Research On Some Problems Of Offshore Wind Farm DC Interconnection System

In the 21st century,environmental problems once again become a global concern.Smog problem has attracted more attention,becomes the focus of public.Reform of electric power and adjustment of energy structure is imperative.In the Thirteenth Five-Year Plan of People's Republic of China,government will adjust the structure of energy sectors,to 2020,the proportion of non-fossil energy sector will increase to 15%,increased to 20%in 2030.With the big resource reserves and the characteristics of geographical dispersion,wind energy becomes a high-profile way of generating new energy field,especially offshore wind energy.China has a vast maritime territory and coastline,so for the study of the offshore wind in a steady growth trend in recent years,which has the significance to the country.The paper puts forward Modular Isolated Direct Current Converter(MIDC)for the offshore wind farm,meanwhile,Modular Multilevel Converter(MMC)has been applied to the system.For offshore wind power grid system,the working principle and state of each part were analyzed.The paper has built a topology structure and described the topology selection for the proposed MIDC converter.Paper mainly studies sub-module failure problems,puts forward the sub module fault redundancy protection strategy and three kinds of redundancy mode,paper analyzes the redundancy protection working conditions.Combination of offshore wind power grid maintenance inconvenience,the paper puts forward the strategy to limite the numbers of the fault sub-modules,mathematics models,at the same time,simulation and verification in PSCAD for sub-module faults.And considering the build system is mainly suitable for the large capacity,high voltage of offshore wind power grid system,so also do the related researchs for shore inverter side,adopts the modular multilevel converter as the main structure of inverter side.Due to the high voltage Level,therefore the paper mades improvement in the traditional Nearest Level Modulation(NLM).The paper puts up the mathematical model based on the modular multilevel inverter structures,and the improved strategy which can restrain the double frequency circulation,at the same time,the steady state and transient simulation in PSCAD.Also,because the offshore wind power grid system away from the coast,thus requiring high reliability and stability.The paper wants to reduce the fault current to the harm of power electronic devices,added Fault Current limiting(FCL)inverter side to the shore inverter side,the paper analyzes the influence to system of adding FCL,builds the mathematical model,illustrates the working principle of FCL,simulation and verification in the steady state,transient state and fault conditions by PSCAD for the inverter side.Finally,the paper introduces the 2250W prototype model with nine sub-modules,introduces the circuit structure one by one section and related devices selection,meanwhile,the paper introduces the control program of DSP,then,some experiments have been finished with prototype model.