11th & 13th harmonics short term

[davva]

A power system model shows that a clients plant will be non compliant with a chinese harmonic code (4% THDv and 3.2% MIH). The primary reason for this is a large 12 pulse drive used to start a compressor, it takes a couple of minutes to run the synchronous motor up to full speed after which the VFD is bypassed.

All the components of the system are on order so I'm told it to late to change anything. Instead the drive vendor is advising that the drive will be compliant with IEC 61000-2-4 Class 3 equipment (allows 10% THDv). Conveniently lowering the bar?

My question is.... bearing in mind the short time use of the VSD, are 11th and 13th harmonics primarily associated with heating effects? or are there sited cases when their presence has had other detrimental effects?

 

[Skogsgurra]

Those higher harmonics rarely have any thermal effects. Investigations on asynchronous motors (the ones most affected by harmonics) show that the effect of 11th and 13th harmonics is next to none.

Some heating in bus-bars due to skin effect can be expected, but since this is for a short time, I see no problem.

The only potential problem I know of is that there might be a problem with resonance in PF correcting capacitors. But that is also very unlikely.

The IEC 61000-2-4 Class 3 equipment actually has a 15 % short term limit. I do not see this standard as lowering the bar (or raising it). It is a very real adaption to industrial reality.

Gunnar Englund

http://www.gke.org

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