Research On Control Strategy And Lightweight Of Flexible DC Converter Valve

Flexible direct current transmission(VSC-HVDC)is a new generation of DC transmission technology following AC transmission and conventional DC transmission.It has a wide range of applications in wind power grid connection,power grid interconnection,urban power supply,islanding and offshore platform power supply.As a core equipment of a flexible HVDC transmission system,there are still many problems in the soft-commutated valve.Compared with the conventional DC transmission converter valve,the flexible capacity converter valve with the same capacity level has large volume/weight,poor seismic performance and high cost.In order to make the soft-commutated valve more suitable for high voltage and large capacity in terms of reliability and economy,the volume and weight of the converter valve need to be reduced.In the converter valve,the capacitor occupies the main volume and weight.By reducing the flow of extra power of the converter valve and reducing the fluctuation of the capacitor voltage of the sub-module,the volume and weight of the converter valve can be effectively reduced.In this paper,the topology and working principle of the hybrid modular multi-level converter valve are introduced.The control strategies of the power,voltage,and current of the converter valve and the circulation suppression method are proposed.The research is based on the full-bridge sub-module of the hybrid bridge arm.The half-bridge sub-modules independently control the optimal pressure equalization control strategy to achieve over-modulation control of the hybrid converter valve.The form of power exchange between the AC/DC side of the converter valve and the sub-modules of the converter valve arm is described.From the perspective of reducing the power fluctuation of the bridge arm,the effect of the modulation ratio on the voltage fluctuation of the sub-module capacitance is analyzed in detail,and the Excellent derating modulation ratio,to achieve the goal of lighter valve.According to the various modulation methods proposed in the paper,circulation suppression control method,sub-module capacitor voltage equalization control,circulation injection suppression capacitor voltage fluctuation and modulation ratio suppression capacitor voltage method,multiple hybrid converter valve simulation models are built in the PSCAD platform.The simulation of different control methods was performed.