Date of Award

Fall 2020

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Enrico Santi

Abstract

Modern DC power distribution systems (DC-PDS) offer high efficiency and flexibility which make them ideal for mission-critical applications such as on-board power systems of All-Electric ships, electric vehicles, More-Electric-Aircrafts, and DC Microgrids. Despite these attractive features, there are still challenges that need to be addressed. The two most important challenges are system stability and load power sharing. The stability and performance are of concern because DC-PDS are typically formed by the interconnection of several feedback-controlled power converters. The resulting interactions can lead to destabilizing dynamics. Likewise, in a DC-PDS there are several source converters that are operating in parallel to supply the total load power. This improves the system reliability through structural redundancy. Improper load sharing, however, leads to overloading of some of the source converters which might result in cascaded failures.

Several stability criteria are proposed in the literature. Among all, the impedance-based approaches are well accepted for stability analysis and stabilizing controllers design. These methods are based on evaluating the system impedances using linear control theory and small-signal dynamic analysis. So, using such methods, stabilization is accomplished in an intuitive and design-oriented manner. However, an important disadvantage of linear methods is that their range of effectiveness is limited to a small-signal region around an operating point of the system wherein the non-linear system can be approximated by a linear one. Likewise, DC-PDS often experience large-signal transients and operating point variations. Thus, linear controllers may fail to preserve the stability and performance for large-signal transients. Therefore, there is a need to develop new methods that guarantee system stability and performance during such large-signal transients.

To solve the problem of load power sharing in DC-PDS, various methods can be found in the literature. Load sharing mechanisms can be categorized as Droop methods and active sharing techniques. In the conventional Droop method, a virtual resistance is added to the output impedance of the source converter and a decentralized load sharing is achieved. Although simple and effective, Droop control causes a variable bus voltage drop which requires additional control measures to achieve tight voltage regulation. Active methods, on the other hand, manage to achieve load sharing at the cost of additional control requirements such as high bandwidth communication links among the source converters which increase the complexity and cost. Thus, it is desirable to develop new methods to solve the problem of proper load sharing in a simple, efficient, and inexpensive manner.

To address the above challenges, in this dissertation, a generic DC-PDS is considered and the system dynamics is studied for small-signal and large-signal operations. Based on this analysis, novel stabilizing control methods are proposed that are implemented in a source converter. The proposed approach manages to guarantees stability and performance for various operating scenarios. Additionally, to solve the load-sharing problem, a novel communication-less current-sharing control scheme is proposed. This method guarantees proper distributed load sharing among several source converters without any bus voltage drop and requiring any physical communication network.

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