Polarization Index and Insulation Power Factor Test 4

[encon5]

Can somebody enlighten me on Polarization Index and Insulation Power Factor Test. Which of the two has more significance in terms of determining the integrity of insulation of a brand new 3 phase oil-filled transformer rated at 23KV/460V. Would appreciate your response.

encon5

 

[oldfieldguy]

The insulation power factor test is THE definitive test for transformer insulation. Properly done, it gives results that can be compared with industry information. Doble Engineering has a database with thousands of samples.

Additionally, the manner in which the test is performed makes it the more easily interpreted for trending information. A good test technician can configure the test to remove questionable elements from the results to better locate problems.

Utility and heavy industrial users have depended on the power factor test for decades.

old field guy

 

[electricpete]

I agree 100%. If the unit is big enough to easily sample, oil analysis can be a very useful test as well.

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[rcw retired EE]

Polarization index readings are sometimes meaningless on new units. The insulation is in such good shape that the ratio of the final reading to the high initial reading is skewed. On old analog Meggar units the needle would be in the almost inifite protion of the scale.
The ratio of infinity to infinity = 1.0, which is a suspect polarization index number. We were trying to ratio very good against excellent.

 

[Zogzog]

I agree with these guys, Pf is the test taht will give the truest condition assesment. PI is just a 10 minute meg test comparing 1 minute with 10 minute readings.

 

[HVDragon]

There is a big difference between these tho measurements. PI=R600/R60 is based on pure polarisation. PF or tandelta /in Europe/ is global macro index. Of course everithing depends of two time constants inside the transformer. One of them - oil constant and the other one - paper constant.
In my experience / I usually test about 100 power transformers per year from 10 to 750 KV and 500 MVA/ I always measure as most as possible isolation characteristics. Every single measurement is like a small part of window. When all of these parts are visible we can say what is condition of the trasformer for sure.
Only one thing - PF has extremely big dependence from moisture and temperature. PI - hasn't, but SV /step voltage/ can help You too.

Somethin terrible is stalking between the high voltage potentials