| At present,the construction scale of offshore wind farms is expanding,HVDC-Flexible system will be an important power transmission method for offshore wind farms in the future.Therefore,our laboratory proposes a novel multi-terminal HVDC-Flexible power transmission system for offshore wind farms.As one of the key equipment,sending end station converter(SESC)is a DC converter.Its tasks are as follows:1)to maintain the stability of its DC low-voltage side voltage;2)with a higher gain,to transmit the power energy generated by the wind turbines to DC grid through DC boost(up to 10 k V or even 30 k V).This paper focus on the design of a non-isolated high-power high-gain bidirectional DC/DC converter as the SESC,and the study of its control.First,according to the task undertaken by the SESC in the system,a non-isolated bidirectional Buck-Boost topology is selected,and its working principle under different working conditions is analyzed in detail,a mathematical model is established,and an interleaved double closed-loop control strategy is adopted.The effectiveness of the control strategy is verified by simulation analysis.A 3k W HVDC-Flexible power transmission system experimental platform is designed for experimental verification.The experimental results show that the designed SESC is feasible.Second,the project requires the SESC to achieve a voltage boost from 1150 V to 10 k V with a gain of 8.7,which is difficult to achieve with conventional DC/DC topologies.In order to achieve high gain,the cascaded construction method is adopted in this paper,and the two-stages cascade Buck-Boost topology is formed in series as the SESC.Through theoretical analysis of the cascade topology,the control strategy of high gain voltage boost was developed,the simulation analysis was completed.Simulating the actual working conditions required by the project,a 69V/600 V SESC prototype with a gain of 8.7 was designed and built.The experimental results verified the feasibility of the designed high-gain SESC.Third,the working voltage level of the second-stage converter of the high-gain SESC is boosted to 10 k V,which means that the withstand voltage value of the IGBT in the converter must meet this working voltage level.Since the existing single IGBT cannot satisfy this level,this paper introduces IGBT series technology,and conducts research on the uneven voltage phenomenon that needs to be solved urgently in IGBT series technology.Taking two IGBTs in series as an example,mathematical model is established.On this basis,by analyzing the mechanism and hazards of the uneven voltage,a novel voltage sharing control circuit is proposed,and its working principle is analyzed.the hardware circuit is designed and an experimental platform is built,and the experimental results verify the feasibility of the designed voltage sharing control circuit.Fourth,the project requires the SESC to achieve MW-scale power transmission,so this paper introduces the parallel technology of DC converter module,and carries out research on the uneven current phenomenon that needs to be solved in the parallel technology.Multiple cascade Buck-Boost converter modules are sed to form the input/output parallel topology as the SESC.Based on this,a mathematical model is established,and the parallel topology control strategy under steady state condition is developed.The causes of uneven current are analyzed,and a new current sharing control strategy is proposed.Simulating the actual working conditions required by the project,a set of parallel system with two converter modules as SESC was built to conduct experiments,and the experimental results verified the feasibility of the designed SESC.Finally,the actual working conditions of 2MW/1150V/10 k V required by the project is simulated and analyzed based on the MATLAB/Simulink simulation platform.The simulation results verify the effectiveness of the proposed current-sharing control strategy. |
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