Rockwell Automation has collaborated with Australian electrical engineering and manufacturing company, Ampcontrol Group to successfully test a solution that detects the impact of earth leakage on mine sites.
Widely used in mining applications, Medium Voltage (MV) variable speed drives (VSDs) generate high frequency voltage, introducing earth leakage currents at frequencies outside the normal power system frequency, which may not be detected by traditional earth leakage protection relays.
Earth leakage or the leakage from an active circuit to ground can create safety issues. Mining regulators in various states of Australia have therefore requested that special attention be given to installations using VSDs.
AS/NZS 2081:2011 is the relevant standard for electrical protection devices for mines and quarries. This standard specifies the performance requirements for protection devices intended for use with electrical supply networks utilising earth fault current limitation techniques (IT networks). Earth fault current limiting devices are used to minimise the risk of touch and step potential reaching harmful levels due to an earth fault. Special mention is also made of VSDs in AS/NZS 4871.1:2012 Appendix E.
Geoff Irvine, Mining Industry Manager at Rockwell Automation explains that the standard is of vital importance to the mining industry, particularly the coal sector. While there has been a lot of discussion within the industry about detecting earth leakage with VSDs, a proven and accepted solution had not been established until now.
To ensure that an earth fault can be detected at the input or output of a VSD, a wide bandwidth earth leakage relay is required. When developing the patented VSDguard earth leakage relay to address earth leakage issues faced by the mining industry, Ampcontrol contacted Rockwell Automation to test the product on the PowerFlex 7000 series of medium voltage VSDs.
Testing of the Ampcontrol VSDguard on a PowerFlex 7000 demo system took place at Rockwell Automation in Canada, to determine if earth leakage currents were within acceptable levels under normal operating conditions, and if earth leakage currents were detectable under earth fault conditions.
The testing also assessed whether VSDguard was capable of detecting earth leakage currents at all frequencies produced by the VSD and tripping under all earth fault conditions. The trial successfully proved the relay could detect earth leakage at different frequencies.
VSDguard works in conjunction with the Powerflex 7000 drive equipped with a Direct to Drive topology, which eliminates common mode filters, removing the possibility of bypassing the Neutral Earth Resistor under earth fault conditions and maintaining the integrity of the earth leakage detection circuit.
Stephen Sneddon, Product Manager of Mining Systems at Ampcontrol commented that the combination will be beneficial for the mining industry in Australia and New Zealand when complying with the AS/NZS 2081:2011 industry standard. The solution will also improve the safety and productivity of a variety of mining applications including ventilation fans, conveyers and pumping applications.