电气工程专业英语课件lecture11-2012.pptx

1、1各位老师：上午好！Lecture 11Specialized English for Electrical Engineering2Lecture 11Reading and TranslationHigh-Voltage Direct-Current TransmissionPart 1 Overview of High-Voltage Direct-Current TransmissionThe year 1954 is generally recognized as the starting date for modern application of High-voltage dir

2、ect current (HVDC) transmission when a DC line of a distance of 100 km began service at 100 kV from the mainland of Sweden to the island of Gotland. Since then, there has been a steady increase in the application of HVDC transmission.Operation of DC line began in 1977 to transmit power from a mine-m

3、outh generating plant at Center, North Dakota to near Duluth, Minnesota, a distance of 740 km. Preliminary studies showed that the DC line including terminal facilities3Lecture 11would cost about 30% less than the comparable AC line and auxiliary equipment. This line operates at 250 kV (500 kV line

4、to line) and transmits 500 MW.DC power can be transmitted in cables over great distances. The capacitance of a cable limits AC power transmission to a few tens of kilometers. Beyond this limit, the reactive power generated by cable capacitance exceeds the rating of the cable itself. Because capacita

5、nce does not come into play under steady-state DC conditions, there is theoretically no limit to the distance that power may be carried this way. As a result, power can be transmitted by cable under large bodies of water, where the use of AC cables is unthinkable. Direct current was chosen to transf

6、er power under the English Channel between Great Britain and France. The use of direct current for this4Lecture 11installation also avoided the difficulty of synchronizing the AC systems of the two countries. Furthermore, underground DC cable may be used to deliver power into large urban centers. Un

7、like overhead lines, underground cable is invisible, free from atmospheric pollution, and solves the problem of securing rights of way.DC transmission has many advantages over alternating current, but DC transmission remains very limited in usage except for long lines because there is no DC device w

8、hich can provide the excellent switching operations and protection of the AC circuit devices. There is also no simple device to change the voltage level, which the transformer accomplishes for AC systems.5Lecture 11No network of DC lines is possible at this time because no circuit breaker is availab

9、le for direct current comparable to the highly developed AC breakers. The AC breaker can extinguish the arc which is formed when the breaker opens because zero current occurs twice in each cycle. The direction and amount of power in the DC line is controlled by the converters in which grid-controlle

10、d mercury-arc devices are being displaced by the semiconductor devices.1. Unlike overhead lines, underground cable is invisible, free from atmospheric pollution, and solves the problem of securing rights of way. 和架空线不同，地下电缆是看不见的，免受大气污染，并解决了安全的公用通道问题。6Lecture 112. DC transmission remains very limited

11、 in usage except for long lines because there is no DC device which can provide the excellent switching operations and protection of the AC circuit devices.直流输电除用于长线（输电）以外在应用上仍然十分有限，这是因为没有直流设备能够提供交流装置所具有的卓越的开关操作和保护功能。3. The direction and amount of power in the DC line is controlled by the converters

12、 in which grid-controlled mercury-arc devicesare being displaced by the semiconductor devices.直流线路上功率的流向和数量用换流器控制，其中栅控汞弧设备正在被半导体装置取代。7Lecture 11New Words and Expressions mine-mouth 矿山口 preliminary 预备的，初步的auxiliary 辅助设备 installation 装置 unthinkable 不能想象的 rights of way 公共事业用地converter 变流器，换流器 mercury-a

13、rc 汞弧semiconductor 半导体8Lecture 11Part 2 Basic DC transmission systemA DC transmission system consists basically of a DC transmission line connecting two AC systems. A converter at one end of the line converts AC power into DC power while a similar converter at the other end reconverts the DC power i

14、nto AC power. One converter acts therefore as a rectifier, the other as an inverter. More exactly, converters at the two ends of the DC lines operate both as rectifiers to change the generated alternating to direct current and as inverters for converting direct to alternating current so that power c

15、an flow in either direction.Stripped of everything but the bare essentials, the transmission system may be represented by the circuit of Fig. 19.1. Converter 1 is a three-phase, six-pulse rectifier that9Lecture 11converts the AC power of line 1 into DC power. The DC power is carried over a 2-conduct

16、or transmission line and reconverted to AC power by means of converter 2, acting as an inverter. Both the rectifier and inverter are line-commutated by the respective line voltages to which they are connected. Consequently, the networks can function at entirely different frequencies without affectin

17、g the power transmission between them. Power flow may be reversed by changing the firing angles and , so that Converter 1 becomes an inverter and Converter 2 a rectifier. Changing the angles reverses the polarity of the conductors, but the direction of current flow remains the same. This mode of ope

18、ration is required because thyristors can only conduct current in one direction.10Lecture 11The DC voltages and at each converter station are identical, except for the drop in the line. The drop is usually so small that we can neglect is, except insofar as it affects losses, efficiency, and conducto

19、r heating.Due to the high voltage encountered in transmission lines, each thyristor shown in Fig. 19.1 is actually composed of several thyristors connected in series. Such a group of thyristors is often called a valve. Thus, a valve for a 50 kV, 1000 A converter would typically be composed of 50 thyristors connected in series. Each converter in Fig. 19.1 would, therefore, contain 300 thyristors. The 50 thyristors in each bridge arm are triggered simultaneously, so together they act like a super-thyristor.