Research On Control Strategy Of Microgrid Stability In Island Mode

With the rapid growth of human demand for energy,renewable energy has gradually become one of the important sources of energy.As the main way for distributed energy to access large power grids,the control strategy and stability of microgrids determine the utilization rate of new energy sources.Compared with AC microgrid,DC microgrid does not need to consider the influence factors of system frequency,voltage phase angle and reactive power,so that the control mode and structure design are simpler and more flexible.The correct control strategy is the basis for maintaining the safe operation of the microgrid,and it is also an important guarantee for the stability of the DC bus voltage and the effective distribution of active power.In this paper,based on the droop control,the outlet converters of the micro-source side and the energy storage system side are modeled respectively,and the effects of the improved droop control and the hybrid energy storage droop control structure on the stable operation of the micro-grid are analyzed.Since the energy storage part has the effect of stabilizing voltage in DC microgrid and plays an important role,the stability and security of its operation is very important.Considering the disadvantage of low charging and discharging efficiency and short life of batteries,this paper chooses a cascaded energy storage system composed of a supercapacitor and a battery.Batteries charge and discharges to buses through bidirectional converters and supercapacitors to maintain bus voltage constant.In this paper,the control structure of this hybrid energy storage system and the droop control principle of two bidirectional converters and the changing process of their working state are introduced in detail.The traditional droop control of DC micro-grid ignores the influence of line impedance,which makes bus voltage keep constant operation,resulting in the output power of micro-power can not be accurately allocated,and the working voltage gap is larger than the ideal value.In this paper,the root cause of this effect is analyzed in depth,and an improved droop control based on hybrid compensation is proposed.This control method uses accurate current distribution instead of power precision as the ultimate goal,takes the difference between the actual current distribution proportion and the target proportion as the input of the fuzzy control,and the output is the change of sag coefficient,so as to offset the influence of line impedance measurement error or disproportion on power precision.At the same time,a central voltage compensator is designed to compensate the bus voltage,so as to enhance and stabilize the DC bus voltage.The simulation results show that under this control strategy,not only the current distribution is more precise,but also the bus voltage can be basically stable at the ideal value,thus realizing the efficient use of energy and stable operation of the microgrid.Based on the above analysis,the stability of the whole DC microgrid is analyzed.Firstly,the small signal mathematical model of two bidirectional converters in hybrid energy storage system is established,and the expression of their equivalent output impedance is deduced.Similarly,the small signal model and equivalent output impedance are established for the micro-source port changer and the load side.Finally,the parallel impedance model of DC micro-grid given in this paper is established.After configuring certain control parameters and line parameters,Bode diagram and Nyquist criterion are used to analyze the influence of the change of droop control coefficient on the stability of the system and the change of operation state of hybrid energy storage on the stability of the system in the improved droop control of micro-source side.In summary,this paper proposes an improved droop control strategy for DC microgrid,which achieves accurate power distribution and improves bus voltage stability.In order to enhance the operation reliability of DC microgrid,the hybrid energy storage system is combined with coordinated control.A system-level stability analysis model of DC microgrid with hybrid energy storage and micro-sources is established,and the influence of system parameters on stability is analyzed,which provides a way of thinking for the stability study of DC microgrid.