Design-Oriented Analysis and Transient Control for VSG With Current Saturation Unit

Under grid fault condition, virtual synchronous generator (VSG) is prone to transient instability due to the existence of current saturation unit (CSU), which has been extensively studied. However, the CSU in existing literatures is characterized by a fixed saturation angle, overlooking the impact of saturation angle on transient instability. To fill this gap, a design-oriented analysis for VSG with CSU is carried out to thoroughly reveal the impact of saturation angle. First, the large signal equivalent model of VSG with CSU is developed, and four types of transient behavior under different saturation angles are qualitatively analyzed to reveal the instability mechanism. Next, the stability boundary analysis is performed to quantitatively evaluate the impact, wherein the feasible region of saturation angle is obtained. The analysis indicates that improper saturation angle and loss of equilibrium point can induce transient angle instability. To limit fault current and guarantee transient angle stability simultaneously, a comprehensive transient control collaboratively adjusting saturation angle and reference power under different fault degree is proposed. The design of control parameters considering voltage support and power transmission is presented. Finally, simulation and experimental tests validate the correctness of theoretical analysis and effectiveness of proposed control.