| In terms of the photovoltaic DC microgrid system,the bus voltage is the core indicator to measure the systematic security and stability.However,as affected by the randomness and fluctuation of the power of photovoltaic power generation as well as the dynamic change of load,there is an unpredictable power disturbance of the PV DC microgrid in the actual operation,especially in the off-grid microgrid which lacks the support of public power grid.When the photovoltaic output does not match the load demand,it is easy to cause bus voltage fluctuation and affect the safe and stable operation of the system.In view of the above phenomena,this paper focuses on the stability of off-grid photovoltaic DC micro-grid under impact load,reveals the factors that affect the stable operation of microgrid,solves the power distribution of multi energy storage converter in parallel operation and the problem of bus voltage stabilization,and then provides theoretical basis and technical support for the development and application of photovoltaic DC microgrid.(1)The control of photovoltaic DC microgrid mostly focuses on the DC bus voltage regulation under the fluctuation of photovoltaic output power and small-scale load disturbance,it seldom involves the large-scale variation of load power,especially the access of impulsive load.In response to this problem,the models and Simulations of the photovoltaic cells,storage batteries and excited DC motor are done in this paper,and their control strategies are studied respectively.Using excited DC motor in open-loop control as the impact load,a photovoltaic DC micro-grid model with impact load characteristics is constructed.(2)Aiming at the stable operation of photovoltaic DC microgrid under small disturbance,the small signal stability determination of microgrid is studied.The small signal models and equivalent impedance models of Boost,Buck and bidirectional DC/DC converters under different control strategies are established by the state space averaging method,the effects of bus voltage and filter capacitance on the impedance characteristics of the converters are studied respectively.The results show that the influence of bus voltage on the impedance characteristics of each converter is small,while the influence of filter capacitance on the impedance characteristics of each converter is large,especially in the middle and high frequency bands,with the increase of filter capacitance,the impedance of each converter tends to decrease.Based on the impedance model of each converter,the stability analysis method based on impedance ratio is used to study the small signal stability of photovoltaic DC microgrid under different operating modes,the worst stability mode is taken as the criterion of the small signal stability of the DC microgrid.(3)Considering the charging and discharging mode of energy storage unit and the dynamic characteristics of load,the stability criterion of photovoltaic DC microgrid under large disturbance is studied.Based on the theory of mixed potential function,the large signal stability of microgrid is studied,the mixed potential function model is established,the criterion of large disturbance stability of the system is deduced,and the main factors affecting the large signal stability of photovoltaic DC microgrid are analyzed.It can be concluded that the more of power of motor load,the worse large signal stability of the system;incorporating resistive load can improve the large signal stability of the system,and the smaller of the resistive load,the better large signal stability of the system;the hybrid energy storage mode of battery and supercapacitor can improve the large signal stability of the system.Finally,the validity and correctness of the conclusions are verified by simulation.(4)The traditional droop control has limitations in improving power allocation accuracy of energy storage unit and reducing bus voltage deviation at the same time,aim at this problem,a droop coefficient compensation method based on correlation parameters is used to improve the traditional droop control.The droop control coefficient is set as the ratio of the initial droop coefficient of the corresponding energy storage unit to the n power of the residual capacity(SOC),by changing the value of power index “n”,the charge and discharge rate of the energy storage system is adjusted in real time,and the accurate power sharing can be realized when multiple energy storage converters are running in parallel.At the same time,on the basis of improving droop control,the secondary control is added to enhance the quality of power.The simulation and experiment show that the improved droop control can control the charging and discharging rate of each energy storage unit in real time and restrain bus voltage fluctuation better.(5)There is an inevitable contradiction of the traditional voltage and current double closed-loop control in improving the dynamic performance and steady-state performance of the system.Under this situation,a fuzzy-PI dual-mode controller is proposed to replace the PI controller of the voltage outer loop,which can automatically switch the working mode according to the deviation of bus voltage.While the bus voltage deviation is large,the fuzzy control is adopted to achieve good transient performance.While the bus voltage deviation is small,the PI controller is utilized for good steady-state performance.The simulation and experimental results show that the fuzzy-PI dual-mode controller combines the advantages of fuzzy control and PI control,it not only improves the dynamic response performance of the system,but also can effectively restrain the large fluctuation and impact of DC bus voltage,it is helpful to improve the robustness of the system. |
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