In the modern power grid, distributed generators are widely connected to the grid via voltage source converters (VSCs). Analyzing the transient stability of VSCs under large disturbances is useful for maintaining the security of the grid. However, this topic is very little studied. In this paper, the transient stability behavior of the droop-controlled VSC is theoretically explained. In particular, it’s shown that transient instability can occur to the droop-controlled VSC when its current is saturated under large disturbances. In addition, the dynamics of the VSC under voltage sags that could incur instability problem is elaborately studied to consider special non-fault disturbances. To deal with this instability problem, a stability enhanced P-f droop control is proposed. Simulations and hardware-in-the-loop experiments verify the validity of the transient stability analysis and the effectiveness of the proposed control scheme.

Under large disturbances, P-f droop-controlled voltage source converters may lose synchronous stability, just like synchronous generator. Moreover, the current limitation leads to a more complicated instability mechanism and decreases stability margin greatly. This letter describes a virtual power angle instability problem and analyzes its mechanism from the virtual power angle characteristic curves of a converter with respect to an infinite bus. Simulation verifies the rationality of the analysis.

Power converters may lose synchronization with the remaining network when integrated in a weak power grid. Such an instability phenomenon [known as grid-synchronization instability (GSI)] features the frequency divergence of phase-locked loop (PLL) and oscillations of the converter's power output. In this paper, we focus on the influences of reactive power control (RPC) methods on GSI. We develop a single-input-single-output model to explicitly reveal how the PLL interacts with the other parts of the converter system in terms of grid synchronization. Then, after deriving the open-loop transfer function and sensitivity function of the entire converter system, we compare the stability margins for different RPC methods. Furthermore, we elaborate on the interactions among RPC, PLL, and voltage feedforward (VFF), and then demonstrate that different design methods of RPC and VFF will lead to different stability margins. The subsequent stability analysis provides insightful guidelines for coordinating the design of multiple control loops, i.e., RPC, VFF and PLL. In particular, we demonstrate how the loop shaping of PLL takes effect in increasing the stability margin and eventually preventing the converter from GSI. The validity of the stability analysis is verified through simulations in MATLAB/Simulink.

With proper control methods, grid-connected power converters can mimic the dynamics of conventional synchronous generators (SGs), i.e., acting as virtual synchronous machines (VSMs) for improving the grid frequency stability and ensuring a smooth transition towards converter-dominated power systems. However, the integration of VSMs will inevitably influence the low-frequency oscillations (LFOs) in power grids which are usually caused by the interactions among SGs. This influence should be of concern especially when the VSM's capacity is similar to that of SG, e.g., operating VSC-HVDC stations as VSMs. This paper investigates how the VSM affects LFOs in power systems by analyzing its equivalent damping torque. It is shown that with proper control design, VSMs can provide considerable positive damping torque to effectively damp LFOs in power grids. Particularly, the influences of virtual impedance and frequency detector (i.e., phase-locked loop) are elaborately studied using the damping torque analysis so as to provide design guidelines to better damp LFOs. Finally, case studies of a two-area test system with integration of a VSM-based VSC-HVDC station verify the validity of the analysis.

In this paper, a generalized short circuit ratio (gSCR) is pro-posed for assessing the strength of multi-infeed LCC-HVDC (MIDC) systems by the theoretical analysis of the relationship between static voltage stability and grid strength. The gSCR can resolve the ambiguity when applying the concept of short circuit ratio (SCR) for single-infeed LCC-HVDC (SIDC) systems to MIDC systems. Moreover, the gSCR is the generalized presen-tation of SCR in MIDC systems. Based on the gSCR, our defined critical gSCR (CgSCR) and boundary gSCR (BgSCR) for MIDC systems have the same functions as the critical SCR (CSCR) and boundary SCR (BSCR) used in grid strength analysis for SIDC systems. Also, the CgSCR and BgSCR have the same values as those of CSCR and BSCR (i.e., $CgSCR=CSCR\approx2$ and $BgSCR=BSCR\approx3$ ) to characterize the stability boundaries of MIDC systems. This allows the analysis experience of using the SCR, CSCR, and BSCR for SIDC systems can be easily transplanted to the application of the gSCR, CgSCR and BgSCR for MIDC systems. The efficacy of the proposed gSCR along with CgSCR and BgSCR is demonstrated by comparisons them with other indices of grid strength assessment for MIDC systems.

Synchronized control (SYNC) is widely adopted for doubly fed induction generator (DFIG)-based wind turbine generators (WTGs) in microgrids and weak grids, which applies P-f droop control to achieve grid synchronization instead of phase-locked loop. The DFIG-based WTG with SYNC will reach a new equilibrium of rotor speed under frequency deviation, resulting in the WTG's acceleration or deceleration. The acceleration/deceleration process can utilize the kinetic energy stored in the rotating mass of WTG to provide active power support for the power grid, but the WTG may lose synchronous stability simultaneously. This stability problem occurs when the equilibrium of rotor speed is lost and the rotor speed exceeds the admissible range during the frequency deviations, which will be particularly analyzed in this paper. It is demonstrated that the synchronous stability can be improved by increasing the P-f droop coefficient. However, increasing the P-f droop coefficient will deteriorate the system's small signal stability. To address this contradiction, a modified synchronized control strategy is proposed. Simulation results verify the effectiveness of the analysis and the proposed control strategy.