Research On Nonlinear Droop Control And Stability Of DC Microgrid

As an important part of primary control in hierarchical control of DC microgrid,droop control is adopted to achieve current or power sharing among multi-sources modules and plug-and-play.Basic droop control uses constant droop coefficient(i.e.,virtual resistance),which improves current sharing accuracy through increasing the droop coefficient but will cause large bus voltage drop.As a result,basic droop control cannot meet the requirements of both the current sharing and bus voltage regulation.Therefore,on the basis of basic droop control,the thesis proposes the piecewise nonlinear droop control strategies(PNDC,including PNDC-I,PNDC-II,and PNDC-III),improves system current sharing and reduces bus voltage deviation.At the same time,in order to ensure the stable operation of the system,the stability of the droop control is analyzed to provide a theoretical basis for system design in the DC microgrid.Firstly,the contradiction is pointed out between improving current sharing performance and reducing bus voltage deviation in basic droop control,and a piecewise nonlinear droop control strategy(PNDC-I)of DC microgrid based on load area control is proposed.Therefore,the load current is divided into several region according to the load condition of the system,and the output characteristic curve of the power converter is designed flexibly according to different load region.Secondly,on the one hand,there is a coupling relationship between the adjustment of the droop coefficient and the bus voltage offset compensation.On the other hand,the bus voltage deviation compensation is limited in the PNDC-I control strategy.Therefore,a piecewise nonlinear droop control strategy(PNDC-II)based on multi-level feedforward compensation of bus voltage and load division control is proposed.The system bus reference voltage is directly compensated in different load ranges,and the slope(droop coefficient)of the output voltage and current characteristics is not changed.So,the bus voltage deviation of the system is improved and the system current sharing performance does not change.Furthermore,on the basis of the PNDC-II control strategy,an improved piecewise nonlinear droop control strategy(PNDC-III)is proposed,where the droop coefficient can be flexibly configured according to partition of load range.In addition,the multi-level feedforward compensation of the bus voltage is introduced to the voltage deviation caused by the different droop characteristics.Therefore,the problem of both current sharing and voltage regulation can be improved in the primary level,which maximally relieve voltage regulation in secondary control,and the optimal design of droop control is achieved in theory.Finally,based on the large signal average model,the existence condition of the system equilibrium point is derived when the system dynamically adjusts droop coefficient.With the help of the small signal mathematical model,the changing trend of the characteristic root is analyzed when droop coefficient and load power change at the same time,and analyzes the characteristics root trajectories of the system when the system load type and line parameters are changed,which provides the theoretical basis for the droop control design of DC microgrid.