基于锁相环的并网VSC暂态失稳机理与控制方法

针对基于锁相环（phase locked loop, PLL）的电压源变流器（voltage source converter, VSC）面临的暂态失稳问题，考虑功率环的影响，对跟网控制型VSC的暂态失稳机理与控制方法进行了全面研究。首先，通过对PLL和功率环进行建模，推导VSC的暂态失稳边界；其次，以电网电压跌落为例，揭示同时考虑功率环和PLL影响时VSC并网系统的暂态失稳机理；然后，提出一种基于d轴重定向的自适应无功控制方法，该方法同时改善VSC功率外环和PLL同步环的暂态稳定运行边界，有效提高基于PLL的VSC并网系统的暂态稳定性；最后，通过硬件在环实验对所提出的暂态失稳机理和控制方法进行实验验证。研究结果表明，功率外环存在时VSC失稳的本质原因是功率环失稳，而非PLL同步失稳。

Transient Instability Mechanism and Control Method of PLL-Based Grid-Connected VSC

The transient instability mechanism and control method of grid-following control voltage source converter (VSC) are comprehensively studied to solve the transient instability of PLL (phase locked loop)-based VSC with consideration of the influence of power loop control. Firstly, by modeling the PLL and power loop, the transient instability boundary of VSC is deduced. Secondly, taking the grid voltage drop as an example, the transient instability mechanism of grid-connected VSC system is revealed with consideration of the influence of both power loop and PLL. And then, an adaptive reactive power control method is proposed based on d-axis reorientation, which can improve the transient stability operation boundary of both VSC power outer loop and PLL synchronous loop, and effectively improve the transient stability of PLL-based grid-connected VSC system. Finally, the proposed transient instability mechanism and control method are verified through hardware in the loop experiment. The research shows that the essential reason for VSC instability in the presence of power outer loop is the instability of power loop, not the synchronous instability of PLL.