DC/DC Converters for Multi-terminal HVDC system for Integrating Offshore Wind Farms
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- Institutt for elkraftteknikk 
The development of far-offshore wind farms and other large-scale renewable energy sources, together with the increasing needs for long distance power transmission is resulting in more HVDC systems being integrated into the traditional AC power network. Due to the capability for operation in isolated AC grids, VSCs are becoming the preferred technology for HVDC systems. Additionally, VSC HVDC allows for more flexible power control within a network than the conventional LCC systems. Moreover, the need for flexible transmission capacity to balance fluctuating power generation from renewable sources over wide geographical areas combined with the corresponding potential benefits in a deregulated power market is expected to favour the concept of VSC based Multi-terminal HVDC system (MTDC). MTDC provides enhanced reliability and functionality and reduces the cost and conversion losses. However, most HVDC transmission schemes are currently constructed as point-to point connections, and there is not yet any clear standardization of voltage levels. Thus, DC/DC converters will become necessary if existing or emerging HVDC links operating with different voltages and different configurations. i.e. monopolar and bipolar should later be interconnected into MTDC configurations. DC/DC converters might also be needed for power flow control in meshed MTDC grids. The goal of this thesis is to summarize the requirements for DC/DC converters in HVDC applications and focus on the modelling and control of DC/DC converters for various applications. The modelling includes both switching and average models. Four types of DC/DC converters intended for different applications are modelled using MATLAB/ Simulink® platform. The functionality of the developed models are demonstrated by simulations in MTDC grid based on the CIGRÉ B4 DC grid test system.