Robust nonlinear control of STATCOMs

Many nonlinear control techniques for STATCOM systems are available nowadays. In this chapter several nonlinear feedback controller design techniques relatively simpler and more robust are to be introduced: input–output feedback linearization (IOL) method, passivity-based control (PBC) method, port-controlled Hamiltonian (PCH) method with dynamics extension method. The IOL method has been applied to STATCOM and it shows uniform transient performance. However, the oscillatory response owing to the lightly damped internal dynamics could negatively affect the life cycle of the system and power quality. A modified IOL control scheme is introduced to improve the damping of internal dynamics of performance while preserving overall system stability. Although the IOL methods improve the performance of type 2 STATCOM systems, these methods are sensitive to parameter uncertainty. Moreover, when the system is working in the inductive operating range, undesired oscillatory transient response appears in the DC voltage due to its lightly damped internal dynamics with the IOL method. The PBC method considers the dynamics characteristics of type 2 STATCOM systems, in particular its passive characteristics. Employing the PBC method improves the robustness of controller implementation and simplifies implementation compared to the IOL method in such a way that it avoids canceling the system nonlinearities exactly. However, since the previous methods are designed based on an approximated model of type 2 STATCOM systems, the closed-loop system has a locally stable equilibrium point. Moreover, the stability region is numerically extensively determined. To overcome the aforementioned problem, PCH with the dynamics extension method is developed for a robust and simple structure of nonlinear controller with the non-approximated model of STATCOM systems in order to improve the performance in time domain and enlarge the stability region.