Question regarding high resistance grounding units off of MCCs with significant drive loads
Question regarding high resistance grounding units off of MCCs with significant drive loads
(OP)
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
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





RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
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
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
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.
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
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?
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
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
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
Sorry, I wrote the relevant sentence incorrectly. I meant that the voltage across the HRG resistor indicates imbalanced phase-to-ground voltage magnitudes.
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
I agree that you are correct, but I do not feel that it is relevant to my situation.
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
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.
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Question regarding high resistance grounding units off of MCCs with significant drive loads
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.