A Lightweight DC Collection and HVDC Transmission System Equipped with DC-DC Converter for Offshore Wind Farms

摘要

In recent years, high voltage dc (HVDC) transmission system is applied to some offshore wind farms. HVDC transmission is suitable for long distance bulk power transmission over submarine cable comparing to the ac transmission. One difficulty in the offshore HVDC system is building a large offshore platform to sustain an ac-dc converter station and heavy step-up transformers. A small light-weight offshore substation is necessary to reduce construction costs for the HVDC system. This paper focuses on a novel system configuration to minimize the offshore facilities in the offshore wind farms. The system applies medium voltage dc (MVDC) for the power collection from each wind turbine to the offshore substation. It needs only one insulated cable for power collection whereas conventional three-phase ac system needs three insulated conductors. In addition, the MVDC is stepped up to the HVDC on the offshore substation not using bulky and heavy transformers but a modular dc-dc converter. The modular dc-dc converter consists of 40 submodules. They are connected in series in their input side and in parallel in their output side in order to boost the dc voltage. Each submodule has a small and lightweight high frequency transformer for insulation, and size of the submodule is minimized. Because the system does not need bulky low frequency transformers, it can realize small and lightweight offshore substation. The control method for the system has been proposed in this paper in order to realize maximum power generation in steady states and continuous operation during grid faults. Appling a proposed control method to the dc-dc converter, the converter can coordinate power flow from the power conditioners to the HVDC inverter. Compared to the conventional ac collection system, the proposed MVDC collection system has advantages in realizing continuous operation under grid faults and output suppression of the wind farm. Simulation results verify the appropriate operating performance of the proposed method.