针对电网三相不平衡时，常用锁相环方法存在电压测量偏移降低，检测准确度和抗低次谐波干扰能力弱的问题，提出一种基于改进型二阶广义积分器（Second Order Generalized Integrator, SOGI）的三相锁相环设计方案。分析了SOGI可提取出电压正序基波分量的原理；通过对SOGI进行频域特性分析，找到传统SOGI抗直流分量和低次谐波干扰能力弱的原因。据此，提出一种能消除直流分量影响的改进型SOGI结构，并以此改进型SOGI为基础通过多个并联来达到消除低次谐波干扰的目的；利用PSCAD软件对基于传统型SOGI的锁相环、基于单个改进型SOGI的锁相环、基于多个改进型SOGI并联的锁相环进行多项对比仿真，仿真结果证明了所提锁相方案的有效性。
英文摘要:
This paper proposes a design scheme of three-phase locked loop (PLL) based on the improved second-order generalized integrator (SOGI) to solve the problem of the reduced voltage measurement offset, low detection accuracy and weak harmonic restraining of traditional PLL in the case of the unbalanced grid voltage in grid-connected PV systems. Firstly, the principle of the SOGI is analyzed, which can effectively extract the fundamental wave component of grid voltage positive sequence. Secondly, the reason of the weak anti-DC component and low harmonic restraining ability in traditional SOGI is found through the analysis of frequency domain characteristic of SOGI. On this basis, an improved structure of SOGI is proposed to eliminate the influence of DC offset. Furthermore, the low-order harmonic interference can be filtered through a structure of parallel SOGIs. So a novel PLL applicable to the case of unbalanced grid which can eliminate both the influence of DC offset and low-order harmonic is shown. Finally, PSCAD software is adopted to conduct a number of comparative simulations for the three types of PLL based on traditional SOGI, single improved SOGI, and multiple improved SOGI parallel. The simulation results verity the effectiveness of the proposed PLL scheme.