Study On FACTS-based Damping Controller Design And Control Strategy

With the development of the large scale power system, interconnected network and diversity of operating conditions make it more complicated to operate and control the power system. Flexible AC Transmission System is an effective technology which can enhance the controllability and flexibility of transmission and distribution system as well as increasing stability and economy of the power system operation. With widely application of FACTS in power system, it is significant to discuss the control method of FACTS and coordination control strategy of multiple FACTS devices. First, power oscillations will occur among the interconnected network, and damage the small disturbance stability of the power system. It becomes an important research field to design FACTS-based damping controller and analyze damping control strategy. Second, besides the conventional function of voltage control, power flow control and reactive power compensation, FACTS can also suppress power oscillations. The impact on the conventional control target should be considered when decide the damping control strategy of FACTS controller. It is the multi-objective coordination problem of the same controller. Third, with the increasing amount and variety of FACTS installed in the power system, the interaction between controllers should be considered when design FACTS controller. It is the coordination control problem of multiple controllers.Immune system is a complicated adaptive system that contains abundant and effective information processing principle, which can cope with different kinds of disturbances and uncertainties of inner body and outside environment. The control problem is similar with the immune response of immune system. The immune regulation mechanism and immune network based coordination mechanism among antigens and antibodies have good performance, which can be applied to the multi-objective coordination control of FACTS and coordination control of multiple controllers in the power system.This dissertation mainly discusses about design of FACTS-based damping controller, and control strategy of FACTS based on artificial immune regulation mechanism. The main research work is summarized as follows:In the first part, torque analysis and energy function analysis are used to explain the mechanism of damping power oscillations by FACTS controller, and relative damping control strategy are poposed. Firstly, the change of generator electromechanical torque after installation of FACTS is analyzed on the basis of power system Phillips-Heffron model. The reason of suppressing the power oscillation is explained from the viewpoint of damping torque analysis. Secondly, oscillation energy function of inter-area mode oscillation is defined which can be regarded as a measure of the deflection away from the steady state position during dynamic oscillation period. The smaller the energy is the more stable situation the power system has. The conversion of energy function during oscillation period is analyzed and damping control strategy of reducing oscillation energy is proposed which aims at reducing oscillation energy to zero as quickly as possible, namely returns to the steady state position after fault. Finally the damping control strategy of SVC and TCSC are proposed.In the second part, Prony analysis is used to identify the transfer function and residue of the system, and FACTS damping controller is designed by compensating the phase of residue. A method of choosing input signal of damping controller is proposed. In course of using the Prony method to analyze simulation signals of the power system, the specific format small disturbance signal is used as input signal. So transfer function can be identified according to the mode of the corresponding input signal and output signal. Oscillation mode and residue can be obtained from the transfer function and used to design damping controller. Thus complicated eigenvalue analysis of large scale power system is not needed. Residue method is used to design the damping controller and set the parameters of controller. Because the power flow along transmission line may have opposite direction under different operation conditions, a method to choose the input signal is proposed that not only the magnitude but also the phase of the residue should be considered so as to guarantee the robustness. A method of adjusting gain of controller according to the transmission power level is proposed, by which optimal damping performance can be obtained under different operating conditions. Simulation studies indicate that the frequency domain response of the identified system is consistent with the real system, and the former is suited to the design of damping controller. Simulations of TCSC damping controller and SVC damping controller show that the damping controller can suppress the oscillation effectively with good robustness. Compared with fixed gain damping controller, varying gain controller has better damping control effect, especially under light load condition.In the third part, artificial immune controller is designed on the basis of immune regulation mechanism and immune FACTS controller is proposed which can realize the coordination of damping control and other objective (voltage control or power flow control). The regulation mechanism of T cells in the process of immune response and the mathematical model of immune regulation are analyzed. The model is modified according to the characteristic of control problems and artificial immune controller is designed. The parameters of controller are discussed. By simulating the different functions of T-cells in different stage of immune response, feedback estimation function is constructed which can adjust the parameters according to the current state and control effect. Thus the coordination of double objective of a single controller can be realized. The proposed artificial immune controller is applied to design FACTS damping controller combined with the damping control strategy of reducing oscillation energy. Simulation results of TCSC and SVC controllers shows that the power oscillations can be damped effectively, as well as the power flow control of TCSC or voltage control of SVC can be achieved.In the last part, a coordination control strategy of multi-controller is proposed on basis of idiotype immune network and coordination mechanism among multi-antigen and multi-antibody. Firstly antigen coupling control strategy is proposed by simulating different response level of antigens in immune response. In multi-variable control system, input signals of controller are adjusted regarding the contribution of different control paths to the whole system performance, so as to eliminate the error as soon as possible. Interactions among multiple controllers are analyzed using relative gain array method. If close-loop operation of a certain controller decreases the control effect of other controllers, the output of the former controller is regarded as disturbance to the latter. Antibodies feedback coupling coordination strategy is proposed by simulating stimulation and suppression among antibodies in immune system. The synthesis of the above two coupling strategy consist the immune coordination control strategy, which utilizes the intrinsic coupling relation among control channels of multi-input and multi-output control system, to realize the coordination control of multiple controllers. Immune coordination strategy based coordination control are carried out for TCSC and SVC, as well as SVC and generator excitation. Simulation results show that compared with decentralized control of each controller, immune coordination control can eliminate the negative interaction between controllers and increase the control effect of the whole system.