Research Of Modular Multilevel Converter Based On Auxiliary Resonant Commutated Pole Submodules

Modular multilevel converter(MMC)has been applied in medium voltage DC(MVDC)distribution network,due to its advantages of high modularity,flexible scalability and excellent quality of output waveforms.However,compared with high-voltage areas such as flexible HVDC transmission,the number of MMC sub-modules(SMs)in MVDC distribution network is significantly reduced,generally 10~30.In order to maintain the quality of output waveform,the switching frequency of switching devices in SMs is high,high switching frequency will cause large switching loss,resulting in a large loss of MMC.If the SMs operate at zero-voltage-/current-switching(ZVS/ZCS)which is called soft-switching,the switching loss of converter can be reduced greatly and the efficiency of the converter can be improved.In this paper,the half-bridge MMC is taken as the research object,and the auxiliary resonant pole(ARCP)soft-switching SMs are used to replace the traditional half-bridge SMs to realize ZVS/ZCS,so as to reduce the switching loss of the converter.Because ARCP SM originates from the soft-switching inverter topology in the field of medium and low voltage,it is not fully compatible when applied directly to MMC.The realization method of softswitching MMC under nearest level modulation(NLM)is studied.The operation principle of ARCP MMC is the same as that of conventional MMC,Essentially.Firstly,the topologies and working principles of MMC and half-bridge SMs are introduced in detail.The working flow of inserting or bypassing SMs controlled by nearest level modulation or carrier phase shift modulation with capacitor voltage balance strategy is described.This paper also introduces the loss classification and loss calculation method of MMC.Subsequently,this paper introduces the topology of ARCP SM and its internal commutation process.The selection principle of resonant element parameters and control parameters is clarified,and the design steps of hardware platform and experimental parameters are introduced.Then the phase shift error between the output voltage of the MMC arm and the step wave of the reference voltage of the arm under NLM is analyzed.In order to eliminate this phase error,it is necessary to select the sub-modules to be inserted and bypassed before the time when the number of inserted SMs changes,but at this time the number SMs to be inserted cannot be determined.A predictive SM choosing algorithm is proposed to realize this goal.A simulation model is built to verify the effectiveness of the predictive choosing algorithm.Finally,a hardware platform of ARCP SM is built.Experiments results verify the correctness of theoretical analysis of soft-switching commutation process and the effectiveness of soft-switching technology in reducing the loss of MMC.