LCL型三电平三相四桥臂逆变器模型预测算法及谐振抑制研究

LCL filter type three-phase four-leg inverters owning advantages, such as resisting unbalanced loads or grid and reducing the output inverter current harmonics and etc, which will become an optimal selection in photovoltaic microgrid systems. Due to the LCL filter type three-phase four-leg multi-level inverter having a complex PWM modulation algorithm and LCL filter resonance, the project focuses on the following: ①To explore a fast model predictive algorithm based on LCL filter T-type three-level three-phase four-leg inverters when the photovoltaic microgrid inverter working in grid-connected model or stand-alone model and two mode switching process, which will improve the model predictive algorithm efficiency. ②It is proposed that the cost function item of suppressing the LCL filter resonance in the cost function of the fast model predictive algorithm is added, which will realize suppressing the LCL filter resonance by selecting the optimizing voltage vectors. On the basis, the differences and internal relations of the proposed nolinear method with the conventional active damping linear method will be explored. ③The theoretical foundation of selecting the cost function and the weight coefficients of the cost funtion of the fast model predictive algorithm is confirmed. By carrying out the project, it will structure the fast model predictive algorithm based on the LCL filter type three-level three-phase four-leg inverter and the method of filter resonance suppression, which has important theoretical and practical significance for photovoltaic microgrid systems. On the basis, the differences and internal relations of the proposed method with the conventional active damping method will be explored. ③The theoretical foundation of selecting the cost function and the weight coefficients of the cost funtion of the fast model predictive algorithm is studied. By carrying out the project, it will structure the theoretical foundation for the fast model predictive algorithm based on the LCL filter type three-level three-phase four-leg inverter and the method of filter resonance suppression, which has important theoretical and practical significance for photovoltaic microgrid systems.

LCL滤波型三相四桥臂逆变器具有抗负载或电网不平衡能力以及减少逆变器输出电流谐波等优点，是光伏微网系统优化选择。针对LCL滤波型三相四桥臂多电平光伏微网逆变器存在PWM调制算法复杂和LCL滤波器谐振问题，本项目重点研究如下内容：①以LCL滤波型T型三电平三相四桥臂逆变器为研究对象，探索光伏微网逆变器并网、离网模式以及两种模式切换过程时快速模型预测算法，提高模型预测算法效率。②提出在模型预测算法的价值函数中增加抑制LCL滤波器谐振的价值项，通过寻优的电压矢量来抑制LCL滤波器谐振。在此基础上，探索所提非线性方法与常规有源阻尼线性方法的区别和内在联系。③确定快速模型预测算法价值函数和价值函数的权重系数选取的理论依据。通过本项目的开展，构建LCL滤波型三电平三相四桥臂光伏微网逆变器模型预测算法和滤波器谐振抑制方法，对光伏微网系统具有重要的理论和实践意义。

本项目以分布式发电系统中的逆变器为研究对象，主要研究内容：（1）针对传统模型预测控制在三电平三相逆变器计算量大以及模型预测控制权重系数选取的繁锁，提出一种快速无权重系数模型预测控制方法。该方法通过无差拍控制得到逆变器期望电压矢量。根据期望电压矢量和三电平逆变器正、负小矢量对中性点电压影响，让远离期望电压矢量和不利于中性点电压平衡的电压矢量不参与预测模型和目标函数在线评估。该方法参与预测模型和目标函数计算的电压矢量由传统的27减少到3个，大大提高了计算效率。同时，无需三电平逆变器中性点电压权重系数的调节，大大简化了模型预测控制算法。（2）针对传统模型预测控制逆变器输出电压、电流频谱比较分散等不足，提出一种定频模型预测控制方法并应用于T型三电平三相并网逆变器。该方法结合传统模型预测控制和空间矢量脉冲宽度调制方法的优点 ，根据期望参考电压矢量和各电压矢量目标函数的大小，确定各电压矢量作用时间和作用顺序。所提方法实现逆变器输出电压、电流频谱特性类似空间脉冲宽度调制方法；同时，系统具有很好动态性能和鲁棒性。（3）当电网高阻抗时，三相光伏并网逆变器的LC滤波器和电网高阻抗形成LCL滤波器。LCL滤波器对高次谐波具有良好的衰减作用，但LCL滤波器为低阻尼三阶系统，容易发生谐振。针对电网电压高阻抗LCL滤波器谐振问题，提出一种虚拟电阻+电容有源阻尼方法。该方法将虚拟电阻和电容串联之后与三相光伏并网逆变器的滤波电容并联。通过滤波电容电压得到虚拟电阻和电容支路的电流，将虚拟电阻和电容支路的电流作为LCL滤波器谐振抑制有源阻尼电流给定。通过逆变侧电流闭环控制，实现对三相光伏并网逆变器电网高阻抗LCL滤波器谐振抑制。通过本项目的开展，构建分布式发电系统逆变器模型预测算法和滤波器谐振抑制方法，对分布式发电系统具有重要的理论和实践意义。