Should I fail this transformer?

[cjones09]

I am PI testing a small dry type transformer and my reading was 157 gigohm after 10 min but PI is less than 1.

Neta says investigate valuess less than 100 megohms but PI "shall not be less than 1.0"

Should I fail this transformer?

[Test Tech from DMV]

What size dry type? What was your final value?


I am PI testing a small dry type transformer and my reading was 157 gigohm after 10 min but PI is less than 1.

Neta says investigate valuess less than 100 megohms but PI "shall not be less than 1.0"

Should I fail this transformer?

[cjones09]

225kVA and the final value was 157 gigohm after 10 min.


What size dry type? What was your final value?

[Test Tech from DMV]

I know some megger values decrease when you physically get close to the test leads. 100+ Gig Ohms, I wouldn't fail it.

[cjones09]

I agree that the value is good but since NETA-ATS specifically says "value shall not be less than 1.0" does that mean it fails no matter what the value is until PI is greater than 1.0? Thanks for the reply.


I know some megger values decrease when you physically get close to the test leads. 100+ Gig Ohms, I wouldn't fail it.

[lester mcmanaway]

Did you have the transformer completely isolated?

[cjones09]

Yes the transformer was isolated. I'm wondering if maybe I have a bad test lead.


Did you have the transformer completely isolated?

[lester mcmanaway]

Yes the transformer was isolated. I'm wondering if maybe I have a bad test lead.

Do an open air test. The megger should max out pretty much right away

[Zog]

Once you get up in the Gig range the PI ratios are not very important, there is a note somewhere regarding this, I will see if I can find it.

[cjones09]

So today I re did the test and the problem was not the test leads but the jumper cables I was using to short the windings together. I did the test without the jumpers and the readings went to full scale. Most PI values were well over 4. I've been taught that jumpers help reduce voltage stress on the windings but it clearly has a big effect on the readings.


Do an open air test. The megger should max out pretty much right away

[cjones09]

Thank you for helping to clarify, Zog!


Once you get up in the Gig range the PI ratios are not very important, there is a note somewhere regarding this, I will see if I can find it.

[quantum]

So today I re did the test and the problem was not the test leads but the jumper cables I was using to short the windings together. I did the test without the jumpers and the readings went to full scale. Most PI values were well over 4. I've been taught that jumpers help reduce voltage stress on the windings but it clearly has a big effect on the readings.

When using jumpers it is really important to make sure they are hanging in free air or if that's not possible, they are at least only touching insulated parts. If you think about it, say you use chopped up car battery jumper cables that are not rated for even a 1kv megger test and these are energized and leaning against the transformer core or ground. At this point you are really testing the insulation of your jumper cables.

[cjones09]

Thanks quantum! This is exactly what happened. How necessary would you say are jumpers when PI testing transformers? I've always been taught to use them but it obviously effects the reading - maybe even in free air?


When using jumpers it is really important to make sure they are hanging in free air or if that's not possible, they are at least only touching insulated parts. If you think about it, say you use chopped up car battery jumper cables that are not rated for even a 1kv megger test and these are energized and leaning against the transformer core or ground. At this point you are really testing the insulation of your jumper cables.

[nscdrgs]

I am PI testing a small dry type transformer and my reading was 157 gigohm after 10 min but PI is less than 1.

Neta says investigate valuess less than 100 megohms but PI "shall not be less than 1.0"

Should I fail this transformer?

Hi,
I had this issue before when the Megger battery was low.
I charged and I retested and was fine.

Another time the weather just changed during the 10 minutes test from cloudy to rain. I retested in same conditions (rain).

[alexlounsbury]

If you think about why a PI less than 1 is failing, it makes more sense. That would mean that the insulation is breaking down under test. The reason it isn't as relevant at such high readings, is that the measurement is extremely close to 0 current flow. Any tiny change in conditions could cause a massive change in results. The insulated leads touch, test set gets moved, air moves around the test area; all of those things will cause an extremely small change in current draw, thus decreasing the measured insulation resistance.

Jumpers aren't necessary when performing insulation resistance on a transformer. The windings are interconnected, and thus all energized. See the attached diagrams of the windings for a Delta Wye transformer. If you were to energize 10kVDC H1 with the positive lead without the connections tied together, you're still going to have 10kVDC on H2 and H3.

[ElectricalTestTech]

I would disagree with the statements about whether or not to short your windings when performing an insulation test. While you are correct in stating that a 3 phase wye or delta winding is already interconnected with jumpers, I disagree and believe you need to still short all of the windings. If you don't short the windings, you polarize the windings and align all of the electrons in a specific direction. The longer the voltage is applied the less movement of electrons is present and the leakage current begins to drop. But you are applying the voltage from one direction. If you use jumpers, you apply the DC voltage evenly across the windings. The idea is to stress the winding evenly from both directions. Depending on the material properties, structure, and condition of the insulation can influence the polarization phenomenon. There also may be dipoles present in the electrical insulation that should have voltage applied from both ends. (A dipole may be represented by a particle having small positive charge at one end and a small negative charge at the other end. When these dipoles are subjected to DC voltage, they are polarized and become aligned with respect to positive and negative polarity of the DC voltage.)

All credible literature that involves transformer testing (Megger, Paul Gill, etc.) mention that all windings should be short circuited.

[Kalbi_Rob]

Once you get up in the Gig range the PI ratios are not very important, there is a note somewhere regarding this, I will see if I can find it.

If a very high initial IR reading >5GΩ is found then further PI tests are not needed.

Reference:
https://electrical-testing-safety.co.uk/regulations/polarisation-index

If the IR1 value (at 40 °C) is greater than 5000 MΩ, the P.I. may be ambiguous and can be disregarded (see
12.2.2).

Reference: IEEE 43-2013 IEEE Recommended Practice for Practice for Insulation Resistance of Electric Machinery Section 11.2 Suitability for operation or continued testing

[Warrengarber]

How low is the PI?




I am PI testing a small dry type transformer and my reading was 157 gigohm after 10 min but PI is less than 1.

Neta says investigate valuess less than 100 megohms but PI "shall not be less than 1.0"

Should I fail this transformer?

[gbriske]

So today I re did the test and the problem was not the test leads but the jumper cables I was using to short the windings together. I did the test without the jumpers and the readings went to full scale. Most PI values were well over 4. I've been taught that jumpers help reduce voltage stress on the windings but it clearly has a big effect on the readings.


Generally when insulation resistance testing or high potential testing you want to make sure your jumper leads are rated for the voltage you are testing to, if they are going to be in close proximity with any bonded/grounded parts, otherwise need to make sure they have enough open air clearance, otherwise they will affect your results by introducing more paths for leakage.

[jflan13]

https://www.ecmweb.com/content/review-polarization-index-and-ieee-std-43-2000
The original article I saw this in was a GE study on PI, but the summary is; If your 1 minute reading is above 5 gig, the PI may not be a useful reading.

The testing at this point is so critical that the standard states, “If the one-minute insulation resistance is above 5,000 Meg-ohms, the calculated PI may not be meaningful. In such cases, the PI may be disregarded as a measure of winding condition.”

[Lukedl83]

So today I re did the test and the problem was not the test leads but the jumper cables I was using to short the windings together. I did the test without the jumpers and the readings went to full scale. Most PI values were well over 4. I've been taught that jumpers help reduce voltage stress on the windings but it clearly has a big effect on the readings.

Where did you find the information on jumpers reducing stress on the insulation when performing a IR/PI test? I could understand AC when energizing these windings, but not so much on a DC test since there is very little resistance between each phase winding.

[Kalbi_Rob]

If a very high initial IR reading >5GΩ is found then further PI tests are not needed.

Reference:
https://electrical-testing-safety.co.uk/regulations/polarisation-index

If the IR1 value (at 40 °C) is greater than 5000 MΩ, the P.I. may be ambiguous and can be disregarded (see
12.2.2).

Reference: IEEE 43-2013 IEEE Recommended Practice for Practice for Insulation Resistance of Electric Machinery Section 11.2 Suitability for operation or continued testing
FYI, IEEE 43-2013 states:

12.2.2 Applicability of polarization index when IR1 is greater than 5000 MΩ
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 submicroampere 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.

[Jgilsen]

So today I re did the test and the problem was not the test leads but the jumper cables I was using to short the windings together. I did the test without the jumpers and the readings went to full scale. Most PI values were well over 4. I've been taught that jumpers help reduce voltage stress on the windings but it clearly has a big effect on the readings.

I have seen the same effect with using jumpers while IR testing MV cables. The cables failed while trying to test a run of 3 at the same time connected with jumpers. I tried testing each cable individually and bolting the cables together and they all passed with both non-jumper methods. The interesting thing was that the jumpers were suspended in open air with seemingly sufficient clearances from other conductors or ground, all testing was performed indoors in a controlled environment. Now I test with as few jumpers as possible.