Transformer power distribution system is basic knowledge for everyone. A transformer is mainly used for this purpose because of its step-up and step-down capability.
A power system basically consists of three components: generation, transmission, and distribution. The local electric company operates a plant that generates several hundreds of megavolt-amperes (MVA), typically at about 18 kV.
Transformer Power Distribution
As Figure.(1) illustrates, three-phase step-up transformers are used to feed the generated power to the transmission line. Why do we need a transformer?
Suppose we need to transmit 100,000 VA over a distance of 50 km. Since S = V I, using a line voltage of 1000 V implies that the transmission line must carry 100 A and this requires a transmission line of a large diameter.
If, on the other hand, we use a line voltage of 10,000 V, the current is only 10 A. The smaller current reduces the required conductor size, producing considerable savings as well as minimizing transmission line I2R losses.
To minimize losses requires a step-up transformer. Without the transformer, the majority of the power generated would be lost on the transmission line.
The ability of the transformer to step up or step down voltage and distribute power economically is one of the major reasons for generating ac rather than dc. Thus, for a given power, the larger the voltage, the better.
Today, 1 MV is the largest voltage in use; the level may increase as a result of research and experiments.
Beyond the generation plant, the power is transmitted for hundreds of miles through an electric network called the power grid. The three phase power in the power grid is conveyed by transmission lines hung overhead from steel towers which come in a variety of sizes and shapes.
The (aluminium-conductor, steel-reinforced) lines typically have overall diameters up to about 40 mm and can carry current of up to 1380 A.
At the substations, distribution transformers are used to step down the voltage. The step-down process is usually carried out in stages. Power may be distributed throughout a locality by means of either overhead or underground cables.
The substations distribute the power to residential, commercial, and industrial customers. At the receiving end, a residential customer is eventually supplied with 120/240 V, while industrial or commercial customers are fed with higher voltages such as 460/208 V.
Residential customers are usually supplied by distribution transformers often mounted on the poles of the electric utility company. When direct current is needed, the alternating current is converted to dc electronically.
Transformer Power Distribution Example
A distribution transformer is used to supply a household as in Figure.(2). The load consists of eight 100-W bulbs, a 350-W TV, and a 15-kW kitchen range. If the secondary side of the transformer has 72 turns, calculate:
(a) the number of turns of the primary winding, and (b) the current Ip in the primary winding.
(a) The dot locations on the winding are not important since we are only interested in the magnitudes of the variables involved. Since
(b) The total power absorbed by the load is
But S = VpIp = VsIs, so that