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Polarization Index (PI) for oil-filled transformers 2

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edison123

Electrical
Oct 23, 2002
4,490
Is there any standard that specifies Polarization Index (PI) for oil-filled transformers ?

I know PI is applicable for solid insulation like generator/motor windings. But trafos ?

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The only thing the NETA MTS says is the PI should be >1.0 for oil filled transformers.
 
Thanks Zogzog. Is there any reference no for this NETA MTS ?

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I've never heard it used with respect to liquid dielectric.

How much peer pressure could there be?
 
is where you can get the specs. There are 29 references listed in that section from ASTM, ANSI, IEEE, etc. IEE 902 (Yellow book) I believe is the main reference.
 
IEEE62-2005 Guide for Diagnostic Field Testing Part1 Transformers (clause 6.1.5.2)recommends a PI (R10/R1 value) of 1 or slightly above 1 for small transformers and 1.1-1.3 for large transformers.A value of less than 1 calls for further investigation.
Strictly PI is not a clear indication of dryness of insulation in oil filled equipment and should be handled with caution.This will be very much affected by the resitivity of oil used.So most of the customers do not insist for minimum values.The advantage of PI is that it is not temperature sensitive ie value will not change with temperature.But there are utilities who insist for minimum values for PI eg one utility in India insist for minimum value of 2 for PI and 1.5 for dielectric absorption factor ie R 60sec/R15 sec for new transformers.With new transformers with new oil having high resistivity it is extremely difficult to achieve such high values.
 
Thx prc.

"With new transformers with new oil having high resistivity it is extremely difficult to achieve such high values."

This is exactly our client is facing (we are doing the generator for them but they came to me with this problem with their new trafo). Since I haven't heard PI mentioned in re trafos, I came here. I will pass your on your tip to them. Thx once again.


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