Overvoltages After Current Chopping in a Three-Phase Inductive Circuit with Isolated Neutral

A theoretical and experimental study of the overvoltages after current chopping in a three phase inductive circuit with isolated neutral is presented. The circuit under investigation contains a cable connection between circuit breaker and inductive load and is supplied from an infinite bus. At first the recovery voltage after chopping in the first pole to clear is treated. Then attention is paid to the resulting dc-potential on the system after interruption in the other two phases. Owing to the fact that the current in the second and the third pole to interrupt does not necessarily chop simultaneously this dc-level may be high. It is further shown that after complete interruption reignition in one phase can generate sharp voltage peaks superimposed on the recovery voltages in the other phases. High dc-levels and peak voltages make heavy demands particularly on the coils of ungrounded transformers and motors, on connecting cables and substations. Theoretical results are confirmed by experiments.