PhD Dissertation Defense

07/25/2014
By Mufeed Mahd

PhD Candidate Vu Tran from Electrical and Computer Engineering will defend the dissertation, "High efficiency grid-connected system using switched-capacitor converter and Lyapunov approach to control design for inverters" on Aug. 4 at noon in Ball Hall 301, ECE Meeting Room.

Committee Members are Professor Mufeed MahD, ECC, PhD PEng (Adviser); Professor Ziyad Salameh PhD, ECE; and Professor Joel Therein, ECE, PhD.

Dissertation Abstract:
This dissertation develops a high efficiency step-up DC-DC converter serial with a grid connected inverter using Lyapunov approach for optimization.

Part I presents a transformer-less switched capacitor buck boost converter that is based on a traditional buck boost topology. The proposed converter achieves high voltage gain and higher efficiency when compared to the conventional buck-boost converter. The average model based on state-space description is analyzed. The simulation results are presented to confirm the capability of the converter to generate high-voltage ratios. The proposed converter is suitable for applications which require high step-up DC-DC converters such as DC micro-grids and solar electrical energy.

Part II presents a Lyapunov approach to control design for grid connected inverters. The control objective is to track a reference current which is proportional to the fundamental harmonic of the grid voltage. By using the internal model principle, the grid voltage and the reference current are described as the outputs of an autonomous linear oscillatory system. The state of this oscillatory system contains all the information for the harmonics of the grid voltage and is estimated via an observer with zero estimation error at steady state. A state space description for the whole system is obtained by combining the state of the inverter circuit and that of the oscillatory system for the grid voltage. Based on the state space description, a Lyapunov approach is developed to design a state-feedback controller for tracking a reference current with minimal tracking error. The design problem is cast into an optimization problem, which can be effectively solved with linear matrix inequality (LMI) toolbox in Matlab. The Lyapunov approach ensures internal stability and makes efficient use of the structural information, such as the total harmonic distortion (THD) of the grid voltage, and the magnitude/phase of the reference current. The effectiveness of the Lyapunov approach is validated via SimPower simulation. A real circuit is built using microcontroller ezDSP28335, the output current obtained is in phase with the grid voltage and has small THD, as we expected.

Part III presents the integration of the two above systems, DC-DC converter and DC-AC inverter to construct one unified system. The models are validated via SimPower simulation. The simulations are conducted to verify the response of the whole system under the varying conditions of input status, different types of loads connected to the output, and the circuit’s protection problem.