Low harmonics offshore hybrid HVDC system based on Vienna Rectifier and voltage source converter with back-up capability

High Voltage Direct Current (HVDC) transmission systems have been extensively discussed as a method to transmit wind power from offshore to onshore grids. However, high capital costs, large footprint and concerns about reliability and maintenance have hindered the adoption of HVDC technology in offshore environments. A hybrid offshore HVDC converter based on a Vienna Rectifier topology in series with a fraction power Voltage Source Converter (VSC) is proposed in this paper to address these challenges. Using this configuration, the offshore system can be operated at lower cost, with low harmonics emission and complexity, and with higher reliability for the offshore system. In addition, thanks to its multifunctional control characteristics, the proposed hybrid topology is also capable of maintaining a controlled and harmonic-free power flow in the offshore network in the event of a failure of the Vienna-rectifier high-power switch. The proposed solution is verified for its performance using PSCAD/EMTDC simulations for both steady-state and dynamic conditions. The obtained results are compared to other low-cost offshore HVDC systems and are used to discuss the advantages and disadvantages of the proposed solution.