基于多步模型预测控制的模块化多电平换流器环流控制策略

Control Strategy of Circulating Current for Modular Multilevel Converters Based on Multistep Model Predictive Control

模块化多电平换流器(MMC)以其结构模块化、低谐波输出以及冗余控制等优点在中高压领域得到了广泛的关注与研究。推导了MMC离散数学模型,提出了基于多步模型预测控制的MMC环流控制方法,能够实现桥臂环流的多步优化控制并有效地降低预测控制计算量。首先利用环流电流离散状态方程进行单步环流预测,再选取满足单步预测效果的投入模块数进行多步环流预测,最终求解出桥臂投入模块数的优化解,实现环流电流的多步优化控制,从而有效地抑制环流中的谐波电流。所提多步预测控制利用单步预测得到的优化解构建多步预测的有限控制集,可以大幅减少多步预测所需要的循环预测次数,有效地降低控制器的计算量。最后在PSCAD/EMTDC中搭建了201电平MMC时域仿真控制系统,仿真结果验证了所提方法的正确性与有效性。

With the attractive features of modularity, low harmonic and redundancy control, modular multilevel converters（ MMC） have received much attention and research in the field of middle and high voltage. To begin with, the discrete state equation is derived for MMC. A new control strategy based on multistep model predictive control is proposed for circulating current suppression existing in MMC. It can realize multistep optimal control for circulation and reduce prediction computation effectively. Firstly, single step prediction can be performed using the circulation discrete state equation, and the number of inserted submodules（ SM） is then determined that meets the requirement to make multistep circulating prediction. The final optimal inserted SM number is chosen to act on the converter for circulating reduction. The proposed multistep model predictive control uses a finite control set（ FCS） generated from single step prediction optimal solution, which greatly improves the efficiency by reducing the computational times of multistep prediction. Finally, the performance of the proposed control strategy is evaluated based on the 201-level MMC time-domain simulation system in PSCAD/EMTDC. The simulation results verify the correctness and validity of the proposed control strategy.