Research On DC Melting Technology And Application Based On Hybrid MMC

In recent years,due to the sudden change of climate,the phenomenon of severe ice coating on the transmission line has occurred from time to time.If the ice can not be removed in time and effectively,it will pose a threat to the safe and stable operation of the entire power system.In modern society,the demand for electric energy is expanding.In order to reduce the heavy losses caused by line icing on national life production and power networks,the research and development of grid de-icing technology is particularly important.Among the existing DC ice melting technologies,modular multilevel converter(MMC)have attracted attention because they meet the requirements of DC ice melting on the operating conditions of inverters.In view of the large volume,high harmonic content and high cost of existing DC ice melting devices,this paper proposes a DC ice melting scheme based on hybrid type MMC,which uses half bridges on the top of the topology.The sub-module(HBSM)and the lower bridge arm are all constructed by a full bridge sub-module(FBSM),which can be used to melt ice when the transmission line is covered with ice,and can perform reactive power compensation to the power grid during normal operation of the power grid.This paper first analyzes the thermal equilibrium process of DC ice melting,critical ice melting current,time required for DC ice melting and several main factors affecting melting ice.Then the topology and control strategy of the full-bridge MMC ice-melting device are introduced,and the effectiveness is verified by simulation.Then the upper arm structure based on hybrid type MMC is analyzed,its equivalent circuit model is established and power analysis is carried out.According to the analysis results,an improved hierarchical control strategy suitable for HBSM is proposed.Because there is a certain relationship between the bridge arm current and the AC-DC side current,the controlstrategy of the bridge arm current is also given to realize the control of the AC-DC side current.The hybrid MMC DC ice-melting scheme model is established in MATLAB/SIMULINK.The simulation results show that the hybrid MMC can output continuously adjustable DC voltage for melting ice in the case of DC ice-melting operation,satisfying different grades and length lines.Melting ice demand.When the line is in normal operation,the device can be used as static compensator to compensate the power grid.The simulation results meet the requirements of dynamic reactive power compensation,and the results are satisfactory.The feasibility of this topology and control strategy is verified.Finally,from the structure,control strategy and the application of melting ice,it is compared with the full bridge type MMC and the hybrid submodule type MMC.