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.

The commutation failure fault usually occurs in the Line Commutated Converter (LCC)-based High Voltage Direct Current (HVDC) transmission system. When commutation failure fault occurs, the voltage of sending Alternate Current (AC) system changes rapidly and the connected Doubly Fed Induction Generator (DFIG)-based wind turbine may be tripped. Thus, the Fault Ride Through (FRT) control strategy of DFIG should be investigated for enhancing the stability of the sending AC system. However, the voltage of the sending AC system during commutation failure isn't changed in rectangular in shape, besides, the voltage presents the “first reduce then rise” characteristic, which is not considered in the existing FRT control strategies. In order to realize the continuous FRT of DFIG during commutation failure, the stator flux and Electromotive Force (EMF) when the stator voltage changes continuously have been analyzed for the first time in this paper. Furthermore, based on the analysis results, an improved continuous FRT control strategy is proposed. The simulation and experiment results validate the effectiveness of the proposed method. The proposed control strategy is not only suitable for the commutation failure condition, but also for the scenario with continuous voltage variation during grid fault, which indicates that the proposed method is general.

The instability issues of grid-connected voltage source converters (VSC) may easily occur during low voltage ride-through (LVRT), especially when connected to a weak AC grid. In this study, a small-signal model of the grid-connected VSC system was developed to deal with the stability problems during deep voltage sags. Based on the model, the interaction between the phase-locked loop (PLL) and current control has been illustrated. In addition, the eigenvalue and modal analysis method was employed to investigate the influencing factors of the VSC system stability, which include the bandwidths of the PLL and current control loop, grid strength, and voltage sags. Furthermore, on the basis of the interaction between PLL and current control loop, a novel additional damping controller which is placed in the active current control loop was proposed and designed. Finally, experiments were conducted to verify the theoretical analysis and proposed control strategy for enhancing VSC system stability during LVRT with a high impedance grid connection.

As the share of wind power in the power system increases, the power grid exhibits the characteristics of a weak grid. The dynamic stability issues of the grid-connected doubly fed induction generator (DFIG)-based wind turbines caused by the interaction between the phase-looked loop (PLL) and rotor current controller (RCC) by high grid impedance have become acute, especially during low-voltage ride-through. In this article, the output impedance model of the DFIG is derived, where the dynamics of the PLL have been first taken into account. In addition, the interaction between the PLL and RCC is analyzed. Furthermore, a novel virtual capacitance control strategy that directly controls the input voltage of the PLL and does not need to change the internal controller parameters and structure is proposed. The proposed control strategy can reduce the effect of the high line impedance by compensating for the PLL output angle deviation. Subsequently, the interaction between the PLL and RCC becomes weaker, and the phase margin of the DFIG system during the severe grid fault increases. Meanwhile, the applicability of the proposed control strategy is presented. Finally, the effectiveness of simulation and experimental results is presented.

In this article, the frequency coupling of weak grid-connected doubly fed induction generator (DFIG) based wind turbines (WT) during asymmetric faults have received little attention. To analyze the frequency coupling phenomenon of the system during asymmetric faults, this article established the complex transfer function model in αβ frame based on the transformation relationship between complex transfer function and transfer matrix. According to the complex transfer function, the frequency coupling mechanism of the DFIG system during asymmetric faults is revealed, and the transfer evolution law of coupling frequencies is described in detail. Finally, the correction of theoretical analysis and the validity of proposed complex transfer models are verified by the simulation and experiment.