Parametric analysis of overvoltage phenomenon during single-phase to earth faults in medium-voltage grids

Because of the more frequent use of underground cables, the capacitive current in medium-voltage power substations is getting higher and when associated to other grid characteristics, it can be the origin of high overvoltage levels during single-phase to earth faults, which can be observed in the entire grid in the two healthy phases. This paper has the objective of demonstrating that four grid parameters have an important impact in overvoltage levels in this particular context: grounding systems, capacitive currents, short-circuit power of the upstream grid and the fault impedance. Through a hybrid steady-state and transient analysis, two parametric studies were conducted. These analyses allow to conclude that in terms of grounding system, the ones creating the higher levels of overvoltage are the ones where the grounding impedance is high, such as impedance-based grounding and compensated grounding in under-compensation mode. It is also possible to infer that the combination of high capacitive currents and low short-circuit impedance of the upstream HV grid can be the source of high overvoltage. At last, it is possible to observe that once the fault impedance is high enough it becomes the principal limiting factor for the voltage level of non-faulty phases.