中文摘要：

首先,建立了ab坐标系下模块化多电平换流器（modular multilevel converter,MMC）交流电压与功率之间的数学模型,结合微分平坦理论提出一种可适用电网正常和故障2种工况下的MMC直接功率平坦控制策略。控制器分为前馈控制和误差补偿2部分：前馈控制根据期望输出及输入控制变量与输出量之间的数学关系,规划系统控制变量的运行轨迹;误差补偿环节消除系统不确定因素和外部干扰引起的实际输出与期望输出间的误差。为获取电网电压不平衡功率参考值,提出一种无需电流正负序分离的功率补偿方法。仿真结果表明,所提出的控制策略在电网电压平衡和故障2种工况下运行良好,且不受电感参数变化和电网频率偏移的影响。

英文摘要：

Firstly, a mathematical model of grid voltage and power in α-β coordinate system for modular multilevel converter（MMC） is established. Then differential flatness theory based on direct power control（DPC） strategy used for normal and fault voltage conditions is proposed. The controller consists of two parts： feed-forward control and error feedback compensation. Feed-forward control plans control variables＇ reference trajectories according to desired output and relationship between output and input control variables. Feedback compensation eliminates error between desired and actual outputs caused by converter uncertainties and disturbance. In order to get power references under unbalanced voltage condition, a power compensation method is presented and it need not to separate positive and negative sequence components of current. Simulation results show that the proposed control strategy works well in both balance and unbalance cases, and it is immune to variation of inductance parameters and grid frequency offset.