Multilevel Converters for Offshore Wind Systems: A comparative study

Offshore Wind systems have recently gained popularity. However, as the distance from shore increase, different challenges arise. The use of HVDC link to shore helps to overcome most of the challenges. Current Source converters have long been used as HVDC converters despite of their large footprint and reactive power consumption challenges. Recent development in the converter led to the use of Voltage Source Converters (VSC). There are a number of VSC topologies proposed by the industry and academic researchers. The purpose of this thesis was to compare the dominant converter topologies and identify their merits. The work was done in three phases. Phase I was initial study and literature review where three dominant topologies were selected at the end. Two level VSC, Three level NPC, and Modular Multilevel Converter (M2C) were selected at the end of Phase I. Phase II involved detailed modelling and simulation of the selected converter topologies. The simulation was mainly focused on harmonics and losses. The M2C was found to produce lower harmonics and losses under the same operating conditions and using the same switching device. The M2C also exhibits modular design which gives it scalability. This is critical in todays power system which changes very frequently. The M2C was selected for next phase analysis because of the above and a number of other reasons. Phase III was the final part which included practical work on the selected topology. This task was undertaken on a small scale setup of M2C with 3 cells. The purpose of the setup was to verify theoretical results obtained from simulation. The results showed good correspondence with the ones obtained from simulation. The overall study pointed in the direction of M2C for future HVDC installations despite of the capacitor ripple and complex control challenges.

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