1、中文 2188 字 EVOLUTION OF ELECTRIC POWER SYSTEMS The commercial use of electricity began in the late1870s when arc lamps were used for lighthouse illumination and street lighting. The first complete electric power system (comprising a generator, cable, fuse, meter, and loads)was built by Thomas Edison-
2、the historic Pearl Street Station in New York City which began operation in September 1882.This was a dc system consisting of a steam-engine-driven dc generator supplying power to 59 customers within an area roughly 1.5km in radios. The load, which consisted entirely of incandescent lamps, was suppl
3、ied at 110 V through an underground cable system. Within a few years similar systems were in operation in most large cities throughout the world. With the development of motors by Frank Sprague in 1884, motor loads were added to such systems. This was the beginning of what would develop into one of
4、the largest industries in the world. In spite of the initial widespread use of dc systems, they were almost completely superseded by ac systems. By 1886, the limitations of dc systems were becoming increasingly apparent .They could deliver power only a short distance from the generators. To keep tra
5、nsmission power losers (RI2) and voltage drops to acceptable levels, voltage levels had to be high for long-distance power transmission. Such high voltages were not acceptable for generation and consumption of power； therefore, a convenient means for voltage transformation became a necessity. The de
6、velopment of the transformation and ac transmission by L. Gaulard and J.D. Gibbs of Paris, France, led to ac electric power systems. George Westinghouse secured rights to these developments in the United States. In 1886, William Stanley, an associate of Westinghouse, developed and tested a commercia
7、lly practical transformer and ac distribution system for 150 lamps at Great Barrington, Massachusetts. In 1889, the first ac transmission line in North America was put into operation in Oregon between Willamette Falls and Portland. It was a single-phase line transmitting power at 4,000 V over a dist
8、ance of 21 km. With the development of polyphase systems by Nikolas Tesla, the ac system became even more attractive. By 1888, Tesla held several patents on ac motors, generators, transformers, and transmission systems. Westinghouse bought the patents to these early inventions, and they formed the b
9、asis of the present-day ac systems. In the 1890s, there was considerable controversy over whether the electric utility industry should be standardized on dc or ac. There were passionate arguments between Edison, who advocated dc, and Westinghouse, who favored ac. By the turn of the century, the ac s
10、ystem had won out over the dc system for the following reasons； Voltage levels can be easily transformed in ac systems, thus providing the flexibility for use of different voltages for generation, transmission, and consumption. AC generators are much simpler than dc generators. AC motors are much si
11、mpler and cheaper than dc motors. The first three-phase line in North America went into operation in 1893-a 2,300 V, 12 km line in southern California. Around this time, ac was chosen at Niagara Falls because dc was not practical for transmitting power to Buffalo, about 30 km away. This decision end
12、ed the ac versus dc controversy and established victory for the ac system. In the early period of ac power transmission, frequency was not standardized. Many different frequencies were in use: 25, 50, 60, 125, and 133 Hz. This posed a problem for interconnection. Eventually 60 Hz was adopted as stan
13、dard in North America, although many other countries use 50 Hz. The increasing need for transmitting larger amounts of power over longer distances created an incentive to use progressively higher voltage levels. The early ac systems used 12,44, and 60 kV(RMS line-to-line).This rose to 165 kV in 1922
14、,220 kV in 1923,287 kV in 1935,330 kV in 1953,and 765 kV was introduced in the United States in 1969. To avoid the proliferation of an unlimited number of voltages, the industry has standardized voltage levels. The standards are 115, 138, 161, and 230 kV for the high voltage (HV) class, and 345, 500
15、 and 765 kV for the extra-high voltage (EHV) class. With the development of mercury arc valves in the early 1950s, high voltage dc (HVDC) transmission systems became economical in special situations. The HVDC transmission is attractive for transmission of large blocks of power over long distances. T
16、he cross-over point beyond which dc transmission may become a competitive to ac transmission is around 500 kV for around 500 km for overhead lines and 50 km for underground or submarine cables. HDVC transmission also provides an asynchronous link between systems where ac interconnection would be imp
17、ractical because of system stability considerations or because nominal frequencies of the systems are different. The first modern commercial application of HVDC transmission occurred in 1954 when the Swedish mainland and the island of Gotland were interconnected by a 96 km submarine cable. With the
18、advent of thyristor valve converters, HVDC transmission became even more attractive. The first application of an HVDC system using thyristor values was at Eel River in 1972-a back-to-back scheme providing an asynchronous tie between the power systems of Quebec and New Brunswick. With the cost and si
19、ze of conversion equipment decreasing and its reliability increasing, there has been a steady increase in the use of HVDC transmission. Interconnection of neighboring utilities usually leads to improved security results from the mutual emergency assistance that the utilities can provide. Improved ec
20、onomy results from the need for less generating reserve capacity in each system. In addition, the interconnection permits the utilities to make economy transfers and thus take advantage of the most economical sources of power. These benefits have been recognized from the beginning and interconnectio
21、ns continue to grow. Almost all the utilities in the United States and Canada are now part of one interconnected system of enormous complexity. The design of such a system and its secure operation are indeed challenging problems. STRUCTURE OF THE POWER SYSTEM Electric power system varies in size and
22、 structural components. However, they all have the same basic characteristics: Are comprised of three-phase ac systems operating essentially at constant voltage. Generation and transmission facilities use three-phase equipment. Industrial loads are invariably three-phase； single-phase residential an
23、d commercial loads are distributed equally among the phases so as to effectively form a balanced three-phase system. Use synchronous machines for electricity. Prime movers convent the primary sources of energy (fossil, nuclear, and hydraulic) to mechanical energy that is, in turn, converted to elect
24、rical energy by synchronous generators. Transmit power over significant distances to consumers spread over a wide area. This requires a transmission system comprising subsystems operating at different voltage levels. Electric power is produced at generating stations (GS) and transmitted to consumers
25、 through a complex network of individual components, including transmission lines, transformers, and switching devices. It is common practice to classify the transmission network into the following subsystems: 1. Transmission system 2. Subtransmission system 3. Distribution system The transmission s
26、ystem interconnects all major generating stations and main load canters in the system. It forms the backbone of the integrated power system and operates at the highest voltage levels (typically, 230kV and above).The generator voltage are usually in the range of 11 to 35 kV. These are stepped up to t
27、he transmission voltage levels, and power is transmitted to transmission substations where the voltage are stepped down to the subtransmission level (typically, 69 kV to 138 kV).The generation and transmission subsystems are often referred to as the bulk power system. The subtransmission system tran
28、smits power in smaller quantities from the transmission substations to the distribution substations. Large industrial customers are commonly supplied directly from the substransmission system. In some systems, there is no clear demarcation between substransmission and transmission circuits. As the s
29、ystem expands and higher voltage levels become necessary for transmission, the older transmission lines are often relegated to subtransmission function. The distribution system represents the final stage in the transfer of power to the individual customers. The primary distribution voltage is typically between 4.0 kV and 34.5 kV. Small industrial customers are supplied by primary feeders at this voltage level. The secondary distribution feeders are supply residential and commercial customers at 120/240V.
