Grid Integration of Offshore Wind Farms using Hybrid HVDC Transmission: Control and Operational Characteristics
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- Institutt for elkraftteknikk 
The transmission of offshore wind energy is one of the most relevant challenges since the best wind conditions are located far from shore. High-voltage DC (HVDC) is the most feasible option for the grid integration of offshore wind farms if the distance to shore is relatively long. Currently, there are two different HVDC technologies: current source converter (CSC)-based HVDC and voltage source converter (VSC)-based HVDC. Among CSCs, the line-commutated converter (LCC)-based HVDC is a well established technology around the world. Few studies have been focused on the application of CSC to integrate offshore wind farms to the main grid. In particular for offshore applications, LCCs have some limitations, namely: the relatively large footprint, and the external commutation voltage required for proper operation. Nevertheless, LCC HVDC transmission has attractive features as well such as higher power capability, lower power losses, and lower costs compared with the other HVDC options. Most of the studies on HVDC for the grid integration of offshore wind turbines have been focused on the VSC technology. VSC uses selfcommutated devices which gives attractive features: independent control of active and reactive power, ability to supply passive grids or weak grids, and ability to operate in island mode. Moreover, VSC has a relatively small footprint. There are also CSCs with self-commutating devices, denoted in this work as pulse-width modulated CSC (PWMCSC), which has the aforementioned features of VSCs plus a bonus, an inherent short-circuit protection capability. This thesis is mainly focused on the application of the combination of CSC and VSC to connect offshore wind turbines to the grid. Two hybrid HVDC alternatives are proposed: The first is a hybrid between a VSC and an LCC. The potential benefits of this concept are lower power losses and costs compared with the solution based only on VSC. There are operational constrains, such as start-up capability and AC fault sensitivity. On the one hand, the start-up of this hybrid HVDC cannot be carried out as a VSC HVDC since the direction of power flow cannot reverse easily. On the other hand, an LCC is inherently susceptible to any lowering of AC voltage, this limitation is inherited by this concept. Simulations results show that these operational constraints can be overcome. The second alternative is a hybrid between a PWM-CSC and an LCC. The potential benefits of this concept include: low power losses, simple AC voltage control and good response to AC and DC faults. This hybrid topology takes advantages of self-commutated converters as well as line-commutated converters. LCC has lower power losses, lower costs and high tolerance to DC faults than VSC, these features remain the same in the hybrid approach. Its force commutated converters has high controllability in the AC side, it has reactive power control capability and good performance to AC faults. Therefore, the proposed hybrid presents a better response to DC faults compared to VSC-based HVDC transmission systems and a better response to AC faults compared to LCC-based HVDC transmission systems. In addition, the resulting AC voltage controller is relatively simpler than the well-known two loops controller used for VSC. The reason behind this characteristic is that the control variable is an AC current in a PWM-CSC instead of an AC voltage. Moreover, the PWM-CSC and LCC are both current sourced converters and therefore the coupling between these technologies can be done effortlessly. The operational characteristics of the two alternatives are studied, and the corresponding control strategies are designed and verified through numerical simulations under various conditions such as wind speed variations and faults on AC and DC sides.