With the increasing penetration of the wind power, the stability issues of the weak AC grid-connected doubly fed induction generator (DFIG)-based wind turbines during low voltage ride through (LVRT) cannot be neglected. In order to explore the instability mechanism of DFIG system during weak grid fault, the small signal state space model is established in this paper. The results of the modal analysis show that the dominant unstable poles are mainly impacted by the phase locked loop (PLL), rotor current control loop (RCCL) and terminal voltage during the fault, where the PLL is the dominant factor. Furthermore, the impact of each factors on the system dynamic stability are comprehensively evaluated, which indicates that the controller bandwidth under normal grid condition is no longer apply to the fault condition due to the interaction between the controller and grid. Then, the optimal current proportion which can significantly improve the system stability is deduced. Finally, based on the analysis, this paper proposes the improved LVRT control schemes from two aspects of either injecting active current or decreasing PLL bandwidth to enhance the system small signal stability. The effectiveness of the proposed LVRT control strategies is validated by the simulation and experiments.

The virtual synchronous generator (VSG) control has been widely used in grid-connected inverter to effectively increase the equivalent inertia of power system. However, the active power oscillation similar to electromechanical transient characteristics of conventional synchronous generator (SG) is introduced to power grid, especially under the condition of multi-virtual synchronizer. In this paper, the dynamic stability of the multiple grid-connected voltage source converters (VSC) controlled by VSG is studied. Firstly, the system linearization model is established to analyze the oscillation risk. In order to improve the damping characteristics of multi-VSG grid-connected systems, an additional power system stabilizer (PSS) method is proposed. Then considering the influence of controller installation location and control parameters on the dynamic stability of the system, the design methods of multi-VSG damping controller based on residue index and hybrid particle swarm optimization algorithm are proposed respectively. Finally, the effectiveness of the damping controller design method is verified through the simulation and test cases of the multi-VSG grid-connected system.

Large-scale wind farms are generally integrated by long-distance transmissions, but power grids cannot sufficiently support the access point voltage of these wind farms. The access point voltage undergoes a stability problem under wind speed variations. However, the reactive power compensation device cannot reconcile the requirements of response speed and compensation capacity. Despite their fast power decoupling control, the reactive power capability of doubly fed induction generators is restricted by active power output. To satisfy the reactive power demand of system under wind speed variations, coordinating the reactive power capability and active power output of wind farm is the key solution, based on which a novel active control idea is proposed. The reactive power capability of wind farm and the reactive power demand of system are both studied, and the controllable conditions of access point voltage are analyzed. Active voltage control strategies, including active adjustment of reactive power reference, active speed control, and active pitch angle intervention according to wind speed ranges, are proposed. In the simulation, the method is verified to adequately consider the reactive power demand, and excavate the wind farm reactive power capability. The method also effectively suppresses the change of grid voltage under wind speed variations.

The voltage-frequency coupled transient stability and risk assessment issues of weak grid-connected large-scale renewable energy plants (REPs) during low voltage ride-through (LVRT) have received little attention to date. The shape and mechanism of voltage-frequency coupled transient instability during LVRT was investigated in detail by the voltage-vector-triangle graphic (VTG) method utilized in this study, which avoids complex mathematical calculations. In addition, the margin of the transmission line's voltage angle (MTVA) was defined in this paper and was then used to quantitatively assess the transient instability risk of the large-scale REPs during LVRT. Furthermore, the expressions of the MTVA were deduced under different conditions and the major influencing factors which resulted in instability risk were determined correspondingly. On this basis, the transient instability risk assessment standard of the large-scale REPs is built and the instability risk minimized scheduling strategy is proposed in this paper, which does not change the internal control structure and parameters of the REPs during LVRT. Finally, both the correctness of the VTG method and the risk assessment standard, and the effectiveness of the scheduling strategy proposed in this paper are validated by experiments and simulations.

In this paper, we propose an improved sub-synchronous resonance (SSR) damping controller (SSRDC) and a related control strategy to mitigate the SSR in a capacitive series-compensated double-fed induction generator based wind farms, based on the damping torque analysis. The proposed strategy is utilized for enhancing the system stability. Compared with the traditional damping controller, the proposed SSRDC is simultaneously embedded into the $d$ -axis and $q$ -axis control channels in the inner current loop of the rotor-side converter. On this basis, combined with the eigenvalue analysis and improved particle swarm optimization algorithm, the optimum gain coefficients of the SSRDC can be determined for maximizing the mitigation effect under different operation conditions of the series compensation levels and wind speeds. Moreover, the effect of the weight coefficients in the objective function on the SSR suppression is also investigated. Finally, the time-domain simulations are carried out to demonstrate the effectiveness of the proposed control strategy for mitigating the SSR and suppressing the connected power system oscillations.

In this paper, the transient synchronization process of the grid-connected voltage source converters (VSC) is studied detailly. Firstly, the phase-locked loop (PLL)-synchronized VSC is modeled according to the rotor motion equation of synchronous generator (SG). Furthermore, the VSC's damping ratio is derived, and the effects of the VSC's control parameters and the operation status on the equivalent power angle (EPA) is investigated. Moreover, the influence of the PLL's parameters and the voltage dips degree on the VSC's transient frequency behavior are analyzed. Analysis result reveals that the VSC's damping ratio decreases once the grid voltage drops, so that the EPA and frequency of the VSC may appear large overshoot and oscillation, which may trigger the frequency protection and deteriorate the VSC's transient stability. In order to ensure that the VSC can smoothly operate to new equilibrium points, an improved PLL is proposed, which can adaptively adjust the VSC's damping ratio for different voltage sags. The proposed method can not only reduce the overshoot of the EPA but also restrain the VSC's frequency dips degree. Finally, the simulations and experimental tests validate the effectiveness of the theoretical analysis.