Current Harmonics from VSD Air Compressor
Current Harmonics from VSD Air Compressor
(OP)
We recently installed a 460V 250kW VSD air compressor (6 Pulse). The machine ran fine for a year. Lately the machine has been tripping the breaker. We also have had problems with other breakers within the same substation tripping. After ruling out mechanical issues we connected a power quality analyzer and recorded the following:


The only other equipment within the substation are four fixed speed 37kW boiler feed pumps. No lights. No computers. Nothing else.
The air compressor manufacturer says their compressor is not the source because it includes the appropriate filters. However when we turn the compressor off the tripping breaker problem goes away.
My questions are, do these filters fail and does anyone have an opinion on what is happening?


The only other equipment within the substation are four fixed speed 37kW boiler feed pumps. No lights. No computers. Nothing else.
The air compressor manufacturer says their compressor is not the source because it includes the appropriate filters. However when we turn the compressor off the tripping breaker problem goes away.
My questions are, do these filters fail and does anyone have an opinion on what is happening?






RE: Current Harmonics from VSD Air Compressor
5th and 7th harmonics are typical. They show everywhere. A k factor equal to 18 - 19 is rather high. If the compressor is a substantial part of the transformer load, such a high k factor trips the protection because the transformer is thermally overloaded by the harmonics.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Current Harmonics from VSD Air Compressor
RE: Current Harmonics from VSD Air Compressor
RE: Current Harmonics from VSD Air Compressor
The compressor does actually have the line reactors Skogsgura. I neglected to include that information. Thanks for your reply.
RE: Current Harmonics from VSD Air Compressor
I wonder if a diode has failed?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Current Harmonics from VSD Air Compressor
I think that you need larger line reactors. Do they have less than 2 percent voltage drop at rated current? Then you need more.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Current Harmonics from VSD Air Compressor
A few questions?
Where were the current readings taken, at the input of the drive?
What was the the voltage THD at the PCC, with the drive turned on and the drive off?
I wouldn't be surprised if there is some reasonant issue.
RE: Current Harmonics from VSD Air Compressor
It does look some inrush currents are present from the small even harmonics present from your graph, but they are low.
Has the VSD been making any unusual noises?
RE: Current Harmonics from VSD Air Compressor
Don't know about the voltage drop. Will check.
Readings were taken at the disconnect, 10 feet from the drive.
Voltage THD with drive on, 1.9%
Voltage THD with drive off, don't know.
The VSD has not been making unusual noises.
RE: Current Harmonics from VSD Air Compressor
The two diagrams you provided in your first post, I take it those are taken of the input current legs of phase A and phase B?
The second diagram (L2) is showing about 20% third harmonic distortion - very strange.
What are the line to line voltages supplying the equipment. A voltage imbalance can cause non-characteristic harmonics, like the third, to appear; although I wouldn't expect anywhere near this level.
Do you have some oscillograms showing the waveshape of the input current on all three incoming legs?
RE: Current Harmonics from VSD Air Compressor
The supply transformer to the switchgear is a 1288 KVA (Forced Air), 480 volt secondary and 5.93%Z. It is feeding 4 circuits. 2 of the circuits have their breakers turned off so the only things being fed are the compressor and a panel. The loads in this panel are fan motors and pump motors. None of these items are on VFDs. They are all on contactors. When one of these items are running we do not see the harmonic distortion problem until the compressor unit comes on.
RE: Current Harmonics from VSD Air Compressor
Is it possible that the drive is causing mis-operation of the earth fault element of the tripping unit? Does your breaker provided any indication of which function initiated the trip?
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