The potential benefits of electrical energy supplied to a number of consumers from a common generating system were recognized shortly after the development of the ‘dynamo’, commonly known as the generator.
The first public power station was put into service in 1882 in London (Holborn). Soon a number of other public supplies for electricity followed in other developed countries. The early systems produced direct current at low-voltage, but their service was limited to highly localized areas and was used mainly for electric lighting. The limitations of d.c. transmission at low voltage became readily apparent. By 1890 the art in the development of an a.c. generator and transformer had been perfected to the point when a.c. supply was becoming common, displacing the earlier d.c. system. The first major a.c. power station was commissioned in 1890 at Deptford, supplying power to central London over a distance of 28 miles at 10 000 V. From the earliest ‘electricity’ days it was realized that to make full use of economic generation the transmission network must be tailored to production with increased interconnection for pooling of generation in an integrated system. In addition, the potential development of hydroelectric power and the need to carry that power over long distances to the centers of consumption were recognized.
Power transfer for large systems, whether in the context of interconnection of large systems or bulk transfers, led engineers invariably to think in terms of high system voltages.
A power plant produces electrical energy in medium (20 000 V) or low (1 000 V) voltage which is then elevated to high voltage (up to 400 kV) by a step-up substation. Electrical power is then transmitted across long distances by high-tension power lines, and the higher the voltage, the more power can be transmitted. A step-down substation converts the high voltage back down to medium voltage and electrical power can then be transported by medium voltage lines to feed medium and low voltage transformers using overhead lines or underground cables. Most of the users are fed in low voltage, but bigger ones, such as factories, commercial buildings, hospitals and so forth, can be directly fed in medium voltage.
Distribution of high ac voltages
Distribution is another section of the electrical power system. Figure 1 shows the major components of the electric power system. The power plants convert the energy stored in the fuel (coal, oil, gas, nuclear) or hydro into electric energy. The energy is supplied through step-up transformers to the electric network. To reduce energy transportation losses, step-up transformers increase the voltage and reduce the current. The high-voltage network, consisting of transmission lines, connects the power plants and high-voltage substations in parallel. The typical voltage of the high-voltage transmission network is between 240 and 765 kV. The high-voltage substations are located near the load centers, for example, outside a large town. This network permits load sharing among power plants and assures a high level of reliability. The failure of a line or power plant will not interrupt the energy supply. The subtransmission system connects the high-voltage substations to the distribution substations. These stations are directly in the load centers. For example, in urban areas, the distance between the distribution stations is around 5 to 10 miles. The typical voltage of the subtransmission system is between 138 and 69 kV. In high load density areas, the subtransmission system uses a network configuration that is similar to the highvoltage network. In medium and low load density areas, the loop or radial connection is used. Figure 65.1 shows a typical radial connection. The distribution system has two parts, primary and secondary. The primary distribution system consists of overhead...