| DC microgrid is an effective way to improve the utilization rate of distributed renewable energy and increase the reliability and security of power supply.It is an important research direction of new energy power generation in the future.Affected by the randomness and fluctuation of distributed generation output power and the dynamic change of load power,it is easy to cause bus voltage fluctuation in DC microgrid and threaten the safe and stable operation of the system.In the meantime,it is necessary to use energy storage device to maintain bus voltage stability and ensure the instantaneous energy balance of the system.Bidirectional DC-DC converter is an important part of energy storage device.Taking bidirectional DC-DC converter as the research object,the research status of bidirectional DC-DC converter in circuit topology and control strategy is introduced.In order to stabilize the DC bus voltage,the following research work has been completed.1.In order to solve the problem of bus voltage fluctuation caused by distributed generation or load fluctuation,considering the inherent nonlinear characteristics of energy storage device,a charging and discharging controller of energy storage device is designed based on nonlinear backstepping method.In view of the time-varying parameters of some capacitors and inductances in the system model,a parameter adaptive backstepping controller is designed.The parameter adaptive principle is used to estimate the unknown parameters online and in real time.Based on the parameter estimation value and Lyapunov theory,the controller is recursively designed to regulate the charging and discharging process of the energy storage device,which ensures the stability of the DC microgrid while improving the system modeling accuracy.2.Aiming at the problem of frequent switching between voltage rise and voltage drop modes of bi-directional half bridge converter when bus voltage fluctuates in a small range,an automatic mode switching control strategy based on voltage current double loop principle is proposed.The outer voltage loop automatically selects the operating mode of the converter according to the bus voltage and outputs the inductance current reference.The inner current loop designs a finite time optimal control based on the affine nonlinear model to achieve the fast tracking reference of the inductance current.The improved double loop control reduces the current impact caused by frequent actions,improves the dynamic performance of the converter,and ensures the smooth and seamless switching of battery charging and discharging.3.Aiming at the problems of state of charge（SOC）imbalance in multiple energy storage devices,based on high pass filter（HPF）and band pass filter（BPF）,an improved power droop SOC equalization control is proposed.The high pass filter is used to achieve SOC balance of battery energy storage units with different capacities and improve the life of the battery pack.The band-pass filter is used for power equalization and voltage compensation to ensure that the DC bus voltage remains within the required range.4.The islanding DC microgrid simulation model is built through MATLAB/Simulink?,and various strategies designed in this paper are simulated and verified.The simulation results verify the effectiveness and applicability of the method proposed in this paper. |
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