Rockwell and Ampcontrol collaborate on earth leakage safety for VSDs

Medium-voltage (MV) variable speed drives (VSDs) are now widely used in mining applications. However, due to the high-frequency voltage generated by these drives, they can introduce earth leakage currents at frequencies outside the normal power system frequency which may not be detected by traditional earth leakage protection relays.

Earth leakage - the leakage from an active circuit to ground - can create safety issues. As a result of this, mining regulators in various states of Australia have requested that special attention be given to installations that use 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.

To this end, 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.

“The standard is of vital importance to the mining industry, particularly the coal sector. There had been a lot of discussion within the industry about detecting earth leakage with VSDs but a proven and accepted solution had not been established - until now,” said Geoff Irvine, mining industry manager at Rockwell Automation.

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 its 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. The aim of the testing was to determine if earth leakage currents are within acceptable levels under normal operating conditions, and if earth leakage currents are detectable under earth fault conditions.

In addition, the testing assessed whether VSDguard is capable of detecting earth leakage currents at all frequencies produced by the VSD and trip under all earth fault conditions. The trial successfully proved the relay does work and can detect earth leakage at different frequencies.

VSDguard works in conjunction with the PowerFlex 7000 drive equipped with a direct-to-drive topology. Direct-to-drive topology eliminates common mode filters, which removes the possibility of bypassing the neutral earth resistor under earth fault conditions and maintains the integrity of the earth leakage detection circuit.

“It is very promising that we have achieved these results by thoroughly testing the Ampcontrol VSDguard with the PowerFlex 7000,” said Stephen Sneddon, product manager of mining systems at Ampcontrol.

“The combination will be beneficial for the mining industry in Australia and New Zealand when complying with the AS/NZS 2081:2011 industry standard and will improve the safety and productivity of a variety of mining applications including ventilation fans, conveyers and pumping applications.”