Transformer for VFD Loads
Transformer for VFD Loads
(OP)
Hi, we are purchasing a transformer and would like some help if possible on deciding whether the transformer should be equipped with an earth screen between the HV and LV windings. The transformer is a conventional 2MVA 11kV/690V Dyn11 unit which will be used to power an MCC switchboard which has two 630kW VFDs. We have specified to the transformer vendor that the transformer needs to be suitable for the VFD duties and they have offered as an option an earth screen between the HV and LV windings to apparently help reduce effects from voltage spikes and RFI generated by the VFDs getting back on the 11kV network. The additional cost for the earth screen is approx $3,000 which isn't great in the scheme of the whole project but I'm not sure if we really need it and also whether there is any downside to having it or if there are any issues we might need to consider during installation. The site uses a common HV/LV earthing system, so if we connect all the earths together including the transformer earth screen then are we defeating the purpose of it anyway. Appreciate any thoughts on this. Thanks in advance.






RE: Transformer for VFD Loads
Of course, spec writers and sales people may think otherwise. But I have never, ever found it necessary since dawn of time (first VFD run in the seventies).
Gunnar Englund
www.gke.org
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RE: Transformer for VFD Loads
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RE: Transformer for VFD Loads
I would be more concerned with harmonic distortion generated by the VFD's when looking at transformer sizing. The best (and cheapest) solution is usually to minimise the THD from source (i.e. from the VFD's) otherwise you will need to increase the rating of the transformers to accommodate for increased heating effects.
RE: Transformer for VFD Loads