Earth Leakage Protection on VSD installation

[bass1mj]

Does anybody have experience with regards to the use of earth leakage protection devices in a VSD installation. We are using a resistance grounding system (IT-system) on our MCCs and need to use Earth Leakage protection on all feeders from the MCC. With the VSD installations, however, we have found that the devices do not operate due to the high frequencies that can be evident on the output of the VSDs. Normal VSD ground fault protection will also not operate as fixed resistor are used in the VSD to determine ground fault setting and this is higher than the earth fault current permitted to flow by the NER.
I found a device by Littlefuse on the net, but would also like to know if anybody have used other makes.

 

[ScottyUK]

Check out Bender's product range. Their people usually have a reasonable technical knowledge too.

 

[LiteYear]

What country? In Australia there are particular requirements associated with AS/NZS 2081 to be wary of.

In any case there are roughly two solutions for wide-bandwidth E/L protection currently on the international market: the Littelfuse EL731 and the Bender RCMA420. The EL731 covers two ranges: DC to 100Hz and 100Hz to 15000Hz. The RCMA420 only goes up to 2000Hz (and so may miss some leakage currents at the switching frequency). Ampcontrol will have a novel device on the market within a few months called VSDguard.

Be wary of being able to set and forget these devices - depending on the VSD configuration there may be a level of higher frequency earth leakage flowing even in unfaulted circumstances. If the relays are not configured and installed correctly, they will likely be subject to nuisance tripping. Additionally there are particularities that may need to be considered - for example, higher frequency fault currents don't necessarily return to the source via the NER. They may return to the VSD via distributed capacitance or filter capacitors. Failure to plan the installation of earth-leakage protection may render them useless in a fault situation.

Speaking on behalf of my employer now, we highly recommend you do some modelling of the network first to determine the most appropriate arrangement of earth leakage protection. We can help with this:

http://www.restech.net.au/assessment

 

[bass1mj]

@ LiteYear. Thank you for your reply. much appreciated.
We are based in South Africa. We have already recognised the problem with regards to the higher frequency fault currents and that is why I am curious to know about these protection devices. We are also aware of the Low impedance earth return paths via the distributed capacitance and have made provision to eliminate this.
We are using a package called PowerTools for network modelling and Analysis, but to be honest I have not yet modelled this installation. Will do and communicate the results through.

@ScottyUK Also thanks for your input.

 

[ozmosis]

bass1mj
I work for the company attached and we typically recommend Bender. A type B RCD is generally recommended for drives due to the points already raised.

http://www.danfoss.com/NR/rdonlyres/9F32A4E6-BF86-4846-939D-94C01F8BB6A7/0/ElectricalHazards.pdf

 

[electrolitic]

All replies are correct. Just a remark. Bender RCMA 420 has a measuring range from 3mA to 500mA.
If you need a higher value, RCMA 423 has a measuring range from 30mA to 3 Amps.
If you have many points to monitor, try RCMS4xx series.