Intermittent Ground Issues

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tankfarms

Member
Hello everyone, first time posting here!

(a side note, how do I attach a picture)

I've been having an intermittent ground issues on one of my 480V switch gears lately. What's frustrating is that it comes and goes. When it happens, it actually hits and clears within seconds, and would just repeat for either minutes or hours. So our alarm registry would actually pick up hundreds of the same "ground alarm" within hours. What's more, when it does happen, it's not the common one like we normally see (i.e. one of the three light bulbs not lit anymore,and you turn off load until you see it's lit again) it makes all three lights flicker at the same time, but after the ground goes away, everything would be back to normal.

I've used a FLUKE Power Analyzer to see what's really going on, just to rule out it's not from the instrumentation from my switchgear. The result is: the switchgear, and the alarm is telling me what it is, I have an intermittent ground (i.e. voltage dip) on phase B. When ground happens, phase B would dip, the phase-gnd voltage would go as low as some 130V, and phase A,C phase-gnd voltage would spike up to some 590V, which makes sense for an ungrounded system like we have.

Now the challenge is to find this ground, which is a pain in the butt, especially given the fact that it comes and goes, and there's no pattern and it's been very erratic. However, the real reason I'm posing is simply to ask:

What if we have an intermittent ground on the primary side of my transformer (2400V/480V), would it cause problem on my secondary side the 480V? Vise versa, could a problem on the secondary side 120V cause voltage spike/dip on the primary side of a 480V/120V transformer?

My plant has ungrounded system on 2400V, 480V. (2400/480 are phase-to-phase voltage)

I ask this because transformer is about the only thing that I haven't been able to check.

Any other advice and ideas would be greatly appreciated!
 
B

benaround

Senior Member
Location
Arizona
Do you have large overhead cranes in the plant ? Do they get their power from spring

loaded shoes that ride in tracks ?

Do you notice any loads starting at the same time this happens ?

You say this is an ungrounded system, so a fault can occur without any disruption, it would

take two phases faulting at the same time or together to cause problems.

Problem on the primary, sure why not, probally a good starting point.
 
SG-1

SG-1

Senior Member
Location
Ware Shoals, South Carolina
How to attach a picture

How to attach a picture

Look below the box you type the post in. Below the Submit/Preview there is another section called Additional Options. Click on Manage Attachments.

Welcome to the forum !


Ever considered changing your system to a High Resistance Ground type ? You retain the benefits of ungrounded & it will control transient overvoltage from a re-striking ground fault. It can also be equipped with a pulser to locate the ground once it becomes solid.
 
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tankfarms

Member
SG-1 said:
Look below the box you type the post in. Below the Submit/Preview there is another section called Additional Options. Click on Manage Attachments.

Welcome to the forum !


Ever considered changing your system to a High Resistance Ground type ? You retain the benefits of ungrounded & it will control transient overvoltage from a re-striking ground fault. It can also be equipped with a pulser to locate the ground once it becomes solid.
Click to expand...

thanks! never heard of this pulser thing, and the "locating the ground" is very attractive. where can I get more literatures?

I'm not even sure if we have an ungrounded ungrounded or high-impedance ungrounded? how would I tell ?
 
T

tankfarms

Member
benaround said:
Do you have large overhead cranes in the plant ? Do they get their power from spring

loaded shoes that ride in tracks ?

Do you notice any loads starting at the same time this happens ?

You say this is an ungrounded system, so a fault can occur without any disruption, it would

take two phases faulting at the same time or together to cause problems.

Problem on the primary, sure why not, probally a good starting point.
Click to expand...

I attached a picture to illustrate the one-line of the system. I do get cranes, but they are at different feeders and the high voltage level.
 
T

tankfarms

Member
I've attached a picture, just curious to know if the primary of transformer 1 and secondary of the transformer 2,3 can be the source of the problem. other than that, I'm just checking loads one by one.
 
zog

zog

Senior Member
Location
Charlotte, NC
You would not see the ground on the other side of a transformer so you can rule that part out. It sounds like you have a ground on one of your motors. First thing I would do would be a megger test on the motors with them isolated from the system. Assuming they are 600V rated motors, use 1000VDC for 10 minutes, record your values at 30 sec, 60 sec, and 10 minutes. Temperature correct them for 40C. Post your results.
 
T

tankfarms

Member
zog said:
You would not see the ground on the other side of a transformer so you can rule that part out. It sounds like you have a ground on one of your motors. First thing I would do would be a megger test on the motors with them isolated from the system. Assuming they are 600V rated motors, use 1000VDC for 10 minutes, record your values at 30 sec, 60 sec, and 10 minutes. Temperature correct them for 40C. Post your results.
Click to expand...

Thank you. Yes, all motors are 480V. In this case, there are quite a few motors, so what I'm doing right now is to turn one motor off one at a time and wait to see if the alarm comes back, if so, move to the next motor and repeat the process. However,this is the interesting part, since the alarm comes and goes, how long do I have to wait after I turn each motor off? I asked that question to myself, well, the maximum "quiet days" for this alarm is about 5 days, so I'll give it a week; so if I turn one motor off and for a week the alarm never come back, I'll go ahead and check the motor. The reason I do this is because I have very limited resources (manpower wise) from my electricians, so I kinda want to do this simple steps before I call them in to meg a motor.

What do you all think?
 
kwired

kwired

Electron manager
Location
NE Nebraska
1000 volt insulation test even for short duration could possibly narrow down what you are looking for pretty easily.

You may still want to get a standard of how you want to test and monitor for future.

Your problem could be in a feeder or branch circuit also. You should test these also. Motor controllers have a tendency to accumulate conductive dusts on them and you could have some trouble with that also.

Test motor and motor branch circuits right at the motor control load terminals (assuming across the line contactor type controllers) at the same time. Any thing with questionable readings can be investigated further - islolate the motor and test separately from the branch circuit to see which is giving the questionable reading.
 
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