Transformer Problems and Meggar Values

10 mva 33/11 kv transformers, the insulation value without oil of hv-lv,hv earth and lv earth is more than 10000 ohms

Dear Guest

The water absorption is too high even your breakdown voltage for the oil is high, drying the winding before fill in the oil the insulation resistance when winding alone should be in GIGA OHM at leat 5 GIGA OHM, your x-mer after oil fill in should be over than 200 Mega Ohm.

10000 ohms = 10 KOhm , its very low and its indicate moisture involving in your winding,

If baking oven is not available may you can dried out by processing vacuum machine or oil purifying machine try 6-8 cycles, depend how much is water involving in your winding

Try and good luck

First thing to be done is to take a sample of the oil and test it as per transformer oil testing procedure . Filter and heat the oil to remove moisture content if any with the help of oil filtration unit. Run the oil through an oil filtration plant for few hours to eliminate the moisture content if any in it. Take insulation test values before and after the filtration.Check weather the value improves.

Since the insulation value of the winding was dropped you must make it a point to take out the winding from the tank and must bake in the oven. must clean the tank also as the insulation failure may be due to contamination of the tank before reinserting the transformer in the tank . Must clean the tank with clean transformer oil. Before refilling the tank with transformer through check should be made to avoid an insulation failure later at site.

You should not fill the oil directly from oil durum with out carrying out insulation test and filtration. You have to carry out all tests including chemical test to arrive at the cause of insulation failure.

First order of the day, Check your Megger. for supply voltage, and than against a standard resistor. Are you sure it's oil in there? It is certainly not humidity problem.

Second order, If your reading are really 10KΩ and 400Ω respectively, Don't even think of connecting the high voltage. or do it if you like explosions.

Third, If you consider even 10KΩ between windings or windings to earth a good start, go back to your text books.

Wangito.

A you said,the oil is new but sometimes the oil can be contaminated with moisture.This reduces the dielectric strenght of the oil.I will advice you to carry out dielectric strenght as well as crackle test to confirm this problem.

Exact problem was faced by us before few years in the case of a 132 kv trfr being commissioned at Oman . The problem was with the oil . OPl ask to verify the oil property with the supplie of the oiil ( even the most reputed ones make mistake sometimes ).

Meanwhile , try some used oils from a running trfr or change the sopurce of oil .

We have found that the oil was faulty in our case .

Thanks

R K Mohapatra-