452 AC POWER SYSTEMS
of an ac power network are identified, and topics related to power transmission and distribution
are introduced.10.1 INTRODUCTION TO POWER SYSTEMS
Thomas A. Edison’s work in 1878 on the electric light led to the concept of a centrally located
power station with distributed electric power for lighting in a surrounding area. The opening
of the historic Pearl Street Station in New York City on September 4, 1882, with dc generators
(dynamos) driven by steam engines, marked the beginning of the electric utility industry. Edison’s
dc systems expanded with the development of three-wire 220-V dc systems. But as transmission
distances and loads continued to grow, voltage problems were encountered. With the advent
of William Stanley’s development of a commercially practical transformer in 1885, alternating
current became more attractive than direct current because of the ability to transmit power at high
voltage with corresponding lower current and lower line-voltage drops. The first single-phase ac
line (21 km at 4 kV) in the United States operated in 1889 between Oregon City and Portland.
Nikola Tesla’s work in 1888 on electric machines made evident the advantages of polyphase
over single-phase systems. The first three-phase line (12 km at 2.3 kV) in the United States
became operational in California during 1893. The three-phase induction motor conceived by
Tesla became the workhorse of the industry.
Most electric energy has been generated by steam-powered (accounting for about 85% of
U.S. generation) and by water-powered, or hydro, turbine plants (accounting for about 10% of
U.S. generation). Gas turbines are also used for short periods to meet peak loads. Steam plants
are fueled primarily by coal, gas, oil, and uranium. While coal is the most widely used fuel in
the United States, due to its abundance, nuclear units of 1280-MW steam-turbine capacity are
in service today. However, rising construction costs, licensing delays, and public concerns have
stopped the growth of nuclear capacity in the United States.
Other types of electric power generation are also prevalent, accounting for about 1% of
U.S. generation. These include wind-turbine generators, solar-cell arrays, tidal power plants, and
geothermal power plants, wherein energy in the form of steam or hot water is extracted from the
earth’s upper crust. Substantial research now under way shows nuclear fusion energy to be the
most promising technology for producing safe, pollution-free, and economical electric energy in
this century and beyond, since the needed fuel (deuterium) consumed in a nuclear fusion reaction
is present in seawater abundantly.
Today the two standard frequencies for the generation, transmission, and distribution of
electric power in the world are 60 Hz (in the United States, Canada, Japan, and Brazil) and 50 Hz
(in Europe, the former Soviet Republics, South America except Brazil, India, and also Japan).
Relatively speaking, the 60-Hz power-system apparatus is generally smaller in size and lighter
in weight than the corresponding 50-Hz equipment with the same ratings. On the other hand,
transmission lines and transformers have lower reactances at 50 Hz than at 60 Hz.
Along with increases in load growth, there have been continuing increases in the size of
generating units and in steam temperatures and pressure, leading to savings in fuel costs and
overall operating costs. Ac transmission voltages in the United States have also been rising
steadily: 115, 138, 161, 230, 345, 500, and now 765 kV. Ultrahigh voltages (UHV) above 1000
kV are now being studied. Some of the reasons for increased transmission voltages are:- Increases in transmission distance and capacity
- Smaller line-voltage drops
- Reduced line losses