Large-Signal Stability Analysis Of DC Microgrid And Research On Power Coordinated Control Strategy

Renewable energy such as wind and solar energy plays an important role in modern energy system.DC microgrid has become a research hotspot because of its advantages of no needs to consider phase angle,frequency and reactive power compensation.In order to solve the problems of fluctuation of DC bus and acceptance of renewable energy,this paper focuses on the stability of DC microgrid and power coordinated control,which has practical significance.The stability of DC bus voltage is the premise of stable operation of the DC microgrid.At present,the research of DC microgrid mainly focuses on the stability of small signal,that is,the research is carried out near the steady point of system operation.In the DC microgrid,varieties of power electronic converters are connected to the DC bus with different control methods to stabilize bus voltage.Among these converters,there are power converters that accurately control the output voltage through a closed loop.They behave as constant power loads(CPL)and have a destabilizing effect from negative incremental resistance(NIK)and their nonlinearities.The large-signal stability of the DC microgrid with multiple constant power loads are studied in this paper.Mixed potential function theory and Lyapunov stability theorem are used to estimate the boundary of power limit.The influences of circuit parameters on the boundary of power limit of two estimation methods are compared.T-S fuzzy modeling method and reverse trajectory tracking technique are used to estimate the region of asymptotic stability(RAS).The estimation precision of two estimation methods as well as influences of circuit parameters on estimating the region of asymptotic stability are also compared.Finally,the effectiveness of estimation methods used for the boundary of power limit and the region of asymptotic stability are verified by simulation results and experiments.Power coordination is of great importance to DC microgrid.Because the output of renewable energy such as photovoltaic and wind power is intermittent and fluctuant,hybrid energy storage system(HESS)combining ultracapacitors and batteries is usually used to solve this issue.According to the characteristics of ultracapacitors and batteries,the low-frequency power fluctuations are assigned to batteries,while the high-frequency power fluctuations are assigned to ultracapacitors.When considering an ultracapacitor group,state-of-charge(SoC)of the ultracapacitors will be unbalanced between each other,on account of the differences of initial states or attributes.A novel distributed control method is proposed in this paper.In general,ultracapacitors control the DC bus voltage and batteries control the SoC of the ultracapacitors.Voltage source converter(VSC)controls the SoC of the battery.Additionally,the distributed control adds the difference between one ultracapacitor's SoC and the average SoC of the ultracapacitor group to its current control loop.As a result,the dynamic stability of SoC among the ultracapacitors can be achieved,and the issues of overcharging and/or over discharging of the ultracapacitors can be avoided.At last,the effectiveness of the proposed control strategy is validated by the simulation model built in Matlab/Simulink.