Electric Power: AC versus DC
Relative to the digital age, the electric utility industry may seem old hat. But power electronics and the power industry have a growing symbiotic relationship. Nearly all power electronics systems draw power from the grid, and utility companies benefit from the applica- tion of power electronics to motor drives and to converters used for high-voltage DC transmission lines. The two fields are very much in a state of constant development of new systems and applications. For that reason, a short review of the history and the present state of the electric utility industry is appropriate for consideration by the power electronics engineer.
Take warning! Alternating currents are dangerous. They are fit only for powering the electric chair. The only similarity between an a-c and a d-c lighting system is that they both start from the same coal pile.
And thus did Thomas Edison try to discourage the growing use of alternating-current electric power that was competing with his DC systems. Edison had pioneered the first true central generating station at Pearl Street, in New York City, with DC. It had the ability to take generators on and off line and had a battery supply for periods of low demand. Distribution was at a few hundred volts, and the area served was confined because of the voltage drop in conductors of a reason- able size. The use of DC at relatively low voltages became a factor that limited the geographic growth of the electric utilities, but DC was well suited to local generation, and the use of electric power grew rap- idly. Direct current motors gradually replaced steam engines for power in many industries. An individual machine could be driven by its own motor instead of having to rely on belting to a line shaft.
Low-speed reciprocating steam engines were the typical prime movers for the early generators, many being double-expansion designs in which a high-pressure cylinder exhausted steam to a low- pressure cylinder to improve efficiency. The double-expansion Corliss engines installed in 1903 for the IRT subway in New York developed
7500 hp at 75 rpm. Generators were driven at a speed higher than the engine by means of pulleys with rope or leather belts. Storage batter- ies usually provided excitation for the generators and were themselves charged from a small generator. DC machines could be paralleled simply by matching the voltage of the incoming machine to the bus voltage and then switching it in. Load sharing was adjusted by field control.
Alternating-current generators had been built for some years, but further use of AC power had been limited by the lack of a suitable AC motor. Low-frequency AC could be used on commutator motors that were basically DC machines, but attempts to operate them on the higher AC frequencies required to minimize lamp flicker were not successful. Furthermore, early AC generators could be paralleled only with difficulty, so each generator had to be connected to an assigned load and be on line at all times. Battery backup or battery supply at light load could not be used. Figure 1.1 shows the difference. Finally, generation and utilization voltages were similar to those with DC, so AC offered no advantage in this regard.