PCB transformers standards

[Gauss2k]

Hello everyone,

We just tested 2 PCB transformers: ratio, winding resistance, PCB dielectric breakdown and insulation test (megger @ 1kV).

All the tests gave good results on both transformers, but the insulation tests we're pretty low on -both- transformers primary side (about 8 to 9 MegOhm for a 25kV primary side).

The test instrument was working well because we tested surge arresters minutes later and they gave high results. The tests were indoor and the humidity level was not high.

Is there any special conditions or internal configuration in PCB transformers that could explain those results?

Thank you!

 

[dpc]

Don't think so. Askarel oil had very good insulating properties. That's one reason why it was so widely used. I would think it would test out comparable to mineral oil, everything else being equal.

 

[rbulsara]

Oil may be ok, the insulation on the winding may not be hence the low insulation resistance results. I don't think this has to do with the oil alone.

What was the dielectric strength of the oil? Insulation test is not the measure of oil's dielectric strength.

 

[Zogzog]

How long did you apply your test voltage? What was the PI result? 1kV is not a high enough test voltage, you should be using at least 5kV.

 

[Gauss2k]

rbulsara: the dielectric strength was about 33 to 35 kV (standard test)

Zogzog: The voltage was applyed for 1 minute. I know the voltage is not high, but that doesn't explain why we got those results. The polarization index was not calculated.

By the way, all cables were disconnected during the test.

 

[Zogzog]

Could be the bushings. you should test a transformer for 10 minutes, your actual IR readings are even less than your 1 minute reading, you still were seeing charging currents at 1 minute making your IR reading higher than actual.

 

[rbulsara]

Many variables:

What was the condition of transfomers before the test? Were they in service?

Why was the test perfromed?

How old are the units? Winding configurations-delta-wye etc?

What was the last time the same test was done?. Is there any history or trending data for the same units? How do they compare?

Wye- connected windings could have tapered insulation (just a guess, I am not a trasfromer manufacturing expert)and if the leads at star point are still connected together, it could give distorted results. I have seen that happening in a old generator windings.

My hunch is this is just an old tranformer and its insulation, somewhere on winding, leads or bushings etc., is deteriorated.

 

[unclebob]

I have done insulation resistance test on many PCB transformers and I have never obtain more than 10 Mohm, either
on high or low voltage side. All tests conducted at 1 KV.

 

[Gauss2k]

rbulsara: The tests were done as an annual inspection and verification. Both transformers were online before the tests and they are online as we speak.

The units are about 30 years old and are delta-wye. We have no history about earlier testing.

Unclebob: MB told me the same thing, but there was no clear explanation to this. I was just wondering if someone else could explain why PCB transformers always seem to have those results. And by the way, PL would like to have your e-mail address so we can have news from you! You know ours, send it there if you want!