Research On DC Delivery System For Offshore Wind Farm

With development of the large-scale offshore wind power,the offshore wind farms are set far away from the mainland,and the distance of power delivery is also increased.The conventional AC transmission is not suitable for offhsore wind power system due to high reactive power of long-distance submarine cables,so the DC transmission becomes a better option for long-distance power delivery for large offshore wind farms.On the other hand,with the increase of the rated power of an offshore wind turbine and the radius of the AC collection system,the all-DC system with DC collection and DC transmission has become a promising solution for the large-scale offshore wind farms.This work focuses on the key technologies of DC transmission with AC collection system and all-DC systems respectively.For an offshore wind farm with an AC collection grid and a DC transmission system,the modular multilevel converter(MMC)has a large number of submodules,high volume,high weight,and complex control systems,which results in high cost.To overcome the problem,this work presents a cost-effective hybrid converter which combines a diode rectifier and a paralleled auxiliary converter.In the normal operation,the active power is delivered by the diode rectifier,and the auxiliary converter controls the AC voltages as well as compensates the reactive power and harmonic currents.This solution highly reduces the submodule number and the cost,and the relibility and efficiency of the system are also improved.The operation and control of this system is analyzed comprehensively.For the large-sacle offshore wind farm,the dc collection grid is applied,and the wind power converter for the DC grid is the key technology.In this work,the MMC is proposed for the high step-up AC/DC power conversion as the DC wind pwer converter,and a novel topology BMMC and its control scheme(Bifurcate Modular Multilevel Converter)are proposed.The BMMC is designed for low-modulaltion-ratio applications,and it has lower submodule capacitance.The operation and fault control of the all-DC system is analyzed.To prove the effectiveness of proposed converters and systems,related models are built on simulation platform and verified.In addition,a flexible experiment platform is also built for system verification.