H.V. Coils are the components of finished transformers. They are made on automatic layer setting winding machines.
A solid cylindrical former of predetermined diameter and length is being used as hase over which is made.
Generally round insulated wire of either copper (Cu) or Aluminum (Al) is used as basic raw material.
The coils are made in number of layers.
The starting and finishing leads of each coil are terminated on either side of the coil.
These leads are properly sleeved and locked at number of points.
Low Voltage Coils
L.V. Coils are also one of the components of transformer. The procedure of making low voltage coil is generally same as described earlier
The shape of the basic raw-material (Al or Cu) is rectangular.
The Test: The “Turn Test” is carried out on the H.V. Coils as per the specifications.
Core Assembly
The basic raw-material is COLD ROLLED GRAIN ORIENTED (CRGO) Silicon Steel
It is in the form of thin sheets & cut to size as per design.
Generally three different shapes of core laminations are used in one assembly.
Notching is performed to increase the magnetic path.
The laminations are put through annealing process.
These laminations are assembled in such a manner that there is no air gap between the joints of two consecutive sheets.
The entire assembly is done on a frame commonly known as core channel. These frames being used as a clamping support of the core assembly.
Core - Coil Assembly
The components produced in the coil winding and core assembly stage are then taken into core-coil assembly stage.
The core assembly is vertically placed with the foot plate touching the ground. the top yoke of the core is removed. The limbs of the core are tightly wrapped with cotton tape and then varnished.
Cylinder made out of insulating press board/ press han paper is wrapped on all the three limbs.
Low Voltage Coil is placed on the insulated core limbs.
Insulating block of specified thickness and number are placed both at the top and bottom of the L.V.Coil.
Cylinder made out of corrugated paper or plain cylinder with oil ducts are provided over L.V.Coil.
H.V. Coils are placed over the cylinder.
Gap between each section of H.V. Coils including top & bottom clearances is maintained with the help of oil ducts, as per the design/drawings.
The Top Yoke is refilled. Top core frame including core bolts and tie rods are fixed in position.
Primary and secondary windings are connected as per the requirements. Phase barrier between H.V. phases are placed as per requirement.
Connections to the tapping switch (if required) are made.
Finally, the component is placed in the oven.
Tank - Up
The core-coil assembly and tank supplied by the fabrication deptt. are taken into tank-up stage.
The core-coil assembly is taken out of the oven and the "Megger test" is carried out.
Only if the megger value is as per the specification, the assembly may be taken for tank-up.cut to size as per design.
The tanks, supplied by fabrication deptt. are brought to tank-up department duly painted.
Fittings like drain valves, HV& LV Bushings, conservator, oil level indicator and explosion vent are fitted in the tanks.
The Core-coil assembly is then placed into the tank and properly locked up.
Pure filtered transformer oil is filled in the tank to immerse the assembly only.
Connections of primary and secondary to the terminal bushings are made. Operating handle for ratio switch is fitted, wherever required.
Painting & Finishing
Cleaning of tanks
The cleaning of tank is done normally by chipping/grinding.
The outside surface of the tank is short blasted to achieve a very fine and smooth finish
Painting of tanks
After cleaning the tanks, a coat of hot oil resistance paint is applied on the internal surface of the tank.
The outside surface is painted with a coat of Red Oxide primer and subsequently with one coat of enamel paint as per customer's requirement.
Finishing
Fittings and accessories as per customer's specification and drawing are checked.
Air Pressure test is subjected to avoid any leakage and seepage on all transformer.
Transformers are filled with oil upto the minimum level marking, wherever necessary.
Loose accessories like, earthling terminals, bimetallic connectors, dial type thermometers are also checked for proper fittings.
