电力电子技术 外文翻译

中文4000字
1 Power Electronic Concepts
Power 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 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 whole power is flowing through the converter. Further, different park solutions using power electronics arealso discussed.
1.1 Criteria for concept evaluation
The most common topologies are selected and discussed in respect to advantages and drawbacks. Very advanced power converters, where many extra devices are necessary in order to get a proper operation, are omitted.
1.2 Power converters
Many 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 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 reduce the disturbances to the grid.
1.2.1 Soft starter
The 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, 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. Connection 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 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 somewhere between the limits sketched in Figure 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 wind turbines.
1.2.2 Capacitor bank
For the power factor compensation of the reactive power in the generator, AC capacitor 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 measured 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 the
nacelle. In order to reduce the current at connection/disconnection of capacitors a coil (L) can 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 rectifier
The diode rectifier is the most common used topology in power electronic applications. For a three-phase system it consists of six diodes. It is shown in Figure 4.
Figure 4. Diode rectifier for three-phase ac/dc conversion
The diode rectifier can only be used in one quadrant, it is simple and it is not
possible to control it. It could be used in some applications with a dc-bus.
1.2.4 The back-to-back PWM-VSI
The back-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 grid 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, 1993), (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.
1.2.4.1 Advantages related to the use of the back-to-back PWM-VSI
The PWM-VSI is the most frequently used three-phase frequency converter. 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 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 decoupling 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 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.
1.2.4.2 Disadvantages of applying the back-to-back PWM-VSI
This 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 DC link capacitor is mentioned as a drawback, since it is heavy 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 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 grid 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 converter
The tandem converter is quite a new topology and a 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 compensate 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 the
following designated the primary converter, and a back-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 inverters 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.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 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 independent 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 total load current il.
1.2.5.1Advantages in the use of the Tandem Converter
The investigation of new converter topologies is commonly justified
by the
search for higher converter efficiency. Advantages of the tandem converter are the low switching frequency of the primary converter, and the low level of 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 converter 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 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 current 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., 1998b).
1.2.5.2 Disadvantages of using the Tandem Converter
An 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 (Trzynadlowski 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 output 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 can 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 converter
Ideally, the matrix converter should be an all silicon solution with no passive components in the power circuit. 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 properly 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 phases must be connected to an input phase at any instant of time. The actual combination of the switches
depends on the modulation strategy.
1.2.6.1 Advantages of using the Matrix Converter
This section summarises some of the advantages of using the matrix converter in the control of an induction wind 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 of the grid voltage, while the period for the conventional inverter equals the output frequency. This reduces the
thermal 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 capacitor 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 (soft switchings) (Wheeler & Grant, 1993).
1.2.6.2 Disadvantages and problems of the matrix converter
A 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 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 unavailability 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-directional 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 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 (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.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,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。

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（(Bogaleaka, 1993), (Knowles-Spittle 网站, 1998), (Pena 网站, 1996), (Yifan & Longya, 1992), (Yifan & Longya, 1995)等。

1.2.4.1 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。

1.2.4.2 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是它的换相开关有时候可能会失灵，在电网整流器和发电机整流器之间进行的整流是在直流联络线的上下限进行强制整流和自然整流。

脉宽调制逆变器是由两个逆变器组成的，逆变调制失败的几率可能更加明显。

电网逆变频率很高时可能需要额外增加电磁干扰滤波器，为了防止对发电机绝缘体和转子电流造成伤害，在使用脉宽调制逆变器时必须通过滤波器来降低电压梯度。

1.2.5 串联逆变器
串联逆变器技术是一种最新发展起来的技术，以前很少有过关于这种逆变器的报导直到最近才被人们重视起来。

不过，这种逆变器下面的用法类似于前面提到的脉宽调制，在脉宽宽调制技术中来补偿因谐波滤波器而发生的谐波畸变是非常有用的。

串联逆变器的应用如图6所示。

图6 串联逆变器在风力发电中的应用示意图
由串联逆变器可组成电流源逆变器，其中串联逆变器主要进行一次逆变，脉宽调制进行二次逆变。

这些串联逆变器构成四个可控的逆变器，通过控制可将自由度数通过输入的正弦波形变为正弦曲线的电流而输出。

然而，在这里人们相信这种逆变器的最大好处是可以通过控制转换装置来控制一次逆变进而控制电流幅度变化模型。

在电流源逆变器中通过周期性的控制开关的闭合状态来输入或
输出电流。

在电流幅度变化模型中，一次逆变开关可以是可关断晶闸管，也可以是绝缘栅双极晶闸管或则是二极管。

与一次逆变不同，二次逆变必须在高频状态下进行开关操作，但是它的关断电流只占总负荷电流很小一部分。

如图7举例说明一次逆变、二次逆变和总负荷电流之间的关系。

图7 一次逆变电流波形（ip）、二次逆变电流波形（is）和总负荷电流波形（il）
为了能利用脉宽调制技术完全控制电流，直流联络线的电压水平必须高于电网电压水平。

前面已经提到，串联逆变器技术很少被人们重视，然而，作为一种独立的控制技术串联有源逆变和脉宽调制是积极和有效的。

1.2.5.1 使用串联逆变器的优点
据调查这种新的逆变技术被普遍认为是一种逆变效率很高的逆变器，使用这种串联逆变器的优点是它的一次逆变关断频率低，二次逆变关断电流水平不高。

实践证明，虽然开关的闭合将使串联逆变器的效率下降70%，(Trzynadlowski 网站, 1998a)。

但由于脉宽调制逆变器的效率远低于串联逆变器，所以总的来说使用串联逆变器后其总平均效率还是提高了。

与通道状态指示器相比，电压通过串联逆变器末端时没有阻断电压，这时在进行二次你变时直流联络线上的电容器将始终保持与输入或输出线路相连(Trzynadlowski 网站, 1998b)。

关于串联逆变器的动态特性，(Trzynadlowski 网站, 1998a)指出串联逆变器要比电流源逆变器和电压源逆变器性能优越。

因为电流大小由电压源逆变决定，而电流移相范围由电流源逆变决定。

除了这些主要功能外，如果在轻载下进行一次逆变，串联逆变器也可以扮演电阻的角色补偿因一次逆变和二次逆变时的电流波形失真，提供阻尼的主要依据是根据负载状况。

1.2.5.2 使用串联逆变器的缺点
串联逆变器的固有缺点是由大量元件组成的，这样就增加了对硬件与软件的成本和复杂性。

为此这种太过复杂的逆变器就得进行精简(Trzynadlowski 网站, 1998a)，但为了减少功率损耗有些额外的设备也是必须的。

当电压经一次逆变到达发电机端时，加在逆变器上的电压将会很高。

因此，在使用脉宽调制逆变时就需要输出滤波器来进行比较和选择。

如图1是逆变器在电力系统中的应用示意图，存在的问题是串联逆变器将使电网电压下降。

通过使用电流源逆变器进行一次逆变，只有0.886%的电网电压可被利用，为了达到相同的功率，这就意味着通过串联逆变器的电网电压（或者是通过接触器的电流）必须增加。

1.2.6 矩阵逆变器
理想的矩阵逆变器必须是由纯净的硅溶液组成，在电力线路中没有任何其它杂质成分。

传统矩阵逆变器技术应用如图8所示。

图8 矩阵逆变器应用示意图
矩阵逆变器最基本的用途是通过合理的连接逆变器的输出端与发电机的输入端来获得电流输出希望值（从补偿处得到），电压输出希望值，频率输出希望值。

为了保护逆变器，下面两条规则必须遵循：在输出支路处的两个（或三个）开关绝对不允许同时打开。

在任何时候所有三相输出必须和单相输入相连。

开关的实际连接要依据调制手段。

1.2.6.1 矩阵逆变器的优点
这部分主要见绍在异步风力发电机中使用矩阵逆变器的优点。

与传统逆变器相比，使用矩阵逆变器的输出频率低，而且发热少。

在矩阵逆变器中，半导体器件在减少输出频率和热量方面起着很重要的作用，至少在每个电网周期内其对频率的抑制是非常有用的，这对矩阵逆变器来说是技术难点。

尽管矩阵逆变器额外增设了六只电力开关，与脉宽调制逆变器相比少了直流联络线电容器，这样就增加了逆变器的工作效率和使用寿命(舒斯特,1998年)。

依靠双相转换开关，矩阵逆变器的开关转损耗要比脉宽调制逆变的少，那是因为自然交换减少了将近一半的开关操作损耗。

（软开关），(惠勒·格兰特,1993)。

1.2.6.2 矩阵逆变器的缺点
矩阵逆变器的缺点是因受自身条件限制它的输出电压比较低。

在调制范围内，矩阵逆变器的输出电压只有输入电压的0.866倍，为了得到相同的输出功率，矩阵逆变器的损耗将是普通逆变器的1.15倍。

有许多文章中报道说使用双相选择开关其效率降低是矩阵逆变器最大的缺点。

有人提出三种方法让双相转换开关在矩阵逆变器继续存在，可以用像脉宽调制中的二极管转换开关、普通发射开关和普通选择开关来代替矩阵逆变器中的双相转换开关。

但是，在相位转换时现实的转换开关没有无穷小的转换时间，这与矩阵逆变器的两条规则是相矛盾的。

有些书籍中至少列出了六种不同的转换方法，Casadei et al., 1994); (Casadei et al., 1995a); (Casadei et al., 1995b); (Casadei et al., 1996); (Enjeti & Wang, 1990); (Nielsen, 1996); (Nielsen et al., 1996); (Oyama et al., 1997); (Zhang et al., 1998a). 其中最简单的转换方法出现在以下这些书中：(Beasant et al., 1990) and (Neft & Schauder, 1988), 但是这些方法中没有一个能够遵循矩阵逆变器的这两条规则。