Should I fail this transformer?

[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...risation-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/revie...ee-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.”