| DC networks have lower line loss and higher power density compared with AC networks.Nowadays,the application fields of DC networks have been greatly expanded with the development of power semiconductor techniques.In the field of power transmission,the high voltage DC transmission technology has developed rapidly in recent yeas,and the mediumvoltage DC power supply and distribution technology is also receiving more and more attention.Modular multilevel converter(MMC)has become the most popular power convertion device thanks to its highly modular structure,better harmonic characterrastic,smaller voltage stress of switching device and lower power loss.However,in the medium voltage DC networks where the bus voltage is not so high,the application of MMC is restricted by the number of submodules.In this case,if the nearest level modulation(NLM)is adopted,the harmonic characteristics of output voltage will be serious affected by the small number of sub-modules.And if the multi-carrier modulation is adopted,there will be a large switching loss.In order to get both good harmonic characteristic and low switching loss in medium voltage MMC application,the following studies are carried out in this paper:Firstly,the basic operating principle of MMC is instructed and its average equivalent model is also decuced.The methmatical analysis of the nearest level modulation,the phaseshifted carrier PWM modulation and the phase-disposition level shifted PWM modulation are performed respectively.The analysis results show that,the quality of output voltage waveform is poor if the number of sub-modules is small under NLM;and under multi-carrier modulation,the quality of output voltage waveform would be better with the increase of the frequency of carrier but the switching loss would be larger at the same time.And in addition,the harmonic characteristic of the output voltage would be better as the number of ouput levels increases under each modulation strategy.Secondly,an improved MMC topology was studied in depth.Considering the large switching loss in its tipycal operation mode,an optimized modulation strategy is proposed.Under the proposed modulation strategy,the switching loss is half of that in traditional modulation strategy and the number of ouput levels will be increased by four times compared with the classical MMC.Then,the mechanism of circulating current is analyzed,and its analytical expression is also derived.The traditional circulating current suppression strategy that based on commonmode voltage compensation is hard to be applied under the proposed modulation strategy which is based on the coordinated control of the upper and lower arms.To solve this problem,a circulating current suppression strategy based on redundant modulation mode is proposed arccording to the hysteresis control.Finally,Hardware-in-the-loop simulation experiments were carried out based on PLECS RT-BOX and TMS320F28069 M digital processors.The experimental results verified the correctness of the relevant theoretical analysis in this paper,and the effectiveness of the proposed modulation strategy,auxiliary sub-module voltage control strategy,and circulating current suppression strategy is also confirmed. |
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