Harmonic Current Distribution in Grounding Systems of Cabled Medium Voltage Grids during Single-Pole Ground Fault

The design of every power supply system must guarantee the safety for human life even in the event of a fault. Due to the system change in the electrical power supply, the fault current contains more and more unknown shares of current harmonics. Especially in medium voltage grids, which are operated with resonant grounding, these are determining for the level of the single-pole ground fault current for determination of permissible touch voltages and compliance with the normative requirements of the European standard EN 50522 must therefore be re-evaluated. In its first part, this concept paper presents the frequency dependent principles of earth loops formed by the grounding system. The focus here is on cabled grids and the influence of connected structures of the low voltage grid. The second part deals with the superposition of these loop currents and the resulting earth currents in case of a line to ground fault. The authors address explicitly the frequency dependence of the current distribution and describe the expected behaviour for current harmonics. The proposed approaches result from processing the state of knowledge, research work and the evaluation of several measurements. The aim is to develop an understanding of the influence of the components connected to a grounding system and to derive generally applicable principles. Therefore, the authors present the results of recent measurements in the last part of the handed paper and point out the possibilities and limits of modeling. It is shown that a dedicated treatment of harmonic currents in the case of a single-pole fault is possible with the methods described. This allows these to be neglected in the estimation of touch voltages under specified circumstances, saving costs for the assessment of grounding systems.