外文翻译---一种新型使用永磁同步发电机和Z源逆变器的变速风力发电系统.doc

毕 业 设 计外 文 文 献 译 文 及 原 文学 生： 曹 文 天 学 号： 200806010211 院 （系）： 电气与信息工程学院 专 业： 电气工程及其自动化 指导教师： 陈 景 文 2012年6月8日一种新型使用永磁同步发电机和Z源逆变器的变速风力发电系统1 介绍风机发出的电作为能源使用在世界上已经有了很显著地增长。随着风能变换系统（WECSs）应用的增加，各种各样适合它们的技术正在发展。正因为有着众多的优势，永磁同步发电机（PMSG）发电系统在风力发电技术发展中已成为一种主流趋势。从风能中获得最大能量以及在电网中得到高品质的电能是风能变换系统的两个主要目标。对于这两个目标，交-直-交变换器是风能变换系统最好的拓扑结构之一。图1展示了一种传统的永磁同步发电机的交-直-交拓扑结构。这个结构包括二极管整流电路，升压直流变换电路和三相逆变电路。在这种拓扑结构中，升压变换电路被控制用来跟踪最大功率点（MPPT），逆变电路用来给电网传递高品质的电能。图1 传统的基于永磁同步电机并带直流升压斩波的风能变换系统Z源逆变器目前被认为替代现有的逆变拓扑结构有着固有的优势，例如电压上升。这个逆变电路在相同的逆变相角（直通状态）中，伴随着两个转换开关的导通可以促进电压的上升能力。本篇论文提出了一种新型的有着Z源逆变电路并且基于永磁同步电机的风能变换系统。这种系统的拓扑结构如图2所示。这种拓扑结构的升压转换电路没有任何的改变。而且，系统的可靠性得到了很大的提升，因为短路通过逆变器中的任何相角都是被允许的。由于没有相角死区时间，逆变输出功率的失真很小。图2 有着Z源逆变电路并且基于永磁同步电机的风能变换系统这篇论文的第二部分介绍了Z源逆变电路并描述从整流电路到Z源逆变电路的操作过程。然后，介绍了功率传递和最大功率点跟踪的系统。2 Z源逆变电路图3展示了Z源逆变电路。在它的直流侧有阻抗网络，连接着电压源与逆变器。阻抗网络由两个电感和两个电容组成。传统的电压源逆变电路有六个有效矢量和两个零矢量。然而，Z源逆变电路仅有一个零矢量（状态）。对于升压来说，它被称为直通矢量。在这种状态下，负载端可以短路通过上下设备的任何一组桥臂，任何两组桥臂，甚至所有的三组桥臂。 图3 电压型Z源逆变器直流电压可以表示成为 （2-1）是电压源，是升压系数，它决定于 （2-2）是间隔一个周期的导通时间。输出的电压峰值向量为 （2-3）是调制系数，电容电压可以表示为 （2-4） （2-5）和之间的关系为 （2-6）电感的电流纹波可以这样计算 （2-7）图4展示了Z源逆变器基本的PWM控制方法。这种方法需要和两个额外的直线作为直通信号。当载波信号高于或低于，逆变电路会产生一个直通矢量。可表示为 （2-8） 图4 Z源逆变器的PWM控制方法在风能变换系统中，带着输入电容（、和）的二极管整流桥作为Z源逆变器的直流源部分。这个结构如图5所示。当二极管整流与逆变器处于直通状态时，输入电容抑制浪涌电压可能会产生线电感。图5 带二极管整流桥的Z源逆变器在任何时刻，只用拥有最大电位差的两相会导通，导通电流从永磁同步发电机侧流向阻抗网络侧。图6展示每个周期六种可能的状态。在任何状态下，一个上桥臂，一个下桥臂和一个与它们相连的电容是导通的。例如，当电位差在a相与b相达到最大，二极管和以及它们相连的电容导通，如图7所示。图6 整流器的六种导通状态图7 当电位差在a相与b相达到最大时的等效电路图在每一个导通周期内，逆变电路有两种工作模式。模式1，逆变电路工作于直通状态。这种模式下，二极管（和）是关断的，直流侧与交流线路被分隔。图8展示这种模式的等效电路。模式2，逆变电路工作于六个有效矢量或两个零矢量当中，因此，可将带二极管（和）的Z源逆变电路看成直流源。图9展示这种模式的等效电路。负载电流在电路工作于零矢量时为零。图8 Z源逆变电路处于第一种模式的等效电路图 图9 Z源逆变电路处于第一种模式的等效电路图3 控制系统控制系统的结构如图10所示。控制系统由两部分组成：1）电网功率的控制，2）最大功率点的跟踪。图10 风能变换系统的控制方框图1）电网功率的控制在同步参考系中的功率方程为 （3-1） （3-2） P和Q分别是有功和无功功率，V是电网电压，i是电网电流。下标d和q分别代表着直轴和交轴分量。如果参考系按照电网电压定向， 就等于零。那么，有功与无功功率就可以表示为 （3-3） （3-4）根据上式，分别控制直轴和交轴电流就可以实现控制有功和无功功率。两条控制路径用来控制这些电流。在第一条路径中，随着无功功率的给定，q轴电流的参考值也给定了。为了获得单位的功率因数，q轴电流的参考值应设为零。在第二条路径中，为了控制有功功率，用一个外部的电容电压控制回路来设定d轴电流的参考值。这使得所有来自整流器的功率被传输到电网。对于这种控制有两种方法：1）电容电压（）的控制 2)直流电压（）的控制。第一种控制方法（控制模型1如图10所示），电容电压保持在参考值不变。在控制回路中，当直通时间改变，和将会改变。然而，另一种方法（控制模型2如图10所示），直流电压（）的参考量被设定。在这种方法中，当直通时间改变，和将会改变。在直通状态下，逆变电路的输入电压为零，这使成为一个很难控制的变量。因此，如公式（2-6）所示，通过控制间接控制。2）最大功率点跟踪风机的机械功率传递公式为 （3-5）是空气密度；是风力机叶片迎风扫掠面积；是风速；是风能利用系数，定义为风力机输出功率和风能功率的比例，取决于叶片的空气动力学特性。图11展示了风速变化时发电机的转速与风力机输出功率之间的联系。可以看出，不同风速时最大功率所对应的发电机转速不同。图11 风速变化时机械功率与转子转速的关系永磁同步发电机的稳态感应电压与转矩方程为 （3-6） （3-7）是转子速度，是定子电流。同时，我们知道 （3-8）是永磁同步发电机的端电压，是其电感。整流后的直流电压为 （3-9）根据式（3-7）、（3-8）、（3-9）可得 （3-10）转矩决定于发电机转速和风速。因此根据式（3-10），对于直流电压会得到一个关于转速和风速的函数式。最后，通过设置直流电压就可以调节发电机转速。A New Variable-Speed Wind Energy Conversion System Using Permanent-Magnet Synchronous Generator and Z-Source Inverter1 INTRODUCTIONWind turbines usages as sources of energy has increased significantly in the world. With growing application of wind energy conversion system(WECSs), various technologies are developed for them. With numerous advantages , permanent-magnet synchronous generator(PMSG) generation system represents an important trend in development of wind power applications. Extracting maximum power from wind and feeding the grid with high-quality electricity are two main objectives for WECSs. To realize these objectives, the ac-dc-ac converter is one of the best topology for WECSs. Fig.1 shows a conventional configuration of ac-dc-ac topology for PMSG. This configuration includes diode rectifier, boost dc-dc converter and three-phase inverter. In this topology, boost converter is controlled for maximum power point tracking(MPPT) and inverter is controlled to deliver high-quality power to the grid. Fig.1. Conventional PMSG-based WECS with dc boost chopper The Z-source inverters have been reported recently as a competitive alternative to existing inverter topologies with many inherent advantages such as voltage boost. This inverter facilitates voltage boost capability with the turning ON of both switches in the same inverter phase leg (shoot-through state).In this paper, a new PMSG-based WECS with Z-source inverter is proposed. The proposed topology is shown in Fig. 2. With this topology, boost converter is omitted without any change in the objectives of WECS. Moreover, reliability of the system is greatly improved, because the short circuit across any phase leg of inverter is allowed. Also, in this configuration, inverter output power distortion is reduced, since there is no need to phase leg dead time.Fig.2. Proposed PMSG-based WECS with Z-source inverterSection II of this paper introduces Z-source inverter and describes operation of rectifier feeding the Z-source inverter. Then, power delivery and MPPT control of system are explained.2 Z-Source InverterThe Z-source inverter is shown in Fig. 3. This inverter has an impedance network on its dc side, which connects the source to the inverter. The impedance network is composed of two inductors and two capacitors. The conventional voltage source inverters have six active vectors and two zero vectors. However, the Z-source inverter has one extra zero vector (state) for boosting voltage that is called shoot-through vector. In this state, load terminals are shorted through both the upper and lower devices of any one phase leg, any two phase legs, or all three phase legs.Fig.3. Voltage-type Z-source inverterThe voltage of dc link can be expressed as （2-1）Where is the source voltage and B is the boost factor that is determined by （2-2）Where is the shoot-through time interval over a switching cycle T. The output peak phase voltage is （2-3）Where M is the modulation index. The capacitors voltage can expressed as （2-4）Where （2-5）Relation between and can be written as （2-6）And current ripple of inductors can be calculated by （2-7）Fig. 4 illustrates the simple PWM control method for Z-source inverter. This method employs two extra straight lines as shoot-through signals, and . When the career signal is greater than or it is smaller than , a shoot-through vector is created by inverter. The value of is calculated by （2-8）Fig.4. PWM control method for Z-source inverterIn the proposed WECS, a diode rectifier bridge with input capacitors (,and ) serves as the dc source feeding the Z-source inverter. This configuration is shown in Fig. 5. The input capacitors suppress voltage surge that may occur due to the line inductance during diode commutation and shoot-through mode of the inverter.Fig.5. Z-source inverter fed with a diode rectifier bridgeAt any instant of time, only two phases that have the largest potential difference may conduct, carrying current from the PMSG side to the impedance network side. Fig. 6 shows six possible states during each cycle. In any state, one of upper diodes, one of lower diodes, and the corresponding capacitor are active. For example, when the potential difference between phases “a” and “b” is the largest, diodes and conduct in series with capacitor , as shown in Fig. 7.Fig.6. Six possible conduction intervals for the rectifierFig.7. Equivalent circuit when the potential difference between phases “a” and “b” is the largest.In each conduction interval, inverter operates in two modes. In mode 1, the inverter is operating in the shoot-through state. In this mode, the diodes ( and ) are off, and the dc link is separated from the ac line. Fig. 8 shows the equivalent circuit in this mode. In mode 2, the inverter is applying one of the six active vectors or two zero vectors, thus acting as a current source viewed from the Z-source circuit with diodes ( and ) being on. Fig. 9 shows the equivalent circuit in this mode. The load current is zero during zero vectors.Fig.8. Equivalent circuit of the Z-source inverter in mode 1 Fig.9. Equivalent circuit of the Z-source inverter in mode 23 CONTROL SYSTEMThe structure of the control system is shown in Fig. 10. The control system is composed of two parts: 1) control of power delivered to the grid and 2) MPPT.Fig.10. Block diagram of proposed WECS control system1）Control of Power Delivered to the GridThe power equations in the synchronous reference frame are given by （3-1） （3-2）where P and Q are active and reactive power, respectively, v is grid voltage, and i is the current to the grid. The subscripts “d” and “q” stand for direct and quadrature components, respectively. If the reference frame is oriented along the grid voltage, will be equal to zero. Then, active and reactive power may be expressed as （3-3） （3-4）According to earlier equations, active and reactive power control can be achieved by controlling direct and quadrature current components, respectively.Two control paths are used to control these currents. In the first path, with given reactive power, the q-axis current reference is set. To obtain unit power factor, the q-axis current reference should be set to 0. In the second path, an outer capacitor voltage control loop is used to set the d-axis current reference for active power control. This assures that all the power coming from the rectifier is transferred to the grid. For this control, two methods are proposed: 1) capacitor voltage () control and 2) dc-link voltage () control.In the first control method (control mode 1 in Fig. 10), capacitor voltage is kept constant at reference value. In the control loop, when shoot-through time changes, and will change. However, in other method (control mode 2 in Fig. 10), a reference value is set for dc-link voltage (). In this method, with changing shoot-through time, and will change. The input voltage of inverter is zero in shoot through state, which makes a difficult variable to control. Consequently, (2-6) is used to control indirectly by controlling . 2）Maximum Power Point TrackingThe mechanical power delivered by a wind turbine is expressed as （3-5）Whereis the air density, A is the area swept out by the turbine b