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8. Drying process and oil filling
8.1 Drying process
To ensure the insulation property of tap-changer, necessary drying must be taken as per below description
(vacuum drying or vapor phase drying).
8.1.1 Vacuum drying
Below parts, which are not allowed to be dried, must be removed before the drying process: tap-changer
top cover, transmission gear box, motor drive unit, gas relay, pressure relief device, bevel gear box, etc.
8.1.1.1
Drying in the oven room
a. The tap-changer is heated up at a rate of 10℃/ hour in the air under atmospheric pressure, till the
temperature reaches highest 110℃.
b. Drying is carried out in the circulating hot air for 20 hours and highest temperature of tap-changer is
110℃.
c. Vacuum drying lasts 50 hours, temperature of tap-changer is 105℃ till 125℃. Maximum residual
pressure 133Pa.
8.1.1.2 Drying in the transformer oil tank
a. By-pass pipe (nominal diameter 25mm) can be connected to the flange connection position between
flanges E2 and Q.
b. The tap-changer is heated up at a rate of 10℃/ hour in the air under atmospheric pressure, till the
temperature reaches highest 110℃.
c. Drying is carried out in the circulating hot air for 20 hours under max. tap-changer temperature 110℃.
d. Vacuum drying lasts 50 hours, temperature of tap-changer is 105℃ till 110℃. Maximum residual
pressure 133Pa.
OLTC top cover, transmission gear box and OLTC accessories are not allowed to be
vacuum dried together with the transformer in the drying oven. Otherwise, they will be
damaged.
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