Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Electrical and Computer Engineering

Supervisor

Badrkhani Ajaei, Firouz

2nd Supervisor

McIsaac, Ken

Co-Supervisor

Abstract

The direct current (DC) microgrid has attracted great attention in the recent years due to its significant advantages over the alternating current (AC) microgrid. These advantages include elimination of unnecessary AC/DC power converters, lower investment cost, lower losses, higher reliability, and resilience to utility-side disturbances. A practical DC microgrid requires an effective control strategy to regulate the DC bus voltages, enable power sharing between the distributed energy resources (DERs), and provide acceptable dynamic response to disturbances. Furthermore, when the power demand of the loads is higher than the power generation of the DERs in the DC microgrid, the power balance cannot be maintained by control actions and the DERs fail to regulate the DC bus voltages. Under such conditions, it is necessary to shed some of the non-critical loads in order to protect the integrity of the DC microgrid. Thus, the DC microgrid also requires an effective load shedding scheme.

This thesis is focused on developing advanced control and load shedding strategies for integrity protection of the DC microgrid. The studies reported in this thesis include developing (i) a versatile DC bus signaling control strategy to achieve coordinated decentralized control of the DERs and loads in the DC microgrid without utilizing costly high-bandwidth communication systems, (ii) an improved mode-adaptive droop control strategy to enable desirable and reliable control mode switching by the DERs under various operating conditions, and (iii) adaptive non-communication based load shedding schemes to enable the DC microgrid to ride through the disturbances that cause large power deficit and voltage sags.

The performances of the proposed integrity protection schemes are investigated under various generation and load disturbances in both grid-connected and islanded operation modes of the DC microgrid. Comprehensive time-domain simulation studies are conducted on a detailed DC microgrid study system using the PSCAD/EMTDC software. The study results indicate that the proposed control strategies: (i) improve power sharing between the DERs, (ii) effectively regulate the DC bus voltages under various operating conditions, (iii) improve the DC microgrid stability and its dynamic response to large disturbances, (iv) do not require an excessively large grid-tie converter or energy storage systems, and (v) enhance the DC microgrid reliability, flexibility, modularity, and expandability.

The study results also indicate that the proposed adaptive load shedding schemes (i) effectively maintain the power balance in the DC microgrid through fast and coordinated shedding of non-critical loads, (ii) prevent the bus voltages in the microgrid from falling below predetermined lower limits, (iii) ensure that the critical loads do not experience excessive steady-state voltage deviations, (iv) minimize the magnitudes and durations of temporary voltage sags caused by sudden disturbances, and (v) increase the reliability of the power supplied to the loads, by preventing over-shedding.

Summary for Lay Audience

The microgrid is an emerging technology that facilitates the integration of distributed energy resources (DERs) in power distribution networks, reduces the energy losses, and improves the quality and reliability of the electrical energy supplied to the consumers. In the recent years, the direct current (DC) microgrid has attracted great attention compared to the alternating current (AC) microgrid. The reason is that the majority of the DERs, e.g., photovoltaics (PVs), fuel cells, and battery energy storage systems (BESSs), provide DC power, and an increasing portion of the emerging loads require DC power, e.g., electric vehicles (EVs), consumer electronics, and LED lighting systems. The DC microgrid offers significant potential advantages over its AC counterpart. These advantages include (i) lower investment cost and power conversion losses due to elimination of unnecessary power converters, (ii) lower cable losses due to absence of skin effect, (iii) higher reliability and resilience to utility-side disturbances, and (iv) elimination of the need for frequency, phase, and reactive power controllers. Hence, the DC microgrid is becoming a popular solution for many applications such as data centers, telecommunication stations, shipboard systems, EV charging stations, smart homes, commercial buildings, and renewable energy parks.

A practical DC microgrid requires effective control and load shedding strategies to protect the integrity of the DC microgrid under disturbances. This thesis is focused on developing advanced control and load shedding strategies for integrity protection of the DC microgrid. The studies reported in this thesis include developing (i) a versatile DC bus signaling control strategy to achieve coordinated decentralized control of the DERs and loads in the DC microgrid without utilizing costly high-bandwidth communication systems, (ii) an improved mode-adaptive droop control strategy to enable desirable and reliable control mode switching by the DERs under various operating conditions, and (iii) adaptive non-communication based load shedding schemes to enable the DC microgrid to ride through the disturbances that cause large power deficit and voltage sags.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Available for download on Sunday, March 01, 2020

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