Electrical Engineering ⇒ Topic : Construction of Transformer

Constructional details of a distribution transformer are shown in Fig. (a) The important parts of the transformer are (1) core, (2) windings, (3) tank,(4) conservator, (5) brushings, (6) breather and (7) radiators.

             FIGURE  (a) Constructional details of a distribution transformer

(1) Core The transformer core is made of silicon steel or sheet steel with 4% silicon. In addition to this, the sheets are laminated and are coated with an oxide layer to reduce the iron losses. The thickness of the lamination is 0.35 mm for 60 Hz operation and 5 mm for 25 Hz operation.The purpose of the core is to provide a magnetic path of low reluctancebetween the two windings so that whenever one winding is excited, the flux established by the winding will link fully with the other winding without any appreciable leakage. The permeability of the material used for the core must have a high value (μ_r > 1000), since the reluctance of the magnetic path is inversely proportional to μ. We also know that in a magnetic circuit flux= mmf/reluctance = primary ampere turns/reluctance. This suggests that for a given value of flux, primary ampere turns required are less when the reluctance is low.

                 A new development in the construction of the transformer core is the spiral arrangement. A spiral core is assembled using a continuous strip of transformer silicon-steel, wound in the form of a circular or elliptical cylinder.Such a construction allows the magnetic flux to follow the grains of the iron in the core, thus enabling the transformer designer to use much higher operating flux densities with lower loss per kg weight. Hence, a spiral core transformer has (a) mechanically a more rigid core, (b) lesser weight and size per unit electrical rating, (c) higher operating flux densities and lower iron losses, and (d) lower cost of manufacture

(2) Windings A conventional transformer has two windings. The winding which receives the electrical energy is called the primary winding and the winding which delivers the electrical energy is known as the secondary winding. Windings are generally made of high grade copper. For carrying higher currents, stranded conductors are used. The windings are provided with insulation such that any one turn will not come into contact with the other turns. Bare copper wires are also given enamel coating in addition to the inter-turn insulation. Single or double layer cotton insulation is generally used. Press board or paper insulation is also sometimes used for supporting the windings. Additional insulation is generally provided for the line-end turns to protect them from lightning and switching over voltages, because under conditions of transient disturbances, the distribution of voltage is not uniform along the winding with 8% of the voltage appearing across the first 10% of the turns from the line end. For large power and distribution transformers, an oil-filled tank is necessary for providing cooling for the windings and core. Like in other electrical machines, during the operation of the transformer there are two types of losses (a) core losses and (b) copper losses.Heat is generated because of energy loss and is roughly proportional to the volume of the material in which the losses occur, whereas the heat dissipation is proportional to the surface area of the same material and the tank. From this, it may be said that to a rough approximation, the ratio of

This ratio must approach unity to limit the temperature-rise. To achieve this,the surface area of the tank is made corrugated. To improve this further,radiators are also used as shown in Fig. (a). Radiators not only increase the surface area but also provide the path for the circulation of the cooling fluid (i.e. transformer oil).

(3) Methods of Cooling of Transformers There are three methods which are commonly used for the cooling of the transformer windings and the corewhich are given below:

(a) Natural radiation                                 :               Used for transformer having low voltage and output ratings (i.e. up to 500 V and 5 kVA)

(b) Oil-filled and self cooled                     :               Used for large sized transformers with ratings up to 132 kV and 1000 MVA

(c)Forced cooling with air-blast ratings    :                Used for machines with higher than 33 kV and 100 MVA.

(4) Conservator Tank When a transformer is oil-filled and self-cooled,the oil in the tank is subjected to heat and thus will naturally expand and contract due to the variations in load current and is also subjected to seasonal variations. The conservator tank provides the means for the oil to settle down by expanding under heavy loads. Also without such a tank, very high pressures will be developed inside it which could lead to the bursting of the tank.

(5) Bushings The purpose of the bushings is to provide proper insulation for the output leads to be taken out from the transformer tank. Bushings used are generally of two types:

 (a)  Porcelain type which are used for voltage ratings of up to 33 kV and

 (b)  Condenser type and oil-filled type which are used for voltage rating higher than 33 kV.

(6) Breather Transformer oil should not be exposed directly to the atmosphere because it may absorb moisture and dust from the environment and may loose its electrical properties in a very short time. To avoid this from happening, a breather is provided. The breather completely prevents the moisture and dust from coming into contact with the oil in the conservator tank when it expands or contracts, depending on the variations in the load

CONSTRUCTION OF TRANSFORMER

There are two types of transformers, viz.

• Core type
• Shell type

In core type, the windings surround a considerable part of the steel core as shown in Figure 1 (a) whereas in shell type, the steel surrounds a major part of the windings as shown in Figure 1 (b).

                                                                    figure (1)

  In iron-core transformers, most of the flux is usually confined to high permeability core.However, there is some flux that leaks through the core legs and non-magnetic material surrounding the core. This flux is called the leakage flux. To reduce this leakage flux in core type transformers, half of the L.V. winding is placed over one leg and the other half is placed over the second leg and similarly for the H.V. winding, L.V. winding is placed adjacent to the steel core and H.V. winding outside so that the insulation cost is minimized. In shell type transformer, the L.V. and H.V. windings are wound over the central limb as shown in Figure 1(b). The L.V. and H.V. windings are sandwiched.

  In core type transformer, the flux has single path whereas in shell type transformer, the flux divides equally in the central limb and returns through the outer two legs