Research On Topology And Control Method Of Modular DC-DC Converter In DC Microgrid

At present,the effective use of environmental protection and energy has aroused widespread concern in society.In order to utilize distributed energy generation more efficiently,flexibly and reliably,research on DC microgrids has become more and more extensive in recent years.The energy storage device contained in the DC microgrid can greatly improve the performance of the entire microgrid system,the dynamic response time is rapidly reduced,and the system loss is greatly reduced.The DC-DC converter is an important power electronic device in the DC microgrid,and is the core device for power conversion and power flow control.After modularizing the DC-DC converter,the capacity expansion of the DC microgrid system can be realized,and the flexibility and reliability of the system can be improved,which has broad application prospects in the DC microgrid.To this end,this paper first analyzes the working principle of several sub-module DC-DC converters,compares the current stress and voltage stress of switching elements with different topologies,and derives the relationship between current and voltage peak coefficients and ripple and duty cycle.According to the parameters of the stress,current and voltage peak coefficient of the switch,the half-bridge DC-DC converter is selected as the optimal sub-module structure,and the topology of the cascaded modular DC-DC converter is proposed.Since the current harmonic content of the single half-bridge DC-DC converter is large,three multi-connections of the half-bridge DC-DC converter are used as sub-modules to effectively reduce the harmonic content and reduce the loss.Then the current ripple,current harmonics and voltage ripple of the sub-module and the single half-bridge DC-DC converter are compared and analyzed,which further proves the superiority of the selected topology.Secondly,combined with the characteristics of the energy storage device and the need to stabilize the peak load of the DC microgrid,the stage charge control strategy of the energy storage device via the modular DC-DC converter and the discharge control strategy based on the low-pass filter principle are proposed.The proposed strategy enables the energy storage system to quickly track load power changes,stabilize peak loads,and extend the life of the energy storage device.Then,according to the different working conditions of the modular DC-DC converter,a control method combining sampling average current and double loop feedback is proposed to accelerate the dynamic response of the modular DC-DC converter.And when the low-voltage side power changes,the sub-module can dynamically switch and stabilize the high-voltage side(DC bus)voltage to ensure stable operation of the DC microgrid.Then,a novel terminal sliding mode variable structure control algorithm is proposed.The Lyapunov "generalized energy" function is introduced,so that multiple sub-modules can achieve current sharing quickly when the excitation signals are not the same and the internal parameters are not matched.It can effectively reduce the loss of modular DC-DC converter.Then,the soft switching characteristics of the modular DC-DC converter are studied,and the opening conditions under ZVZCS are derived and the range of soft opening is derived.Finally,the simulation model of the modular DC-DC converter is built by Matlab/simulink simulation software.The correctness and effectiveness of the proposed control strategy are verified by simulation.Through the research of this paper,the topology and control methods of modular DC-DC converters are analyzed.The optimal sub-module topology is proposed and the stress,current harmonic and voltage ripple expressions are derived,which fully proves the selected topology.rationality.A series of control strategies are proposed for the topology.The feasibility is fully verified by simulation,which lays a solid theoretical foundation for the subsequent research and promotion of modular DC-DC converters.