| In this paper, an equivalent model of a wind farm based on real time digital simulator(RTDS) is built to study the stability of wind-thermal-bundled UHVDC power transmission system. The eletromechanical decoupling characteristic of doubly-fed induction generator(DFIG) based on vector control and the results it leads are analyzed. For a single machine infinite bus system, the power angle of DFIG is defined and the transient behavior is analyzed, and the physical essence of the power angle transient behavior of the eletromechanical decoupling characteristic of DFIG, which is an electromagnetic transient with a power angle rapidly changing behavior, is revealed. The external manifestation of the power angle rapidly changing behavior is that the active power of the system will rapidly recovered after the disturbance. The research shows that, on the one hand, during a constant power transmission mode of UHVDC, the wind turbines do not provide any mechanical inertia so the equivalent mechanical inertia of the wind-thermal-bundled source at the sending end is decreased, which then reduced the overall stability of the UHVDC transmission system. On the other hand, because of the power angle rapidly changing behavior and the rapidly recovery of active power from the wind turbines, the adjustment of unbalanced power of the sending end system after the disturbance is only supported by the thermal power units, whose stability is decreased when the DC sending end system is paralleled, and the inhibiting ability of the system for frequency fluctuating is weakened when the DC sending end system is isolated. The analysis is verified through the simulation based on RTDS.Based on this research, the wide area additional control strategy with DFIG as the controlled object and the DC additional control strategy with the DC system as the controlled object are proposed respectively, with the prototype for the combined additional control designed. The functions of the prototype include the dynamically adjustment of the active power from the wind farm to inhibit the frequency fluctuating, the mining for the reactive power of wind farm itself to inhibit the power frequency overvoltage during the system failures, and the dynamical change of the DC active power command to achieve dynamical balance of active power. The efficiency of the prototype is verified through the simulation based on RTDS, and the results shows that this combined additional control strategy effectively enhances the overall stability of the wind-thermal-bundled UHVDC transmission system in both paralleled and isolated mode. |
