Switching Overvoltages in Single Phase Ground Fault Mode in Networks 6–10 kV

Abstract

The article considers issues of switching overvoltages in mode of arc single-phase ground fault in 6-10kV networks. Single-phase ground faults account for majority of emergency outages in industrial enterprises. Number of single-phase ground faults due to switching overvoltages has been steadily increasing in recent years. Switching of electric motors and power transformers occurs in most cases with high-speed circuit breakers, in which creates the effect of current chopping, in particular with vacuum circuit breakers. Effective limitation of switching overvoltages allows to significantly reduce accidents in industrial enterprises. Issue of switching overvoltages has been studied for many years by both Russian researchers and international community. Analysis of switching overvoltages occurring in mode of arc single-phase ground fault was carried out. Oscillograms of overvoltages at switching of synchronous motor by vacuum circuit breakers in emergency-free mode and in mode of arc single-phase ground fault are given. Oscillograms of switching overvoltages were obtained during real measurements. From the analysis performed and the presented oscillograms, it can be seen that, in the single-phase ground fault mode, connecting electric motors to network can lead to a breakdown of motor windings insulation due to appearance of ultra-high switching overvoltages. Disconnection of electric motors in mode of arc single-phase ground fault must be selective. This places high demands on selectivity of single-phase ground fault protection.