| DC microgrid system is usually composed of renewable energy,load and energy storage units.It has many advantages such as less converter,less loss,no frequency and phase problems.With the access of a large number of loads and converters,the structure and control strategy of DC microgrid are becoming more and more complex.Among them,the motor load and converter load of most closedloop control can be regarded as constant power load.Due to its unique negative impedance characteristics,the disturbance signal will be amplified when disturbance occurs,which will seriously affect the stability of the system.Therefore,the stable operation of DC microgrid is increasingly worthy of attention.Energy storage unit is an essential part of DC microgrid,which can stabilize the DC bus voltage and balance powers between sources and loads.Hybrid energy storage system(HESS)combining highenergy density storage batteries and high-power-density supercapacitors(SC)are commonly utilized in DC microgrid,which greatly improves the working performance and service life of the energy storage unit.As an important part of hybrid energy storage system,the control strategy of energy storage converter has a great influence on the stability of DC microgrid system.In order to improve the large signal stability of DC microgrid system and compensate the negative impedance characteristics of constant power load,based on the hybrid potential function method,this paper proposes the stability control strategy of parallel system of energy storage converter with hybrid energy storage unit.The stability control strategy reveals the design constraints of the current innerloop proportional control parameters of the battery bidirectional DC / DC converter,the power outerloop proportional control parameters of the supercapacitor bidirectional DC / DC converter,the power of the constant power load,the DC bus voltage,the voltage stabilizing capacitor,the filtering time constant of the low-pass filter with power distribution,the battery side inductance,the battery charge and discharge factor and other parameters.Firstly,according to the control strategy and segmental feature,the constant power load model,the new energy generation unit model and the hybrid energy storage unit model are established respectively,and the equivalent simplified model of DC microgrid system is obtained.Then,the large signal nonlinear energy model of the system is established by using the mixed potential function method,and the control parameters of the parallel energy storage converter are fully considered.The stability control strategy of energy storage converter parallel system is obtained by using the stability theorem.Finally,the simulation model is built to verify the correctness of the obtained stability control strategy.With the increasing scale of DC microgrid system and the increasing capacity of energy storage unit,single battery energy storage cannot meet the capacity requirements of microgrid.Therefore,this paper further studies the parallel energy storage system with n battery energy storage converters and 1supercapacitor energy storage converter.The large signal model of n+1 parallel energy storage converter is established,the stability criterion is derived and the control strategy is obtained.The stability control strategy is optimized and improved by the Thevenin theorem.The stability control strategy takes into account the influence of the output ratio of each power source on the system stability,making the obtained criterion more accurate and more realistic.Simulation and experiment results show the effectiveness of the derived stability control strategy for n+1 parallel energy storage converter. |
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