Overvoltage and insulation co-ordination in underground mine power systems

Abstract

In recent years mining electrical equipment on various coal mine sites in Queensland have experienced failures on 3.3 kV system insulation. The reported failures have occurred at the load side of the transfo1mer in underground substations. This raised concerns that switching surges associated with vacuum circuit breakers (VCBs) were adversely interacting with systems that used neutral earthing reactor (NER) as a means of earth fault limitation. This implied a need to study the performance of NERs in conjunction with switching ofVCBs, in order to identify the expected overvoltage levels. The series of tests carried out on 1.1 kV and 3.3 kV systems revealed the levels of switching overvoltages that should not result in equipment failure, provided that the system insulation complies with standard insulation levels recommended in the Standard. System tests were simulated using Transient Network Analyser (TNA) to give predictions of overvoltage levels for worst case conditions. The levels of harmonic voltages for a typical mine power system were examined using Electrical Power System Analysis Suite (ERA CS). As a result the possible levels of overvoltages in underground mine power systems were identified and means of overvoltage protection were recommended. The recommendation is also given for the development of overvoltage type test for mining equipment including NERs