电气专业中英文文献

1、外文文献1 power electronic conceptspower electronics is a rapidly developing technology. components are tting higher current and voltage ratings, the power losses decrease and the devices become more reliable. the devices are also very easy tocontrol with a mega scale power amplification. the prices are

2、 still going down pr. kva and power converters are becoming attractive as a mean to improve the performance of a wind turbine. this chapter will discuss the standard power converter topologies from the simplest converters for starting up the turbine to advanced power converter topologies, where the

3、whole power is flowing through the converter. further, different park solutions using power electronics arealso discussed.1.1 criteria for concept evaluationthe most common topologies are selected and discussed in respect to advantages and drawbacks. very advanced power converters, where many extra

4、devices are necessary in order to get a proper operation, are omitted.1.2 power convertersmany different power converters can be used in wind turbine applications. in the case of using an induction generator, the power converter has to convert from a fixed voltage and frequency to a variable voltage

5、 and frequency. this may be implemented in many different ways, as it will be seen in the next section. other generator types can demand other complex protection. however, the most used topology so far is a soft-starter, which is used during start up in order to limit the in-rush current and thereby

6、 reduce the disturbances to the grid.1.2.1 soft starterthe soft starter is a power converter, which has been introduced to fixed speed wind turbines to reduce the transient current during connection or disconnection of the generator to the grid. when the generator speed exceeds the synchronous speed

7、, the soft-starter is connected. using firing angle control of the thyristors in the soft starter the generator is smoothly connected to the grid over a predefined number of grid periods. an example of connection diagram for the softstarter with a generator is presented in figure1. figure 1. connect

8、ion diagram of soft starter with generators.the commutating devices are two thyristors for each phase. these are connected in anti-parallel. the relationship between the firing angle () and the resulting amplification of the soft starter is non-linear and depends additionally on the power factor of

9、the connected element. in the case of a resistive load, may vary between 0 (full on) and 90 (full off) degrees, in the case of a purely inductive load between 90 (full on) and 180 (full off) degrees. for any power factor between 0 and 90 degrees, will be somewhere between the limits sketched in figu

10、re 2.figure 2. control characteristic for a fully controlled soft starter.when the generator is completely connected to the grid a contactor (kbyp) bypass the soft-starter in order to reduce the losses during normal operation. the soft-starter is very cheap and it is a standard converter in many win

11、d turbines.1.2.2 capacitor bankfor the power factor compensation of the reactive power in the generator, accapacitor banks are used, as shown in figure 3. the generators are normally compensated into whole power range. the switching of capacitors is done as a function of the average value of measure

12、d reactive power during a certain period.figure 3. capacitor bank configuration for power factor compensation in a wind turbine.the capacitor banks are usually mounted in the bottom of the tower or in thenacelle. in order to reduce the current at connection/disconnection of capacitors a coil (l) can

13、 be connected in series. the capacitors may be heavy loaded and damaged in the case of over-voltages to the grid and thereby they may increase the maintenance cost.1.2.3 diode rectifierthe diode rectifier is the most common used topology in power electronic applications. for a three-phase system it

14、consists of six diodes. it is shown in figure 4.figure 4. diode rectifier for three-phase ac/dc conversionthe diode rectifier can only be used in one quadrant, it is simple and it is notpossible to control it. it could be used in some applications with a dc-bus.1.2.4 the back-to-back pwm-vsithe back

15、-to-back pwm-vsi is a bi-directional power converter consisting of two conventional pwm-vsi. the topology is shown in figure 5.to achieve full control of the grid current, the dc-link voltage must be boosted to a level higher than the amplitude of the grid line-line voltage. the power flow of the gr

16、id side converter is controlled in order to keep the dc-link voltage constant, while the control of the generator side is set to suit the magnetization demand and the reference speed. the control of the back-to-back pwm-vsi in the wind turbine application is described in several papers (bogalecka, 1

17、993), (knowles-spittle et al., 1998), (pena et al., 1996), (yifan & longya, 1992), (yifan & longya, 1995).figure 5. the back-to-back pwm-vsi converter topology. advantages related to the use of the back-to-back pwm-vsithe pwm-vsi is the most frequently used three-phase frequency conve

18、rter. as a consequence of this, the knowledge available in the field is extensive and well established. the literature and the available documentation exceed that for any of the other converters considered in this survey. furthermore, many manufacturers produce components especially designed for use

19、 in this type of converter (e.g., a transistor-pack comprising six bridge coupled transistors and anti paralleled diodes). due to this, the component costs can be low compared to converters requiring components designed for a niche production.a technical advantage of the pwm-vsi is the capacitor dec

20、oupling between the grid inverter and the generator inverter. besides affording some protection, this decoupling offers separate control of the two inverters, allowing compensation of asymmetry both on the generator side and on the grid side, independently.the inclusion of a boost inductance in the

21、dc-link circuit increases the component count, but a positive effect is that the boost inductance reduces the demands on the performance of the grid side harmonic filter, and offers some protection of the converter against abnormal conditions on the grid. disadvantages of applying the back-to

22、-back pwm-vsithis section highlights some of the reported disadvantages of the back-to-back pwm-vsi which justify the search for a more suitable alternative converter:in several papers concerning adjustable speed drives, the presence of the dclink capacitor is mentioned as a drawback, since it is he

23、avy and bulky, it increases the costs and maybe of most importance, - it reduces the overall lifetime of the system. (wen-song & ying-yu, 1998); (kim & sul, 1993); (siyoung kim et al., 1998).another important drawback of the back-to-back pwm-vsi is the switching losses. every commutation in

24、both the grid inverter and the generator inverter between the upper and lower dc-link branch is associated with a hard switching and a natural commutation. since the back-to-back pwm-vsi consists of two inverters, the switching losses might be even more pronounced. the high switching speed to the gr

25、id may also require extra emi-filters.to prevent high stresses on the generator insulation and to avoid bearing current problems (salo & tuusa, 1999), the voltage gradient may have to be limited by applying an output filter.1.2.5 tandem converterthe tandem converter is quite a new topology and a

26、 few papers only have treated it up till now (marques & verdelho, 1998); (trzynadlowski et al., 1998a); (trzynadlowski et al., 1998b). however, the idea behind the converter is similar to those presented in (zhang et al., 1998b), where the pwm-vsi is used as an active harmonic filter to compensa

27、te harmonic distortion. the topology of the tandem converter is shown in figure 6.figure 6. the tandem converter topology used in an induction generator wind turbine system.the tandem converter consists of a current source converter, csc, in thefollowing designated the primary converter, and a back-

28、to-back pwm-vsi, designated the secondary converter. since the tandem converter consists of four controllable inverters, several degrees of freedom exist which enable sinusoidal input and sinusoidal output currents. however, in this context it is believed that the most advantageous control of the in

29、verters is to control the primary converter to operate in square-wave current mode. here, the switches in the csc are turned on and off only once per fundamental period of the input- and output current respectively. in square wave current mode, the switches in the primary converter may either be gto

30、.s, or a series connection of an igbt and a diode.unlike the primary converter, the secondary converter has to operate at a high switching frequency, but the switched current is only a small fraction of the total load current. figure 7 illustrates the current waveform for the primary converter, the

31、secondary converter, is, and the total load current il.in order to achieve full control of the current to/from the back-to-back pwmvsi, the dc-link voltage is boosted to a level above the grid voltage. as mentioned, the control of the tandem converter is treated in only a few papers. however, the in

32、dependent control of the csc and the back-to-back pwm-vsi are both well established, (mutschler & meinhardt, 1998); (nikolic & jeftenic, 1998); (salo & tuusa, 1997); (salo & tuusa, 1999).figure 7. current waveform for the primary converter, ip, the secondary converter, is, and the to

33、tal load current il. advantages in the use of the tandem converterthe investigation of new converter topologies is commonly justified by thesearch for higher converter efficiency. advantages of the tandem converter are the low switching frequency of the primary converter, and the low level of

34、 the switched current in the secondary converter. it is stated that the switching losses of a tandem inverter may be reduced by 70%, (trzynadlowski et al., 1998a) in comparison with those of an equivalent vsi, and even though the conduction losses are higher for the tandem converter, the overall con

35、verter efficiency may be increased.compared to the csi, the voltage across the terminals of the tandem converter contains no voltage spikes since the dc-link capacitor of the secondary converter is always connected between each pair of input- and output lines (trzynadlowski et al., 1998b).concerning

36、 the dynamic properties, (trzynadlowski et al., 1998a) states that the overall performance of the tandem converter is superior to both the csc and the vsi. this is because current magnitude commands are handled by the voltage source converter, while phase-shift current commands are handled by the cu

37、rrent source converter (zhang et al., 1998b).besides the main function, which is to compensate the current distortion introduced by the primary converter, the secondary converter may also act like an active resistor, providing damping of the primary inverter in light load conditions (zhang et al., 1

38、998b). disadvantages of using the tandem converteran inherent obstacle to applying the tandem converter is the high number of components and sensors required. this increases the costs and complexity of both hardware and software. the complexity is justified by the redundancy of the system (tr

39、zynadlowski et al., 1998a), however the system is only truly redundant if a reduction in power capability and performance is acceptable.since the voltage across the generator terminals is set by the secondary inverter, the voltage stresses at the converter are high. therefore the demands on the outp

40、ut filter are comparable to those when applying the back-to-back pwm-vsi.in the system shown in figure 38, a problem for the tandem converter in comparison with the back-to-back pwm-vsi is the reduced generator voltage. by applying the csi as the primary converter, only 0.866% of the grid voltage ca

41、n be utilized. this means that the generator currents (and also the current through the switches) for the tandem converter must be higher in order to achieve the same power.1.2.6 matrix converterideally, the matrix converter should be an all silicon solution with no passive components in the power c

42、ircuit. the ideal conventional matrix converter topology is shown in figure 8.figure 8. the conventional matrix converter topology.the basic idea of the matrix converter is that a desired input current (to/from the supply), a desired output voltage and a desired output frequency may be obtained by p

43、roperly connecting the output terminals of the converter to the input terminals of the converter. in order to protect the converter, the following two control rules must be complied with: two (or three) switches in an output leg are never allowed to be on at the same time. all of the three output ph

44、ases must be connected to an input phase at any instant of time. the actual combination of the switches depends on the modulation strategy. advantages of using the matrix converterthis section summarises some of the advantages of using the matrix converter in the control of an induction wind

45、turbine generator. for a low output frequency of the converter the thermal stresses of the semiconductors in a conventional inverter are higher than those in a matrix converter. this arises from the fact that the semiconductors in a matrix converter are equally stressed, at least during every period

46、 of the grid voltage, while the period for the conventional inverter equals the output frequency. this reduces thethermal design problems for the matrix converter. although the matrix converter includes six additional power switches compared to the back-to-back pwm-vsi, the absence of the dc-link ca

47、pacitor may increase the efficiency and the lifetime for the converter (schuster, 1998). depending on the realization of the bi-directional switches, the switching losses of the matrix inverter may be less than those of the pwm-vsi, because the half of the switchings become natural commutations (sof

48、t switchings) (wheeler & grant, 1993). disadvantages and problems of the matrix convertera disadvantage of the matrix converter is the intrinsic limitation of the output voltage. without entering the over-modulation range, the maximum output voltage of the matrix converter is 0.866 times

49、the input voltage. to achieve the same output power as the back-to-back pwm-vsi, the output current of the matrix converter has to be 1.15 times higher, giving rise to higher conducting losses in the converter (wheeler & grant, 1993).in many of the papers concerning the matrix converter, the una

50、vailability of a true bi-directional switch is mentioned as one of the major obstacles for the propagation of the matrix converter. in the literature, three proposals for realizing a bi-directional switch exists. the diode embedded switch (neft & schauder, 1988) which acts like a true bi-directi

51、onal switch, the common emitter switch and the common collector switch (beasant et al., 1989). since real switches do not have infinitesimal switching times (which is not desirable either) the commutation between two input phases constitutes a contradiction between the two basic control rules of the

52、 matrix converter. in the literature at least six different commutation strategies are reported, (beasant et al., 1990); (burany, 1989); (jung & gyu, 1991); (hey et al., 1995); (kwon et al., 1998); (neft & schauder, 1988). the most simple of the commutation strategies are those reported in (

53、beasant et al., 1990) and (neft & schauder, 1988), but neither of these strategies complies with the basic control rules. 译 文1 电力电子技术的内容电力电子技术是一门正在快速发展的技术,电力电子元器件有很高的额定电流和额定电压,它的功率减小元件变得更加可靠、耐用.这种元件还可以用来控制比它功率大很多倍的元件。电力电子元件的价格不高而且还在继续下降,由它发展而成的变流技术逐渐被应用在风力发电中。这一章将讨论标准的变流器技术从简单转换以启动风力机推进变流器技术的发展.

54、进一步说,利用电力电子技术解决各种问题的渠道还在探索之中。1.1电力电子概念评价标准的选择很多普通的电力电子技术被讨论和研究是为了了解它们的优缺点，现在正在发展的逆变器中增设有很多额外的元件是必要的，以获得正常的操作和运行结果。1.2 功率变换器有各种各样的功率变换器被应用在风力发电中。在使用电力电子产品时,功率变换器可以改变其电压和频率.当然,目前有很多方法可以实现上述功能,具体内容在下一节中讲到。其它类型发电机要求有很多复杂的其它保护，但是,到目前为止应用最多的技术是软启动,利用软启动可以限制并网时的冲击电流从而可以减少冲击电流对电网的干扰。1.2.1 软启动器 软启动器是一种功率转换器,

55、它已被应用在衡速风力机中以减少发电机并网或脱网时引起的冲击电流。当发电机转速超过同步转速时软启动装置开始启动,同过控制晶闸管的导通角将发电机缓慢并入电网。如图1所示是具有软启动的发电机并网原理图。图1 具有软启动的异步发电机并网示意图软并网装置是由在发电机与电网每相之间串接两只反并联的晶闸管组成，软并网装置的导通角和功率放大系数是非线性关系。如果是纯电阻性负载，则导通角变化范围在0°90°之间。如果是纯电感性负载，则导通角变化范围在90°180°之间。如图2是晶闸管导通角范围示意图。图2 晶闸管导通角区间示意图软启动装置能够限制在发电机并入电网时引起的冲

56、击电流，而且软启动装置是一种既经济又可靠的启动装置，在风力发电中得到了广泛的应用。1.2.2 电容器组在风力发电中经常使用电容器组补偿无功功率以提高发电机的功率因数，如图3所示。图3 风力发电中用电容器组补偿功功率示意图在风力发电中，通常发电机需要在整个功率范围内进行补偿。把电容器并联在一起来测量特定周期内的无功功率平均值，它们通常被安装在塔架或大机床的低部用来限制发电机并网时的冲击电流。因为电容器可吸收电网过电压从而保护发电机和电网不受过电压的损害，减少了系统运行的维修费用。1.2.3 二极管整流器 二极管整流器是最常见的电力电子器件。在三相交流系统中的整流装置有六只二极管组成，如图4所示。

57、图4 利用二极管进行三相交直转换示意图 二极管整流装置只能用在一个象限，简单而且不可控，它有时也被应用在直流母线上。1.2.4 脉宽调制变频技术 脉宽调制变频是由两只普通的双相变流器连接在一起组成的。如图5所示。图5 脉宽调制变频器组成框图为了能够完全控制电网电流，支流联络线电压必须提高到比电网电压幅值更高的水平。而控制电网潮流是为了保持支流联络线电压恒定。控制发电机能够适应磁化需求和额定转速。脉宽调制技术在风力发电中的应用在其它一些文章中也有见绍，如（(bogalecka, 1993), (knowles-spittle 网站, 1998), (pena 网站, 1996), (yifan

58、& longya, 1992), (yifan & longya, 1995)等。 脉宽调制技术的优点 脉宽调制技术使用最频繁的三相变频器。因此，脉宽调制被广泛应用而且效果很好。报导关于这种变频器的文献资料和文件要远比报导其它转换器的多，此外，许多生产厂家的一些特殊设备也都使用这类转换器，（如六相桥式晶体管变频电路就是由晶体管和二极管反向并联而组成的）。由于这种产品的成本比较低，有利于设计和生产体积更小的产品。一种新的脉宽调制技术是在电网逆变器和发电机逆变器之间进行电容去耦，除了上述作用外它还能提供一些保护功能。这种去耦可在电网逆变器和发电机逆变器之间进行隔离操作以补偿电网和发电机之间的不平衡程度。但缺点是增加了直流联络线回路中的增益电感的数目，不过这种电感对电网斜波滤波器性能的要求降低拉，而且当电网出现异常情况时可以保护逆变器不受损害。 脉宽调制在使用中的缺点 这一节重点介绍脉宽调制逆变器在使用中存在的缺点，以便进行选择更加符合实际条件的逆变器。在一些书中介绍了另一种可供选择的逆变器频率驱动逆变器，可是现在直流联络线存在的问题是虽然它的功率稳定但它的体积大，这样就增加了成本，更重要的是它的存在可能会降低电网运行寿命。 脉宽调制的另一个严重的缺陷