Degree of drying is directly affect insulation and other specifications of transformer. Therefore, drying insulation systems is the firstly factor to extending transformer life.

After drying Transformer oil filtration which be affected by bump, somethings need to do firstly:

1) whether exposed time in air is over time;

2) whether the damp degree and insulation degree over the requirements;

3) how is color change of silicone air dryer;

4) does wind re-winding;

5) whether transformer oil excessive humidity;

6) whether oil leakage;

If there is one of above problems, the on-site drying is needed.

Drying methods are as follows:

1) hot oil circulation.

It is more effective to less serious moisture.

It is more effective to less serious moisture.

It use hot oil as medium, through eddy current, heating transformer then insulation fiber, make moisture evaporation. This method requires vacuum pumping every few hours. The vacuum level of tank should be under l33Pa. Temperture of circulating hot oil should maintain at 60 ~ 70 degree Celsius.

2) Drying by heat oil spray.

It is a commonly method used for on-site drying, suitable for medium and large oil-immersed transformers.

This method is to using a special nozzle, spraying atomization transformer oil which heating above temperature 1O0 degree Celsius, on body of transformer to make it heating.

                         
Then pumping vacuum to transpire the moisture on body. The needy devices are simple. The method has effective for large transformer on-site drying.

3) Field winding charged drying.

It need short secondary winding of transformer, and energize the primary winding, to drying insulation systems by electricity heating effect through primary and secondary windings.

This method need use vacuum oil filtration machine to transpire moisture in oil. Comparing the ground insulation resistance from high, midium and low phrase, after drying, the dielectric loss and leakage current of insulation resistance are decrease obviously.

4) Vacuum drying by load on fuel tank and eddy current method.
This method is energized field winding on tank to produce vortex of energy. This vortex of energy can increase the internal temperature of tank, so as to heating body.

The turns per coil, current and power-on time are as to make temperature of tank at 90 ~ 100 degree Celsius.

5) Zero sequence current drying.

It is to choose proper three-phase windings, connected in series or in parallel. The windings of other voltage level are open circuit. Adding zero sequence voltage to choosed winding, to generate zero sequence flux in iron core. The flux closed circuit through tank produces eddy current loss to drying the body.

This method is suitable for medium and small distribution transformers drying.

5 the maintenance for large transformer windings on-site

On-site repair for large transformer must ensure the following:

1) Keeping body of transformer from damp;

2) the winding and insulation devices need be placed in the vacuum container. Vacuum must be maintained at 133Pa;

3) If two phase windings need replacement or removal, the order of winding should be inside first then outside,to avoid wire insulation damaged, as well as to ensure the tight;

4) winding along each phase;

5) If winding need soking, the pure insulating oil is used. After that, using clean cloth to wipe impurity on upper and lower yoke, insulation tube, bracket on oil way;


6) After replacement, compressing 0.2Pa air to tank, which is to avoid moisture sucked;

7) To forced Oil Circulation Transformer , it need pump vacuum to transpire moisture after oil injection. Besides the fuel tank, other parts have same need are submersible pump, oil purifier, cooler, cooler group support, set tank, high pressure and neutral current transformer bushing elevated seat, gas following appliances, storage cabinets, pressure release valve, oil-load tap-room.

On-site releasing and filling oil need assistance from vacuum oil filtration machine. Transformer oil after vacuum degassing  re-fill into transformer, it is better to testing the specifications after a while. The purpose of testing is to know whether blinster resoled in oil. Cause it would be the hidden peril if corona discharge happened.

The most important requirement to change windings of  220kV and 500kV such large transformer on-site, is quicker, quicker. The exposed air time of insulation paper must be less than 14hours. Cause during 12 ~ 14h, moisture usually entered limited on surface of paper, which is easy to transpire by vacuum.

According maintenance experience, the time of pumping vacuumn is beeter longer than exposed time in air.

Replacement of transformer windings are better with two steps: repair the windings in day time, suck vacuum of tank at night. It avoids humidity in to tank. After completion of maintenance, nitrogen or dry air should be immediately filled into the tank.

In addition, other insulation components which will replaced on site should be kept dry.

Leads must be properly placed, are not squeezed or damaged.

After repair work done, partial discharge test is suggestted on-site. This is to ensure the re-running reliability of transformer.