| DC microgrid has the advantages of less grid connection and convenient access to distributed new energy,so it has a broad application prospect.However,due to the low inertia problems caused by the high proportion of new energy and the access of power electronic equipment,the DC bus voltage is very sensitive to the disturbance caused by load change,new energy output fluctuation and other factors.The energy storage system has the advantages of two-way regulation,flexible and controllable.Adjusting the charge and discharge power of the energy storage system can provide necessary inertia support for the DC system and improve the stability of DC voltage.However,DC microgrid not only has instantaneous power shock,but also has long-term energy release requirements,but also needs to take into account the economy and safety of energy storage,a single form of energy storage can not fully meet the above requirements.The construction of hybrid energy storage systems(Hybrid energy storage system,HESS),giving play to their complementary advantages in energy density and power density,can not only ensure the safety of energy storage,but also provide effective and controllable inertia support for DC power grid.Therefore,this paper studies the power allocation and virtual inertia control strategy of DC microgrid hybrid energy storage system.Firstly,the mathematical model of DC microgrid system with hybrid energy storage is established.Based on the equivalent control model and equivalent circuit model of energy storage converter with two kinds of droop control,the transient response of DC bus voltage to load sudden change under V-I droop control and I-V droop control is quantitatively calculated.Furthermore,the voltage regulation model of DC microgrid system is established,and the feasible path to enhance the inertia of DC system is given.Then,a virtual inertia control strategy of hybrid energy storage system based on power adaptive adjustment is proposed,which uses nested function to flexibly adjust the output power of supercapacitor to provide flexible and adjustable virtual capacitor for the system,ensure that the supercapacitor energy storage can provide sudden power at the moment of the sudden change of power in the network,so as to avoid the impact of the sudden change of power in the network on the energy storage of storage battery and DC microgrid.In order to prolong the service time of the supercapacitor and the service life of the battery,a hierarchical operation control scheme of multi-type hybrid energy storage is proposed,and the operating condition of the hybrid energy storage system is further optimized.The simulation results verify the correctness of the method for calculating the peak value of the transient voltage change of the DC bus voltage under the V-I droop control and the effectiveness of the control strategy.Finally,in order to solve the problem that the traditional frequency division response strategy does not have inertia support capability,based on the concept of frequency domain coordination impedance,a hybrid energy storage virtual inertia control strategy based on frequency domain impedance is proposed.through analyzing and shaping the frequency domain impedance of battery and supercapacitor,the action time of energy storage is further optimized,so that the supercapacitor can better respond to short-term high power fluctuation and realize the frequency division response of hybrid energy storage power at the same time.Provide flexible and adjustable inertia time constant for the system.The simulation results verify the correctness of the theoretical analysis of I-V droop control voltage regulation and the effectiveness of the control strategy.By establishing the small signal model of DC microgrid with hybrid energy storage.system,the influence of key parameters on stability is revealed and the selection.range of parameters is obtained. |
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