Research On Control And Stability Analysis Method Of Microgrid Based On Improved Droop Characteristics

With the continuous development of distributed power sources and the maturity of power electronics technology,renewable energy such as wind power and photovoltaics are beingconnected to the power grid on a large scale.The microgrid can integrate loads and micro power supplies through power electronic devices,and can provide both electric energy and thermal energy,which can meet the user’s demands for power reliability and power safety.In addition,there are many advantages of the microgrid in consuming distributed energy and improving power supply reliability.Therefore,the microgrid will inevitably become an important form of access in the future distribution network,which is important for the transition from traditional power grid to smart grid.The microgrid has become one of the important topics for scholars at home and abroad.Based on the analysis of the traditional control structure of microgrid,this paper studies the improved droop control strategy considering line loads,and studies the stability analysis of microgrid based on improved droop control.The main research contents of the thesis can be summarized as follows:(1)The paper introduces the basic control methods commonly used in microgrids,and gives the block diagrams and control characteristics analysis of inverter control based on PQ control,droop control and VF control,and analyzes a control method of bidirectional droop for AC/DC microgrid interconnection.In addition,the stability analysis method of microgrid and the mathematical basis of Lyapunov stability analysis method are introduced,and the distributed power supply stability analysis model of microgrid is established based on the control topology of droop method.(2)Aiming at the state of charge(SOC)equalization of multiple energy storage units in microgrid,an improved droop control of microgrid considering the SOC balance of each energy storage unit is proposed.Firstly,the output current characteristics and SOC changes during the charging and discharging process of the energy storage unit under the traditional droop control are studied.On the basis of the traditional droop control,the SOC equalization factor is introduced to control the output current of the DC/DC converter,which can help achieve the goal of SOC equalization of energy storage units with different states of charge.In addition,the influence of line impedances on SOC equalization effect is analyzed,and an adaptive control method is proposed to eliminate the SOC difference caused by the difference in line impedance.Due to the introduction of SOC equalization factor,the droop coefficient becomes a variable coefficient,which will affect the stability of the system.For this reason,the characteristic equations for analyzing the stability of the system are given based on the control strategy and control structure diagram.The Lawrence stability criterion verifies the stability of the proposed control strategy.Simulation examples in PSCAD verify the effectiveness of the proposed method.(3)A microgrid stability analysis method based on bifurcation theory is proposed for microgrid systems with virtual synchronous droop control.Firstly,the unified state space equations of microgrid with distributed power,lines and loads are established.Secondly,based on the established state space equations,the eigenvalue analysis method is used to study the dominant factors affecting the stability of the microgrid system.Besides,the bifurcation theory is utilized to study the key control parameters under virtual synchronous control mode and the influence of system topology parameters on system stability,and the boundary conditions of micro-grid parameters under the premise of ensuring the stability are determined.Finally,the simulation model is established in PSCAD to verufy the validity of the results.(4)Aiming at the AC-DC hybrid microgrid with bidirectional droop control,a system stability analysis method based on matrix perturbation theory is proposed.Firstly,by introducing the matrix perturbation theory based on complex mode,the perturbation component solution method for parameter perturbation of the system is given.Then,according to the distribution of the system eigenvalues,the isolated eigenvalue and the heavy eigenvalue are studied respectively.In these two cases,the method of solving the eigenvalue perturbation and the eigenvector perturbation of the system are proposed.Based on the small-signal analysis model of the AC-DC hybrid microgrid,the accuracy of the perturbation method and the actual eigenvalues is compared,and the system is generated in the control parameters.In addition,the low stability margin eigenvalues during perturbation are analyzed,which gives certain guiding significance for the selection of parameters in microgrid controllers.Finally,a simulation model is established in PSCAD,and the effectiveness of the proposed method is verified.