Research On High Step-Up Ratio Modular DC-DC Converter And Fault Protection Method

With the development of offshore wind power towards long distance and large capacity,the transmission and grid-connection scheme based on medium voltage(MV)DC collection and high voltage(HV)DC delivery is becoming more and more competitive.The HV large capacity DC-DC converter connecting the MV DC bus and the HV DC bus is the core equipment.Considering the construction cost of offshore wind farms,the volume and weight of offshore platforms should be as small as possible.In addition,the high cost of offshore operation and maintenance makes the reliability and fault protection of the system particularly important.Therefore,the above-mentioned HV large capacity DC-DC converter needs to meet the following requirements: high step-up ratio,high power density,high reliability and equipping with fault protection capability.Firstly,a high step-up ratio modular DC-DC converter(HSRM-DC)satisfying the above requirements is proposed,which uses submodule capacitors as energy transmission medium to transfer the energy from the MV DC port to the HV DC port,and it presents attractive characteristics such as high step-up ratio,high device utilization rate,small size,and low cost.In addition,the key parameters of HSRM-DC topology are designed,and the corresponding control strategy is proposed to ensure the stable operation of the HSRM-DC.In order to promote the reliability of the HSRM-DC,the characteristics of the internal thyristor fault are analyzed in detail,and then the fault ride-through method is designed to ensure that the power transmission during the fault stage is not interrupted.Meanwhile,the fault current and the voltage fluctuation of the submodule capacitors during the fault ride-through process are calculated to ensure the stable fault ride-through of the HSRM-DC.On the basis of ensuring the reliability of HSRM-DC,the external DC shortcircuit fault is studied in this paper.By establishing the equivalent model and using the state variable method to solve the equivalent circuit,the analytic solution of the fault current and capacitor voltage is obtained.And the corresponding fault protection method is designed to realize HSRM-DC external DC short-circuit fault protection.Based on theoretical analysis,the simulation model parameters and experimental prototype parameters are designed,and a simulation model with port voltages of 35 k V/250 k V and transmission power of 100 MW and an experimental prototype with port voltages of 120V/480 V and transmission power of 2400 W are built.Simulation and experimental verification are carried out under the steadystate operation condition,the internal thyristor fault condition and the external DC short-circuit fault condition respectively.The simulation and experimental results are consistent with the corresponding theoretical analysis.