电网不平衡下基于SOGI的MMC环流抑制策略

模块化多电平换流器（modular multilevel converter，MMC）的设计和分析通常是在电网平衡下进行，但在实际运行中交流电网是不平衡。电网不平衡将导致MMC桥臂环流的直流分量不再相等，含有2倍频的正序、负序和零序分量，使系统损耗增加，影响系统性能。首先，本文根据MMC的数学模型和相单元的瞬时功率分析环流的产生机理，得到电网不平衡时环流各分量的等效电路。然后，提出一种电网不平衡下基于二阶广义积分器（second order generalized integrator，SOGI）的环流抑制策略，该策略将三相环流中2次谐波分量与不相等的直流分量分离后单独抑制，采用比例积分（proportional integral，PI）控制经SOGI提取桥臂环流的正序、负序2倍频分量，采用准比例谐振（proportional resonant，PR）控制环流的零序2倍频分量。最后，在PSCAD/EMTDC平台中搭建217电平的MMC系统模型，将本文所提环流抑制策略与传统环流抑制策略、采用准比例谐振器控制环流抑制策略和采用比例积分谐振（proportional integral resonant，PIR）控制环流抑制策略进行仿真实验对比。实验结果表明：在单相非金属接地故障时，与其他3种环流抑制策略作对比，本文策略能将环流的2倍频分量抑制到0.000 6 kA，将谐波畸变率降低到0.03%，表明所提策略的优越性；同时在两相非金属接地故障时，本文策略能将环流的2倍频分量抑制到0.003 9 kA，谐波畸变率降低到0.22%。仿真实验结果验证了本文方法的有效性。

Circulating Current Suppression Method of Modular Multilevel Converter Based on SOGI Under Unbalanced Grid Conditions

The design and analysis of modular multilevel converter (MMC) is usually carried out under the condition of power grid balance, but the AC power grid is unbalanced in actual operation. The imbalance of the power grid will cause the DC components of the circulating current of the MMC bridge arm to be no longer equal, including the positive sequence, negative sequence, and zero sequence components of the double frequency, which will increase the system loss and affect the system performance. In this paper, based on the mathematical model of MMC and the instantaneous power of phase units, the generation mechanism of circulating current was analyzed, and the equivalent circuit of each component of circulating current was obtained when the power grid is unbalanced. Then, a circulating current suppression strategy based on second order generalized integrator (SOGI) was proposed under unbalanced power grid. This strategy can suppress the second harmonic component of three-phase circulating current and the unequal DC component separately. Based on SOGI, the proposed strategy uses proportional integral (PI) control to extract the positive sequence and negative sequence second harmonic components of bridge arm circulating current. Meanwhile, quasi proportional resonance (PR) is applied to control the zero sequence second harmonic component of the circulation. Finally, a 217 level MMC system model was built in PSCAD/EMTDC platform, and through simulation experiments, the circulation suppression strategy proposed in this paper was compared with three typical strategies, i.e. the traditional circulation suppression strategy, the circulation strategy controlled by quasi proportional resonator and the circulation suppression strategy controlled by proportional integral resonator (PIR) . The experimental results showed that in the case of single-phase non-metallic ground fault, compared with the other three circulating current suppression strategies, the proposed strategy can suppress the second harmonic component of circulating current to 0.000 6 kA and reduce the harmonic distortion rate to 0.03%, which verifies its superiority. Furthermore, in the case of two-phase non-metallic ground fault, the proposed strategy can suppress the second harmonic component of the circulating current to 0.003 9 kA and reduce the harmonic distortion rate to 0.22%. The simulation results verify the effectiveness of this method.