Influence of harmonic voltages on single line to ground faults in distribution networks with isolated neutral or resonant earthing

摘要

Due to the relatively high content of the 5^th and 7^th harmonic that is accepted in the system voltage of MV distribution networks, the single line to ground fault (SLG) current in large distribution networks may be dominated by the harmonic SLG fault current contribution. The harmonic SLG fault currents can cause dangerous touch and step voltages if the impedance to earth is not sufficient low. The treatment of the neutral point plays an essential role, whether the distribution network consists of pure overhead lines (OH), pure underground cables (UC) or is a mixed system. The specific treatment of the neutral in distribution networks, isolated neutral and resonant earthing (RE), can for some configurations mitigate the harmonic SLG fault current content. This paper will therefore investigate the harmonic SLG fault current content with different treatment of the neutral point in OH and UC systems. The frequency dependency of the OH and UC parameters and of the soil resistivity in the earth return path are taken into account in the calculations, and the results are compared with measurements from a well defined distribution network to show and explain the influence of harmonic SLG fault currents. The measurements are carried out on a 10kV distribution network mainly supplying heavy fishing industry in Hirtshals, Denmark.