TRANSFORMERS – CONSTRUCTION
A transformer mainly consists of three basic parts: a primary winding that receives the electrical energy from the applied voltage source, and a secondary winding that receives the induced electrical energy, and a core that provides a low reluctance circuit for magnetic lines of force.
Windings
The windings, primary and secondary, are the coils of conductive wires, as a coil of conductors creates a higher magnetic flux compared to the flux created by a single conductor.
Windings rated for higher voltages with greater number of turns are designated as high voltage (HV) windings. Windings for lower voltages are called Low Voltage (LV) windings. The high-voltage winding is made up of many turns of relatively thin copper wire, while the low-voltage winding is made up of relatively few turns of heavy copper wire. The current on the HV side will be less because the product VI is a constant. Furthermore, the high voltage winding needs better insulation properties to withstand higher voltages. HV also needs more clearance for the core, yoke or body.
The material used in the windings is application specific. Insulated solid copper wire is used for small power and signal transformers, while rectangular conductors/strips of copper or aluminum are used for larger power transformers. RF transformers use Litz to minimize losses due to the skin effect.
Taps (or external connections) can be provided from intermediate points in the windings.
Double-winding transformers use separate primary and secondary windings, while autotransformers use a single tapped winding.
Winding insulation
To ensure that the current travels through the core along the wound conductor, and not through a turn-to-turn short circuit, the winding materials are enamelled, thus providing insulation. Additionally, several other methods are used to provide insulation. The type of insulation has a definite influence on the size and operating temperature of the unit.
Currently four classes of insulation are used
· Class 130 isolation system transformers.
· Class 150 isolation system transformers.
· Class 200 isolation system transformers.
· Class 220 isolation system transformers.
When properly charged and installed in an environment not exceeding 40 °C , Class 130, Class 150, Class 200 and Class 220 transformers will operate at no more than 60 ° C, 80 ° C, 130 ° C and 150 °C temperature rise in °C in the winding, respectively .
The insulation used for the electrical conductors of a transformer is varnish or enamel. In larger power transformers, the conductors are insulated with non-impregnated paper/cloth and the assembly is immersed in a tank containing oil; Transformer oil acts as an insulator and also as a coolant.
· Refrigerator
Due to 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 thermal energy. Mechanism must be provided to remove thermal energy from the transformer and dissipate it into the surrounding air, otherwise excessively high temperatures may destroy the transformer insulation. To remove the heat generated in a transformer, refrigerant is used.
Various types of cooling mechanisms used are
· Air-cooled transformers (or dry-type transformers)
The windings are surrounded by air at atmospheric pressure. Heat is removed by natural convection and radiation. Air-cooled self-cooled transformers are used in systems with a capacity of 3000 kVA and voltages up to 15,000 V.
· Air jet cooled transformers
In this type of transformer, the core and windings are enclosed in a metal casing through which air circulates using a blower. They are used for large power transformers in ratings up to 15,000 kVA and voltages up to 35,000 V.
· Liquid-immersed and self-cooled transformers:
In self-cooled liquid-immersed transformers, the core and windings are immersed in an insulating liquid and enclosed in a metal tank. The liquid conducts heat from the core to the surface of the tank and then the heat is removed by natural convection and radiation.
· Gas-steam transformers
In gas-to-vapor transformers, the transformer is insulated with an amount of gas required for starting, together with a vaporizable liquid that provides insulation and cooling during operation.
· Shielding
To avoid any capacitive effect on the transformers (due to the proximity of the primary and secondary windings), an electrostatic shield is used between the windings. Transformers can be shielded by magnetic or electrostatic shields, or both, to prevent interference from other devices.
· Terminals
Small transformers have leads taken from the unit for circuit connections. Larger transformers may have screw terminals, busbars, or high-voltage insulated bushings.