Variable Speed Drives generate high frequency currents that circulate through the cabled and physical earth. With standard earth leakage protection relays and electrical system assessments based only on consideration of faults driven by the supply system (50Hz), protection performance is unlikely to be adequate when variable speed drives are used in mining applications.
Computer modelling has indicated that when powered from a common supply the combined effects of multiple VSDs and EMI filters during fault conditions on the load side of the VSD can excite resonances that cause large circulating currents to flow between the EMI filters. This modelling suggests that currents several times the maximum value of the limitation device can flow at the substation NER.
Therefore, during an earth fault on the output side of an operating VSD high fault currents could also have the effect of generating higher than expected prospective touch potentials at machinery associated with the supply system, not just at the location of the earth fault. There is also an increased risk of sparking between machines, if machines are in close proximity to each other at the time an earth fault occurs on the VSD supplied motor or cable.
ResTech has been investigating this issue for a number of years and are uniquely positioned to help you. We offer:
- Independent assessments of your system to help you understand the effect your earth protection system has on the touch potentials on electrical equipment and ensure your protection system is compliant with the step and touch potentials specified by AS/NZS4871.1:2010. Contact us to find out more.
- A two day seminar exploring the principles and pittfalls associated with earth leakage protection in the high resistance earthed networks common in mining environments. Contact us to find out more.
For more information on this issue refer to Mine Safety Bulletin No: SB11-04 issued 21/12/11