TRANSFORMERS – CONSTRUCTION
A transformer primarily consists of three basic parts- a primary winding which receives the electrical energy from the applied voltage source, and a secondary winding which receives the induced electrical energy and a core which provides a circuit of low reluctance for magnetic lines of force.
Windings, primary as well as secondary, are the coils of conducting wires as a coil of conductors create a higher magnetic flux compared to the flux created by a single conductor.
Windings rated for higher voltages with more number of turns are designated as High Voltage (HV) winding. The windings for lower voltages are called Low Voltage (LV) winding. The HV winding is composed of many turns of relatively fine copper wire, while the LV winding is composed of relatively few turns of heavy copper wire. The current on the HV side will be lower as V-I product is a constant. Also the HV winding needs better insulation properties to withstand higher voltages across it. HV also needs more clearance to the core, yoke or the body.
The material used for the windings is application specific. Insulated solid copper wire is used for small power and signal transformers whereas copper or aluminium rectangular/strip conductors are used for larger power transformers. RF transformers use Litz so as to minimise losses due to skin effect.
Tappings (or external connections) may be provided from the intermediate points on the windings.
Double-wound transformers use separate primary and secondary windings, while autotransformers use single winding with tapping.
To ensure that the current travels around the core along the coiled conductor, and not through a turn-to-turn-short circuit, winding materials are enamelled thereby providing insulation. In addition, various other methods are used to provide insulation. The type of insulation has a definite bearing on the size and operating temperature of the unit.
Currently four classes of insulations are used
· Class 130 insulation-system transformers.
· Class 150 insulation-system transformers.
· Class 200 insulation-system transformers.
· Class 220 insulation-system transformers.
When properly loaded and installed in an ambient not over 40°C, Class 130, Class 150, Class 200 and Class 220 transformers will operate at not more than a 60°C, 80°C, 130°C and 150°C temperature rise on the winding respectively.
The insulation used for the electrical conductors in a transformer is varnish or enamel. In larger power transformers the conductors are insulated using un-impregnated paper / cloth and the assembly is immersed in a tank containing oil; the transformer oil acts as an insulator and also as a coolant.
Because of the resistance of its windings and the hysteresis and eddy currents in the iron core, a certain amount of the electrical energy delivered to a transformer is transformed into heat energy. The mechanism must be provided for removing the heat energy from the transformer and dissipating it into the surrounding air otherwise, excessively high temperatures may destroy the insulation of the transformer. To remove the heat generated in a transformer, coolant is used.
Various types of cooling mechanisms used are
· Self-air–cooled transformers (or dry-type transformers)
The windings are surrounded by air at atmospheric pressure. The heat is removed by natural convection and radiation. Self-air–cooled transformers are used in systems with 3000-kVA capacity and voltages up to 15,000 V.
· Air-blast–cooled transformers
In this type of transformers, the core and windings are enclosed in a metal enclosure through which air is circulated by means of a blower. These are used for large power transformers in ratings up to 15,000 kVA and voltages up to 35,000 V.
· Liquid-immersed, self-cooled transformers:
In liquid-immersed, self-cooled transformers, the core and windings are immersed in an insulating liquid and enclosed in a metal tank. Liquid conducts away the heat from the core to the tank surface and then, the heat is removed by natural convection and by radiation.
· Gas-vapor transformers
In Gas-vapor transformers, the transformer is insulated with a quantity of gas necessary for start-up, along with a vaporizable liquid which provides insulation and cooling during operation
To avoid any capacitive effect in the transformers (due to the proximity of primary and secondary windings), an electrostatic shield is used between the windings. Transformers may be shielded by magnetic or electrostatic shields, or both to prevent interference from other devices
Small transformers have leads brought out of the unit for circuit connections. Larger transformers may have bolted terminals, bus bars or high-voltage insulated bushings.
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