Introduction To Power System

Introduction to Power System: 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.

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
  • Reduced right-of-way requirements
  • Lower capital and operating costs.

In association with ac transmission, there have been other significant developments:

  • Suspension insulator
  • High-speed relay system
  • High-speed, extra-high-voltage (EHV) circuit breakers
  • EHV surge arrester to protect from lightning strokes and other surges
  • Communications via power-line carrier, microwave, and fiber optics
  • Energy control centers with supervisory control and data acquisition (SCADA) and with automatic generation control (AGC)
  • Extensive use of microprocessors for various tasks.

Along with ac transmission in the United States, there have been modern high-voltage dc (HVDC) transmission lines: the ±400-kV, 1360-km Pacific Intertie line between Oregon and California in 1970 as well as four other HVDC lines up to 400 kV and five back-to-back ac–dc links as of 1991.Atotal of 30HVDClines up to 533 kV are in place worldwide. For anHVDCline interconnected with an ac system, solid-state converters at both ends of the dc line are needed to operate as rectifiers and inverters. Studies in the United States have shown that overhead HVDC transmission is economical for transmission distances longer than about 600 km. Also, HVDC links seem to improve the overall system stability.

  • Better maintenance of continuity of service
  • Increase in reliability and improved economy
  • Reduction of reserve requirements
  • Scheduling power transfers taking advantage of energy-cost differences in respective areas, load diversity, and seasonal conditions
  • Shared ownership of larger and more efficient generating units.

Some of the disadvantages of interconnected operations are:

  • Increased fault currents during short circuits
  • Occasional domino effect leading to a regional blackout (such as the one that occurred in 1965 in the northern United States) due to an initial disturbance in some part of the interconnected grid system.