Studies Of Operation Characteristics Analysis And Control Methods Of Hybrid AC/DC Microgrid Under Complicated Operation Conditions

As the environmentally efficient and flexible power generation way,the distributed generation such as photovoltaic and wind power give full play to superiority in replacing traditional fossil energy,relieving environmental pollution and climate warming.The AC and DC microgrid provides an effective mean to autonomously integrate the multi-type distributed generations,storage units and loads,and has become the attention focus around the world.However,the fluctuation of distributed generation and load power,the low inertia characteristics caused by large amount of grid-connected power electronic equipments,the unbalanced grid voltage and the complicated conveters seriously threaten the safe and reliable operation of the microgrid system and restrict the sustainable development of distributed energy.Therefore,the theoretical and systematical analysis of the operation characteristics and discussion of reasonable control methods and strategies of AC and DC microgrid under complicated operation state are possesses essential theoretical and practical significance significant to ensure a safe and reliable microgrid system,and to guide the development of smart power distribution systems toward more economical,safe,and environmental-friendly directions.This paper mainly focuses on the efficient and reliable operation of ac and dc microgrid under complicated operation conditions,and conducts overall study by theoretical characteristics analysis,control method research and simulation case verifications.In islanding hybrid AC/DC microgrid,a well-designed control of the bidirectional AC/DC converter can achieve independent power distribution and improve the anti-interference ability of AC and DC buses.In this paper,through the equivalence of AC and DC subgrids in the hybrid microgrid to the integrated source,the active power droop control equation of the AC side and DC side of the bidirectional AC/DC converter is given.Then proportional sharing based outer loop power control strategy is proposed to realize the power balance and independent allocation in the hybrid microgrid.Meanwhile,owing to that the convention inner loop Proportional-Integral(PI)control cannot get ideal voltage dynamic response,the voltage fluctuation of the bidirectional AC/DC converter is analyzed.Then an enhanced inner loop control through the disturbance observer and disturbance suppression is proposed,which can track the reference va lue with no steady error and improve the robust stability of the microgrid.Simulation results show the validity and efficiency of the proposed control strategies.The inertia characteristics of the hybrid AC/DC microgrid makes it unable to use inherent moment of inertia for improvement of transient characteristics during power fluctuation.In this study,the virtual inertia equation and the virtual capacitance equation are firstly established through simulating the inertia and damping characteristics of synchronous generators.Then the improved control based on virtual synchronization machine(VSM)is proposed by integrating the active power balance equation and eliminating the steady-state error with PI controller.Next,the small signal and large signal characteristics analysis of the converter are conducted to analyse the VSM control feature with different control parameters.On the basis of the small signal analysis,a proportional power sharing principle to select the control parameters is given to make DGs share the power commensurate with their power ratings.Finally,simulation cases are carried out to show the validity and efficiency of the proposed control strategy.When the dc part is disconnected from the AC/DC hybrid microgrid,the isolated dc microgrid is formed.In the droop-controlled DC microgrid with conventional droop control,the allocation precision of load power will be reduced due to the inconsistent line parameters of each DG and the existing of local load.Therefore,the maximum efficiency of the distributed generation can hardly be realized and the overload condition may even happen.Besides,the voltage drop in the lines will furtherly reduce the voltage quality of the DC bus.Therefore,the accurate power allocation strategy and the zero steady-state error voltage control of the islanding DC microgrid based on adaptive droop control are proposed by introducing the adaptive virtual resistance and dc voltage recovery convergence coefficient in this paper.The influence of local load is also considered in the power allocation strategy.Meanwhile,response characteristics analysis of the proposed control of the DC/DC converter is conducted and the influence of key control parameters on system stability is discussed.Simulation results show the validity and efficiency of the proposed control strategies.Under unbalanced grid voltage conditions,the power of the grid-connected converter will produce the double frequency power ripples.The mathematical control equ ations of the double frequency ripple power of converter under unbalanced operating conditions are firstly derived and established,where the dynamic behaviors of the double frequency ripples in active and reactive power are regarded as being driven by cur rent-relevant components and voltage-relevant components,respectively.Based on the equations,a robust passivity-based power compensation control with disturbance observer for the converter of the grid-connected microgrid under unbalanced grid voltage is proposed.The passivity-based control is responsible for the tracking control of current-relevant components with the expected value,while the disturbance observer focuses on the compensation of the voltage-relevant components.Theoretical stability anal ysis and simulation cases show the effectiveness and superiority of the proposed strategy.The AC and DC side current fluctuation occurs of the modular multilevel converter(MMC)of the grid-connected microgrid under asymmetric arm conditions.In this study,The performance characteristics analysis of MMC is conducted based on the average switching function current model,which demonstrates that fundamental and double frequency ripples exist in the arm circulating current under asymmetric arm parameter conditions.Besides,there will be dc and double frequency components in the ac side current,while fundamental and double frequency ripples also appear in the dc side current.According to the analysis results,the arm current control based on an improved Proportional Integral-Resonant(PIR)controller via the feedback adjustment control(FAC)method is proposed to suppress the asymmetric components with significantly enhanced dynamic response by introducing the integration and feedback loops of the FAC me thod to the conventional PIR controller.Simulation cases verify the correctness of the theoretical analysis and the validity of the proposed control.