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8.1.2 Vapor phase drying
8.1.2.1
Vapor phase drying in the vacuum tank
Before drying process, below parts must be removed and
placed outside the drying oven, tap-changer top cover,
transmission gear box, motor drive unit, protective relay,
pressure relief device, bevel gear box, temperature sensor,
etc. If it is vapor phase drying, be sure to open the oil
drain bolt at the bottom of the oil compartment before
drying, so that the condensate can be discharged from the
oil compartment.
a. Turn clockwise to loosen the oil drain bolt between the oil compartment bottom and the tap selector
mechanism.
(See Figure-8-2)
b. Remove the tap-changer top cover and store it outside the vacuum oven.
c. Remove all accessories except the tap-changer body and put them outside the drying oven.
d. Pass in kerosene vapor around 90℃ and keep this temperature constant for 3 to 4 hours.
e. Increase the kerosene gas temperature at a rate of 10°C/hour till the specified final temperature, but
maximum temperature of the tap-changer cannot exceed 125°C.
f. In vacuum drying, the temperature of the tap-changer is 105°C to a maximum of 125°C. The highest
residual pressure is 133Pa. The drying duration is the same as the transformer, but at least 50 hours.
g. Close the oil drain bolt (torque 18~20N.m.) after drying process.
8.1.2.1.2 Before drying in the oil tank of the transformer
,
the diverter switch insert should be lifted out, and
the oil drain bolt at the bottom of the oil compartment should be loosened with a special wrench from in the oil
compartment to discharge the condensate from the oil compartment.
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.
Figure-8-1
8.1.2.1.1 Loosening oil drain stopper
Use a special wrench to loosen the oil drain bolt counterclockwise. Complete loosening of oil drain
bolt is not allowed. (See Figure-8-1)
Top cover, transmission gear box and accessories of the on-load tap-changer
are not allowed to be dried in the vacuum tank together with the transformer,
otherwise these parts will be damaged.
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