Bus Voltage Hierarchical Control And Small Signal Stability Analysis In DC Microgrid

In the DC microgrid with photovoltaic cells,whether the power balance of the system affects the voltage stability of the microgrid.Therefore,whether the bus voltage is fluctuating is considered to be the unique indicator for measuring the power balance of the DC microgrid system.At present,the voltage stability of the DC microgrid is mainly solved by paralleling with the large power grid to solve the system power shortage.There is less research on the island operation of the DC microgrid.Therefore,it is very necessary to study the system of the DC microgrid island operation.Establish a simulation platform for the DC microgrid,and establish a mathematical model for the photovoltaic power generation unit,battery unit,supercapacitor unit and load simplification.And analyze the basic principles ancd working methods of boost module,buck module and bidirectional DC/DC module.Based on the MATALB/Simulink experimental simulation platform,the unit models of DC microgrid are established and their output characteristics are studied.Finally,a simulation model of the DC microgrid is built on the platform to prepare for further analysis of the DC microgrid.Taking the bus voltage of DC microgrid as the research object,the traditional DC microgrid bus voltage control mode is improved when the DC microgrid is operated.According to the influence of the output power and load power of the renewable unit on the DC tbus voltage fluctuation,in order to maintain the stability of the DC bus voltage,a combination of the battery and the supercapacitor is used to realize the voltage grading adjustment of the photovoltaic energy storage DC bus.According to the different operating states of each module of the system corresponding to different DC bus voltages,the system is divided into five operating modes.Simulation experiments are carried out on the dc micro-grid system for this regulation mode to verify the effectiveness of the improved graded regulation strategy for photovoltaic energy storage dc bus voltages.In a DC distributed system,the interaction between separately designed subsystems may have an impact on the stability of the system.The micro-grid of DC bus voltage grading control is a multi-point balance system.The stability of different control modes may be different.The stability of small signal modeling and analysis of different operating modes of the system.Because the closed-loop impedance of each unit of the system is different in different operation modes,the equivalent micro-source output impedance and equivalent load input impedance of each operation mode are calculated,and the impedance ratio criterion is used to assist Nyquist plots to analyze the stability of different operation modes of dc micro-grid.The DC microgrid has a resonant peak in the operating mode 1-1 and a small stability margin.A damping circuit is added to the load circuit to eliminate the filtering parameters and the resonance peak of the supercapacitor unit.The parameters of the damping loop are determined by the filtering parameters.The effects of different damping parameters on the stability of small signals are analyzed.The simulation results show that the damping circuit improves the stability of small signals only when the optimal parameters are taken.It can reduce the fluctuation of bus voltage and effectively improve the DC microgrid the problem of voltage fluctuations.