Research On Modulation Strategy Of Modular Multilevel Converters In HVDC

With the rapidly development of economy,the demand for energy has increased greatly.As China's energy resources are mainly distributed in the West and northeast,the country on the strategic level has designed the strategyabout "west to East,North and south for each other".The excellent characteristics of HVDC makes it one of the best programs to achieve this strategy[1,2].The key equipment of DC transmission is converter,such as modular multilevel converter(MMC).In this paper,the modulation strategy of the modular multilevel converter is studied.The main purpose is to simplify the modulation method of the converter and reduce the harmonics.Based on the research of the modulation strategy of the modular multilevel converter,an improved modulation strategy and an equal angle modulation strategy are proposed.In theory,the improved nearest level modulation strategy is the strategy,which the auxiliary sub module with a plurality of cascade is equivalent to the original masters module.Because the cascade auxiliary sub module can be equivalent to the master module,that is prevent the basic skeleton ladder the same,so that the DC voltage will be the constant,Due to using sub structure the module of a plurality of cascade,each step can be refined,the step number will be increase,aim at reduce the output voltage of the harmonic component.According to the analysis of the improved nearest level modulation strategy theory,through MATLAB and PSCAD is validated by simulation,it can be concluded that the improved nearest level modulation strategy can reduce the harmonic component of the output voltage.Theoretically speaking,angle modulation strategy is in the process of modulation sine wave approximation,with the increase of the equivalent angle of trigger pulse is generated,gradually improve the sub module capacitor voltage value,the output waveform sine wave modulation.Because the modulation mode adopts the equivalent angle pulse triggering mode,the comparison process of equivalent voltage is reduced,and the loss time is reduced,and the control method is simplified.According to the principle of equal angle modulation strategy,a PSCAD simulation model is built.It is proved that the pulse triggering modulation strategy with equivalent angle can reduce the simulation time and simplify the control process.Two kinds of modulation methods and angle modulation strategy--two module corresponding modulation strategy and multi module corresponding modulation strategy is carried out by using multi module corresponding modulation strategy,the DC voltage and the AC voltage waveform will be in two modules corresponding modulation strategies under good waveform.