Research On VSC-HVDC System Control Strateies To AC System Asymmetric Fault

VSC-HVDC is a new type dc transmission technology with more flexible and controllable features and easier to form the multiterminal systems, which is an important part of an/dc hybrid grid technology and distributed energy grid connection. Therefore, in order to guarantee the safe and stable operation of VSC-HVDC system, it need to carry on the corresponding analysis on the system dynamic characteristics under the ac fault. With the ac fault, the control performance of VSC-HVDC system determines its operating characteristics, so in this case, research on protective control strategy of VSC-HVDC system has certain practical significance and value.Steady state mathematical model of VSC-HVDC system was established under balanced condition of ac side. A traditional PI vector steady-state control strategy was also derived, and the influence of ac fault to the performances of this controller was studied. Based on the steady state mathematical model, and combined with the backstepping control method and variable structure control method, a predictable uncertainty bound backstepping variable structure control strategy was proposed to improve the dynamic performance of the VSC-HVDC under disturbance. The results show that the proposed control strategy can further improve the dynamic performance of VSC-HVDC system, and enhance the robustness of the system.The mathematical model of VSC-HVDC system in positive and negative sequence coordinates was established under asymmetric fault of grid based on the instantaneous symmetrical component method, and dynamic performance of VSC-HVDC system was analyzed. The momentary jump phase Angle exist in the asymmetric fault, and negative sequence current component can affect the dynamic performance of PLL which also lead to the second harmonic power fluctuations in transmission lines. These factors has a harmful effect on the transient response performance of VSC-HVDC system, therefore two conventional asymmetric control strategies were studied. Results show that the dc-link voltage ripple restraint control strategy can restrain system reactive power fluctuations, is suitable for short distance transmission, while the negative sequence current restraint control strategy reducing the influence of asymmetric fault to system by inhibiting the fault negative sequence current of converter station, but still cannot completely eliminate the power fluctuations.In order to further eliminate the influence of ac system asymmetric fault, a power compensation asymmetric control strategy based on the theory of Lyapunov asymptotic stability was proposed. Here attribute the influence of Asymmetric faults on the system to system uncertainties, by increasing the stability margin of the control system, realize the effect of fault to weaken or eliminate. Under the proposed control strategy, the operation conditions of VSC-HVDC system were simulated in the Matlab/Simulink environment for short-circuit ground faults at two-phase AC-side. The results show that the second harmonic power fluctuations of transmission lines can be effectively suppressed by the proposed control strategy with good control performance.