Power Transmission

Transmission networks operate at high voltage levels such as 400 kV and 275M because the transmission of large blocks of energy is more efficient at high voltages (Weedy, 1987). Step-up transformers in generating substations arc responsible for increasing the voltage up to transmission levels and step-down transformers located in distribution substations are responsible for decreasing the voltage to more manageable levels such as 66 kV or 11 kV.

High-voltage transmission is carried by means of AC overhead transmission lines and DC overhead transmission lines and cables. Ancillary equipment such as switch-gear, protective equipment and reactive power support equipment is needed for the correct functioning of the transmission system.

High-voltage transmission networks are usually ‘meshed’ to provide redundant paths for reliability. Figure 1.5 shows a simple power network.

Fig. 1.5 Meshed transmission network.

Under certain operating conditions, redundant paths may give rise to circulating power and extra losses. Flexible alternating current transmission systems controllers are able to prevent circulating currents in meshed networks (IEEE/CIGRE, 1995).

Overhead transmission lines are used in high-voltage transmission and in distribution applications. They are built in double circuit, three-phase configuration in the same tower, as shown in Figure 1.6.

Fig. 1.6 Double circuit transmission line.

They are also built in single circuit, three-phase configurations, as shown in Figure 1.7. Fig. 1.7 Single circuit transmission line.

Single and double circuit transmission lines may form busy transmission corridors. In some cases as many as six three-phase circuits may be carried on just one tower. In high-voltage transmission lines, each phase consists of two or four conductors per phase, depending on their rated voltage, in order to reduce the total series impedance of the line and to increase transmission capacity. One or two sky wires am used for protection purposes against lightning strikes.

Underground cables arc used in populated areas where overhead transmission lines are impractical. Cables arc manufactured in a variety of forms to serve different applications. Figure 1.8 shows shielded, three-phase and single-phase cables; both have a metallic screen to help confine the electromagnetic fields.

Fig. 1.8 (a) Three-conductor, shielded, compact sector; and (b) one-conductor, shielded.

Belted cables are generally used for three-phase, low-voltage operation up to approximately 5 kV, whilst three-conductor, shielded, compact sector cables are most commonly used in three-phase applications at the 5 46 kV voltage range. At higher voltages either gas or oil filled cables are used.

Power transformers are used in many different ways. Some of the most obvious applications are:

1. as step-up transformers to increase the operating voltage from generating levels to transmission levels;
2. as step-down transformers to decrease the operating voltage from transmission levels to utilization levels;
3. as control devices to redirect power flows and to modulate voltage magnitude at a specific point of the network;
4. as 'interfaces' between power electronics equipment and the transmission network.

For most practical purposes, power transformers may be seen as consisting of one or more iron cores and two or three copper windings per phase. The three-phase windings may be connected in a number of ways, e.g. star—star, star—delta and delta—delta.

Modern three-phase power transformers use one of the following magnetic core types: three single-phase units, a three-phase unit with three legs or a three-phase unit with five legs.

Reactive power equipment is an essential component of the transmission system (Miller, 1982). It is used for voltage regulation, stability enhancement and for increasing power transfers. These functions are normally carried out with mechanically controlled shunt and series banks of capacitors and non-linear reactors. However, when there is an economic and technical justification, the reactive power support is provided by electronic means as opposed to mechanical means, enabling near instantaneous control of reactive power, voltage magnitude and transmission line impedance at the point of compensation.

The well-established SVC and the STATCOM, a more recent development, is the equipment used to provide reactive power compensation (Hingorani and Gyugyi, 2000). Figure 1.9 shows a three-phase, delta connected, thyristor-controlled reactor (TCR) connected to the secondary side of a two-winding, three-legged transformer. Figure 1.10 shows a similar arrangement but for a three-phase STATCOM using GTO switches. In lower power applications, IGBT switches may be used instead.

Although the end function of series capacitors is to provide reactive power to the compensated transmission line, its role in power system compensation is better understood as that of a series reactance compensator, which reduces the electrical length of the line. Figure 1.11(a) illustrates one phase of a mechanically controlled, series bank of capacitors whereas Figure 1.11(6) illustrates its electronically controlled counterpart (Kinney et al., 1994). It should be pointed out that the latter has the ability to exert instantaneous active power flow control.

Several other power electronic controllers have been built to provide adaptive control to key parameters of the power system besides voltage magnitude, reactive power and transmission line impedance. For instance, the electronic phase shifter is used to enable instantaneous active power flow control. Nowadays, a single piece of equipment is capable of controlling voltage magnitude and active and reactive power. This is the UPFC, the most sophisticated power controller ever built (Gyugyi, 1992). In its simplest form, the UPFC comprises two back-to-back VSCs, sharing a DC capacitor. As illustrated in Figure 1.12, one VSC of the UPFC is connected in shunt and the second VSC is connected in series with the power network.

Fig. 1.9 Three-phase thyristor-controlled reactor connected in delta.

Fig. 1.10 Three-phase GTO-based STATCOM.

Fig. 1.11 One phase of a series capacitor. (a) mechanically controlled; and (b) electronically contralled.

HVDC Light is a very recent development in electric power transmission. It has many technical and economical characteristics, which make it an ideal candidate for a variety of transmission applications where conventional HVDC is unable to compete. For instance, it can be used to supply passive loads, to provide reactive power support and to improve the quality of supply in AC networks. The installation contributes no short-circuit current and may be operated with no transformers. It is said that it has brought down the economical power range of HVDC transmission to only a few megawatts (Asplund et al., 1998). The HVDC Light at HellsjÖn is reputed to be the world's first installation and is rated at 3 MW and ±1OkV DC. At present, the technology enables power ratings of up to 200 MW. In its simplest form, it comprises two STATCOMs linked by a DC cable, as illustrated in Figure 1.13.

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