弱电网下考虑锁相环影响的并网逆变器改进控制方法

由于电网阻抗的存在,并网逆变器的控制系统与电网阻抗相互耦合,弱电网条件会影响并网逆变器的稳定性。并网逆变器控制系统中通常使用锁相环来获取电网同步信息,其动态特性是影响系统稳定运行的关键因素。分析弱电网情况下锁相环输出对系统稳定性的影响,在此基础上提出一种提高系统稳定性的控制方法。在同步旋转坐标系下建立了包括电流环、锁相环和滤波器等环节的三相并网变换器阻抗模型,分析不同电网阻抗和锁相环带宽与并网逆变器稳定性的内在联系。结合阻抗模型中系统电压通过锁相环对电流环的影响,提出一种改进的前馈控制方法来减小锁相环输出影响,前馈环节中包括系统电压、锁相环动态特性和滤波器等环节。分析表明,改进的控制方法能够有效提高并网逆变器在弱电网条件下运行的稳定性。实验证明了所提方法的正确性。

An Improved Control Method for Grid-connected Inverters Considering Impact of Phase-locked Loop Under Weak Grid Condition

The coupling effect between the control system of grid-connected inverters and grid impedance influences the stability of grid-connected inverter system under the weak grid condition due to grid impedance. The control system of grid-connected inverters usually uses the phase-locked loop(PLL)to acquire the synchronization information of grid. The dynamic characteristic of PLL is the key factor that affects the stability of inverter system. The effect of PLL on system stability under weak grid condition is analyzed. Besides, a method to improve the stability of inverters is proposed based on the analysis above. The output impedance modeling of a three-phase grid-connected inverter including the current loop, PLL and filter is derived under the synchronization reference frame. The internal relationship between the stability of inverter system with different grid impedance and the bandwidth of PLL is analyzed. Considering the influences caused by the system voltage perturbation on the current loop through PLL in the impedance model, an improved feed-forward control method to reduce the output effect of PLL is proposed. The feed-forward control strategy takes the system voltage, the dynamic of PLL and filter into account. The analysis result shows that the proposed control method can effectively improve the stability of grid-connected inverter under weak grid condition. The experimental results verify the correctness of the proposed control method.