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Topic Title: Transformer sizing on VSD application
Topic Summary: How to calculate, include PF and harmonics?
Created On: 12 November 2012 12:40 PM
Status: Read Only
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 12 November 2012 12:40 PM
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Posts: 24
Joined: 11 January 2007

How do you size a transformer feeding a single Variable Speed Drive?

Example looking to retrofit Direct on line 3.3kV driven pump with a LV VSD package.

3.3kV / 690V Step down transformer
250kW VSD 690V system
New 690V motor

VSD probably an Active Front End / Regenerative type, so minimum harmonic current distortion ~ 2/3%, with a Power Factor close to 1 with modern AC drives.

Following doc suggests no over-rating:

In the past on a simular application we have installed 315kVA transformer (as offered by supplier)

Is this oversizing practice due to heating from harmonic currents? If a PF close to 1 the real power and apparent power are practically equal are they not? 250kW = 250kVA, could you use a 250kVA transformer?

Thanks for any help or advice
 13 November 2012 03:31 PM
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Posts: 91
Joined: 18 October 2002

Fundamentally, the transformer shouldn't need to be oversized, since the VSD (if it has an active regenerative grid-side) should operate with minimal harmonic currents, almost unity power factor, and almost zero unbalance.

However, you might want to overrate the xfmr just a bit. This might be due to
- temperature (what is the operating environment/cooling?)
- grid voltage. If this is a bit low at your site for any reason, then the VSD will still demand up to pu power and this could lead to currents >1pu
- grid voltage/freq. If grid voltage every goes high, or frequency every goes low, then perhaps overrating the xfmr a bit (in voltage, and hence power for the same current rating) is a good idea, to avoid saturation.


Dr. Andrew Roscoe

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