Three Stage Power Electronic Transformer based MVAC Collection System and its Control System Design for Offshore Wind Power Generation Mills

Full-scale Power Electronic Transformer (PET)-based Wind Power Generation Mills (WTPGMs) become more attractive due to the advancements in power electronics. Typically for cell voltage/power balancing, large number of sensors, and complex control circuitry make control design and implementation challenging, resulting in lower system reliability. This paper proposes a simple yet effective closed-loop control architecture and integrated dynamic modelling of three-stage (LV AC-DC/DC-DC/MV DC-AC) PET-based MVAC collection system for offshore WTPGMs. The proposed control directly regulates nacelle DC-link voltage from MV-stage, irrespective of wind conditions. It eliminates the voltage sensors at CHB cells and nacelle DC-link. The proposed technique achieves natural cell voltage balancing in MV-stage, and it is independent of the number of cells. The proposed modelling incorporates the effect of DC cable parameters and allows to implement entire control algorithm using single voltage sensor on common LVDC-link. Additionally, the proposed solution reduces maintenance costs and complexity by installing the converters appropriately. This configuration of the PET has never been used in the existing MVAC collection systems. The proposed scheme is experimentally validated using a 1.2kV(MVAC)/400V(LVDC)/200V(LVAC), 7.2kW PET-based prototype system. The reliability and stability of entire system are also verified during worst-case wind scenarios.