Transformer for VFD Loads 3

[markjp]

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.

 

[Skogsgurra]

If experience counts, you do not need the screen.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

It never hurts, but it probably isn't an absolute as it used to be. The issue of having a shielded (screened) transformer was a bigger deal when you had DC drives or really old AC drives where the front-end converter / rectifier was using thyristors, because there was a lot of common mode noise associated with switching them. But hardly anyone does that any more. Even the few that use SCRs on the front-end rectifier only do so to ramp the voltage into the DC bus when first initialized, so the noise is minor and temporary, as opposed to constant as it was in the past. So generally, people don't need to worry about it as much any more.




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[ozmosis]

I would say not either. RFI needs as short a path to earth as possible and I see no point in bypassing the galvanic isolation your transformer provides, by inserting an earth cable. Push it to earth on your secondary.
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.

 

[markjp]

Thanks for the feedback guys. If we were to get the transformer fitted with an earth screen, do you think there might be any special considerations with the earth connections? Is it okay to connect the earth screen to the common HV/LV earth bar beside the transformer?