Plant & Equipment

Applying the brakes to earth leakage

Medium voltage (MV) variable speed drives (VSDs) are widely used in extractive 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. 
Australian and New Zealand Standard 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 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.
 ?The standard is of vital importance to the mining industry,? Rockwell Automation mining industry manager Geoff Irvine said. ?The industry?s talked a lot about detecting earth leakage with VSDs but a proven, accepted solution hadn?t been established until now.?
To ensure 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, 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 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.
The testing also assessed whether VSDguard could detect 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 that eliminates common mode filters.
?It is very promising that we have achieved these results by thoroughly testing the Ampcontrol VSDguard with the PowerFlex 7000,? Ampcontrol mining systems product manager Stephen Sneddon said. 
?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, conveyors and pumping applications.?
Source: Rockwell Automation

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