DRU Based Low-Cost Mid-and Long-Distance Offshore Wind Power Transmission System

The development potential of offshore wind power is huge,while the required offshore wind power transmission distance and capacity can be hardly satisfied by the traditional high voltage alternating current transmission technology as the offshore wind farm sites extend to far deep sea areas.High voltage direct current(HVDC)and low frequency alternating current(LFAC)transmission systems have the capability of mid-and long-distance offshore wind power transmission,while the economy and reliability of either scheme need to be further investigated in the actual project planning.On the one hand,the HVDC scheme and the LFAC scheme have to add the offshore converter station and the onshore AC/AC converter station respectively,increasing the terminal capital cost.On the other hand,more significant impacts on the land grid from the offshore wind power transmission system fault ride-through(FRT)performances may be observed as the grid-connected capacity increases.In view of the economic and reliability problems mentioned above for mid-and long-distance offshore wind power transmission system,following studies are carried out in this paper:(1)The diode rectifier unit(DRU)based low-cost offshore wind power transmission schemes are proposed.The basic principle of DRU is firstly introduced,including the converter topology and the steady-state mathematical model.Then,the LFAC transmission scheme with an AC/AC converter of the DRU-MMC topology is proposed.The topological characteristics and the main circuit parameters calculation method for DRU-MMC are elaborated.Meanwhile,the DRU-HVDC transmission scheme with medium frequency AC collection is proposed.The HVDC transmission system miniaturization mechanism based on the offshore DRU converter station and medium frequency AC collection system is analyzed.Finally,the cost advantages of the proposed low-cost offshore wind power transmission scheme are illustrated by the economic analysises in case studies.(2)A grid-forming control strategy suitable for permanent magnetic synchronous generator based wind turbine is proposed,which supports the stable operation of the proposed low-cost offshore wind power transmission system.Firstly,the operating principle of grid-forming controlled wind turbine is introduced,and the selection basis for grid-forming controlled wind turbine type is analyzed according to the application scenario.Then,the control system design of the grid-forming controlled wind turbine is elaborated in detail.Then,the machine side converter should maintaining the DC link voltage,while the grid side converter needs to implement both the maximum power point tracking control and the offshore AC system voltage/frequency control.Further,the existence of the system steady-state operation point is analyzed,and tbe application of wind turbine grid-forming control strategy into the DRU based low-cost offshore wind power transmission system is validated in theory.Finally,the effectiveness of the proposed control strategy is verified by electromagnetic transient simulations.(3)An active MMC based fault ride through strategy for the low-cost offshore wind power LFAC transmission system is proposed.The converter topology of active MMC is firstly introduced.And the fault ride through principle of the active offshore wind power transmission system is also explained.Then,the mathematical model of the active MMC is derived and the equivalent circuit model is estabilshed.On this basis,the designed control system is elaborated,where the active MMC based bi-directional AC fault isolation strategy and the state of charge(SOC)balance control strategy among energy storage units are proposed.Finally,the proposed scheme is further validated by electromagnetic transient simulations.(4)The full-state arm average value model(FSAAVM)of the active MMC is proposed to support high-efficient electromagnetic transient simulation of the active offshore wind power transmission system.Firstly,the converter arm average value mathematical model is derived.The equivalent circuit models for different operating states of the active MMC are also obtained.On this basis,the component discretized companion modeling theory is utilized to establish the FSAAVM which supports electromagnetic transient simulations of the active MMC under different operating conditions.Finally,the active offshore wind power LFAC transmission system is considered in case studies,where the simulation results derived from the FSAAVM and other typical modeling method are compared to verify the accuracy and efficiency of the proposed modeling method.