The majority of inverter-based resources (IBRs) currently operate as grid-following inverters (GFLIs). These inverters exhibit certain stability issues when integrated in low-strength areas of the grid. To enhance the grid strength in a GFLI-dominant area, virtual synchronous generators (VSGs) are installed. If the VSG and the GFLI are investigated as a paralleled system, despite the benefits brought by the VSG, in some cases, the transient angle instability process, caused by a voltage sag, is accelerated. Therefore, this paper studies the transient angle stability of a paralleled VSG-GFLI system via investigating the voltage at a common bus between the two IBRs. Subsequently, a stable region of the voltage angles of the two IBRs is determined. Based on this region, the stability margin of the paralleled system can be determined and used to quantitatively evaluate the system stability. Moreover, a control loop is proposed to improve the transient stability of the paralleled system. The proposed controller can adaptively reduce the VSG power set-point to avoid a complete failure of the VSG even when no stable equilibrium point exists during a voltage sag. The stability investigation and performance evaluation of the proposed method are conducted in PSCAD/EMTDC and experimentally validated.

A current limiter (CL), which protects virtual synchronous generators (VSGs) from overcurrents, may significantly alter the operation of the VSGs when activated. In this letter, a transient stability analysis of VSGs equipped with quadrature- (q-) prioritized CL is performed. This study shows that in addition to the instability caused by the positive feedback of the primary controller, a VSG could become unstable after large disturbances due to the failure of the inner voltage controller. In this study, a stability criterion is provided for the voltage control loop.