1.1 Generation and transmission of electric energy

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 ccurrent at low-voltage, but their service was limited to highly localized areas and were 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 centres 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. Figure 1.1 lists some of the major a.c. transmission systems in chronological order of their installations, with tentative projections to the end of this century.

The electric power (P) transmitted on an overhead a.c. line increases approximately with the surge impedance loading or the square of the system’s operating voltage. Thus for a transmission line of surge impedance ZL (250 Ω) at an operating voltage V, the power transfer capability is approximately P = V2/ZL, which for an overhead a.c. system leads to the following results:

The rapidly increasing transmission voltage level in recent decades is a result of the growing demand for electrical energy, coupled with the development of large hydroelectric power stations at sites far remote from centres of industrial activity and the need to transmit the energy over long distances to the centres. However, environmental concerns have imposed limitations on system expansion resulting in the need to better utilize existing transmission systems. This has led to the development of Flexible A.C. Transmission Systems (FACTS) which are based on newly developing high-power electronic devices such as GTOs and IGBTs. Examples of FACTS systems include Thyristor Controlled Series Capacitors and STATCOMS. The FACTS devices improve the utilization of a transmission system by increasing power transfer capability.

Although the majority of the world’s electric transmission is carried on a.c. systems, high-voltage direct current (HVDC) transmission by overhead lines, submarine cables, and back-to-back installations provides an attractive alternative for bulk power transfer. HVDC permits a higher power density on a given right-of-way as compared to a.c. transmission and thus helps the electric utilities in meeting the environmental requirements imposed on the transmission of electric power. HVDC also provides an attractive technical and economic solution for interconnecting asynchronous a.c. systems and for bulk power transfer requiring long cables.

Table 1.1 summarizes a number of major HVDC schemes in order of their in-service dates. Figure 1.2 provides a graphic illustration of how HVDC transmission voltages have developed. As seen in Figure 1.2 the prevailing d.c. voltage for overhead line installations is 500 kV. This ‘settling’ of d.c. voltage has come about based on technical performance, power transfer requirements, environmental and economic considerations. Current trends indicate that d.c. voltage levels will not increase dramatically in the near future.

1.2 Voltage stresses

Normal operating voltage does not severely stress the power system’s insulation and only in special circumstances, for example under pollution conditions, may operating voltages cause problems to external insulation. Nevertheless, the operating voltage determines the dimensions of the insulation which forms part of the generation, transmission and distribution equipment. The voltage stresses on power systems arise from various overvoltages. These may be of external or internal origin. External overvoltages are associated with lightning discharges and are not dependent on the voltage of the system. As a result, the importance of stresses produced by lightning decreases as the operating voltage increases. Internal overvoltages are generated by changes in the operating conditions of the system such as switching operations, a fault on the system or fluctuations in the load or generations.

Their magnitude depends on the rated voltage, the instance at which a change in operating conditions occurs, the complexity of the system and so on. Since the change in the system’s conditions is usually associated with switching operations, these overvoltages are generally referred to as switching overvoltages.

In designing the system’s insulation the two areas of specific importance are:

The balance between the electric stresses on the insulation and the dielectric strength of this insulation falls within the framework of insulation coordination and will be discussed in Chapter 8 .

1.3 Testing voltages

Power systems equipment must withstand not only the rated voltage (Vm), which corresponds to the highest voltage of a particular system, but also overvoltages. Accordingly, it is necessary to test h.v. equipment during its development stage and prior to commissioning. The magnitude and type of test voltage varies with the rated voltage of a particular apparatus. The standard methods of measurement of high-voltage and the basic techniques for application to all types of apparatus for alternating voltages, direct voltages, switching impulse voltages and lightning impulse voltages are laid down in the relevant national and international standards.