Modeling of transient processes at ground faults in the electrical network with a high content of harmonics

The paper presents analytical investigations on determination of influence of higher harmonics existing in a ground fault current on the characteristics of transient processes at arcing ground faults. As initial data, results of real oscillography of ground fault currents in the operational 6 kV distribution network are used. It is shown that there are harmonics of high amplitudes in a ground fault current which are 2.6 times greater than a residual 50 Hz reactive current at a single-phase fault location. Calculations using the detailed mathematical model of the 6 kV network confirm the fact that negative influence of arcing overvoltages on equipment insulation increases due to the high content of harmonics in a fault current which are not compensated by Petersen coils and worsen conditions for successful arc quenching. It is noted that a ground fault current may cross zero many times without current extinction due to harmonic distortion. Due to high-frequency overvoltages, probability for restrikes increases. It results in expanding of a fault area in cable insulation and reducing of time for transition of single phase-to-ground fault into stable arcing with possible breakdowns of phase-to-phase insulation. To prove these statements, resistances in a fault place taking into account harmonic distortions of an arc current are calculated.