The role of global earthing systems to ensure the reliability of electrical networks

For a safe and reliable operation of distribution networks, the coordination of high and low voltage earthing systems (substations, transmission lines and cable systems, earthing systems of substations, earthing systems of customers) is very important. Because of the age and the technical condition of existing cable distribution networks (reduced insulation capacity), alternative protection concepts (e.g. SHPE short-time healthy phase earthing, SNE short-time neutral earthing) must be used to handle single earth faults to prevent double earth faults. These concepts with short and high fault currents, make the detection of single earth fault easier than in resonant grounded grids, can reduce supply interruptions and their duration, but increase the requirements on earthing systems regarding personal safety. Based on an example the impact on step and touch voltages, when changing from resonant to solid earthed medium voltage systems will be analyzed, considering the influence of global earthing systems. Based on the potential-coefficient simulation method, the earth potential rise and the resulting step and touch voltages are calculated in global earthing systems in case of substation and cable earth faults. Furthermore, the impact of the induced voltage in accompanying signal and protection lines, caused by the high fault currents, will be shown. The results of the study show that in urban areas in case of a medium voltage earth fault inside a MV/LV-substation, the normatively permissible step and touch voltage limits, caused by the fault currents, which reach up to several kA, are usually below the limits. Nevertheless, because of the high fault currents, high induced voltages are occurring, which can be dangerous for personal safety and can damage sensitive electronic equipment. In suburban and rural areas, depending on the fault location (e.g. cable faults) — even inside global earthing systems — unacceptable step and touch voltage may occur in case of single earth faults. So the paper shows that global earthing systems cannot ensure the safety of persons for any fault situation and what are the new challenges for earthing systems if high fault-current protection concepts are applied.