Research On DC Melting And Reactive Power Compensation Based On Half-Bridge MMC

In recent years,the rapid development of smart grids and UHV power grids has led to a growing scale of grid structures for power systems,and more and more attention has been paid to the efficient,stable,and economical operation of power systems.However,the unexpected snow and ice disasters pose a great threat to the operation of the power grid.When the lines are covered with ice,the power supply is interrupted,and even some lines cannot tolerate heavy snow and ice,resulting in disconnection faults.The place where the first-level load occupies a relatively heavy place brings economic losses,and it is difficult for people to ensure normal life.Reactive power that reflects the high-quality operation of the system has drawn widespread attention from people of all walks of life.In response to these problems faced by the power grid,the article proposes a modular multi-level converter(MMC)composed of half-bridge unit modules that can be used as ice melting devices in the event of ice transmission lines in winter.It also can be used as a reactive power compensation device during non-icing.Therefore,the structural characteristics of modular multi-level converters make it a promising application in future engineering applications.First of all,in order to study the dynamic process of heat dissipation and heat absorption on the surface under the conditions of line icing,the experts observed and calculated the current flowing through the line just after melting and the time it took to completely drop the ice based on the principle of heat balance and the principle of DC melting.After experimental verification,the factors affecting the magnitude of the ice melting current and the falling time of the ice layer are obtained,which provides a basis for erection of transmission lines to avoid ice disasters and improve the quality of transmission lines.Secondly,analyze and research Half-bridge MMC,and analyze its working principle and mathematical model according to its equivalent circuit structure diagram.According to the sub-module circuit structure,the working mode of each switch state is described.This article studies the half-bridge sub-modules and the output voltage can be switched between 0 andV_c two states.Thirdly,aiming at the application of MMC in direct current melting,a carrier phase-shift modulation strategy suitable for MMC is proposed.Based on this strategy,the capacitor voltage of the MMC is balanced and controlled;When Half-bridge MMC is used in reactive power compensation mode,real-time and accurate detection of reactive current directly affects the compensation effect of static synchronous compensator.After a comparative analysis,the detection of reactive current can be quickly and accurately measured by applying the instantaneous reactive power theory.In combination with the fuzzy PID control method,the system can be compensated in time.Finally,a simulation model was built in MATLAB/SIMULINK to simulate the operation of the MMC in the DC ice melting and reactive power compensation modes,and the control strategies applied in the article,such as the DC side voltage control in the case of ice melting and the effectiveness of the AC side control and compensation at the time of reactive power compensation was verified and the results confirmed the correctness of the proposed scheme.