| With the increasing penetration of renewable power sources and the fast development of semiconductor devices and power equipment manufacturing technologies,DC grid technology has become a hot spot of concern.Similar to transformers in AC power systems,high-voltage and high-capacity DC-DC converters are the core equipments for voltage matching and interconnection networking in DC grids.Having the advantages of high modularity and high voltage handling capability,modular multilevel dc-dc converter(MMDC)is an effective solution for realizing grid-level DC-DC conversion.However,unlike traditional DC-DC converters,utilizing submodules to synthesize the desired voltage,MMDC contains a large number of energy buffer units and complex power interaction processes.The energy stability of each buffer unit and the balance between them are the prerequisites for normal operation of MMDC.Up to now,the research on MMDC is still in its infancy,and there is no targeted and systematic research on the energy control of MMDC.Concentrated on the energy control requirements of MMDC including submodule and arm energy balancing control and AC-link energy optimization,this paper systematically studies the energy control principles and their implementation methods for MMDC.The main research contents are as follows:(1)The basic operating principles and energy control methods of MMDC.According to the circuit topology and operating principle of the converter,the mathematical models of nonisolated and isolated MMDC are established respectively.Based on the models,the internal power transmission principle and the internal flow path of MMDC are analyzed,then the power interaction process of the energy buffer units is revealed.Based on the control degree of freedom analysis,the general energy control methods of MMDC are studied.The adjusting-control-pulse method applicable for the submodule energy balancing control,the injecting-compensation-current method applicable for the arm energy balancing control,and the adding-auxiliary-loop method applicable for the internal energy balancing of nonisolated MMDC are proposed.(2)The topology improvement and energy self-balancing operation of nonisolated MMDC.Firstly,the inherent limitations of the conventional energy balancing control method for nonisolated MMDC are analyzed.In order to avoid the problems of large AC circulation and filtering demand in the conventional control strategy,an improved nonisolated MMDC topology with energy self-balancing capability along with its operating and control principles are proposed based on the adding-auxiliary-loop method.The main advantages of the proposed topology are: 1)Utilizing the quasi-resonant circuit to transfer the balancing power loop between the submodules and arms,the AC current flows through the arms and the DC side is avoided.2)The self-balancing of energy among the submodules and between the arms is realized by switching the adjacent submodules in a complementary,resulting in a simple and reliable control system;3)The amplitude of the square wave voltage applied to the input inductor is limited to the submodule voltage,greatly reducing the size of the input filter inductor;4)Soft-switching operation of the majority switches are realized,leading to lower switching losses and electromagnetic interference.(3)The modulation strategy,operation analysis and submodule energy balancing control of isolated MMDC.The inter-submodule control pulse phase shifting modulation method is proposed to realize the quasi-two-level operation of isolated MMDC.This modulation method can generate quasi-square-wave voltages at the AC side with their rising and falling edges modified into staircase waveforms,achieving small dv/dt,and at the same time enables zero-level turn-on operation of the switches,thereby synthesizing the technical advantages a modular multilevel converter and the conventional two-level DC-DC converter.Combined with the submodule energy analysis under the quasi-two-level modulation mode,the submodule energy balance control strategy for isolated MMDC is proposed based on the adjusting-control-pulse method.The constrained condition for the control strategy to realize soft-switching operation is analyzed.There is no need to sample the arm current in the proposed energy control strategy,and the voltage sampling and control frequency can be much lower than the switching frequency,which can effectively reduce the complexity and computational burden of the digital control system.(4)The asymmetric parameter operation and arm energy balancing control of isolated MMDC.Firstly,the generating mechanism of power difference and energy imbalance between the upper and lower arms of isolated MMDC are revealed.The effects of the arm energy imbalance on the operating state of the converter are analyzed.The inter-arm phase shift modulation strategy for isolated MMDC is proposed,where a small phase shift angle is introduced between the upper and lower arms to establish a compensation power transmission path between the arms.Based on the injecting-compensation-current method and the operating mode and transmission power analysis of isolated MMDC under the proposed modulation strategy,two arm energy balancing control strategies for the converter are proposed,including the hysteresis control algorithm and the symbol-adaptive closed-loop control algorithm.The main technical advantages of the proposed modulation strategy and control strategies are as follows: 1)The quasi-multilevel operation is achieved when combining with the inter-submodule phase shift modulation,which doubles the number of steps during the transition period of the AC-side voltages and further reduces the dv/dt;2)There is no need to sample any current in the arm energy balancing control algorithm,which can effectively reduce the complexity of the digital control system;3)Organic combination of the two control algorithms are realized.Utilizing the hysteresis control as a backup algorithm for the symbol-adaptive closed-loop control improves the reliability of the control system.(5)The arm-voltage decoupling control and AC link energy optimization of Isolated MMDC.Firstly,the limitation of conventional DC-voltage-arm-voltage coupling control of isolated MMDC under the quasi two-level modulation mode is pointed out,that is,when the primary and secondary voltages of the transformer are not matched,the AC-link energy oscillates with a large amplitude.By utilizing the coupled arm inductor as the voltage-boosting unit to build a virtual buck-boost circuit in the common-mode current loop,the DC-voltage-arm-voltage decoupling control strategy of isolated MMDC is proposed.Compared with the conventional control method,the advantages of the proposed one are as follows: 1)The amplitudes of the primary and secondary voltages of the isolation transformer are kept matched whether the DC side voltage is higher than or lower than the rated voltage,the AC-link energy is thus optimized,reducing the AC-side current stress and improving the system efficiency.2)With the wide-range variation of the DC side voltage-even higher than the rated voltage,this control mode can maintain the voltage stress of the submodules,effectively expanding the voltage range of the converter.The effectiveness of the proposed control and optimization strategies are verified by simulation and experiment results. |
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