Transformer testing
- Thread starter siva
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zog
Senior Member
- Location
- Charlotte, NC
Re: Transformer testing
Well the TTR results should match the nameplate data within 0.5%, and should be done on each tap.
Winding resistance varys greatly but should be the same for all three phases. Winding resistance is a pretty useless test anyhow IMHO.
They should also conduct a PI test and a PF test. If this an oil filled transformer he should also draw an oil sample and have it tested, including a DGA analysis.
Well the TTR results should match the nameplate data within 0.5%, and should be done on each tap.
Winding resistance varys greatly but should be the same for all three phases. Winding resistance is a pretty useless test anyhow IMHO.
They should also conduct a PI test and a PF test. If this an oil filled transformer he should also draw an oil sample and have it tested, including a DGA analysis.
Re: Transformer testing
Hi Scott Peterson
Thanks for your information.Here we did the insulation test and voltage ratio test.These are dry type 3000 KVA 11 KV/415 V transformer.Winding insulation resistance results are(Using Megger)
For HT winding - Applied 5000 V for 1 minutes
U-GND= >5 T ohm, V-GND= >5 T ohm, W-GND=3.31 T ohm
For LT Winding - Applied 500 V for 1 minutes
U-GND=155 G ohm, V-GND=193 G ohm, W-GND=236 G ohm
Can we accept the difference like greater than 5 terra ohm versus 3.31 terra ohm?
Reading for second transformer is
For HT winding - Applied 5000 V for 1 minutes
U-GND= 3.14 Tohm, V-GND= 3.93 Tohm,W-GND >5 Tohm
For LT Winding - Applied 500 V for 1 minutes
U-GND >500 G ohm, V-GND >500 G ohm, W-GND >500 G ohm.Both transformers are 3000 KVA and supposed to be identical..
Thanks
Siva
Hi Scott Peterson
Thanks for your information.Here we did the insulation test and voltage ratio test.These are dry type 3000 KVA 11 KV/415 V transformer.Winding insulation resistance results are(Using Megger)
For HT winding - Applied 5000 V for 1 minutes
U-GND= >5 T ohm, V-GND= >5 T ohm, W-GND=3.31 T ohm
For LT Winding - Applied 500 V for 1 minutes
U-GND=155 G ohm, V-GND=193 G ohm, W-GND=236 G ohm
Can we accept the difference like greater than 5 terra ohm versus 3.31 terra ohm?
Reading for second transformer is
For HT winding - Applied 5000 V for 1 minutes
U-GND= 3.14 Tohm, V-GND= 3.93 Tohm,W-GND >5 Tohm
For LT Winding - Applied 500 V for 1 minutes
U-GND >500 G ohm, V-GND >500 G ohm, W-GND >500 G ohm.Both transformers are 3000 KVA and supposed to be identical..
Thanks
Siva
zog
Senior Member
- Location
- Charlotte, NC
Re: Transformer testing
Those readings all look good (>25G), however you should do a 10 minute test and compare the 10 minute reading to the 1 minute reading. This test is called a polarization index test (PI) and gives you a better indacation of the condition of your insulation.
Those readings all look good (>25G), however you should do a 10 minute test and compare the 10 minute reading to the 1 minute reading. This test is called a polarization index test (PI) and gives you a better indacation of the condition of your insulation.
friebel
Senior Member
- Location
- Pennsville, N.J.
Re: Transformer testing
Zog, I do agree with you when you suggested that he do a polarization check of the transformer with his megger. Yes, you are correct, you take a reading at 1 minute, and then take a reading at 10 minutes. Then you divide the 1 minute reading into the 10 minute reading and the result is as follows: (A) Less than 1 reading, (Dangerous)
(B)1.0 to 2, reading, (Questionable)
(C)2 to 4 reading, (Good)
(D)Above 4 reading, (Excellent)
The above information is taken from the Megger Insulation Handbook, called " A Stitch in Time".
If you send me a Private Message, I can give you the name and phone number of the Electrical Engineer that works for the AVO company, formerly the Biddle Megger Company, and mention my name, and I know that he would be glad to send you the booklets.
Zog, I do agree with you when you suggested that he do a polarization check of the transformer with his megger. Yes, you are correct, you take a reading at 1 minute, and then take a reading at 10 minutes. Then you divide the 1 minute reading into the 10 minute reading and the result is as follows: (A) Less than 1 reading, (Dangerous)
(B)1.0 to 2, reading, (Questionable)
(C)2 to 4 reading, (Good)
(D)Above 4 reading, (Excellent)
The above information is taken from the Megger Insulation Handbook, called " A Stitch in Time".
If you send me a Private Message, I can give you the name and phone number of the Electrical Engineer that works for the AVO company, formerly the Biddle Megger Company, and mention my name, and I know that he would be glad to send you the booklets.
Re: Transformer testing
Hi
I understood that these values are above the acceptable level.But as I said these transformers are identical ones.Still how these difference is coming like
Transformer -A (LT winding)
U-GND=155 G ohm, V-GND=193 G ohm, W-GND=236 G ohm
Transformer -B (LT winding)
U-GND >500 G ohm,V-GND >500 G ohm,W-GND >500 G ohm
and also for the same transformer for the third winding difference is approx 2 terra ohm...U-GND= >5 T ohm, V-GND= >5 T ohm, W-GND=3.31 T ohm.
Is there any specific reason for this or is it a common thing in manufacturing..?
Thanks
Siva
Hi
I understood that these values are above the acceptable level.But as I said these transformers are identical ones.Still how these difference is coming like
Transformer -A (LT winding)
U-GND=155 G ohm, V-GND=193 G ohm, W-GND=236 G ohm
Transformer -B (LT winding)
U-GND >500 G ohm,V-GND >500 G ohm,W-GND >500 G ohm
and also for the same transformer for the third winding difference is approx 2 terra ohm...U-GND= >5 T ohm, V-GND= >5 T ohm, W-GND=3.31 T ohm.
Is there any specific reason for this or is it a common thing in manufacturing..?
Thanks
Siva
Re: Transformer testing
Make sure that you get a PF test also....very important!!!!
This is mainly due to manufacturing, they look OK, but I would keep an eye out in the longterm. While the transformers are identical, the manufacturing process allows those variations, evern in the best "controlled climate" facilities. Because of the ratings of the units, they are probably not manufactured in a "controlled climate", those are usually reserved for large power transformer manufacturing.
Make sure that you get a PF test also....very important!!!!
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