Polarization index in MV motors and genrators 1

[Zhk1983]

As IEEE 43-2000(Cl no 12.2.2) says , If 1 min IR is greater than 5000Mega-Ohm then Polarization index may or may not be indication of insulation condition.

Can any one elaborate what is phenomena behind this.



regards

Zafar

 

[electricpete]

According to "A New Polarization Index Test?" by Richard Nailen in Electrical Apparatus magazine, December 2001 edition, the problem was that many end users had test equipment which was not capable of reliably/accurately reading higher than 5 gigaaohms. Their meter would be near max R at both 1 minute and 10 minute and they would write down something like 5000M/5000M = 1 and fail the test. That's about all he says on the subject.

But it raises in my mind a question why we wouldn't apply a P.I. limit at higher insulation resistance levels IF we know our test equipment is operating within its reliable indicating range. Maybe they are trying to provide allowance for very small degree of resistive tracking in vicinity of test leads (small enough to be insignificant at lower insulation resistance levels but dominate at this level)? Or is there some characteristic of the insulation itself that suggests PI should not be relevant when IR very high (haven't heard of that). Beats me.


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

 

[ScottyUK]

I agree ePete - making reliable measurements at very high insulation resistance values in a real-world plant environment becomes tricky. For example, surface contamination of the test leads themselves can cause strange results which don't seem to make sense. I spent an entertaining hour or so chasing ghost readings until I finally cleaned the leads with an alcohol wipe, at which point order was restored. It is interesting how few people know how to use the guard terminal of the larger meggers, which is pretty much essential for readings in the 1T[Ω] range.

 

[electricpete]

Yes, and those problems at the leads can also be somewhat voltage sensitive and interfere with a dc step voltage test...

An interesting one I heard about from our repair shop recently was dc step voltage test of 13.2kv stator to around 30kvdc… they had very non-linear results during initial test with unlugged extra-long motor T-leads tied to each other with bare wire at some distance far away from any ground (I think it was 3 feet from ground). Clamping the T-lead ends removed the non-linearity. From what I can gather, there must have been some corona originating at some of the strands where they feathered apart at the ends (corona is enhanced by small effective conductor diameter and by the end/tip of the conductor), even though the leads were very far from ground. Also again the resistance of the newly rewound stator was very high so as you point out it didn't take much current (from corona) to affect the reading.

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

 

[prc]

The standard was revised in 2013. Under 12.2.2, it says "When the insulation resistance reading obtained after the voltage has been applied for 1 min (IR1) is higher than 5000 MΩ, based on the magnitude of applied direct voltage, the total measured current (IT) can be in the sub microampere range (see Figure 3). At this level of required test instrument sensitivity, small changes in the supply voltage, ambient humidity, test connections, and other non-related components can greatly affect the total current measured during the 1 min –10 min interval required for a P.I. Because of these phenomena, when the IR1 is higher than 5000 MΩ, the P.I. may or may not be an indication of the insulation condition and is therefore not recommended as an assessment tool."

 

[Zhk1983]

Thanks everyone for your valuable inputs.