电气工程专业英语课件lecture11-2012

1、1,各位老师： 上午好！,Lecture 11,Specialized English for Electrical Engineering,2,Lecture 11,Reading and Translation High-Voltage Direct-Current Transmission Part 1 Overview of High-Voltage Direct-Current Transmission The year 1954 is generally recognized as the starting date for modern application of High-v

2、oltage direct 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 fr

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

4、 (500 kV line 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. B

5、ecause capacitance 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 c

6、hosen to transfer power under the English Channel between Great Britain and France. The use of direct current for this,4,Lecture 11,installation 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

7、 urban centers. Unlike 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 ther

8、e is no DC device which 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.,5,Lecture 11,No network of DC lines is possible at this time because no circ

9、uit breaker is available 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 i

10、n which grid-controlled 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。,6,Lecture 11,2. DC transm

11、ission remains very limited 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

12、is controlled by the converters in which grid-controlled mercury-arc devices are 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。,7,Lecture 11,New 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

13、 way 公共事业用地 converter 变流器，换流器 mercury-arc 汞弧 semiconductor 半导体,8,Lecture 11,Part 2 Basic DC transmission system A 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 converte

14、r at the other end reconverts the DC power into 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 d

15、irect to alternating current so that power can 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 that,9,Lecture 11,converts the AC power of line 1 into DC

16、 power. The DC power is carried over a 2-conductor 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

17、 entirely different frequencies without affecting 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

18、 current flow remains the same. This mode of operation is required because thyristors can only conduct current in one direction.,10,Lecture 11,The 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 ins

19、ofar as it affects losses, efficiency, and conductor 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, 10

20、00 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.,11,Lecture 11,1. More exactly, conver

21、ters 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 can flow in either direction. 更为准确地说，直流线路两端的换流器都既可作为整流器将 产生的交流变为直流，也可作为逆变器将直流转换为交流， 从而功率可以向每个方向流动。 New Wor

22、ds and Expressions rectifier 整流器 inverter 逆变器 strip of 剥夺 line-commutated 线换向的 respective 分别的 thyristor 晶闸管 valve 阀 bridge arm 桥臂 trigger 触发 simultaneously 同时地,12,Lecture 11,Part 3 HVDC system Configuration and Components HVDC links may be broadly classified into three categories: monopolar links, b

23、ipolar links and homopolar links. The basic configuration of a monopolar link is shown in Fig. 19.2. It uses one conductor, usually of negative polarity. The return path is provided by ground or water. Cost considerations often lead to the use of such systems, particularly for cable transmission. Th

24、is type of configuration may also be the first stage in the development of a bipolar system. Instead of ground return, a metallic return may be used in situations where the earth resistivity is too high or possible interference with underground/underwater metallic structures is objectionable. The co

25、nductor forming the metallic return is at low voltage.,13,Lecture 11,The bipolar link configuration is shown in Fig. 19.3. it has two conductors, one positive and the other negative. Each terminal has two converters of equal rated voltage, connected in series on the DC side. The junctions between th

26、e converters are grounded. Normally, the currents in the two poles are equal, and there is no ground current. The two poles can operate independently. If one pole is isolated due to a fault on its conductor, the other pole can operate with ground and thus carry half the rated load or more by using t

27、he overload capabilities of its converters and line. From the viewpoint of lightning performance, a bipolar HVDC line is considered to be effectively equivalent to a double-circuit AC transmission line. Under normal operation, it will cause considerably less harmonic interference on nearby facilitie

28、s than the monopolar system. Reversal of power-flow direction is achieved by changing the polarities of the two poles through controls.,14,Lecture 11,In situations where ground currents are not tolerable or when a ground electrode is not feasible for reasons such as high earth resistivity, a third c

29、onductor is used as a metallic neutral. It serves as the return path when one pole is out of service or when there is imbalance during bipolar operation. The third conductor requires low insulation and may also serve as a shield wire for overhead lines. If it is fully insulated, it can serve as a sp

30、are. The homopolar link, whose configuration is shown in Fig. 19.4, has two or more conductors, all having the same polarity. Usually a negative polarity is preferred because it causes less radio interference due to corona. The return path for such a system is through ground. When there is a fault o

31、n one conductor, the entire converter is available for feeding the remaining conductors which, having some overload capability,15,Lecture 11,can carry more than the normal power. In contrast, for a bipolar scheme usually not feasible. Homopolar configuration offers an advantage in this regard in sit

32、uations where continuous ground current is acceptable. The ground current can have side effects on gas or oil pipes, so configurations using ground return may not always be acceptable. Each of the above HVDC system configurations usually has cascaded groups of several converters, each having a trans

33、former bank and a group of valves. The converters are connected in parallel on the AC side (transformer) and in series on the DC side (valve) to give the desirable level of voltage from pole to ground. Back-to-back HVDC systems (used for asynchronous ties) may be designed for monopolar or bipolar op

34、eration with a different number of valve groups per pole, depending on the purpose of the interconnection and the desired reliability.,16,Lecture 11,Most point-to-point (two terminal) HVDC links involving lines are bipolar, with monopolar operation used only during contingencies. They are normally d

35、esigned to provide maximum independence between poles to avoid bipolar shutdowns. A multiterminal HVDC system is formed when the DC system is to be connected to more than two nodes on the AC network. The main components associated with an HVDC system are shown in Fig. 19.5, using a bipolar system as

36、 an example. The components for other configurations are essentially the same as those shown in the figure. In order to function properly, an HVDC system must have auxiliary components, in addition to the basic converters. The most important components are DC smoothing reactors,17,Lecture 11,Harmoni

37、c filters on the DC side (DC filter), Converter transformers, Reactive power source, Harmonic filters on the AC side (AC filter), Ground electrodes, Microwave communications link between the converter stations (not shown), Circuit Breakers (CB). 1. Instead of ground return, a metallic return may be

38、used in situations where the earth resistivity is too high or possible interference with underground/underwater metallic structures is objectionable. 在地电阻率太大或者不允许对地下/水下金属结构产生干扰 时，可以用金属回路取代地回路。,18,Lecture 11,2. When there is a fault on one conductor, the entire converter is available for feeding the

39、remaining conductors which, having some overload capability, can carry more than the normal power. 当一根导线上有故障时，换流器可为余下的线路供电，这些导 线具有一定的过载能力，能承受比正常情况更大的功率。 3. Back-to-back HVDC systems (used for asynchronous ties) may be designed for monopolar or bipolar operation with a different number of valve group

40、s per pole, depending on the purpose of the interconnection and the desired reliability. 背靠背HVDC系统（用于不同步的联络线）可设计为单极或双极 运行，每极有不同数目的阀组，这取决于互联的目的和期望的 可靠性。,19,Lecture 11,New Words and Expressions monopolar 单极的 bipolar 双极的 homopolar 同极的 junction 连接处 double-circuit 双回路 reversal 颠倒，反向 feasible 可行的 spare 备用品 side effect 副作用 cascade 级联的 transformer bank 变压器组,20,Lecture 11,Part 4 Advantages and Disadvantages of DC Transmission The advantages of DC transmission: i) The cost of DC transmission over long distances is lower; ii) Voltage regulation is less of a problem since at ze