Research On Coordination And Active Disturbance Rejection Control Method For Multi-inverter System In Microgrid

As an important means to alleviate the impact of high-proportion new energy integration into power grid,microgrid is an important support to large power grid,and plays a more and more important role in the process of power production and consumption.However,whenever in grid connected mode or island mode,there still exist many problems in high power-electronic-based microgrid due to its particularity on its networking form,such as insufficient reserve of mechanical rotary inertia,power quality interaction with its connected power grid,limited power supply voltage support capacity,etc.These problems will restrain the improvement of new energy consumption level and hinder the process of commercial popularization for microgrid.Therefore,in the context of following the sustainable energy development strategy and high power quality demand,these problems need to be effectively solved.Responding to these problems,this paper makes full use of the flexibility of inverter in distributed generation system in microgrid,and makes in-depth research on key technical issues such as harmonic cooperative control,inertia support control,high-quality power supply voltage control and distributed cooperative control of voltage and frequency in grid-connected/islanded microgrid,aiming to improve the safe and stable operation ability of microgrid and the“friendliness”level of microgrid when connected to power grid.The main work and contributions of this paper are as follows:1.Focusing on the power quality problem of droop-controlled microgrid in grid-connected operation,a coordinated control strategy based on microgrid inverters and a reactance-integrated hybrid active power filter is proposed.Firstly,the output impedance characteristics of droop-controlled inverter in microgrid and the key power quality problems in grid-connected mode are analyzed.Then,the harmonic impedance enhancement control strategy is proposed to effectively reduce the burden on harmonics for the grid-connected droop-controlled inverter,and improve its rejection capability against background voltage harmonics in the grid.In order to improve the harmonic“friendliness”of microgrid to the grid,a RIHAF featured with low capacity and integrated design is proposed to suppress the harmonics flowing into the grid in a centralized manner.The working principle,control strategy,integrated design method,DC side voltage control and dynamic reactive power compensation principle of the RIHAF are introduced.By establishing the harmonic equivalent circuit model of the grid-connected microgrid system,the filtering mechanism of the proposed coordinated control strategy is analyzed.Finally,simulation and experimental results verify the effectiveness of the proposed strategy.2.Focusing on the problem that mechanical inertia level declines due to high proportion of new energy generation integrated into the grid,the active inertia support control technique of grid-connected inverter without energy storage is proposed.Firstly,by establishing the mathematical model of traditional synchronous generator and three-phase grid-connected inverter,the mapping relationship between physical structure and control structure of three-phase grid-connected inverter and synchronous generator is revealed.Based on the derived one,the active inertia support control strategy coupled with dynamic characteristics of DC side capacitor of grid-connected inverter is proposed.To improve the stability of the grid-connected system,a physical damping resistance is introduced into the DC side of the grid-connected inverter,which is similar to the damping principle of synchronous generator.The small-signal model of the proposed system under the DC side physical resistance damping mode is established.The principle of DC side series/parallel damping resistance to improve system damping characteristics is analyzed,and the advantages and disadvantages of DC side series/parallel damping resistance are compared.On the basis of the DC side series resistance damping control method,the DC side virtual resistance damping control mode is further proposed,and the small-signal model of the system is established to demonstrate its consistency with the physical resistance damping control mode in improving the system stability margin.The output impedance model of three-phase grid-connected inverter under the proposed control strategy is established,and the influence of reactive power control loop on the output impedance of inverter is also analyzed,and the adaptability of grid connected inverter to weak grid is analyzed based on impedance ratio criterion.Finally,simulation and experimental results verify the effectiveness of the proposed method.3.Focusing on the problems that frequent load disturbance and insufficient rigidity of power supply voltage support exist in islanded microgrid,the application of linear active disturbance rejection control(LADRC)in voltage control of master inverter in microgrid is deeply studied to realize high-quality supply voltage support control of islanded microgrid.Firstly,the basic principle,structure and function of ADRC are systematically described and the general form of LADRC is given.Based on this general form,the design process,control parameter setting method and equivalent form of LADRC in frequency domain are introduced.Then,the mathematical model of the master inverter in dq coordinate system is established,and the total disturbance in the model is defined.Based on the mathematical model,the 2LESO-ADRC and3LESO-ADRC voltage controllers of the master inverter are established respectively.From the perspective of frequency domain analysis,the modified control models after2LESO and 3LESO compensation are established respectively.By analyzing the closed-loop transfer function of the modified control object,the constraint relationship among the control input gain b₀,the observer bandwidth?₀ and the system parameters is obtained based on which the value range of the control input gain is obtained,which provides effective reference for reasonable parameter tuning and engineering application of ADRC.The system control transfer function covering the feedback control law is established and the PID equivalent forms of 2LESO-ADRC and 3LESO-ADRC control systems are obtained,respectively,and their stability capabilities are compared and analyzed.Finally,the experimental results verify the improved effect of 3LESO-ADRC on the control characteristics of the master inverter.4.Focusing on the problem of voltage and frequency deviation in droop-controlled islanded microgrid,a distributed coordinated control method of voltage and frequency based on consensus protocol is proposed,which improves the disturbance rejection ability of microgrid and overcomes the problems of high communication cost and low reliability in traditional centralized control method.The proposed microgrid distributed coordinated control method includes a voltage distributed secondary controller and a frequency distributed secondary controller.For the voltage distributed secondary controller:Firstly,the nonlinear large-signal model considering droop control loop,voltage and current double closed-loop and inverter main circuit is established.Then,LADRC is used to observe and compensate the nonlinear and unknown model dynamics in the large signal model,and the quasi linear second-order system is obtained based on which the voltage distributed secondary controller is established.Then,the proof of controller convergence and the design basis of important control parameters are given in time domain,and the influence of communication delay on voltage consistency convergence is analyzed by Chebyshev discretization method;For frequency distributed secondary controller:the consistency control algorithm of frequency and active power is given,and the control method is given with the help of Lyapunov theorem.The convergence of frequency and active power is proved.Finally,the simulation results verify the excellent performance of the proposed control method in voltage and frequency restoration,communication fault robustness,plug and play,disturbance rejection and the correctness of relevant theoretical analysis.