构 网 型 与 跟 网 型 变 流 器 混 联 并 网 系 统 配 比 关 键 影 响 因 素 分 析

The grid-following converters have the risk of sub-synchronous oscillation (SSO) instability in the weak power grid. The grid-forming converters can improve the stability of the hybrid system in the sub-synchronous frequency range in the weak power grid. The proper configuration of grid-forming units in renewable energy power plants can enhance the system stability. Therefore, the ratio of grid-forming and grid-following converters is a key factor influencing the system stability. Aiming at the small-signal stability issue of in the hybrid grid-connected system of grid-forming and grid-following converters, this paper studies the influence of converter control system characteristics on the boundary conditions for the ratio of the grid-forming converter. First, the state space model of the hybrid grid-connected system of grid-forming and grid-following converters is established. On this basis, this paper analyzes the influence of key parameters of grid-following/grid-forming control systems and the voltage loop control structures of grid-forming converter on the system stability. From the perspective of maintaining the small-signal stability of the system in the sub-synchronous frequency range, this paper analyzes the key influence factors for the lower limit of the ratio of grid-forming converters in the hybrid system. The stability boundary under the constraint of each influence factor is obtained. In addition, the frequency-domain impedance method is used to explain the influence mechanism of each influence factor. Finally, the accuracy of the theoretical analysis is verified based on the RT-LAB semi-physical platform.