Date of Award

2016

Document Type

Open Access Thesis

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Enrico Santi

Abstract

Due to recent advances in power electronics technology, DC power distribution systems offer distinct advantages over traditional AC systems for many applications such as electric vehicles, more electric aircrafts and industrial applications. For example, for the All-Electric ship proposed by the U.S. Navy the preferred design option is the adoption of a Medium Voltage DC power distribution system, due to the high power level required on board and the highly dynamic nature of the electric loads.

These DC power distribution systems consist of generation units, energy storage systems and different loads connected to one or more DC busses through switching power converters, providing numerous advantages in performance and efficiency. However, the growth of such systems comes with new challenges in the design and control areas. One problem is the potential instability caused by the interaction among feedback-controlled converters connected to the same DC bus.

Many criteria have been developed in the past to evaluate system stability. Additionally, passive or active solutions can be implemented to improve stability margins. One previously proposed solution is to implement Positive Feed-Forward (PFF) control in the load-side converter; with this technique it is possible to introduce a virtual damping impedance at the DC bus. A recently proposed design approach for PFF control is based on the Passivity Based Stability Criterion (PBSC), which analyzes passivity of the overall bus impedance to determine whether the system is stable or unstable.

However, since the PBSC does not provide direct information about system’s dynamic performance, the PFF control design based on PBSC might lead to lightly damped systems. Therefore, a disturbance in the system may result in long-lasting lightly damped bus voltage oscillations. Moreover, in order to study the system dynamic performance it is necessary to know the bus impedance. A method has been proposed that uses digital network analyzer techniques and an additional converter that acts as a source for current injection to perturb the bus.

The present work provides original contributions in this area. First of all, the effect of the dominant poles of the bus impedance on the system dynamic performance is analyzed. A new closed-form design procedure is proposed for PFF control based on the desired location of these dominant poles that ensures a desired dynamic response with appropriate damping.

Regarding bus impedance identification using a switching converter for perturbation injection, a new technique is proposed that eliminates the need for an external converter to provide the excitation. The technique combines measurements performed by existing converters to reconstruct the overall bus impedance. Additionally, an improved perturbation technique utilizes multiple injections to eliminate the problems of injected disturbance rejection by the converter feedback loop at low frequency and the problem of attenuation due to reduced loop gain at high frequencies.

The proposed methods are validated using time domain simulations, in which the bus impedance of a single-bus DC power distribution system is estimated and then utilized for the design of a PFF controller to improve the dynamic characteristics.

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