三电平并网变流器中点电位自平衡特性分析及拓扑改进

Self-balancing Characteristic Analysis of Neutral Point Potential and Improved Topology of Three-level Grid-connected Converter

传统三电平并网变流器中点电位自平衡时间较长,中点电位可能产生较大偏移,使并网电流低次谐波增加,常需采用中点电位控制防止中点电位偏移。为充分发挥拓扑的自平衡能力,省去中点电位控制算法,文中首先定量分析了载波同相层叠脉宽调制输出电压的谐波分布特性,在此基础上,给出了传统拓扑中点电位自平衡机理及影响因素。针对传统拓扑自平衡时间的制约因素,提出了三电平并网变流器中点电位自平衡改进拓扑,通过新增零序通路在载波频率处产生串联谐振,减小阻抗,从而显著提升自平衡速度。最后,仿真和实验结果表明改进拓扑能快速有效地将中点电位稳定于直流侧电源中点,且自平衡速度不受直流侧电压不平衡度影响,中点电位平衡后并网电流质量较高。相对传统拓扑,自平衡速度提高10倍以上,与传统中点电位控制速度相当,因此,可省去中点电位控制。尽管所提拓扑需增加一个小电容,但其无需电压偏差检测,无需增加或调整控制算法,适用于全功率因数范围,易于实现。

The self-balancing time of the conventional three-level grid-connected converter is long and the neutral point potential may drift largely, which leads to the increase of low-order harmonic components in grid-connected current. Therefore, the neutral point potential control method is usually employed to address this issue. In order to make full use of the self-balancing ability of the topology and cancel the control method of neutral point potential, this paper quantitatively analyzes the harmonic distribution characteristics of the output voltage by using phase disposition pulse width modulation. On this basis, the self-balancing characteristic of neutral point potential and its influencing factors in the conventional grid-connected converter are given. Aiming at constraints of self-balancing time of conventional topology, an improved topology for self-balancing of neutral point potential is proposed, which adds a zero-sequence path to generate series resonance at the carrier frequency. The proposed topology reduces the zero-sequence path impedance and improves the self-balancing speed significantly. Finally, the simulation and experimental results show that the proposed topology can not only achieve high grid-connected current quality but also stabilize the neutral point potential rapidly at the neutral point of DC-side power source, and the balancing speed is not affected by the unbalance degree of DC-side voltage. Compared with the conventional topology, the self-balancing speed is improved by more than 10 times, which is equal to the speed of conventional control of neutral point potential. Thus, the control of neutral point potential can be removed. Although a small capacitor needs to be added in the proposed topology, the proposed self-balancing scheme does not need to measure voltage deviation or modify control algorithm. The proposed topology could be adopted in full range of power factors, which is easy to realize.