The Research Of Islanded Microgrid Based On Droop Control In Aspect Of The Output Impedance Profiles

With new policy promoted by Chinese government to support energy storage industry,microgrid based on energy storage has been widely established in China.And power-electronic-interfaced equipment is also widely for the grid frequency and voltage regulation.The power-electronic-interfaced equipment usually simulates the drooped profile of synchronous generator.But the droop-based control microgrid is different from the traditional power system of mechanical generators,which has low level electro-mechanical inertia,severe voltage sag and frequency fluctuations,and poor anti-interference ability.The micro-grid is lack of synchronous and damping torque,frequency and active are coupled naturally,voltage stability depends on reactive power demand,and the relationships of two couplings are appropriated confusion.Therefore,the stability problems of islanded microgrid are more severe than that of the conventional power system.In aspect of inverter output profiles,it is an effective method to study the stability of the droop-based microgrid.Therefore,the stability of the droop-based islanded microgrid will be researched by inverter output profiles,the main researches are as follows:1.The output impedance model of droop inverter is established by the small signal modeling method.The sensitivity of droop inverter output impedance model is analyzed with key parameters distribution.It is revealed that the droop control mainly affects the output impedance profiles at lower frequency,and the output impedance of other frequency are mainly affected by the voltage outer loop control parameters and capacitance value of LC filter.It is constructed that three-parameter approximation model,based on the voltage outer loop control parameters and the LC filter capacitance Cf,for simplifying output impedance model and proving the correctness of the above conclusions.The simplified impedance model can also be used in the stability analysis of aggregated droop inverters.2.The influence of voltage outer loop integrator on power sharing is studied.A power sharing enhancement is proposed to improve microgrid voltage drop caused by the traditional virtual impedance control,which bases on adaptive integral parameter control.In addition,parameter sensitivity analysis is researched when the QPR control(Quasi Proportional Resonant Control)is used.It is revealed that the difference of the control parameters of the QPR control has little influence on the fundamental frequency output impedance,and can improve the impedance profiles at other frequency regions.It means that is the QPR control can improve the power sharing performance among inverters,and can also damp the resonance of the related frequency regions.3.Based on the proposed impedance model,the harmonic interactions of aggregated droop-based inverters are studied,and a research direction based on the output impedance of the inverter is provided to study the stability of aggregated droop inverters,based on the output impedance model.And the frequency region,where occurs unstable issue,should be founded at first.Then,the output impedance should be reshaped by related parameters or developed the harmonics suppression controller,based on the output impedance characteristics.4.Three experimental prototypes based on the drooping inverter were developed,hardware and software of the droop control was designed.The single-inverter output impedance measurement was carried out.Power sharing experiments based on the proposed adaptive voltage loop integrator control,resonance experiments under reactive power compensation and circulated resonant experiments among aggregated inverters were also carried out.The correctness of the output impedance model,the power sharing enhanced method and resonance suppression method are verified.