Polarization Index Test

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pabrey630

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Good day to everyone!

We have conducted PI test to generator stator winding. The rating of generator is 2000kW. We have applied 500V to the winding and we got a PI ratio of 1.49. According to some references, this value is dubious. Can you tell me what is the standard or acceptable value for this kind of test. Was it stated in IEEE or ANSI as well?
 
Re: Polarization Index Test

Polarization Index Acceptable Values.
To: pabrey630
You did not mention the Ratio that you used to get the reading of 1.49.
The two ratios are: 60/30 second ratio, and 10/1 minute ratio.
I am quoting from a manual from Megger, called
"A Stitch in Time".
If you were using the 60/30 second ratio, the reading of 1.49 is listed as GOOD.
Using the 10/1 minute ratio, the reading of 1.49 is Questionable.
I could send you the information where you can contact the Megger Company, and I am sure that they would send the manual that I have quoted the information to you.
 
Re: Polarization Index Test

A PI test is a 10 minute reading/ 1 minute reading. The 60 sec/30 sec ratio is called a DAR (Dielectric absorbtion ratio). Your result is questionable for a PI test but the results are not accurate if you use too low of a test voltage, what is the voltage rating of the generator? If it is 480V you should have used a minimum of 1000VDC for your PI test.
 
Re: Polarization Index Test

Thanks for your replies.

The voltage rating of generator is 4160V, 2000kW, 3 phase synchronous generator. We used 10:1 minute ratio. We applied 500V of megger tester between the stator winding and to ground.
 
Re: Polarization Index Test

You minimum test voltage for you PI test should have been 2500V, a 500V test will not give you accurate results for your PI test or IR readings.
 
Dielectric-absorption test

Dielectric-absorption test

zog said:
You minimum test voltage for you PI test should have been 2500V, a 500V test will not give you accurate results for your PI test or IR readings.
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Here on the company we have 30/12kV power transformers and 12kV/127V distribution transformers. You mentioned that for a device operating on 480V AC you should have a testing voltage of 1000V DC. Now my question is the following: What are the minimum test voltages that we can use on these transformers to still obtain reliable results and if there a formule or rule to define the minimum test voltage.
 
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