Research On The Phase-shifted Carrier Modulation Scheme For Modular Multilevel Converters

With the further development and utilization of energy,the security and stability of accessing grid for wind power,solar and the other new energy have become an important problem to be solved.The development of modern power electronics technology makes the flexible HVDC technology have a broad application prospect in it.As an innovative and high efficient cascaded multilevel converter,Modular Multilevel Converter(MMC)is gaining more and more attention,it presents great advantages of high transmission efficiency,low power loss,high quality of AC output waveform and scalability.Phase-shifted carrier(PSC)modulation scheme is one of the most commonly used modulation schemes for MMC,Research on the harmonic characteristics of the PSC scheme for MMC has great significance to improve the performance of converter.The main content of this paper is as follows:Based on Kirchhoff's law,the mathematic model of MMC is established,and then proposed the MMC control strategy,the circulating current suppression strategy and the sub-module's(SM)capacitor voltage balance strategy.By comparing the advantages and disadvantages of different modulation techniques for MMC,it's found that PSC modulation scheme has better performance on MMC with less SMs.Based on the different topologies of the half-bridge sub-module(HBSM)and the full-bridge submodule(FBSM),appropriate PSC modulation schemes are proposed respectively.Based on the double Fourier analysis method,the half-bridge MMC and the fullbridge MMC are analyzed respectively,and obtained the double Fourier series representation of the output voltage and the circulating current.It's pointed that the phase shift angle between the upper and lower arm carrier has great influence on the harmonic characteristics of MMC.a proper phase shift angle can be calculated to reduce the harmonics of the AC output voltage and the circulating current,leading to a reduction of the filter size.And it's found that MMC can generate 2N+1 levels when there is a phase shift angle between the upper and lower arm carrier.The validity of the Fourier analysis is verified by the simulation.Based on the double Fourier analysis of the hybrid MMC,the existing problems of the traditional PSC modulation scheme for the hybrid MMC are studied.An improved PSC modulation scheme is proposed to deal with these issues.By reducing the carrier frequency of the FBSM to half of the HBSM's,the distribution of switching loss can be optimized.Applying a proper phase shift angle between the triangular carrier of the HBSM and the FBSM,harmonic elimination can be achieved,leaving only Nth carrier harmonic multiples and its sideband harmonic,improving the harmonic characteristics of the converter.The double Fourier analysis and the improved PSC modulation scheme are verified by the simulation.