Hot Dip Method of Varnishing for a Transformer
Q. Explain the hot dip method of varnishing for a transformer.
Vacuum impregnation is carried out in a vacuum impregnation plant; which consists of a double jacketed vacuum impregnating chamber.It is having atop removing cover. There is separate varnished storing tank. A compressor cum vacuum-exhauster which can create a desired pressure or vacuum by using proper valves connected to the chamber.
- The wound armature, stator of rotor is placed in the chamber by operating the top-cover.
- Then it is tightly closed.
- The chamber is then heated up 100°C by circulating steam or hot oil all through the jacket of chamber and the temperature is maintained for nearly four hours.
- At the same time, air from the chamber is pumped out by vacuum exhaustor and vacuum is the maintained, which will help in vapourising the moisture present in the coils and removing the same completely.
- Varnish from the varnish tank is allowed to flow into the chamber till the varnish in the chamber comes upto a level in which windings are completely immersed. The valve of varnish tank is closed. A pressure from compressor about 1.4 kg / cm² to 2.1 kg / cm² is applied above varnish surface. It will make the varnish to be forced in all the porous (air packets) etc. Spaces in the interior of the coils. The pressure is maintained for an hour or half.
- The valve of the varnish tank is opened and varnish is made to flow back to the tank under air pressure till excess varnish also gets drained out. Then the valve is closed.
- Then the armature / stator is baked in the chamber at 100 to 110°C for nearly 8 hours, till the varnish is completely dried out or it becomes dry bone.
- The air pressure and circulating steam or oil in the jacket is then stopped and the armature or stator is then removed by opening the top cover.