| With increasing interest in the integration of renewable energy and energy storage systems into the electric grid, DC interconnections are attracting interest as a versatile solution to realize wind farm systems. This concept has received attention towards the application of large wind farm interconnections as a DC collection network offers several advantages over traditional AC technology. A wind farm whose turbines interface with a DC collection bus can exhibit improvements in overall farm efficiency, lower installation costs and provide a platform which is more readily scalable than AC technology. However, such a system requires appropriate power converters at tens to hundreds of MW scale to interface between generators typically rated at several kilovolts AC, the utility grid at several hundred kV and a medium voltage DC collection bus at several tens of kilovolts. While well established power converter topologies to meet this task have significant drawbacks, the recently developed modular multilevel converter (MMLC) of the bridge of bridge converter (BoBC) family has the potential to overcome these drawbacks and provide considerable advantages. This thesis presents an examination the BoBC from the point of view of power semiconductor requirements, reactive component requirements, operating losses, fault tolerance, multi-terminal operation, modularity, complexity, etc. The ad-hoc BoBC circuit topology presented in the literature is extended to a systematic and complete description that enables the application of canonical analysis tools for switching power converters, leading to an elegant cascaded boost-buck converter model. An appropriate modulation and control approach based on the analytical model is proposed for the BoBC. Using this controller, a candidate wind farm utilizing a chain configured DC interconnection realized with BoBCs is presented along with suitable AC droop based controls and impedance matching based stability evaluation. The analytical design methodologies are verified with detailed computer simulations and laboratory scale experimental results. |
