Polarization Index

[buddy91082]

For PI results for newly installed dry type transformers less than 2 (I have results @ 1.9) what are strategies and/or remedies aside from having the manufacturer replace the transformer to get a beter result? Or do we just have a monitoring program implemented to watch insulation?

Thanks.
b

 

[glendale]

What is the final (10 minute) insulation resistance?

 

[buddy91082]

331,000 megohms.

1 minute is 169,000 megohms. The values look good to me but the ratio doesn't indiacte that.
I am begining to belive the PI is not accurate indication of IR conditions.

Thanks.
b

 

[ScottyUK]

Neither is a particularly great result. The values are low and the PI is low.

Have you made absolutely certain that everything is clean? Terminations and bushings obviously should be clean. How about your test leads? It is well worth running a cloth dampened with a mild solvent - isopropyl alcohol for example - down the length of the test leads before doing any measurements. You will be amazed how much crap adheres to silicone leads, even ones which look clean. Dirty leads can give all sorts of screwy readings - I have been lead a merry dance with this in the past, especially when chasing readings in the G[Ω] range.

 

[electricpete]

I'm coming from rotating equipment background, can't recall typical values for transformers.

But 169,000 megaohms @ 1 minute seems pretty darned high to me. Certainly it is for motors.

And in the motor world, IEEE43 for motors does not require PI is the 1-minute reading is above 5,000 mega-ohm because the reading tends to get inaccurate at very high resistances since this equates to low measurement currents.

=====================================
(2B)+(2B)' ?

 

[glendale]

A couple of other questions:
What are the primary and secondary voltages of the transformer, and is the PI result you quote the LV winding to earth?

 

[GTstartup]

I think if you told the manufacturer that your transformer tested at 170 GigaOhms and you want a replacement he might have something to say about that.

 

[buddy91082]

primary voltage: 4160v
secondary voltage: 480v

The PI results are from the LV winding.

 

[glendale]

I believe there is no merit in carrying out a PI test on the 480V LV winding, I am suprised you managed to get a PI as high as 2. I only carry out PI tests on 6.6kV and above using a 5kV DC megger. I would recommend that you energise the transformer as the insulation resistances you have recorded are perfectly acceptable.

 

[buddy91082]

Correction. Values are from high to low side tested at 5kVdc.

I agree that the measured values alone are an indication of good insulation. But when you apply the PI ratio, that would point to a "fair" condition. I am starting to see that PI ration cannot be relied upon.

Thanks.
b

 

[ScottyUK]

I am going to book myself in for an eye test. I have mis-read the 169,000 M[Ω] as 169 M[Ω].

If the eye test comes back OK I am going to book myself in for a brain test, see if they can find one...


Apologies for my earlier post - disregard everything I said.

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