1、1,各位老師： 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-Curre

2、nt Transmission The year 1954 is generally recognized as the starting date for modern application of High-voltage direct current (HVDC) transmission when a DC line of a distance of 100 km began service at 10

3、0 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

4、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 t

5、he comparable AC line and auxiliary equipment. This line operates at ±250 kV (500 kV line to line) and transmits 500 MW. DC power can be transmitted in cables over great distances. The capacitance of a

6、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 capacitance does not come into play und

7、er 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

8、 unthinkable. Direct current was chosen 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 synchroniz

9、ing the AC systems of the two countries. Furthermore, underground DC cable may be used to deliver power into large urban centers. Unlike overhead lines, underground cable is invisible, free from atmospheric pollution, an

10、d 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 d

11、evice 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,Lectur

12、e 11,No network of DC lines is possible at this time because no circuit breaker is available for direct current comparable to the highly developed AC breakers. The AC breaker can extinguish the arc which is formed when t

13、he 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-controlled mercury-arc devices are being displaced by the s

14、emiconductor devices. 1. Unlike overhead lines, underground cable is invisible, free from atmospheric pollution, and solves the problem of securing rights of way. 和架空線不同，地下電纜是看不見(jiàn)的，免受大

15、氣污染，并解 決了安全的公用通道問(wèn)題。,6,Lecture 11,2. DC transmission remains very limited in usage except for long lines because there is no DC device which can provide the excellent switching operations and prot

16、ection of the AC circuit devices. 直流輸電除用于長(zhǎng)線（輸電）以外在應(yīng)用上仍然十分有限， 這是因?yàn)闆](méi)有直流設(shè)備能夠提供交流裝置所具有的卓越的開(kāi) 關(guān)操作和保護(hù)功能。 3. The direction and amount of power in the DC line is controlled by the converters in

17、which grid-controlled mercury-arc devices are being displaced by the semiconductor devices. 直流線路上功率的流向和數(shù)量用換流器控制，其中柵控汞弧設(shè) 備正在被半導(dǎo)體裝置取代。,7,Lecture 11,New Words and Expressions mine-mouth 礦山口

18、 preliminary 預(yù)備的，初步的 auxiliary 輔助設(shè)備 installation 裝置 unthinkable 不能想象的 rights of way 公共事業(yè)用地 converter 變流器，換流器 mercury-arc 汞弧 semiconductor 半導(dǎo)體,8,Lecture 11,

19、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 s

20、imilar converter 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 rec

21、tifiers 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. Stripped of everything but the bare essen

22、tials, 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 power. The DC power is carried ov

23、er 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 connect

24、ed. Consequently, the networks can function at entirely different frequencies without affecting the power transmission between them. Power flow may be reversed by changing the firing angles and , so tha

25、t 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 operation is required because t

26、hyristors 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 ne

27、glect is, except insofar 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 th

28、yristors 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

29、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, converters at the two ends of the D

30、C 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 direct

31、ion. 更為準(zhǔn)確地說(shuō)，直流線路兩端的換流器都既可作為整流器將 產(chǎn)生的交流變?yōu)橹绷?，也可作為逆變器將直流轉(zhuǎn)換為交流， 從而功率可以向每個(gè)方向流動(dòng)。 New Words and Expressions rectifier 整流器 inverter 逆變器 strip of 剝奪

32、 line-commutated 線換向的 respective 分別的 thyristor 晶閘管 valve 閥 bridge arm 橋臂 trigger 觸發(fā) simultaneously

33、同時(shí)地,12,Lecture 11,Part 3 HVDC system Configuration and Components HVDC links may be broadly classified into three categories: monopolar links, bipolar links and homopolar links. The basic configu

34、ration 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, particular

35、ly for cable transmission. This 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

36、is too high or possible interference with underground/underwater metallic structures is objectionable. The conductor forming the metallic return is at low voltage.,13,Lecture 11,The bipolar link configuration is shown in

37、 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 the converters are grounded. Norm

38、ally, 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

39、thus carry half the rated load or more by using the overload capabilities of its converters and line. From the viewpoint of lightning performance, a bipolar HVDC line is considered to be effectively equivalent

40、 to a double-circuit AC transmission line. Under normal operation, it will cause considerably less harmonic interference on nearby facilities than the monopolar system. Reversal of power-flow direction is achieved by cha

41、nging 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

42、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

43、a shield wire for overhead lines. If it is fully insulated, it can serve as a spare. The homopolar link, whose configuration is shown in Fig. 19.4, has two or more conductors, all having the same polarity. Usu

44、ally 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 on one conductor, the entire converter is availabl

45、e 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

46、 advantage in this regard in situations 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.

47、 Each of the above HVDC system configurations usually has cascaded groups of several converters, each having a transformer bank and a group of valves. The converters are connected in parallel on the AC side (trans

48、former) 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 operatio

49、n 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, wi

50、th monopolar operation used only during contingencies. They are normally designed to provide maximum independence between poles to avoid bipolar shutdowns. A multiterminal HVDC system is formed when the DC sys

51、tem 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 an example. The components for other config

52、urations 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

53、are DC smoothing reactors,,17,Lecture 11,Harmonic filters on the DC side (DC filter), Converter transformers, Reactive power source, Harmonic filters on the AC side (AC filter), Ground electrodes, Microwave communication

54、s link between the converter stations (not shown), Circuit Breakers (CB). 1. Instead of ground return, a metallic return may be used in situations where the earth resistivity is too high or possible

55、 interference with underground/underwater metallic structures is objectionable. 在地電阻率太大或者不允許對(duì)地下/水下金屬結(jié)構(gòu)產(chǎn)生干擾 時(shí)，可以用金屬回路取代地回路。,18,Lecture 11,2. When there is a fault on one conductor, the entire

56、 converter is available for feeding the remaining conductors which, having some overload capability, can carry more than the normal power. 當(dāng)一根導(dǎo)線上有故障時(shí)，換流器可為余下的線路供電，這些導(dǎo) 線具有一定的過(guò)載能力，能承受比正

57、常情況更大的功率。 3. Back-to-back HVDC systems (used for asynchronous ties) may be designed for monopolar or bipolar operation with a different number of valve groups per pole, depending on the purpose of

58、 the interconnection and the desired reliability. 背靠背HVDC系統(tǒng)（用于不同步的聯(lián)絡(luò)線）可設(shè)計(jì)為單極或雙極 運(yùn)行，每極有不同數(shù)目的閥組，這取決于互聯(lián)的目的和期望的 可靠性。,19,Lecture 11,New Words and Expressions monopolar 單極的 bipola

59、r 雙極的 homopolar 同極的 junction 連接處 double-circuit 雙回路 reversal 顛倒，反向 feasible 可行的 spare 備用品 side effect 副作用

60、cascade 級(jí)聯(lián)的 transformer bank 變壓器組,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;

61、 ii) Voltage regulation is less of a problem since at zero frequency is no longer a factor, whereas it is the chief contributor to voltage drop in an AC line; iii) The reliability i

62、s higher due to the possibility of monopolar operation in an emergency when one side of a bipolar line becomes grounded; iv) DC power can be controlled much more quickly; it means

63、 power in the megawatt range can be reversed in a DC line in less than one second on one side and DC short circuit currents can be limited to much lower values than those,21,Lecture 11,encoun

64、tered on AC networks on the other; V) The corona loss in a DC line operating at a voltage corresponding to the peak value of the equivalent alternating voltage is substantially less th

65、an for the AC line. The disadvantages of DC transmission: i) The much more onerous conditions for circuit breaking when the current does not reduce to zero twice a cycle; ii) Voltag

66、e transformation has to be provided on the AC sides of the system; iii) Rectifiers and inverters absorb reactive power which must be supplied locally; iv) DC converting sta

67、tions are much more expensive than conventional AC substations.,22,Lecture 11,SUMMARY OF GLOSSARY 1. 換流器 converter rectifier 整流器 inverter

68、 逆變器 2. HVDC鏈的分類 categories of HVDC links monopolar link 單極型 bipolar link 雙極型 homopolar link 同極型 3. HVDC連接 HVDC c