Project

To transfer energy from collected offshore wind farms over a long distance, HVDC transmission is preferred over HVAC in terms of efficiency and economy. Several multi-stage configurations have been proposed in the literature. However, the multi-stage configuration generally results in a large size due to a large number of conversion stages, relatively high cost, and low efficiency and power density. Also, the independent control of several converters and communication among the sources make the system complex. To overcome these disadvantages, multi-port modular DC/DC topologies have been suggested. Multiport converters are highly non-linear MIMO systems with many control variables. Also, the coupling between the control variables makes modeling and control system design complicated. Despite such complexity, advanced control techniques have not been comprehensively studied. Moreover, most controller design work on multiport converters has not considered the uncertainties of the converter model. In this Ph.D. study, a robust controller is implemented for multi-port modular DC/DC converter for offshore wind farms application.

HVDC offshore wind farms with MVDC power collection have recently aroused researchers' interest as these systems offer lower losses and fabrication expenses. Numerous potential MVDC converters could be used in the power collection stage of offshore wind farms; however, when it comes to the technology level, these DC/DC converters are still immature since no substantial studies concerning their control exist. Thus, this Ph.D. project aims to address the research gap to enhance the performance as well as the efficiency of an MVDC converter. The novel switching and control technique proposed in this project together with the significant features of wide bandgap switches provide the condition based on which the MVDC converter could operate at higher switching frequencies than what is already possible. Hence, the controlled MVDC converter will be smaller in size and lighter in weight compared to the conventional ones which reduces the LCOE and provides better possibilities for modularity.

The aim is to obtain knowledge of nonlinear loads which could cause unwanted dynamics or stability issues for the DC-microgrid. This leads to the investigation of the nonlinear loads: Constant Power Loads and Reversible Solid Oxide Cells.
The design of a suitable DC-DC converters and control systems are investigated to fulfill performance requirements and mitigate stability issues.