Analysis of modular multilevel converters for HVDC connection of offshore wind power plants

Commercial offshore Wind Power Plants (WPP) started in 2002 and the number of installations increases each year, as well as their distances from the coast. For projects more than 70 km far from the coast, the trend is to use HVDC systems, since, in this way, problems with reactive power losses on the insulated cables are avoided. In addition, Modular Multilevel Converters (MMC) are strong candidates to be used in these HVDC systems, since they overcome the main limitation of Line Commutated Converters (LCC). This paper presents a dynamic model for the MMC as well as describes the AC grid forming rectifier and the DC Bus regulator inverter controllers required to implement an offshore HVDC link. The internal control of MMC variables is carried out using the energy concept which simplifies the controllers design. Finally, simulation results are presented to validate the theoretical analysis.