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1ZSE 5492-118 en, Rev. 4

7 Oil lling

7.5 Restoring the gas cushion

WARNING

Before any work is carried out on the on-load tap-changer:

Make sure that the transformer is disconnected and that earthing is properly carried

out. Obtain a signed certicate from the engineer in charge.

After oil lling under vaccum a gas cushion has to be formed.

Proceed as follows:

1. Shut the conservator valve.

2. Connect a pump to the oil valve and start to suck out oil. Open the air vent to the on-

load tap-changer and let air in. Stop the pumping after about 2 litres of oil has been

sucked out from the on-load tap-changer.

WARNING

The oil in the selector switch housing may be hot. Be cautious!

3. Shut the air vent and the oil valve and disconnect the pump.

4. Open the conservator valve and an appropriate gas cushion will be formed.

If the on-load tap-changer has been lled with degassed oil and has a very low frequency

of operation, the gas cushion may have been absorbed by the oil. To avoid fault trip by the

pressure relay, the gas cushion should be restored as above.

CAUTION

To operate the on-load tap-changer with a too small or no gas cushion means a risk for a

trip of the pressure relay.

The procedure above is used for on-load tap-changers without oil lter unit for continuous oil

ltration. In case the on-load tap-changer is equipped with an oil lter unit for continuous oil

ltration from ABB, and it is installed according to our recommendations, follow the instruc-

tions in ”Oil lter unit for on-load tap-changers, manual” for restoring the gas cushion.

CAUTION

Avoid to do the restoring in damp weather since moisture will get into the selector switch

housing. If the restoring has to be done in such weather, the incoming air has to be dehy-

drated.

ABB UBBRT User Manual

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