A Modular IGBT Based Current Flow Controller for Multi-terminal HVDC Grids

Offshore wind turbines are preferred rather than onshore ones for their numerous advantages, such as land saving, higher wind speeds and higher power generation. However, AC power transmission would fail to deliver the generated power economically over distances longer than 80 kilometres using submarine cables. The more feasible option is to use High Voltage DC (HVDC) power transmission for offshore wind generation. Unlike AC transmission systems that have established power and current flow control methods, DC power transmission systems have only reliable power flow control techniques for point to point systems, which makes it one of the challenges preventing realisation of Multi Terminal HVDC grids (MT-HVDC) as cables may be subjected to higher currents causing overloading and thermal problems. Different HVDC power flow control schemes are suggested by controlling the AC/DC converters such as voltage droop control and voltage margin control. Other methods of power and current flow control based on the connection of new power electronic equipment to the grid have been also proposed.
This thesis presents operation and control of an IGBT based Current Flow Controller (CFC) for MT-HVDC grid applications. The CFC is studied in its preliminary two-port configuration and possible modes of operation and dynamic models are produced. An extended topology is proposed to allow the CFC to be connected to more than two cables at a time. Although the proposed extended CFC topology is simple in construction and gave acceptable results in most case studies, it has shown some drawbacks in certain case studies where controlled currents have significant differences in magnitudes. To resolve this problem, a generalized Modular CFC (MCFC) topology is proposed which allows each current to be controlled independently and overcome the extended topology’s drawback. Moreover, a reduced count switch count topology is proposed which reduces the MCFC cost by half in cases of unidirectional current flow control.
All proposed control strategies and topologies are validated using both computer simulation through MATLAB/SIMULINK and PSCAD/EMTDC software packages and experimental validation through Rapid Control Prototyping (RCP) with the aid of Opal RT real time simulator. Studies carried throughout this thesis show that the proposed MCFC may play an important role in current flow control applications in MT-HVDC grids due to its low cost, small footprint and accurate performance.