不平衡电网电压下模块化多电平换流器的环流抑制策略

A Circulating Current Suppressing Method for Modular Multilevel Converter Under Unbalanced Grid Voltage

模块化多电平换流器(MMC)内部环流的存在增大了桥臂电流的峰值和有效值,增加了子模块电容电压的波动幅度,影响电力电子器件的安全运行,因此,有必要对其进行有效抑制。通过对相单元瞬时能量进行推导和分析,指出电网电压不平衡时三相环流可能存在2倍频的正序、负序和零序分量,三相环流的直流分量不一定彼此相等。推导了三相环流直流分量即环流参考值的计算公式,并提出基于比例积分(PI)调节器和矢量比例积分(VPI)调节器并联的新型环流抑制策略,给出了相关控制参数的整定方法。新型环流抑制策略可直接在abc三相静止坐标系下执行,因而无需坐标变换和锁相环,控制结构简单,可同时消除2倍频正序、负序和零序环流,并且在电网电压平衡和不平衡时均适用。在PSCAD/EMTDC中搭建了21电平MMC仿真模型,验证了该环流抑制策略的正确性和有效性。

The circulating current within modular multilevel converters increases the peak / RMS value of the arm currents and the magnitude of the sub-module voltage fluctuation,and thus affects the safe operation of power electronic devices. Therefore,it is necessary to eliminate the circulating current. It is pointed that,under the unbalanced grid voltage,the second three-phase harmonic circulating current can be decomposed into positive,negative and, zero sequence components and the dc component of the three-phase circulating current is not necessary equal to each other by deriving and analyzing the phase unit instantaneous energy. The formulas of the dc component of the circulating current, i. e. the circulating current reference, are derived and a novel circulating current suppressing method, which is based on proportional-integral（ PI） regulator and vector proportional-integral（ VPI） regulator in parallel, is proposed. The tuning principle of the related control parameters is given. The novel circulating current suppressing method can be executed in the abc stationary reference frame so that coordinate transformation is not needed and thus it has a simple control structure. Besides that,the positive,negative,and zero sequence components of the second harmonic circulating current can be eliminated simultaneously and the method can achieve good control performance under balanced and unbalanced grid voltages. A 21-level MMC simulation model is set up in the PSCAD / EMTDC in order to validate the correctness and effectiveness of the proposed method.