DC "Step" Hi-Pot test - Noise Interference? 3

[electricpete]

We perform a DC Hi-pot test of motors in discrete voltage steps (controlled overvoltage test per IEEE95).

After each voltage step we wait approx 1 minute for the current to "stabilize" (most of the capacitive charging current is gone after a minute).

We plot current vs voltage to try to detect impending failure and stop the test before damage occurs (unsuccessful on two occasions, successful on one occasion).

One problem I note is that for very clean dry insulation (approx 10k meg-ohms), we see a whole lot of bouncing around of the digital readout. This makes it difficult to get an accurate trendline sensitive to small changes.

I suspect the problem lies in trying to measure very small currents (0.1 to 1 microamps) ... possibly near the lower sensitivity of the instrument. I haven't investigated that yet.

Other observations from a recent test:
- Oscillation seemed to get more as the voltage was raised. (this is the opposite of what I would expect for noise-induced oscillation since signal to noise ratio improves as voltage increases.....unless noise is coming from the instrument power suppply?).
- I saw a worker vacuuming near the motor base (I know... not a good safety practice during a hi-pot). I asked him to turn off the vacuum cleaner. Saw a huge oscillation the moment he turned it off. But many minutes later oscillations remained...presumably from other sources.

Has anyone heard of similar problems or steps taken to prevent meter oscillation during hi-pot?

 

[THooper]

electricpete, I have never ran into this problem before, but I will ask around in our field service department and see if they have...

 

[resqcapt19]

I seem to remember that a testing contractor that I hired to do some acceptance testing used a battery powered inverter to power the hi-pot test set. He told me that this was to eliminate oscillations in the results by stabilizing the input voltage. In the NETA (national electrical testing association) specs for hi-pot testing of medium voltage cables(600-69kV) the first step is: "insure that the input voltage to the test set is regulated".
Don(resqcapt19)

 

[THooper]

electricpete, I spoke with several technicians that said this has happen to them on occasions when measuring such a very small current, and contributed to the lower end sensitivity of the equipment, like you suspected...

kind regards,
motorhead

 

[electricpete]

Thanks to all. I remember reading now about the ac supply voltage effect.

resqcapt19 - If the ac supply voltage changes slightly, then the dc output voltage will change slightly. It may not be noticeable on the voltage meter but could be much more noticeable on the current meter due to large connected capacitance. (i = c * dv/dt). Based on your coments, I looked at IEEE95-1972 Section 4.8.1 which states: "The ac supply to the DC equipment should be free from intermittent loads and transients.... must have constant nonfluctuating voltage... Regulating transformers may be used". I'll have to go see if we had any special transformer... if we were just plugging into the outlet with no special regulation within the hipot set itself, that may be the problem.

Any other ideas?

 

[busbar]

From a practical standpoint, DC step-voltage tests (on cables, at least) can be maddening without a stable source. The tester is supposed to carefully watch leakage current, in order to terminate the test if it suddenly increases. If readings are unstable, this is harder to observe, likely leading to an embarrassing test-induced failure.

 

[electricpete]

You have described our situation exactly,busbar.

The test set is Biddle 220070. Looking at the documentation, I don't see any discussion of internal voltage regulating transformer at all. But the troubleshooting guide does show several causes of "erratic output current", including "line voltage transients".

That is my prime suspect at this point, since I noticed the current swing particularly high when a nearby vacuum cleaner was switched off.

Does anyone know of a portable regulating transformer to connect between the site power outlet and the instrument power input?

Thanks again to resqcapt19 for focusing me on this issue. I'll report back results of retest if I can get my hands on external regulating transformer.

 

[busbar]

Electricpete, it's a small world. In the past, I've spent (seemingly) man-years babysitting that very same test set. I used an AC regulator by California Instruments, and I don’t know if something a Sola ferroresonant unit would work. Out in the field (literally, like near a cable trench), I’ve seen some use an old Tripp-Lite inverter and a car battery. I’d be interested to know of others’ experiences with this.

 

[jbartos]

Suggestion: When using various digital meters, one must make certain that they are immune to the level of surrounding Electromagnetic Interference. This may mean to avoid using some less expensive digital meters for a few dollars.

 

[electricpete]

jbartos - It sounds a little high-tech to call it electromagnetic interference.

I believe it is a simple case of sensitivity to variations in ac input voltage carried through to instrument output voltage. For a large motor with large capacitance to ground, only a very small dv/dt is required to create a capacitive current which will swamp the 0.2 microamp resistive leakage current that we're trying to measure.

I did locate a newer instrument (High Voltage Instruments) which incorporates a ferroresonant transformer directly within the instrument. Review of the Biddle diagram showed it didn't have anything like that. We should be retrying the test within a week using the High Voltage Instrument test set. I'll let you know how it turns out.

Thanks again to busbar and resqcapt!