Research On Control And Protection Technology Of MMC Based DC/DC Converter

In the DC grid,the large-capacity DC/DC converters are indispensable key equipments for interconnecting different voltage levels,large-scale renewable energy collection,and realizing the development of medium voltage DC distribution network to low voltage DC microgrid.With the development of modular multilevel converter(MMC)technology to the field of DC/DC conversion,MMC based DC/DC converters have wide application prospects in DC power grid due to their advantages of strong voltage scalability,high reliability and fault current limiting capability.The topologies,control strategies and protection measures during faults of the typical MMC-DAB DC/DC converters whose overall structure is dual active bridge(DAB)in MMC based DC/DC converters are studied in this paper,and the main work and innovative achievements are as follows:1.Based on studying the two typical topologies of MMC based DC/DC converters--MMC-MMC-DAB and MMC-H bridge-DAB converter,a novel input-series-output-series modular multilevel dual-active bridge(ISOS-M2DAB)DC/DC converter is proposed.According to the operating principles,mathematical model and equivalent circuit of MMC,the operating principles of the primary-side and secondary-side converters of those DC/DC converters are analyzed,the equivalent circuit of the DC/DC converter is obtained,and the principle of phase-shift control is derived.Then according to the different hierarchical control structures of the basic control system of MMC based DC/DC converters,the detailed overall control diagrams of MMC-DAB and ISOS-M2 DAB DC/DC converters are obtained.In addition,the complete power characteristic expressions of MMC-DAB and ISOS-M2 DAB DC/DC converters are derived,the graphs of power characteristics are depicted,and the advantages over the traditional DAB DC/DC converters are compared and analyzed.2.According to the generating mechanism of circulating currents of the MMC system,it is derived that compared with the DC side of the three-phase MMC system is not affected by circulating currents,the single-phase full-bridge MMC system has the characteristic that the DC side will generate double-frequency ripple due to the inflow of the circulating currents,thereby the reliability is reduced.Then a circulating currents suppression strategy based on orthogonal virtual vector specifically for single-phase full-bridge MMC system is proposed in this paper,whose control diagram is simple and it can be applied to both single-phase fullbridge MMC grid-connected system and MMC-DAB DC/DC converter,the effective reduction of the double frequency circulating currents is realized,thereby improving the systemetical reliability.Moreover,by analyzing the equivalent circuit of circulating currents of ISOSM2 DAB DC/DC converter and combining structural characteristics of that DC/DC converter,an inter-module arithmetic phase interleaving control(IAPIC)circulating current suppression strategy is presented in this paper,which eliminates the double frequency circulating currents through simple control theoretically.Furthermore,the paper also indicates that the proposed IAPIC control strategy can weaken even ripples of DC output voltage,which is beneficial to the systemetical miniaturization and compact design.3.The simulation proves that the MMC-MMC-DAB DC/DC converter with half-bridge submodule has the ability to isolate the bipolar short-circuit DC fault and limit the fault current through blocking the MMCs on both sides of the system.To ensure the normal power transmission in the non-faulty zone,the method of optimizing the topologies of MMC submodules or converters is further discussed,in order to realize the isolation and current limiting during bipolar short-circuit fault by using the blocking single-side MMC on the fault side.In order to make the current limiting control free from the shortcomings of the above passive current limiting methods limited by the DC fault identification speed,a novel active current limiting control method based on the reduction of numbers of the MMC submodules in switching on states is proposed in this paper,which reduces the numbers of submodules which are in switching on states in each phase unit after the fault occurs.Simulation results show that it reduces the peak value of the short circuit current greatly,increases the peak time of the short circuit current,and thus saves more time for the submodules blocking protection action,and improves the safety margin of the device and systemetical reliability.4.The hardware experimental prototype of MMC based DC/DC converters based on dSPACE1103 real-time simulation controller is built,and the basic control of MMC-H bridgeDAB DC/DC converter,the basic control of ISOS-M2 DAB DC/DC converter,the circulating currents suppression strategy based on orthogonal virtual vector for single-phase full-bridge MMC system,and the circulating current suppression strategy based on IAPIC control proposed in this paper are verified by using the prototype.The experimental results are consistent with the simulation results,which proves the correctness of these controls strategies.