Gas CB Testing

[climb1]

We are Doble testing a gas CB for power factor. We are getting a 'bad' result on one of the bushings at full pressure. All of the others are getting 'good' ratings. When we take the pressure down to 5psi, it passes. We are getting a new tank of gas from the manufacturer.

Has anyone run into conflicting results like this? The manufacturer does not use Doble testing, and I am wondering what other opinions there are.

thanks

 

[Elmir]

What is the make and model of the circuit breaker?

Are the individual poles all connected together through external plumbing?

You could conduct diagnostic testing to determine path of leakage current.

What are the results you are obtaining for the tests you are conducting?

Elmir

 

[climb1]

I am attaching the results. The first test was completed in the field, and the second was done at our testing facility. We ran multiple tests and would get the 'bad' results from the #5 bushing with full pressure, and 'good'
results at 5psi.

Any ideas?

 

[climb1]

Elmir,
I forgot, yes they are all connected through common gas manifold.

 

[ausphil]

Strange that the OPEN GND5 test (which I assume is the one you are most concerned about) has a capacitance about half of the other good circuit breaker, and you OPEN UST 5 is about 3 times the value of the other unit.

Have you tested the dew point and purity of the SF6? However, my assumption is that it would be fine.

Was the unit sufficently cleaned before filled on site?

Does it change again when you fully re-pressurise it? ie is it repeatable?

Its almost like there is a blockage or something that is loose, that with pressure changes the configuration of the insulation system, to give those significant chages in capacitance, and also power factor.

Also, what voltage are we talking about?

There may have been damage during transport that may be showing up here. We recently bought a standard capacitor for DDF testing (up to 250kV) that gave 2 different readings when in operation, due to loose hardwear inside the tube from transport.

Has the manufacturer commented on what could be wrong?


ausphil

 

[climb1]

We have had the gas tested. dp and purity both good.
We have cleaned the bushing with alcohol.
It is repeatable, in that when we lower pressure we get the same results from #5.
It's a 72.5kV unit.
The manuf reluctantly sent a bottle of gas, as they think that there may be a flaw with the bushing. They are sending a field eng this week.

 

[ANGIBEW25]

Hi climb 1:
I am of the opinion that you have a defective bushing. Do you have a composite or a porcelin type bushing? You are not supposed to use alcohol on the composite type.
Has any other electrical testing been performed besides Power Factor, Contact Resistance and Dew point.
I suspect that the pressurized SF6 gas is entering the bushing and displacing the bushings internal insulating medium.
I would pressurize the tank again and check with a gas detector all around the bushing very carefully. You can try this field trick:
1. Enclose the exterior of the bushing with a large plastic garbage bag. Allow for an opening that you can insert a gas leak detector and reseal the bag.
2. Fill the breaker with SF6 gas.
3. Wait several minutes, and check the bushing for leaks.

If you find a gas leak anywhere on the exterior of the bushing you have your answer.
Good Luck.

Regards,
Ang

 

[climb1]

Angibew25,

Thanks that is a great idea. It is a porcelin bushing. The only further testing is the power frequency testing from the manufacturer and by our facility. Both passed with ANSI/IEC standard.

I will let you know what we find.

thanks

 

[Elmir]

Climb 1,

I reviewed your test results and have a couple questions.

I suspect that the results you are getting for PF may have to do with the test conditions or test setup although the fact that capacitance varies greatly is disturbing when circuit breaker gas pressure is changed.

I see the first test was completed in the morning (7:39am) with relative humidity at 74%.

The second test conducted at your testing facility was completed with relative humidity at 39%.

The wide range of power factor readings in the first test suggest a test condition problem with moisture. There may be condensation on the surface of the bushing which has not evaporated at the time of testing which can lead to questionable test results.

The only thing I wouldd recommend at this point if you have not already done so is to perform diagnostic testing using the gaurd circuit of your test set. Guard different portions of bushing #5 with the breaker open and the test run in GST mode.

You can also perform hot collar tests on different portions of the bushing and run the test in UST mode with your LV lead connected to the conductor.

If this does not give any clues I would move onto diagnosing the insulation support system within the breaker.

I would also perform a GST test on each pole with the circuit breaker closed and compare results.

Try hanging your test lead in free air after you obtain a questionable test result to ensure there are no problems with the HV cable.

These are some suggestions. The most important thing is to make sure that you run a series of tests so you can compate the questionable results on pole 5 to other known good poles.

Good luck and let us know your progress.

Elmir

 

[climb1]

We received new gas from the manuf and testing results all showed good. We are considering this one closed.