# Electrical power Electricity transmission

As considered earlier, renewable energy supplies that are mechanical in origin, e.g. hydro, wave and wind, are usually best distributed by electricity. In this way electricity is a carrier or vector of energy, and not necessarily the main end-use requirement.

Figure 16.6 Electrical transmission. (a) Power transmission to a load of resistance Rl, through a wire of finite resistance Rw. (b) More likely realisation, with generated voltage transformed up for transmission and then down for consumption.

Consider two alternative systems transmitting the same useful power P to a load RL at different voltages V1; V2 in a wire of the same resistance per unit length (Figure 16.6). The corresponding currents are I1 = P/V1 and I2 = P/V2, and therefore the ratio of power lost in the two systems is p2_ _ iJ2rw = (L\ VVkY = VL

p2 i22Rw v vj U; V12

Thus significantly less power is dissipated in the system working at high voltage. The low voltage system can have the same loss as the high voltage system only with thick, and therefore expensive, cable. For instance if electricity is to be transmitted at domestic mains voltage (~110 or ~220V), the cost of cabling becomes prohibitive for distances greater than about 200 m. The difficulty becomes even greater at very low voltage, ~12V.

These factors govern the design of all electrical power networks. Normal rotating generators work best at voltages <10kV. The ease with which alternating current (AC) can be transformed to larger or smaller voltage explains why AC transmission systems have been standard for all but the smallest networks. As indicated in Figure 16.6(b), power is generated at low voltage, stepped up for transmission, and then down again to a safer voltage for consumption. It is important to note, however, that solid-state power electronic components increasingly allow DC/AC/DC conversion at large power and reasonable cost, consequently transforming DC voltage is not so difficult as in the past.

The transmission voltage is limited by dielectric breakdown of the air around the overhead cables and by the insulation of the cables from the metal towers that are at earth potential. Improvements in insulation have allowed transmission voltages for long lines to increase from 6000 V in the year 1900 to over 200 000 V today. Grids using even larger voltages are now being constructed, but will probably make only a marginal improvement in costs. The same is true for very high voltage direct current (DC) systems, which have certain advantages in transmission, for instance having no inductive loss and larger power density for the same peak voltage, but require more expensive interconnection equipment. Superconducting lines of zero resistance are, in principle, attractive but can operate only at very low 'cryogenic' temperatures. Maintaining such low temperatures is difficult over large distances and such lines are not yet a commercial proposition.