TRANSFORMERS – CONSTRUCTION

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

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.

Winding Insulation

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.

·         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

 

·         Shielding

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

·         Terminals

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.

Cores

Any material inside a coil, used to serve as a form to support it, is called a core. Cores are made of different materials with permeability ranging from 1 to over 10000. The higher permeability aid in providing low reluctance path of the ?ux and the ?ux lines mostly con?ne themselves to the core. The permeability of air is 1 whereas the permeability of common “ferro-magnetic” materials is about 300 for ordinary steel, about 5,000 for 4% silicon transformer steel, and up to about 100,000 for some nickel-iron-molybdenum alloys. Because such materials concentrate magnetic flux, they greatly increase the inductance of a coil. Coil inductance is directly proportional to the square of the number of turns and also, direct proportional to the permeability of the core. Silicon steel, hot rolled grain oriented steel, Cold Rolled Grain Oriented (CRGO), etc. are some of the material used in the form of thin laminations for the core; the laminations (in the form of E & I, C & I or O) are coated with a layer of insulating varnish, oxide or phosphate.

Ferrite cores are best suited for high frequency applications and steel laminations are best suited for low frequency applications. For lower frequencies, core material selection is governed by core saturation considerations. Eddy current losses are low so steel laminations can be considered. For higher frequencies, core material selection is governed by core loss considerations. Eddy currents can be significant. In such applications, ferrites are commonly used.

Numeric Codes representing the power handling ability have been assigned to the cores by the manufacturers; the assigned number is the product of its window area and the core cross-section area. The codes are available for laminations, C cores, pot cores, powder cores, and Toroidal tape-wound cores.