PhD Oral Exam - Ahmad Malkawi, Electrical and Computer Engineering
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
The existing power system is based on a centralized approach. Large power plants produce AC electricity that is transmitted over long distances for distribution to the consumers. To meet a higher load demand, the entire system has to be upgraded, which is costly and acquiring rights of way can take decades. Another approach, called distributed generation, is the deployment of smaller generation units closer to the users. This can be based on renewable energy sources (RESs) that mitigate the environmental impact of power generation. However, the stochastic nature of RESs can lead to power quality issues in the distribution system. This can be addressed with the addition of energy storage units and controlling the system as a cluster or a microgrid.
This concept can be extended for small buildings and residences, called nanogrids, offering a means for the realization of net-zero energy homes (NZEHs). These can be AC or DC, but the latter looks more promising since most RESs suitable for NZEHs provide a DC output and DC-DC interfaces tend to present a higher efficiency than their DC-AC counterparts. DC nanogrids also favor the integration of electric vehicles (EVs) and are compatible with modern, electronically controlled, appliances. To date, there are no standards concerning the number of buses and voltage levels of DC nanogrids. The control structure of DC micro and nanogrids, can be based on a hierarchical approach where the primary control level relies on locally measured quantities. This allows a decentralized operation of interfaces using the DC bus voltage as a communication means and V vs. I curves, with specific parameters, for coordination of operation, a method known as DC bus signaling (DBS).
There are several aspects of DC nanogrids for NZEHs that deserve further investigation and are addressed in this thesis. These include a means for a smooth transition of the modes of operation of RESs, such as photovoltaic (PV), which employ V vs. I curves with three regions. This can minimize the DC bus voltage variations as the system adjusts to variations in load demand and power generation due to varying solar irradiances. The use of supercapacitors (SCs) along with batteries in hybrid energy storage systems (HESSs) can mitigate the impact of high and fast current variations on the losses and lifetime of the battery units. However, by controlling the HESS as a single unit, one forfeits the potential contribution of the SC and its high power capabilities to dynamically improve voltage regulation in a DC nanogrid. This can be achieved by controlling the SC and battery independently without sacrificing the support the battery receives from the SC. Finally, although dual DC bus nanogrids have been advocated by industry associations, they are conceived to have power sources and storage units only in the high voltage (HV) bus. The low voltage (LV) bus is fed through a unidirectional converter, making it vulnerable to a fault in a single element. This thesis proposes the deployment of generation and storage in both buses, with a bidirectional interface for optimizing power balance in both buses. The techniques proposed in this thesis are verified by means of simulation or experimental results.