10.1 INTRODUCTION TO POWER SYSTEMS 453- 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. A total of 30 HVDC lines up to 533 kV are in place worldwide. For an HVDC line
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.
An interconnected system (in contrast with isolated systems) has many advantages: - 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.
Present and Future Trends
According to the Edison Electric Institute, electricity’s share of U.S. primary energy was almost
36% in 1989, and it is likely to reach 46% by the year 2010. The growth rate in the use of electric
energy in the United States is projected to increase by about 2.4% per year for the near future,
in spite of conservation practices, more efficient use of electricity, and a slackening population
growth. One should also be aware of large growth of power systems internationally.
Because of the large amount of U.S. coal reserves, there is the continuing shift away from
the use of gas and oil and toward increasing use of coal. Unless the construction time and cost
per kW can be reduced significantly, no new nuclear units will be commissioned. Also, safety