Grounding measurements in urban areas - comparison of low and high voltage measurements in common grounding systems

Common grounding systems (global earthing systems) are not very clearly defined in standards. One definition of a common grounding system is that no dangerous touch voltages caused by ground fault currents occur. This means that a common grounding system does not depend only on the grounding system itself but also on the type and operation of the electrical grid and the protection schemes. In a resonant grounded network only low ground fault currents appear because of the compensation, therefore it is difficult to detect and clear single ground faults if desired or necessary. To provide a reliable localization of a ground fault a higher current can be injected intentionally but it must be proved that no dangerous touch and step voltages occur. To prove this in a typical urban grid, different measurements were carried out. Measurements at low and high current levels and an additional simulation are compared and discussed.

I. Nomenclature

CGS common grounding system (“global earthing system”) EPR ground potential rise (voltage to remote ground) Transformer's impedance Petersen (arc suppression) coil's impedance Transformer,s short circuit power Impedance of MV cable Zero sequence impedance Fault current at MV tests Injected current at LV tests Currents in MV cable-shields Currents in the PEN conductor of the LV grid Current control resistor in feeding substation Earthing resistance of local grounding system Earthing resistance of buried ground wires