Underground » Detection and Prevention of Fires and Explosions
In recent years mining electrical equipment on various coal mine sites in Qld have experienced failures of 3.3kV system insulation. The reported failures have occurred at the load side of the transformer in underground substations. It was also suspected that switching surges associated with vacuum circuit breakers (VCB) were adversely interacting with systems that used neutral earthing reactors (NER) as a means of earth fault current limitation.
With the above background Safety in Mines and Testing Research Station (SIMTARS) and Queensland University of Technology (QUT) approached ACARP for a research grant to study the performance of NERs in conjunction with switching of VCBs.
In this report we describe our investigations into the causes of overvoltages in underground mine power distribution systems. We investigated the effects of NERs on transient overvoltages resulting from the operation of VCBs by conducting a series of full scale surface tests on both 1.1kV and 3.3kV distribution systems and low voltage simulations using transient network analyser (TNA) components.
The results from the 1.1kV system tests have shown that peak transient overvoltage magnitudes of 7.66 p.u. could be expected when a VCB disconnects the system while in a single phase-to-ground fault condition. Further investigation also showed that the NERs in service did not comply with the requirements of Australian Standard AS 2081.5-1988 regarding power factor levels.
The transient overvoltages recorded on 3.3kV mining distribution substations did not exceed 6 p.u. which was measured. The worst transient overvoltages were found under rapid VCB switching (jogging) conditions. A maximum transient overvoltage of 3.8 p.u. was on switching off a line to ground fault and the highest overvoltage in other conditions was 3.4 p.u. With normal operational procedures no excessive overvoltages should be expected.
It is concluded that none of the overvoltages found in mine distribution systems are intolerable provided that the equipment used complies with the BIL levels specified in Australian Standards AS 1824.1-1985.