High-Voltage Engineering

a branch of electrical engineering that encompasses the study and application of the electrical phenomena occurring in various mediums at high voltages. A tension of 250 volts (V) or higher relative to ground is considered high. The construction of major electric power plants near fuel deposits or large rivers and the transmission of the electric power produced (for example, over wires) to industrial regions that are sometimes at a great distance from the main energy source is economically feasible. The transmission of large amounts of electric power over long distances at low voltage is virtually impossible because of power losses; hence the rated voltages of electric power networks are rising as electrification spreads. Rated voltages rose particularly quickly in the USSR during the implementation of the GOELRO (State Commission for the Electrification of Russia) plan and in the mid-1950’s during the construction of the Unified High-voltage Network (EVS) in the European part of the country (see Figure 1).

Figure 1. Graphs of growth of highest rated voltages (in kV) of electric power grids in the USSR: (1) AC lines, (2) DC lines

Russian and Soviet scientists have played a great role in the development of high-voltage engineering. The first high-voltage laboratory in Russia was established by Professor M. A. Shatelen in 1911 at the St. Petersburg Polytechnical Institute. In the Soviet Union there are dozens of major laboratories—attached to scientific research institutes, plants, and higher educational institutions—studying problems of high-voltage engineering. A great deal of work has been done in this field by B. I. Ugrimov, A. A. Smurov, A. A. Gorev, A. A. Chernyshev, L. I. Sirotinskii, and V. M. Khrushchov and their staffs, as well as by the scientific school headed by Academician A. F. Ioffe. A large number of monographs and textbooks on high-voltage engineering have been published.

The main problem in high-voltage engineering is the construction of reliable high-voltage insulation with minimum dimensions at low cost. Each type of insulating device has fixed long-term and short-term dielectric strength, the values of which determine the size and cost of insulation. The short-term dielectric strength of insulation is a measure of its ability to withstand short-term voltage surges (overvoltages) occurring in electrical systems as a result of various transient phenomena (for example, when individual parts of the system are switched on or off, or in case of short circuits) or lightning strikes on the transmission lines or other current-carrying elements. Overvoltages of the first type are known as system-generated overvoltages and usually last a few hundredths of a second. Overvoltages of the second type are known as lightning surges and do not last more than a few ten-thousandths of a second.

The most common dielectric in electrical systems is the ordinary air that surrounds the cables of power transmission lines and other external insulating elements—for example, support, partition, and suspension insulators. The specific dielectric strength of air (the ratio of the breakdown voltage to the distance between the electrodes) falls sharply as the distance between the electrodes increases (see Figure 2), so that the ground clearance of electric transmission lines must

Figure 2. Specific electric strength (kV/cm) of the “conductor-plane” gap in air at 20°C and 760 mm Hg

increase faster than the rated voltage. This circumstance may place a limit on increases in the rated voltage of overhead transmission lines; this limit appears to be approximately 1,500 kV relative to ground, corresponding to a rated voltage of 2,000 kV for three-phase AC lines and 3,000 kV for DC lines. At this voltage, each line can carry a current of several gigawatts over distances of the order of 1,000 km and more. A further increase in transmitted power will apparently be made possible by the use of new types of transmission lines, the most promising of which are gas-filled and superconducting or cryogenic cables, as well as through transmission of electric power by wave guides at frequencies of the order of several dozen gigahertz.

The dielectric strength of air is closely dependent on the duration of the phenomenon only for short periods of time (less than 100 microseconds); therefore it is approximately the same for lightning surges and system-generated overvoltages (for dry, clean insulators exposed to air). If the surface of an insulator is dirty or dampened by rain or mist, the dielectric strength of the insulator is reduced and depends on the duration of the voltage phenomenon. Hence, air gaps on power transmission lines—for example, the distance between a cable and the ground or the supports—are governed solely by overvoltages, whereas the number and type of insulators on which the cables are hung is also dependent on the operating voltage. The magnitude of overvoltages, the extent to which insulators are soiled, and the strength of the wind, which moves cables from their normal positions and closer to the supports, vary widely. For this reason, statistical techniques are used to select the insulation for power lines. The internal insulation of electrical machines and apparatus—for example, the insulation of transformer windings from the grounded core or casing—usually consists of a combination of different types of insulating materials. The most common is a combination of insulating mineral oil and items made of cellulose (paper, insulating cardboard, pressboard, bakelite, and so on). In constructing insulators, care is taken to equalize the electrical fields by using rounded electrodes, taking advantage of the difference in the dielectric constants of the insulating materials, or using induced distribution of the voltage throughout the insulation. The short-term specific dielectric strength of internal insulation, like that of air, decreases with increasing distance between the electrodes, so that it is usually helpful to split the insulation into a number of relatively thin layers connected in series.

The long-term dielectric strength of internal insulation determines its service life under normal operating conditions. The principal factors leading to the gradual deterioration in the initial properties of insulation are mechanical effects (for example, as a result of the electrodynamic forces between current-carrying parts in case of short circuits), a rise in temperature, dampness and dirt, and overvoltages. Partial discharges in the gas pockets that form in the body of the insulation, which are one of the main causes of aging of insulation, are of special importance. Normal operating conditions are understood to be the limitation of the above factors to a particular level, which will ensure that the insulation lasts out its design life. A system of preventive tests of the insulation, during which it is possible by measuring a number of typical parameters (leakage resistance, the tangent of the dielectric loss angle, capacitance at two frequencies or two temperatures, and the rate of partial discharges) to assess the condition of the insulation and to effect a timely determination of the duration and nature of any necessary repairs, is of great importance in extending the service life of insulation. The preventive testing system also includes a test at increased voltage, which is obligatory after the insulation has undergone repair.

The dimensions of internal insulation are governed by the intensity of the lightning surges and system-generated over-voltages to which it may be subjected—that is, by its short-term dielectric strength, which for equipment with a rated voltage of 220-500 kW, is approximately 2.5-3 times greater than the maximum operating voltage. Thus, since overvoltages may be very frequent, one of the main requirements in high-voltage engineering is the study of overvoltages in order to limit their amplitude; this is usually achieved through the use of lightning arresters or switched barrier-layer rectifiers. In superhigh-voltage systems (1,200 kV and over), overvoltages will be limited to 1.5-1.8 times the rated voltage; and the main influence on the dimensions of insulation will be its long-term strength—that is, the gradual aging of the insulation under the influence of the operating voltage and the external phenomena mentioned above. The possibility of using compressed gas as internal insulation is of great interest in connection with this; the gas has minimal dielectric losses and is far less susceptible to aging. The most promising insulating gases are considered to be elegaz (sulfur hexafluoride, SF₆) and Freon (dichlorodifluoromethane, CCl₂F₂), whose dielectric strength is approximately 2.5 times greater than that of air. At a pressure of several dozen meganewtons per sq m (MN/m²; 1 MN/m² = 10 kilograms-force per sq cm), the short-term dielectric strength of Freon and elegaz is no lower than that of such traditional dielectrics as porcelain and transformer oil (see Figure 3). Switching systems for voltages up to 220 kV, in which all the equipment operates in an elegaz atmosphere at a pressure of 0.3-0.4 MN/m², already exist.

Figure 3. Breakdown voltage for various dielectrics in a uniform field: (1) porcelain, (2) transformer oil, (3) elegaz (0.1 MN/m²), (4) elegaz (0.7 MN/m²)

Such devices combine very well with transmission lines using gas-filled cables, and they have a promising future, especially in densely populated areas.

Another major problem of high-voltage engineering is the study of corona discharge on overhead transmission lines; it is accompanied by power loss and high-frequency radiation, which interferes with radio reception in the vicinity of the transmission line. Since the intensity of corona discharge is determined by the electric field intensity on the surface of the wires, corona losses and radio interference are reduced if the diameter of the cables is increased. For this purpose, so-called split cables are often used instead of single cables. Split cables, consisting of two, three, or four individual conductors at a distance of 50 cm from each other, are used in lines carrying 330-750 kV. Split cables made up of six or eight individual conductors at appreciable distances from each other would be used on lines carrying 1,100-1,200 kV to reduce the characteristic impedance of the line and to increase its carrying capacity.

Corona losses and the level of radio interference are substantially lower for direct current than for alternating current, and herein lies one of the advantages of DC transmission lines. Their principal advantage, however, is the possibility of the connection of asynchronously functioning electrical systems, thereby eliminating the problem of stability; the distance over which direct current can be transmitted at a constant voltage is limited only by economic considerations. Consequently, the use of direct current at 1,500 kV (± 750 kV relative to ground) is planned for the Ekibastuz-Central Zone transmission line, the first superlong electric transmission line in the Soviet Union. The main difficulty in the successful implementation of DC transmission is associated with the construction of rectifiers and inverters, in which powerful controlled semiconductors and mercury-arc rectifiers are used. It is expected that direct current lines will form the basis of the Unified High-voltage Network of the USSR.

An important branch of high-voltage engineering is concerned with the development of high-voltage devices for testing of insulation and other purposes. Test transformers, often connected in grids, are used as a source of industrial-frequency alternating current (50 Hz). Grid transformers are made for tensions up to 3,000 kV. High DC voltages (up to 6,000 kV) are obtained by using electrostatic generators or rectifiers connected in series, usually using high-voltage semiconducting diodes. Pulse voltage generators, which generate overvoltages with an amplitude of up to 10 MV, have been developed to imitate lightning surges. System pulse generators, providing a voltage pulse lasting up to 0.01 sec, also became widespread during the 1960’s. Pulse current generators operating at medium voltages (up to 200 kV) and amplitude of current impulse up to several million amperes were originally used for testing grounding connections and lightning arresters. The field of application of pulse current generators (which are often called storage capacitances) became far broader: they are used in magnetic-pulse treatment of metals, in equipment utilizing the electrohydraulic effect, in laser pumping circuits, and to produce high-temperature plasma. One type of pulse current generator (the so-called Gorev circuit) is used to test the insulating ability of circuit breakers. High voltages of high frequency are obtained from valve oscillators or Tesla transformers.

The construction of high-voltage testing equipment has also necessitated the development of special measuring equipment. The simplest device for measurement of high voltages is the spherical discharger. High voltages are also measured with electrostatic and rotary voltmeters; voltage pulses are measured by electron oscillographs with a voltage divider across the input. Large pulse currents are usually measured by electron oscillographs, in which the voltage is directed onto their plates from shunts or air-core transformers (Rogovskii belt) that are connected in the curcuit in series. When taking high-voltage readings, strong electromagnetic fields, which distort the results, must be taken into account. To eliminate this distortion, the measuring instruments and the feed cables are carefully shielded, and grounding devices and other measures are used to reduce spurious inductance and capacitance. Recording devices such as automatic oscillographs and peak-load voltmeters have been developed to measure voltages and currents in operating electrical systems; their large-scale use makes it possible to obtain sufficiently reliable statistical information on lightning surges and currents.

One of the self-contained branches of high-voltage engineering is so-called ion technology, which is associated with aerosols, whose particles are charged as a result of friction or corona discharge. By means of a powerful electrical field the movement of the charged particles can be controlled and the desired technological process (electrolytic gas purification, electromagnetic mixing or separation, electrolytic painting, and so on) can be carried out. An example of the application of ion technology is the use of corona-discharge precipitators at steam power plants to remove ash and other suspended particles from the exhaust gas of steam-boiler fireboxes.