Transformer oil Filtration
The transformer oil due to its repeated use comes in contact with moisture, dust, dirt, temperature, sludge formation and becomes impure and it is to be reconditioned so that it can be used again and again. For this purpose the oil is to be filtered in a filter plant. Due to filtration; moisture and dissolved gases, sediments, dirt, dust carbon deposits are removed from the oil and hence it becomes ready for re-use. Filtration improves dielectric strength and insulating properties of the oil.
There are various methods of filtering
- Centrifugal oil purifier system.
- Filter pack purifier system.
- Activated earth media filter system.
Centrifugal oil purifier
This is most popular, economical method and gives reliable results. There are three steps involved in it (i) Heating of oil (ii) Applying centrifugal forces (ii) Evaporation. These three possesses are shown by the line diagram in the following sketch.
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| Figure A |
P1 = input pump
P2 = Discharge pump
P4 = Vacuum pump
M = Motors to drive pump/shaft
T1 = Transformer tank with used oil
T2 = Heater tank
T3 = Spray tank
T4 = vacuum tank
V = valves for closing/opening
The contaminated (used) oil from the transformer tank T1, is taken out by opening the drain valve. The flow is regulated by the second valve near to it. It is pumped by pump P1, which is driven by the motor M and admitted to tank T2, where it is heated by immersion heaters at temperature of about 60–70°C. The hot oil is admitted to tank T3, into the centre of bowl. The bowl has many numbers of conical shaped vanes fitted on the shaft which is driven by the motor at very high speed upto 8000 r.p.m. The heavy particles in the form of impurities are thrown out due to heavy centrifugal forces produced by vanes rotating at very high speed. These impurity particles remain stuck on the walls of bowl. Due to heavy centrifugal forces an extremely fine spray is formed due this, the moisture is evaporated. Impurities are taken out by opening value provide on tank T3, at the lower level. The process is accelerated due to high temperature and high vacuum created in tanks T2, and T4. The purified oil is pumped from T4, and delivered to transformer tank.
Varnishing of insulation
The windings, cores, and other parts air varnished to fill air pockets, voids, gaps for perfect insulation.
Methods of vanishing
1. Air-dry vanish by brush
Varnishing for rewinding, repairs, small machines are carried out by using air dry varnish applied by brushes.
2. Air varnish by spray
For medium work air-dry varnish is applied by spray so that it goes into air pockets, slot gaps, voids. For this purpose also thin air drying varnish is used.
3. Hot dip-method of varnishing
This method gives better results than the first two methods. In this method the windings of rotors, armatures stators, poles, etc. are heated upto the temperature of 80° to 100° in the suitable ovens in which temperature control setting arrangement is provided. Over heating should be avoided. This evaporates the moisture. The thin varnish is ready in the tank. The heated part is immediately dipped fully in the tank and kept for atleast 30 minutes. During this period the varnish fills up the voids, slot gaps, air pockets and covers the winding and inter-turn gaps. The parts are then lifted out of the tank and hang or put on the iron-grill for some time so that excess varnish runs down. The parts are kept in the oven again for drying purpose for 4 to 6 Hrs.at the temperature of 90 to 110°C. The parts are taken out and kept out for natural cooling in the air.
4. Varnishing by “Vacuum Impregnation”
The processes involved are (1) Heating of coils (ii) Varnishing (iii) Drying. The completed process is shown by the following block-diagram.
H = Electric heater
VIC = Vacuum
impregnation chamber
P = Pump
C = Compressor
T = Thermometer
L = Lit
VT = Vanish tank
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| Figure B |
The machine parts such as rotor/ startor to be varnished are kept in VIC by lifting the lit ‘L’ and closed. VIC is heated by taking heat from heating chamber ‘H’. The temperature of about 100°C is maintained for about 3 to 5 hours. During this period the air is pumped out by exhauster so that moisture present in the coils are evapourated and drained out completely. The varnish from the tank ‘VT‘ is allowed in the VIC chamber till the machine part is fully covered with varnish. The valve ‘V’ is then closed. Then compressor creates a pressure of about 1.5 to 2 kg/ m² above the varnish surface. Due to this pressure the varnish is forced to go in the air pockets, voids, gaps in the winding and machine slots. The pressure is maintained far atleast 30 minutes. After this the varnish is taken out by opening the valve from chamber to varnish tank. The excess varnish is also drained out from the windings. The valve is closed. The varnished winding and machine parts in the impregnation chamber are dryed at 100°C by passing heat from hot tank to the VIC for about 8 hrs. The parts are taken out by opening the lit of chamber.
Prevention of insulation of machine during inactivity
The insulation of machine deteriorates and losses it insulting properties if proper care is not taken during the machine kept idle or inactive for a prolonged period. This is due to absorption of moisture, deposition of dirt, dust, oils, etc. During inactivity, following precautions should be taken to protect it.
- Keep clean the insulated and other parts of machine by soft cloth, blower, vacuum cleaner to remove dirt, dust.
- Arrange hot air circulation or electric bulbs to heat up the insulation so moisture will not be accumulated.
- Cover the machine with water proof cloth when not in use.
- Keep off the brushes from commutator and slip-rings to avoid pitting.
- Keep rotating parts free from oil grease, dirt, dust.
- Try to run the machine if possible in the idle period even if not required.
- Pour varnish if necessary.