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Harmonics And Vfds


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#1 marke

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Posted 31 December 2007 - 02:36 AM

One of the areas that I operate in, has a high density of VFDs on pumps on relatively weak supplies. The result is that the high levels of harmonics on the VFD inputs has accumulate in the supply and is causing a high THD of the supply voltage. While we do have supply regulations covering harmonics, in this instance, the harmonics are higher than they should be.

There is an option of using zig zag transformers and six phase rectifiers as a means of reducing the harmonics drawn by drives however in this case, the drives are already installed.

There is a transformer for each drive and sizes range up to 200KW.

One thought that I had, was that for future installations, and there are new installations going in all the time, that the new supply transformers be designed with a zig or a zag winding to give a phase shift, and install equal loading on the leading and lagging phase shifts. This should act like a twelve pulse input on one drive, only it will be across two drives.

Any thoughts on this??

Best regards,

#2 mariomaggi

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Posted 01 January 2008 - 10:47 AM

Dear Marke,
happy new year - first of all.

Your solution is valid if both drives are energized together, otherwise you will lose the benefit.
If local rules are not so stringent, it is possible to use this solution, accepting to have more harmonics when one load is disconnected.

Regards
Mario

Mario Maggi - Italy - http://www.evlist.ithttps://www.axu.it


#3 marke

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Posted 01 January 2008 - 07:02 PM

Hi Mario

Compliments of the season to you also.

This region has around 2000 VFDs installed at present and the harmonics are high. New VFDs are going in daily, so my though was to recommend phase shift transformers for all new installations and this would over time help to reduce harmonic levels, which is probably a better solution that installing future drives with filters etc and leave existing harmonics in place.

Drives are for controlling irrigation pumps and there is one transformer per pump, so it is practical to do and all drives tend to run at the same time.

Have a good day,
Best regards,

#4 anu_rags

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Posted 02 January 2008 - 01:50 PM

Hi Mark,

At one such place, we were required to provide drives that meet IEEE519 standards for current and voltage. We had 4 no drives of equal size. A 12 pulse drive wont have been able to match the harmonics standards. We were looking for a phase shifting transformer for quase 24 pulse system with 6pulse drives. I was not able to find any suitable manufacturer of such transformers. Have you come across any such trafo?

Regards,
Anurag




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