Current Harmonics from VSD Air Compressor

[JRLAKE]

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?

 

[Skogsgurra]

You have very typical VFD harmonics. The filters that the compressor manufacturer mentions are probably EMI filters. They do not influence harmonics. For that, you need line reactors.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[GMIEE]

Inrush current?

 

[GMIEE]

Also, JRLAKE, may I ask what power quality analyzer that is you used?

 

[JRLAKE]

A Fluke Model 1735 GMIEE. The measurements were taken at a steady state, around 55 Hz.


The compressor does actually have the line reactors Skogsgura. I neglected to include that information. Thanks for your reply.

 

[itsmoked]

So this compressor has a VFD that is taking in the six-pulse power?

I wonder if a diode has failed?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Can't be. That would create lots of third harmonics - and probably second. Most VFDs, I would say all, these days have supervision that sets off alarms when diodes die.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sibeen]

The harmonics are way worse than I'd expect with even an unfiltered 6 pulse input stage. 90% is way over the top. 30 -35% I could see, but not 90%.

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.

 

[marks1080]

I didnt think a manfacturer could appropriatly apply a conventional filter (LRC) to equipment as the filter paramenters would also depend on the load?

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?

 

[JRLAKE]

Thanks for the responses.

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.

 

[sibeen]

JRLAKE, a few other questions.

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?

 

[JRLAKE]

Some more info:

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.

 

[ScottyUK]

The harmonic spectra show the drive little more than idling at 40A/phase when full load would be nearer 350A/phase. The input rectifier conduction angle will be very short causing higher percentage distortion even though the actual values are small relative to drive rating. Do you have a harmonic spectrum for the drive working somewhere near rating? I expect the spectrum will be rather different.

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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