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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 Linear : Threading : Single : Branch
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 12 November 2012 12:40 PM stuartllewellyn 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: http://www05.abb.com/global/sc...le/lvd-eotn13u-en.pdf 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 aroscoe 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. Andrew ------------------------- Dr. Andrew Roscoehttp://personal.strath.ac.uk/andrew.j.roscoe
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