Transformer very low megger readings 2

[capuchi]

hi guys,

We have two brand new, dry type to be put on service. T-1 is a 750 KVA and T-2 is a 500 KVA. Both are 460/230 Volts, 3 phase delta –Y. Using a 1000V megger, the insulation resistance rdg of T-1 are : Pri-Grnd= 160, Pri-Sec=259 and Sec-Grnd=158; while the T-2 has Pri-Grnd=2.86, Pri-Sec=3.9 and Sec-Grnd=2.9 All readings in Megohms. I am apprehensive putting into service the T-2 owing to the very very low insulation resistance readings as compared to T-1. Is this sign of moisture in T-2? Do you think it can safely be placed in service? I intend to use infra-red lamp to dry the T-2 and monitor the insulation resistance. Will this be effective? Another alternative drying methods?
Please note that T-2 while stored indoor with enclosures intact, however rainwater got its way into the room. Would appreciate any advise or recommendations.
Thanks in advance,
capuchi

 

[oldfieldguy]

capuchi--

You're right to be apprehensive. Identical units should show essentially identical results. While 2.8 megohms might be marginally acceptable for an older unit that has been in service and is dirty and damp, that reading much lower than I'd expect for a new unit.

Your idea about applying heat to your T-2 is not a bad one at all. I'd do that at least overnight, but do make sure that the temperature remains under 200 degrees F or 95 degrees C. After you remove the heat, megger it immediately, then again after it cools off. The first megger reading may be rather low due to the heat, but the second reading should be closer to acceptable.

If the readings do not improve significantly, I'd have a talk with my vendor.

old field guy

 

[HVDragon]

It's dangerous to use test voltage more that nominal voltage of the transformer.
You did't say nothing about the test temperature.
The isolation of T2 is very low. Try to clean the bushings,mark the temperature and do the measurements again, but these 3 measurement circuits : HV-LV+G ; LV-HV+G ; HV+LV - G.
If You can test the transformer at 2 different temperatures with minimum 20C degrees we will know where is the problem

Something terrible is stalking between the high voltage potentials