| Over the past decade improvements in semiconductor technology have led to faster power electronic switches with higher ratings. These devices have been used to improve the performance of various power converter systems. In particular, the Voltage Source Converter (VSC) has been widely used in power systems to facilitate power flow control and improve power quality. The research described in this dissertation mainly relates to two important power quality applications of the VSC: the Static Synchronous Compensator (STATCOM) which performs power factor compensation (PFC), and the Active Power Filter (APF) which attenuates system harmonics. Both applications can be formulated as trajectory tracking problems for a nonlinear dynamic system, and the approaches taken here involve a direct model-based compensation for the nonlinearity in order to establish a formal statement regarding the stability of the closed-loop system. An emphasis of these designs is placed on adaptive control to directly account for uncertainties in the system parameter.;First, the thesis focuses on the STATCOM application of the VSC. We present two different third order models based on different power balances. Using these models we propose a nonlinear flatness-based control which guarantees tracking of constant dc voltages and q-axis current. The particular novelty of the approach lies in the use of steady state relations to improve existing work. Next we present the state-of-the art in nonlinear adaptive control theory and apply an existing approach to PFC while dealing with uncertainty in two system parameters. We show that it is not possible to apply existing theory to an important choice of uncertainty in parameters. Hence, we propose two alternate adaptive designs which directly account for uncertainty in three parameters. The first design is a so-called adaptive PI vector control which is reminiscent of the classical vector control method. The other approach is the so-called full adaptive control. A Lyapunov function is derived to demonstrate these designs provide asymptotic convergence of the tracking error. Two designs are presented for the APF application: a traditional PI control and an adaptive control. The adaptive control shows improved tracking of power factor, harmonic compensation, and robustness to model error. In parallel to the control theory derived and simulation work, experimental validation was performed on a VSC test stand. |
