On Decentralized Control of Power Electronics Using Nonlinear Oscillators

Abstract

This dissertation develops theoretical tools and hardware prototypes for the decentralized control of power electronic circuits connected through an electrical network. The research thrust is timely and relevant given that renewable generation, storage devices, and electric vehicles continue to be rapidly integrated via these power electronic interfaces into various power systems with ad-hoc control architectures. In particular, controllers are developed for two key applications of inverter control for dc-ac conversion for standalone and grid-connected microgrids, and switch interleaving for multiphase dc-dc conversion, where decentralized control lends a way to ensure robust, efficient and modular operation. The control philosophy derives from the rich subfield of coupled oscillator theory and focuses on a particular class of second order systems called Lienard-type oscillators. Depending upon the nature of coupling, such oscillators demonstrate emergent patterns of sustained oscillations that can be leveraged to engineer steady-state behavior with stability certificates. Based upon this premise, the core idea is to program the second order nonlinear differential equation onto a micro-controller and use its states to construct switching signals for the power-electronic converters. To close the loop, the output current is used to design a local feedback strategy that guarantees desirable steady-state behavior : synchronized solutions are of interest in inverter systems and phase-balanced solutions are of interest in interleaving switching waveforms for multiphase systems. Theoretical stability proofs based on Lyapunov and passivity arguments along with extensive hardware results are presented to demonstrate the suitability of the proposed paradigm. In the case of inverters, the work establishes a link between oscillator-based control and the classical droop laws that affords a comprehensive design procedure for synthesis of oscillators which incorporates steady state regulation, control of harmonic content and rate of convergence. Furthermore, a completely communication-free switch interleaving for dc-dc converters has a distinct advantage over the state-of-the-art methods that are at best distributed in nature and have a single point of failure.