veritas
Electrical
- Oct 30, 2003
- 467
For the harmonics gurus,
I have read in quite a few texts on power system harmonics that a VFD is a non-linear load and so is a source of harmonics. Thinking about it I would say that this is misnomer (the way I see it anyway). The VFD is not a source or sink of electrical power. Considering a 6-pulse VFD, the current drawn by the VFD from the line is two pulses every halfwave so 4 pulses per cycle. Yes, they are not sinusoidal and can be represented as the sum of the fundamental and 5, 7, 11, 13, 17, 19, etc harmonics. These are positive phase sequence harmonics that are supplied from the source to the VFD load.
This I have verified using PTW. For a single load on the system the I_THD% at the cable supplying the VFD is nearly the same as the I_THD% at the utility point way upstream (all other loads, capbanks, etc. out of service). When the other load is put into service, the I_THD% at the source decreases dramatically. The reason is that it still supplies the harmonic current as before, but to this is added load of all the DOL drives, other VFD's, lights and power, etc. So the fundamental increases but the harmonics not as much.
Am I on the right track so far?
The real conundrum for me is the effect of a pf correction capbank on harmonics. For example, without the bank, the cable supplying the MCC has I_THD% = 42%. Once the bank is in service I get I_THD% = 18.7% for that same cable. Why? I've also noticed that the capbank has a series inductor and so it is a tuned filter to the 3.77 harmonic. This makes a huge difference. When I model the capbank without the inductor the I_THD% in the supply cable shoots up to 155% as there is 5th harmonic resonance.
I'm trying to get my head around why the capbank has such an influence on the harmonics. It draws leading fundamental current from the source. And then...
Thanks.
I have read in quite a few texts on power system harmonics that a VFD is a non-linear load and so is a source of harmonics. Thinking about it I would say that this is misnomer (the way I see it anyway). The VFD is not a source or sink of electrical power. Considering a 6-pulse VFD, the current drawn by the VFD from the line is two pulses every halfwave so 4 pulses per cycle. Yes, they are not sinusoidal and can be represented as the sum of the fundamental and 5, 7, 11, 13, 17, 19, etc harmonics. These are positive phase sequence harmonics that are supplied from the source to the VFD load.
This I have verified using PTW. For a single load on the system the I_THD% at the cable supplying the VFD is nearly the same as the I_THD% at the utility point way upstream (all other loads, capbanks, etc. out of service). When the other load is put into service, the I_THD% at the source decreases dramatically. The reason is that it still supplies the harmonic current as before, but to this is added load of all the DOL drives, other VFD's, lights and power, etc. So the fundamental increases but the harmonics not as much.
Am I on the right track so far?
The real conundrum for me is the effect of a pf correction capbank on harmonics. For example, without the bank, the cable supplying the MCC has I_THD% = 42%. Once the bank is in service I get I_THD% = 18.7% for that same cable. Why? I've also noticed that the capbank has a series inductor and so it is a tuned filter to the 3.77 harmonic. This makes a huge difference. When I model the capbank without the inductor the I_THD% in the supply cable shoots up to 155% as there is 5th harmonic resonance.
I'm trying to get my head around why the capbank has such an influence on the harmonics. It draws leading fundamental current from the source. And then...
Thanks.
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