Date of Award

Spring 1-1-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Dragan Maksimovic

Second Advisor

Hanh-Phuc Le

Third Advisor

Khurram Afridi

Fourth Advisor

Milan Ilic

Fifth Advisor

Brian Johnson

Abstract

Stacked DC-AC converters offer several unique advantages including i) the ability to create power systems with voltage level specifications many orders above the device voltage ratings, ii) flexibility in system design, and iii) modularity for system expansion. The benefits of stacked DC-AC system can be further extended by making the converters bi-directional using circuit design and control techniques. This thesis identifies and analyses the challenges that come along with the advantages of stacked DC-AC systems. These challenges are then addressed by introducing circuit architectures and control techniques. Simple, yet powerful models are presented to understand the overall system operation.

Unlike DC-DC stacking, DC-AC stacking requires line frequency information to synchronize the AC ports and build up the voltage across the stack. Rather than using phase-locked loop (PLL) which requires additional wiring or using power-line communication (PLC) which requires additional hardware, this thesis describes the design and implementation of a self-synchronizing stacked DC-AC converter system that achieves synchronization without additional wiring or hardware or any means of communication. The research presented in this thesis can be applied to off-grid systems for supporting standalone loads using either PV or batteries at the DC port.

A virtual droop control technique that enables the stacked system to process power flow in both the directions is also presented. Such a control scheme when implemented, makes the DC-AC converter operate as an AC-battery. One straightforward application of this technique is energy storage integration with the grid.

There are significant challenges in extending the stacked architecture to the case of DC to three-phase AC systems. Unlike single-phase stacking, having an isolated DC source such as a battery or a PV panel at the DC port is not a sufficient condition for three-phase AC stacking. A new multilevel DC to three-phase AC architecture is presented, which includes phase-to-phase isolation using a multi-port active-bridge DC-DC converter. This approach achieves required isolation using compact high-frequency transformers and eliminates the need for bulk energy storage or bulky line-frequency transformers. Furthermore, distributed control techniques are developed to achieve system level goals from local measurements.

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