Research On Interaction Analysis And Coordinated Control Of The Multiple FACTS Controllers In Power System

With the development of the electric power system and the increasing requirement of power system operation, the technique of Flexible AC Transmission System (FACTS) has been widely applied. FACTS can enhance the capacity of transmission line and improve the transient and dynamic stability of the system efficiently. Recently, with the wide applications of flexible AC transmission system (FACTS) controllers in power systems, a potential risk, the interaction between FACTS controllers, is emerging and may impose negative influence on power system operation and control. Manintaining the joint-operation performance of multiple FACTS controllers faces a new challenge.At present, the impact of the interaction among power system controls, especially FACTS controllers, is on the elementary stage, there still exist many problems to be explored thoroughly. Therefore, exploring the new theory, new technology and new method, enhancing the level of the security of the power system through the coordination control of multi-FACTS elements has enormous economic and project significance. The dissertation consists of four main parts as follows:In the first part, with the consideration of four types of FACTS devices, i.e., SVC, STATCOM, TCSC and UPFC, a general procedure for dynamical modelling of a large-scale multi-machine power system installed with multiple FATCS devices is proposed. Based on the mathematical model developed, the relative gain array (RGA) theory is applied to assess the interaction between FACTS controllers. The proposed method can quantify the interaction among the FACTS devices and indicate the optimal placement for devices to weaken the influence of interaction. Based on the RGA analysis method, the influence caused by the electrical distance between two FACTS devices is investigated emphatically.The detailed simulation results in two case studies accord with the analysis results by RGA method, which demonstrates the effectiveness of RGA method in analyzing interactions of FATCS controllers.In the second part, an analytical approach based on normal form theory is proposed for the analysis of non-linear interactions among the multiple FACTS controllers in power systems. With this technique, considering the effect of the nonlinear term in practical system, it is possible to obtain the simplest form of a set of non-linear differential equations and hence, to identify and study the nature of system oscillations. Compared with conventional linear analysis methodologies, the analysis results obtained by normal form method are more exact. Moreover, a non-linear interaction index is developed to investigate the interactions among multiple FACTS controllers. The simulation results validate the proposed approach.In the third part, the mechanism analysis of interation among multiple functional controllers of UPFC, i.e., power flow control, DC voltage control and AC voltage control, is studied. Based on the UPFC power flow model proposed, the effect on several system variables caused by the changing of pre-specified values, i.e., active power and reactive power of transmission line and voltage magnititude of bus in which UPFC is installed, is studied. By non-linear simulation of the system, it is shown that there exist negative interactions among different control channels of an UPFC. From the control theory point of view, the influence of series inverter's magnitude and angle on the interaction is also researched. In addition, it is concluded that the interaction among UPFC controllers is caused by the confliction between active power controller of series inverter and AC voltage controller of shunt inverter, and if the connection between series and shunt inverters is removed, the joint operation performance of multiple controller interaction is recovered.In the fourth part, the coordination design of multiple FACTS controllers is investigated in multi-machine power systems, which is formulated as a multi-objective optimization problem. The multi-objective Evolutionary Algorithm (MOEA) based approach is developed to solve the problem by optimizing FACTS controller parameters. By simultaneously optimizing the different objective functions of the multi-objective controller design, the best performance of joint-operation among multiple FACTS controllers can be obtained. The coordinated operation problem of TCSC, SVC and Power System Stabilizer (PSS) is investigated by mechanism analysis and time-domain simulation. The Pareto optimal solutions can be obtained with the use of MOEA by optimizing FACTS controller parameters. Application of the multi-objective evolutionary algorithms in power system can provide feasible schemes to power system operation and its control, therefore, the final decision can be made according to the different design requirements.