Question regarding high resistance grounding units off of MCCs with significant drive loads 1

[lookingtolearn1]

Hi all,

Today I noticed that a voltmeter measuring the voltage across a high-resistance ground off of a 480V bus is fluctuating between 5V-45V, with about 10 seconds between each low and high point. The bus feeds several small VSD-powered motors (30 hp or less). Is there any reason to be concerned about the high-resistance ground voltage reaching 45V? It seems like this is indicative of a significant phase voltage imbalance, which could lead to increased negative-sequence current presence in the motor loads.

The bus does not have a THD detector or anything else that indicates how prevalent harmonics are compared to the normal waveform.

Your input is greatly appreciated.

Thanks

 

[jraef]

... a voltmeter measuring the voltage across a high-resistance ground off of a 480V bus is fluctuating between 5V-45V

Not sure what you mean here. You have a 480V resistance grounded service? When you set up the VFDs, did you read the instructions as to disconnecting the line MOVs and/or EMC filters if you do?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[lookingtolearn1]

Not sure what you mean here. You have a 480V resistance grounded service? When you set up the VFDs, did you read the instructions as to disconnecting the line MOVs and/or EMC filters if you do?

Sorry, the voltmeter is measuring the voltage across the high-resistance ground resistor. The high-resistance grounding unit is connected to the neutral point of a 480V wye transformer secondary.

I was not the one that installed the VFDs, but I am responsible for maintaining electrical equipment in and being fed out of this substation.

I have not checked to see if there are PF-correction cap banks fed off this bus, but this may be causing resonance.

 

[jraef]

If the VFDs were not properly reconfigured for the HRG system connection, the common mode noise filter inside may be giving an alternate path for ground current to flow, making the HRG system basically ineffective, UNTIL such time as that filter cannot handle the ground current and it fails. The beginnings of that failure could be giving you fits here. When you have an ungrounded Delta or an HRG Wye system, you must remove the ground reference for the filters on the VFDs. Otherwise the ground current in your entire system may be building up a charge and then discharging through the MOV in the filter that is referenced to ground, which may be what you are seeing in those readings. That is, at the same time, stressing that MOV.




"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[lookingtolearn1]

I will look into what you said and follow up. Thanks, jraef!

The beginnings of that failure could be giving you fits here.

It seems like you are saying that there may be a separate issue that is causing the voltage across the HRG resistor to reach 45V, correct? Or are you saying that the deteriorating filter may be causing the increased voltage?

 

[Parchie]

It seems like you are saying that there may be a separate issue that is causing the voltage across the HRG resistor to reach 45V, correct? Or are you saying that the deteriorating filter may be causing the increased voltage?

I guess what jraef is saying is for you to check whether or not you are grounding of your VFD filters.

 

[lookingtolearn1]

From looking at the drive instruction manuals, I do not believe any VFD filters are grounded. The only grounding points are for the ground terminal on the VFD and for external braking resistors.

 

[lookingtolearn1]

Some cable runs from the drive to the motor load are over 150 meters. There are line reactors for each of the VFDs, but I am not sure what their %Z is.

The phase-to-ground voltage tells me that that the phase voltages are imbalanced. The somewhat periodic and smooth shift between 5V and 45V tells me that the voltage imbalance is changing periodic. I most likely cause that I have thought of is the presence of some type of harmonic resonance between the L and C components of the line reactors and the long cable runes.

Do you have any other thoughts on the topic?

Thanks

 

[davidbeach]

In an HRG system, unbalanced phase-ground voltages do not mean unbalanced phase-phase voltages. They just mean that the triangle has moved and that ground isn't in the center of the triangle. For a solid phase-ground fault you'll still have (near) proper phase-phase voltages, but one phase-ground voltage will be zero and the other two will be the phase-phase voltage.

 

[lookingtolearn1]

You are correct.

Sorry, I wrote the relevant sentence incorrectly. I meant that the voltage across the HRG resistor indicates imbalanced phase-to-ground voltage magnitudes.

 

[lookingtolearn1]

Actually, I apologize again. My previous statement more accurately represents what I mean. I do not expect that there is a ground fault, so the voltage across the HRG resistor tells me that the phase-to-phase voltages are imbalanced.

I agree that you are correct, but I do not feel that it is relevant to my situation.

 

[davidbeach]

If all you have is phase-phase connected loads (as is the only option on an HRG system) then there is nothing you can do with your loads to put current into the neutral. You have to have a complete circuit. If there's something going on with capacitances you must have a extreme amount of very high frequency harmonics; otherwise you can't get to 45A of charging current.

You really need to dig into those VFD manuals and find the paragraph about operation on delta or ungrounded systems. There's either going to be a list of modifications required or a statement telling you not to do it. Listen to jraef.

 

[jraef]

What brand and model f VFDs do you have?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[lookingtolearn1]

They are GE Fuji Electric G11 drives.

I looked through a PQM on the bus and found that the phase voltage THD was proportional to the voltage across the grounding resistor. At a ground voltage of ~45V, the voltage THD reached ~21%. Both of these would increase and decrease simultaneously. The voltage THD was approximately equal for each phase. The voltage magnitude would stay about the same, but the phase shifted up on each phase up to +/- 10 deg. The shift for each direction was not always in the same direction, so there were times that two phases had 100 deg between them and times that they had 140 deg between them.

I also took a look at current phasors. These had a phase shift of up to +/- 25 deg. The significant difference between the magnitude of phase shifting between the voltage and current phasors seems abnormal. I would expect both to exhibit similar +/- phase shifts.