Doctoral Dissertations

Date of Award

12-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Electrical Engineering

Major Professor

Fei Wang

Committee Members

Leon M. Tolbert, Hua Bai, Yiwei Ma

Abstract

In this dissertation, the modeling, design, and control of power electronic converters are conducted for the unbalanced loads and groundings in the microgrid.

First, from the system planning perspective, a detailed model for a grid-forming inverter is proposed considering the control impacts for unbalanced power flow analyses. The proposed model is applied for the unbalanced power flow analysis in the IEEE 13-bus and 34-systems. The results demonstrate that the proposed model can achieve a significant increase in the negative sequence (NS) voltage calculation accuracy.

Second, a comprehensive analysis of NS current distribution among different grid-forming sources is conducted. The analysis considers different grid-forming source types and control implementations. The analysis is verified through simulation and experimental testing. A NS impedance design approach is proposed to realize the NS current distribution among different grid-forming sources (inverters and generators) in a microgrid with multiple source locations considering the topology impacts. The proposed method is demonstrated through simulation and experimental testing on a converter-based hardware testbed. To verify the proposed approach, a novel synchronous generator emulator is also proposed considering the unbalanced load impacts.

Third, to deal with the potential loss of grounding in the islanded microgrid, two novel grounding schemes are proposed, which are an inverter-based grounding scheme and a controllable DER transformer-based grounding scheme. Simulation and experimental demonstration of two grounding schemes are also conducted.

Then, the unbalanced load impacts on the power conditioning system (PCS) for a transformer-less asynchronous microgrid are analyzed. The PCS utilizes the modular multi-level converter (MMC) topology. Based on the analyses, the design and control solutions for the MMC are proposed and demonstrated on an actual 10 kV SiC MOSFET-based MMC up to 25 kV dc-link voltage.

Moreover, the unbalanced load impacts on DER’s interfacing converter are analyzed. Based on the DER’s ripple capability, the control strategy is proposed for the dc/dc stage to eliminate or partially support the second-order frequency current from the unbalanced load. The design of the dc-link capacitors is also conducted. The dc-link capacitor design and the proposed control algorithms are verified through simulation and experimental testing.

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