Application Of Optimal Variable Aim Cooperative Control Strategy In AC/DC Systems

In nowadays, the tendency of our power industry is "Interconnection in the whole country, transmission electric power from West to East and exchange power between South and North". To meet this leaning, high voltage direct current (HVDC) transmission system becomes a major style of the interconnection in power network, because it is appropriate for long distance and large capability transmission. The application of HVDC has changed the structure of origin systems, so the challenges about applicability of origin controllers emerge, and the focus fixes on how to cooperate HVDC controllers and origin controllers in ac systems.It has been proved that the additional power modulation of HVDC can improve the stability of AC/DC systems. Based on the research about the cooperative control between FACTS and generator excitation, this paper aims at the cooperative control between power modulation of HVDC and generator excitation for advancing the stability of AC/DC systems.Optimal variable aim control (OVAC) has tracking ability and adaptability, and can deduce closed solution. Thus closed control structure is available, which fits for application on line and avoids the inadaptable for serious disturbance caused by linearized model. First, according to a single machine infinite bus system, the cooperative controls of power modulation of HVDC and generator excitation are developed adopting OVAC theory and direct feedback linearized optimal controlmethod respectively. The simulation results suggest cooperative control is more effective than power modulation of HVDC alone to improve stability of AC/DC systems. And compared to direct feedback linearized optimal cooperative control, OVAC cooperative control is better.To deep the research, OVAC is used in multi-machine AC/DC systems. On the basis of establishing an observation decoupled state space model of system by localized states estimation, an OVAC cooperative control for power modulation of HVDC and generator excitation is deduced. The control is easy to be implemented because it need only local information, driving subsystem firstly to an artificial medial equilibrium point and secondly to a desirable equilibrium point. And if all subsystems are in their dynamic equilibriums, the stable equilibrium state entirely is achieved. The simulation results of test system show that the proposed method can improve not only the power angle stability, but also the performance of DC systems.At the last part of this paper, combining the researches above, an OVAC hierarchical cooperative control strategy is presented for the AC/DC systems, which have multi-feed HVDC. The structure of the control strategy includes AC/DC cooperative layer and DC cooperative layer. The coordination among power modulations of multi-feed HVDC is considered as well as coordination among power modulation of HVDC and generator excitation. Also the simulation results of test system indicate the strategy is efficient for mending the stability of systems.