitive current from distribution network, so, in this case, the reliability of harmonic based earth fault protection is low.Provided harmonic source only exists in load side. In this case, the maxim magnitude of harmonic current at fault point is also much lower than that of capacitive current from distribution network. But there is a strong relation between reliability and fault point position: to feeders with harmonic source, much higher the reliability is as much closer to the harmonic source the fault point is, while the reliability decreases rapidly as the fault point is close to bus; to feeders without harmonic source, the reliability is nearly the same as it on the bus, so the reliability is low.Provided harmonic source only exists at fault point. In case of smooth burning, magnitude of harmonic voltage is large, but harmonic current is very small. In case of fault point intermittent burning, transient process is obvious. As an output of Fourier transformation, the magnitude of harmonic current is large, so the reliability is high compared with that of smooth burning.In fact, the magnitudes of harmonic current in system side, load side and smooth burning of fault point are all very small and change with fault point locations in a considerable range, therefore the reliability is very low. In case of fault point intermittent burning, the reliability is high, but it is essentially a particular applicationof transient based earth fault protection, and the reliability is lower than that oftraditional transient based earth fault protection.Relevant conclusions are verified by simulation results and field data.