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7. Transformer ratio test and transformer DC resistance
measurement
It is suggested that the ratio test and DC resistance measurement should be carried out before drying.
7.1 Transformation ratio test (upper head flange and diverter switch insert are required for bell type
tap changer)
When turning the drive shaft of the tap changer transmission gear box, a short pipe with an inner diameter
of 25 mm (a connecting bolt with a diameter of 12 mm has been screwed into the pipe) can be used, and
then a hand wheel or hand crank can be used.
Incomplete tap change operation can cause damage to the OLTC!
It is suggested that the ratio test and DC resistance measurement should be carried
out before drying.
Operation without oil will lead to damage of OLTC.
• Without oil, tap-changer cannot be operated more than 10 cycles before drying.
• For the first operation of tap -hanger after transformer being dried, make sure
tap selector is fully immersed in oil and the diverter switch oil compartment is
full of oil.
For bell-type tap-changer, head flange, diverter switch and top cover need to
be assembled temporarily.
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