ungrounded low impedance system ULIS- distortion

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blair

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Hi to all reading this!! I am new here and am seeking some input to a puzzler at work!

Our plant uses the typical ULIS. We noticed recently two of our load centers 600v/3ph showing a full scale deflection ground - 2 amps. When we turned 2 VFD's off (one on either bank - seperate metering), we observed both drop to normal levels. (incidently 1 VFD is a newer digital 1336 plus II - 25 amp, the other an older Relcon 400amp.

Q1. Is it correct that the harmonic distortion on our system would be likely a zero sequence current or a negitive sequence current? or a combination of A0, A1 & A2?? Note we are using the 3 light method and there is no indication of dimming on any phase.

Q2. The fact that there is an apparent ground - (FS deflection max grnd current - also phase to ground voltages 345 volts as expected), but no dimming . . . are we in a potential critical fault situation?

Also noteworthy, the other day we had an intersting grounded condition where we found 600v to ground on 2 phases and 16.5 volts to ground on the 3rd. We cleared that fault and voltages returned to normal. I have done some study on this but would appreciate any insight that you may be able to provide. With the information I receive I will be applying a power quality meter and testing to prove the theory.

Thanks again and looking forward to "hitting the ground" running.

Blair in the northern coastal rainforest of NW Canada!
 

bennie

Esteemed Member
Re: ungrounded low impedance system ULIS- distortion

It appears the VFD's have a filter breakdown.

The lights, remaining bright, indicate no ground.
 

stuartw

Member
Location
Arkansas
Re: ungrounded low impedance system ULIS- distortion

I have seen, and still see neutral currents on high resistance grounded systems that have a PWM drive. I have seen this with a number of drive manufacturers and sizes. I have never encountered an ULIS system.
The theory should be the same however. First of all you do not have a ground on your system. Non-linear load phase currents create neutral currents. (All 3 phases of power do not sum to 0 so current flows through neutral to 0 at transformer).
Most drive manufacturers will not admit to this occurring. I have an article from Allen Bradley discussing this, circa 1990.
Potential problems:
Is neutral wire size sufficient to carry the load? (Not all 60hz).
Is the ground fault sensor suitable for sensing damaging ground fault current? Because of this current, a ground fault sensor will trip sooner (nuisance tripping or alarms). You could replace the ground fault sensor with 1 designed for harmonic currents or modify the sensor with a high frequency capacitor bypass circuit (need to contact OEM of sensor).
Since we have a high resistance grounding system, we set our alarm just above the normal neutral current. This ensures we get an alarm when we have a ground on 1 phase. It enables us to clear the ground prior to a phase to phase failure.
I've seen all kinds of voltages when 1 phase grounds. Always, 2 phases will go high and 1 low. Sometimes the low phase will go to 0 V (dead ground) or some value above 0. Phase to phase voltage will be unaffected.
I almost forgot there is another way to solve this "problem." You can put the VFDs on an isolation transformer. It moves the neutral current to the isolation transformer and gets it off the main.
A full study of your system would have to be done to determine if harmonics from the drives are a problem on your distribution system.

Hope that Helps

Stuart
 

arneykaner

Member
Location
Illinois
Re: ungrounded low impedance system ULIS- distortion

The magnitude of phase voltage distortion, that you discribed shows a typical picture of a ground fault in ungrounded/high resistance ground
system. harmonics cannot create a voltage distortion of this magnitude. Harmonics in a 3-phase simmetrical rectifier system could neither create a significant neutral current (disbalance), unless there is something wrong with equipment.
The disbalance neutral current is an effect of either unbalanced voltage, or unbalanced impedance between phases. The cure: perform the full manufacturer recommended check of your drives. I would agree with Bennie - there is probably an internal fault in the equipment. Is there any equipment, either internal or external to the drive, that is 1Ph and connected between two "hots"?

To answer your question about components: harmonics cannot create zero sequence or negative components - only a short circuit or fault to the ground in an ungrounded system does.
 
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