A Multi-Model Approach to Design a Robust Fixed-Order Controller to Improve Power System Stability

The rapid increase in power system grid has resulted in additional challenges to reliable power transfer between interconnected systems of a large power network. Large-scale penetration of intermittent renewable energy increases uncertainty and variability in power systems operation. For secure operation of power systems under conditions of variability, it is imperative that power system damping controllers are robust. Electromechanical oscillations in the range of 0.2 Hz to 1 Hz are categorized as inter-area modes. These modes arise due primarily to the weak interconnections characterized by long transmission lines between different operating areas of an interconnected power system. One of the main challenges to secure operation of interconnected power systems is the damping of these inter-area modes. This dissertation introduces two multi-model approaches (loop shaping and H∞) to designing a fixed-order robust supplementary damping controller to damp inter-area oscillations. The designed fixed-order supplementary damping controller adjusts the voltage reference set point of the Static Var Compensator (SVC). The two main objectives of the controller design are damping low-frequency oscillations and enhancing power system stability. The proposed approaches are based on the shaping of the open-loop transfer function in the Nyquist diagram through minimizing the quadratic error between the actual and the desired open-loop transfer functions in the frequency domain. The H∞ constraints are linearized with the help of a desired open-loop transfer function. This condition can be achieved by using convex optimization methods. Convexity of the problem formulation ensures global optimality. One of the advantages of the proposed approach is the consideration of multi-model uncertainty. Also, in contrast to the methods that have been studied in literature, the proposed approach deals with full-order model (i.e., model reduction is not required) with lower controller order. In addition, most of the current robust methods are heavily dependent on selecting some weighting filters: such filters are not required in the loop-shaping approach. The proposed approaches are compared with different existing techniques in order to design a robust controller based on H∞ and H2 under pole placement. With large-scale power systems, it is difficult to handle large number of states to obtain the system model. Thus, it becomes necessary to use only input/output data measured from the system, and this data can be utilized to construct the mathematical model of the plant. In this research, the mentioned approaches are offered in order to design a robust controller based only on data by using system identification techniques. The mentioned techniques are applied to the two-area four-machines system and 68 bus system. The effectiveness and robustness of the proposed method in damping inter-area oscillations are validated using case studies