Supervisor:
Professor Josep M. Guerrero

Co-Supervisor:
Professor Juan C. Vasquez
Associate Professor Baoze Wei

Assessment Committee:
Associate Professor Jayakrishnan Pillai, AAU Energy (Chair)
Professor Olav Bjarte Fosso, Norwegian University of Science and Technology (NTNU)
Associate Professor Muzamir Isa, Universiti Malaysia Perlis (UniMAP)

Moderator:
Associate Professor Sanjay Chaudhary

Abstract:

Electric ships are obtaining much attention in reducing greenhouse gas emissions and achieving clean water transportation. In most cases, shipboard power systems are regarded as shipboard microgrids (SMGs) due to their islanding operation. The concept of DC SMG is popular in academia and industry since it has many merits compared with the AC counterpart, such as reduced fuel consumption, flexibility in generation integration, and compact system design. Some research on DC SMGs has been done in recent years, while there are still several open questions for high-power, efficient, flexible, and reliable ship operation. Thus, this thesis introduces a comprehensive overview of medium voltage DC (MVDC) SMGs, presenting the system configuration and control issues and highlighting the technical particularities compared to microgrids in other applications. In light of the investigation, two topics are focused on in this thesis: the proper control methods on the generation side and the power supply subsystem for pulsed power loads (PPLs), which is a particular load type in ships.
The overview of MVDC SMGs presents the hardware configuration of electric ships based on the DC network and the associated system control issues. The general system architecture and power components are reviewed and analyzed, and then the suitable applications of each type are discussed. Regarding control issues, the overall control schemes for the generators and the energy storage systems (ESSs) in ships are presented. Besides, due to the stability issues as well as the protection systems in MVDC SMGs are still rarely studied, particular concerns on these two topics for maritime application are discussed.
The coordinated control of synchronous generators and ESSs in electric ships is developed. The DC network enables variable-speed operation of generators, therefore allowing the optimization of the generator speed to achieve different objectives, which is managed in the energy management system (EMS). Additionally, the integration of ESS makes it flexible in power sharing. However, these studies do not consider the cases of the ships operating in different operation modes under various load conditions. To fill this research gap, the coordinated control should be well designed to fit each mode. This thesis presents a coordination method for synchronous generators and batteries. In the proposed approach, hybrid-electric and all-electric operation modes are considered to meet different operation scenarios, and flexible mode switching can be achieved. Besides, the state of charge balance between battery packs is considered to avoid overcharge and over-discharge of batteries.
The power supply for PPLs is a challenge in DC SMG control, as the PPL absorbs enormous transient power and requires a fast dynamic response. Shipboard PPLs could be high-power pulse radars and electromagnetic railguns, which are vital for naval vessels. The confidentiality of military applications makes it difficult to obtain the technical details of existing PPLs. This thesis studies the impacts of the PPL parameters on the bus voltage and based on these, the control method of a pulsed power supply (PPS) system is developed. An improved approach based on sliding mode control (SMC) is adopted. Simulation results verify the effectiveness in terms of dynamic response and robustness.
In summary, the contributions of this Ph.D. project aims to explore the overall configuration and control of DC SMGs, and then present coordinated control strategies for generators and ESSs as well as the control of the power supply subsystem in the presence of high power PPLs. These works are anticipated to pave the way for the broader implementation of MVDC SMGs in industry and naval applications.