1 An Effective Trade-off between Stability and Voltage Regulation Hassan Bevrani * Member Takashi Hiyama * Member This paper addresses a new robust control methodology to enhance the power system stability and voltage regulation as an integrated design approach. The automatic voltage regulation (AVR) and power system stabilizer (PSS) design problems are reduced to solve a single H∞ based static output feedback control problem. To determine the optimal gains, an iterative linear matrix inequalities (LMI) algorithm is used. A four-machine infinite-bus system example is given to demonstrate the efficiency of developed approach. The proposed robust technique is shown to maintain the robust performance and minimize the effects of disturbances, properly. Keywords: H∞ control, static output feedback, LMI, voltage regulation, power system stabilizer, robust performance 1. Introduction Power systems continuously experience changes in operating conditions due to variations in generation/load and a wide range of disturbances (1) . Power system stability and voltage regulation have been considered as an important problem for secure system operation over the years. Currently, because of expanding physical setups, functionality and complexity of power systems, the mentioned problem becomes a more significant than the past. That is why in recent years a great deal of attention has been paid to application of advanced control techniques in power system as one of the more promising application areas. Conventionally, the automatic voltage regulation and power system stabilizer (AVR-PSS) design is considered as a sequential design including two separate stages. Firstly, the AVR is designed to meet the specified voltage regulation performance and then the PSS is designed to satisfy the stability and required damping performance. It is well known that the stability and voltage regulation are ascribed to different model descriptions, and it has been long recognized that AVR and PSS have inherent conflicting objectives (2) . That is why the successful achievement of both goals using nonintegrated design approach turns out to be very difficult. Therefore, it is reasonable to realize a compromise between the desired stability and regulation performances by a unique controller. In the last two decades, some studies have considered an integrated design approach to AVR and PSS design using domain partitioning (3) , robust pole-replacement (4) and adaptive control (5) . Recently, several control methods have been made to coordinate the various requirements for stabilization and voltage regulation within the one controller (6~11) . A desensitized controller based on Linear Quadratic Gaussian (LQG) optimal technique is used in Ref. (6). An approach used in Ref. (2, 7) involves use of Internal Model Control (IMC) method to make a trade-off between voltage regulation and power system stabilization. Although all above approaches have used linear control techniques, because of complexity of control structure, numerous unknown design parameters and neglecting real constraints, they are not well suited to meet the design objectives for a multi-machine power system. Some proposed scenarios apply a switching strategy of two different kinds of controller to cover the different behavior of system operation during transient period and post- transient period (8~10) . The performance of these schemes essentially depends upon the selection of switching time. Moreover, using different control surfaces through a highly nonlinear structure increases the complexity of designed controllers. As a preliminary step of this work, the authors have addressed the problem of a robust control methodology to enhance the stability and voltage regulation of a single-machine infinite bus in the presence of conventional PSS and AVR equipments (11) . In this paper, the stabilization and voltage regulation considering the practical constraints for feasibility are formulated via an H∞ static output feedback (H∞-SOF) control problem which it can be easily solved using an iterative linear matrix inequalities (LMI) algorithm. The resulting controller is not only robust but it also allows direct and effective trade-off between voltage regulation and damping performance. The proposed controller uses the measurable signals and has merely proportional gains; so gives considerable promise for implementation, especially in a multi-machine system. In fact the proposed control strategy attempts to make a bridge between the simplicity of control structure and robustness of stability and performance to satisfy the simultaneous AVR and PSS tasks. In order to show the effectiveness of this methodology, it is applied to a four-machine infinite-bus system. The obtained results are compared with a full-order dynamic H∞ output feedback control design. * Department of Computer Science and Electrical Eng., Kumamoto University, Kumamoto 860-8555. Paper Author Personal Copy Author Personal Copy