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Drying out of power transformers


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By the process of Transformer oil filtration we remove the sludge and moisture in the transformer oil, but we can not remove the moisture present in the transformer winding. For removing the moisture present in the transformer winding transformer drying out is necessary



DRYING OUT PROCESS OF POWER TRANSFORMERS

By the process of Transformer oil filtration we remove the sludge and moisture in the transformer oil, but we can not remove the moisture present in the transformer winding. For removing the moisture in the transformer winding transformer drying out is necessary

For carrying out the drying out process at site, following procedure is adopted

1)Test the oil in transformer prior to drying out and record for
a) Moisture content(ppm)
b) Break Down Voltage in KV
2)Take the Insulation Resistance (IR Values), LV Winding to Earth, HV Winding to Earth and HV winding to LV winding with oil prior to dry out.
3)Measure the dew point of N2 in cylinders.
4)Drain oil from transformer under N2 cushion.
5)Measure the insulation resistance (IR Values) without oil.
6)Place the space heaters around the transformer body and lag the same with glass wool to prevent thermal transfer/loss.

FIRST CYCLE
7) Fill N2 in main tank and pressurize the same up to 3 PSI, hold the pressure for three to four hrs and observe the dropping of pressure, if there is drop in pressure check and attend the leakages, then measure dew point. Switch on the space heaters and measure dew point after 24 hrs.
8)When wall temperature reaches 60 Deg Centigrade and OTI at 45 Degree Centigrade, pull vacuum to 3 Torr and hold for 24 hrs.

SECOND CYCLE
9)Start N2 filling with the heaters ON. Pressurize transformer to 3 PSI and hold for 24 hrs. Measure dew point after 24 hrs.
10)When tank wall temperature reaches 80 Degree Centigrade and OTC at 50 Degree Centigrade, pull vacuum to 3 Torr and hold for 24 hrs.

THIRD CYCLE:
11)Start N2 filling and when tank wall temperature is 43 Degree Centigrade, OTI at 35 Degree Centigrade Minimum, switch ON the heaters. Pressurize the transformer to 3 PSI and hold for 24 hrs. Measure the dew point after 24 hrs. If the dew point attained is –25 Degree Centigrade of lower, discontinue further N2 purging cycles, evacuate the transformer and complete further processing as per Erection and Maintenance Manual. If the dew point is not attained –25 Degree Centigrade, continue N2 purging and evacuation cycles for further repetitions till the dew point is achieved. Then evacuation and processing should be carried out as per Erection and Maintenance Manual.
12)Note down the insulation resistance (IR Values) at the end of each cycle
13)Test the oil filtered in storage tank for
a) Moisture content shall be less than 10 PPM &
b) Break down voltage shall be more than 70 KV.
14)Fill oil in transformer if parameters achieved are as per above and oil has the temperature of 50 Degree Centigrade
15)After oil filling, filter for 2 – 3 passes and test the oil as per 10 above. Ensure filtration through DOF for 2 passes.
16)Fill oil cooler bank and give 2 – 3 passes and test the oil as per 10 above.
17)Mix the transformer and cooler bank oil. If transformer has cooling pumps, run the pumps individually for 8 hrs.
18)Allow setting as per Erection & Maintenance Manual and sample oil for testing as per 10 above.
19)Conduct low voltage testing of transformer including insulation resistance and check the supervisory controls.
20)Thoroughly conduct air releasing from all ports in right sequence i.e. bottom and top.
21)Energize the transformer.

Observations :- After end of each cycle IR values of winding increases and at end of drying out the IR values of Transformer winding are considerably increased
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