Insulation Resistance Of Power Transformer

Insulation values are good.What type of transformer it is? What type of cooling?

Before any good evaluation can be done, more info will be needed:

What are the HV and LV winding voltages of the transformer? Was the same test instrument used for all the measurements? Was it calibrated recently, to know that its measurements are accurate? What were the weather conditions when each set of measurements were taken, particularly the relative humidity?

500MΩ to 90MΩ and back to 200MΩ is not too big a change (in the same order of magnitude), and could just be due to humidity. If the bushings were not wiped clean and dry, or the instrument's test leads were carelessly draped during the test, it is possible that this would cause the change in readings. There are multiple possibilities for this amount of variation.

here are the infos that you were asking me sir.

69000/13200

DYn1

the test instrument used was insulation resistance tester, Megger, 5kV

when the transformer was to be energized after a long time of deenergization and then reconditioned, the test result that i have given above was with a hot weather condition. (500Mohm)

then the result which gave 90Mohm was tested during night time at around 10PM

then the 200Mohm result was taken at 12 noon

The polarization index measurement is good if to judge the quality of the insulation. The Polarization index is the ratio of IR test after 10 Minutes to IR after 01 Minute . If polarization index is more than 3, IR value of the transformer is OK.

CR4 Admin: removed link to virus-infected site

WARNING!!!

LINKS IN THE ABOVE POST GO TO A VIRUS INFECTED SITE.

DO NOT CLICK ON THEM!

REPORTED TO ADMINS.

Dear prometu,

You have not mentioned HV and LV voltage rating, connection diagram of the transformer. What is the voltage rating of the insulation tester( meggar)? Had you clean properly all the terminals, bushings,etc. before insulation testing? During oil filteration it mandatory to use vacuum oil filteration plant for this large transformer; but the same is not mentioned.

Low values of insulation indicates that due non use for long time, moistures ingressed in transformer insulations ( specially paper insulation) and cores. After energisation and use of the transformer insulation values are coming down due increase of oil temperature; after cooling down for five hours time the insulation values are again going upto the level of 200 Meg ohm.

For upgradation of insulation values in simplest method you can do the following:

Short the all secondary terminals and inject three phase impedance volt( depending of percentage impedance of transformer) so that full load current will flow in secondary side and continue this for long hours say, 10 to 12 hours so that sufficient heating in windings will be occured(note the oil temperatue and winding temperature )and ingressed moistures will be out to some extent. After withdrwal of voltage measure insulation values in hot and cold condition.

Regards,

Manindra

here are the infos that you were asking me sir.

69000/13200

DYn1

the test instrument used was insulation resistance tester, Megger, 5kV

when the transformer was to be energized after a long time of deenergization and then reconditioned, the test result that i have given above was with a hot weather condition. (500Mohm)

then the result which gave 90Mohm was tested during night time at around 10PM

then the 200Mohm result was taken at 12 noon

during filtration we have used vacuum filtering machine. the method that you have mentioned at the last part of your message ins't familiar to us at all. what we are planning now is that we will bake the winding of the power transformer but i know that it would cost us a lot and waste some longer time.

but you still haven't answered me with what i am asking sir. i am asking about the standard values of the insulation resistance of a power transformer.

There is no single "standard value" of insulation resistance, although various standards give a range of acceptable values. For a 69kV transformer, these values are on the low end of what I would like to see, but they don't appear in danger of imminent failure. You didn't tell how old the transformer is, but you say it is reconditioned, which implies it is older so I am not surprised that the values are not what they could be for a new unit.

As for the range of test values, they vary by less than a factor of 10, so I would expect that it probably relates to the fact that the transformer was cycled from de-energized (cold) to energized (hot) back to de-energized again. There is always an equilibrium between moisture dissolved in the oil and held in the winding insulation. Heating the transformer drives water from the winding papers into the oil, and cooling draws it back into the paper (or if cold enough, condenses it to sit on the floor of the tank). That in itself would cause the difference in the readings, as the "hot" reading (middle one) would have more moisture in the oil.

You gave the oil analysis BDV of 45kV, which is acceptable, but I would suspect it would vary somewhat if you had sampled it at each of the shut-downs. My guess is that at the 90MΩ IR reading, the BDV may have been somewhat lower due to more moisture in the oil, but it's only a guess.

If you really want a good evaluation of the insulation quality of the transformer, you should go beyond the DC tests done with a megger, and have Doble (TM) insulation power factor tests (also known as dissipation factor) done. It is an AC test that can give a more detailed evaluation of the insulation quality of the transformer components (oil, windings, and bushings) and point to specific possible problem areas so that targeted correction can be made. Find a qualified testing company and have them do the tests for you if you want to be more confident in your transformer.

Dear prometu,

Please note that for old and used transformer, in respect of feild engineering practice,for voltage rating 66Kv/69Kv, the minimum acceptable value of IR is given below when carried out by 5Kv meggar for minimum period of one minute:

30 deg. celcius - 600 Megohm

40 deg.celcius - 300 Megohms

50 deg. celcius - 150 Megohms

60 deg. celcius - 75 Megohms

Regards,

Manindra

I know that Iam 9 years away from the thread date, however it was a lot of satisfaction for me personally as we did short circuit the LT winding of our 70 MVA, 132 KV /11 KV to accelerate the IR value for a healthy value. My opinion did match to the opinion of the thread's opinion which boosts my confidence.

regards

M.R.V.Rajesh