7500 Amps, 180 VAC, Megger problems

[TwoBlocked]

Got this resistance sintering process using ungrounded secondary transformers with ground fault detection. 7500 amps, 180 VAC, single phase. One or two of the three banks is normally running in a batch system. As you can imagine, there's induced voltage/current going on probably in every bit of steel in the room.

One bank has been occasionally showing partial ground faults. When trying to troubleshoot using a megger (both digital and crank) the conductive parts show dead grounds. Using a digital ohmmeter, will get both positive and negative readings depending on which lead is taken to ground. The numerical difference is around 500k ohms (like +200k and -300k).

Anybody experience this sort of thing? Could it be some sort of partial inductive grounding on one leg?

 

[zbang]

It's a not-so-long shot, but many oxides can form a diode, and different readings when the polarity is reversed can indicate that. Looks for corrosion between supposedly-insulated conductors and grounded parts.

"With enough RF, even an unclean thought can be a detector."

 

[TwoBlocked]

It's a not-so-long shot, but many oxides can form a diode, and different readings when the polarity is reversed can indicate that. Looks for corrosion between supposedly-insulated conductors and grounded parts.

"With enough RF, even an unclean thought can be a detector."

Ah, maybe.... Also much of the equipment and conductors are cooled with deionized water, but also treated with "something".

 

[winnie]

Testing with a meter of the wrong impedance is always going to have problems.

1) All insulation leaks current. The process will have a definition of excessive leakage. You need to find out what level of leakage you are testing to.

It wouldn't surprise me if a 7500A system leaks several amps as part of normal and acceptable operations.

2) After you figure out what sort of impedance and voltage you need to test to, then you can pick a meter that is matched to the job.

3) You might look at tools not considered meters, for example a variable lab power supply. An Ohmmeter simply applies a test voltage (or current), measures the current (or voltage) and calculates ohm's law to report a value. You can do the same test with a power supply at 30V...if 1mA flows, you have a 30K resistance.

4) You might be able to use the ambient induction as the power source for your measurements: first measure the voltage between two points, and then measure the current flow with the two points shorted together.

Jon

 

[TwoBlocked]

Testing with a meter of the wrong impedance is always going to have problems.

.....

3) You might look at tools not considered meters, for example a variable lab power supply. An Ohmmeter simply applies a test voltage (or current), measures the current (or voltage) and calculates ohm's law to report a value. You can do the same test with a power supply at 30V...if 1mA flows, you have a 30K resistance.

.....

Jon

Thanks, Jon. maybe use an incandescent lamp (as a current limiter) powered by a small isolation transformer. Measure the current and the voltage across the resistance and use ohms law. Yeah, maybe.

 

[zbang]

It wouldn't surprise me if a 7500A system leaks several amps as part of normal and acceptable operations.

That's bothered me since yesterday. Maybe I'm missing something but how would a high-amperage system leak more at a given voltage than something lower? Especially when presumably the overall system has fewer and shorter conductors than the usual distribution system.

All I come up with is bigger conductors -> more insulators -> more potential leakage paths.

 

[winnie]

I'm thinking high temperature systems with water cooling for the conductors. Lots of 'insulators' that are all slightly conductive.

Think about the problems with GFCIs and electric ranges, but multiply the scale by a factor of 150 then add water.

Jon

 

[TwoBlocked]

Forgive me for not going into detail about my employer's process. But here is something interesting. When one bank is running, but another bank is powered down and the busses grounded, up to 30 amps can be measured in the grounding cables with a clamp-on.

 

[winnie]

That could either be galvanic coupling from the 'live' bank to the grounded bank, or it could be magnetic coupling.

-Jon

 

[TwoBlocked]

I am sure its magnetic induction. There are places that will make a meter's magnetic fob stand straight out and vibrate.