1 Introduction
With the strengthening of environmental awareness and the demand for renewable energy, wind power generation technology has been increasingly valued. Due to the instability and randomness of wind energy, the electric energy generated by wind turbines is alternating current with random changes in voltage and frequency. Only after effective power transformation measures are taken can wind power be sent to the grid. In order to improve the operation performance of wind power generation system, variable speed wind power generation system based on AC-DC-AC converter has been developed in recent years. In the AC-DC-AC variable-speed wind power generation system, the control technology of inverter is the key, and research work in this field has been carried out at home and abroad.
2 Introduction to AC-DC-AC variable speed wind power generation system
For the AC-DC-AC variable speed wind power generation system, the rectifier and inverter in the figure are diode rectifier and PWM inverter based on fully controlled devices. In order to solve the problems of too low voltage amplitude, too fast frequency change, large DC ripple and voltage spike after rectification at low wind speed, a DC link is added between the rectifier and the inverter, which has the function of boosting and stabilizing the voltage. The inverter converts the DC into AC suitable for the combined grid conditions and then directly connects it to the grid through the transformer or directly.
The most remarkable feature of this AC-DC-AC system is that a buffer circuit is connected between the wind turbine generator and the power grid. There is no current impact during grid connection. The inverter can not only adjust the voltage and frequency, but also adjust the output power. It is a stable grid connection mode.
3 Control scheme of PWM inverter
Topology of PWM inverter. The input of the inverter is connected with the output end of the DC stabilized voltage. The voltage at the input end is the voltage value udc after the DC stabilized voltage. The output end is connected to the grid through the filter inductor. For the grid-connected inverter system of wind power generation, the output phase voltage, phase current and the grid electromotive force meet the vector relationship shown in Figure 2b. For the infinite public grid, the grid-connected inverter is used as the current source to transmit electric energy to the grid. Therefore, the output power can be controlled by controlling the output current of the inverter. It can be seen from Figure 2b that in order to avoid harmonic pollution in the public power grid, the output current of each phase of the inverter must be reversed from the grid voltage to achieve the unit power factor output of the inverter. To achieve this goal, an inverter control system is designed as shown in Figure 3.
4 Test of grid-connected inverter
The waveform of battery voltage and phase a current is shown in Figure 6b. Figure 6c shows the spectrum of output current. The experimental results show that under the condition of stable battery voltage, the output current of the inverter is a stable sine wave, and the phase is opposite to the grid voltage, so the unit power factor transmission of electric energy is realized. The inverter output current frequency is basically 50Hz. The harmonic content meets the grid connection requirements.
1 引言
随着环保意识的加强以及对于可再生能源的需求，风力发电技术日益受到重视。由于风能具有不稳定性和随机性，风力发电机发出的电能是电压、频率随机变化的交流电，必须采取有效的电力变换措施后才能够将风电送入电网。为了改进风力发电机发电系统的运行性能，近年来发展了基于交-直-交变流器的变速风力发电系统。在交-直-交变速风力发电系统中，逆变器的控制技术是关键，国内外纷纷展开这方面的研究工作。
2 交-直-交变速风力发电系统简介
交-直-交变速风力发电系统，图中整流器和逆变器分别采用二极管整流器及基于全控型器件的PWM逆变器。为了解决在低风速时整流以后的电压幅值过低、频率变化太快、直流纹波较大、电压尖刺等问题，在整流器与逆变器之间加入了直流环节部分，该环节具有升压和稳压功能逆变器将直流转换成适合并网条件的交流后再通过变压器或直接并入电网。
这种交-直-交系统最显著的特点是在风力发电机和电网之间连接了缓冲电路，在并网时无电流冲击，逆变器不仅可以调节电压、频率，而且可以调节输出功率，是一种稳定的并网方式。
3 PWM逆变器的控制方案
PWM逆变器的拓扑结构。逆变器输入与直流稳压的输出端相连，其输入端的电压为直流稳压后的电压值udc，输出端通过滤波电感上后并入电网，对于风力发电并网逆变器系统，输出相电压、相电流与电网电动势满足图2b所示矢量关系。 对于无穷大公共电网，该并网逆变器作为电流源向电网输送电能。因此通过对逆变器输出电流的控制即可达到控制输出功率的目的。由图2b可知，为了不对公用电网产生谐波污染，必须使逆变器各相输出电流与电网电压反相，以实现逆变器的单位功率因数输出。为了实现这一目的，设计了如图3所示的逆变器控制系统。
4 并网逆变器的试验
蓄电池电压与a相电流波形图，图6b为a相电压与电流波形图。图6c为输出电流的频谱图。实验结果表明，在蓄电池电压稳定的条件下，逆变器输出电流是稳定的正弦波，且与电网电压相位相反，因而实现了单位功率因数传送电能。逆变器输出电流频率基本是50Hz。谐波含量达到了并网要求。
集中逆变
集中逆变一般用与大型光伏发电站（>10kW）的系统中，很多并行的光伏组串被连到同一台集中逆变器的直流输入端，一般功率大的使用三相的IGBT功率模块，功率较小的使用场效应晶体管，同时使用DSP转换控制器来改善所产出电能的质量，使它非常接近于正弦波电流。最大特点是系统的功率高，成本低。但受光伏组串的匹配和部分遮影的影响，导致整个光伏系统的效率和电产能。同时整个光伏系统的发电可靠性受某一光伏单元组工作状态不良的影响。最新的研究方向是运用空间矢量的调制控制，以及开发新的逆变器的拓扑连接，以获得部分负载情况下的高的效率。
组串逆变
组串逆变器已成为现在国际市场上最流行的逆变器。组串逆变器是基于模块化概念基础上的，每个光伏组串（1kW-5kW）通过一个逆变器，在直流端具有最大功率峰值跟踪，在交流端并联并网。许多大型光伏电厂使用组串逆变器。优点是不受组串间模块差异和遮影的影响，同时减少了光伏组件最佳工作点
与逆变器不匹配的情况，从而增加了发电量。技术上的这些优势不仅降低了系统成本，也增加了系统的可靠性。同时，在组串间引入“主-从”的概念，使得在系统在单串电能不能使单个逆变器工作的情况下，将几组光伏组串联系在一起，让其中一个或几个工作，从而产出更多的电能。最新的概念为几个逆变器相互组成一个“团队”来代替“主-从”的概念，使得系统的可靠性又进了一步。目前，无变压器式组串逆变器已占了主导地位。
组件逆变
组件逆变器是将每个光伏组件与一个逆变器相连，同时每个组件有一个单独的最大功率峰值跟踪，这样组件与逆变器的配合更好。通常用于50W到400W的光伏发电站，总效率低于组串逆变器。由于是在交流处并联，这就增加了交流侧的连线的复杂性，维护困难。另一需要解决的是怎样更有效的与电网并网，简单的办法是直接通过普通的交流电插座进行并网，这样就可以减少成本和设备的安装，但往往各地的电网的安全标准也许不允许这样做，电力公司有可能反对发电装置直接和普通家庭用户的普通插座相连。另一和安全有关的因素是是否需要使用隔离变压器（高频或低频），或者允许使用无变压器式的逆变器。这一逆变器在玻璃幕太阳能并网逆变器 光伏并网逆变器墙中使用最为广泛。