Research On Key Control Techniques Of Dual-mode Three-level Grid-connected Inverter In Micro-grid

Micro-grid is a small power grid system that integrates multiple new energy distributed generation units,control centers and local loads.It can operate steadily and effectively solve the uncertain generation problem caused by intermittent new energy to large power grid.Micro-grid can grid-connected operate with large power grid and can also take off into the island operation mode.When the large power grid fails,it enters the island operation mode,will continue the supply for the local load and give full play to the advantages of the new energy distributed generation.Micro-grid usually have multiple DG(distributed generation).In island operation,key technologies of micro-grid need to be solved are how to adjust and maintain the stability of the voltage amplitude and frequency of micro-grid,how to control the DG to be "plug and play" and how to control the DG to share the load according to its own capacity.Based on the background of the micro-grid composed of multiple photovoltaic inverters,from a single inverter,this paper studies the key control strategies,such as "plug and play",adaptive stability of voltage and frequency,and reasonable load sharing and so on.It studies the active synchronization method of fast synchronization with the large grid,thus realizing the parallel operation,stand-alone operation,smooth switching of grid-connected and off-grid.In this paper,a diode clamped three level inverter is selected as the main circuit topology.First,the working principle of the inverter is analyzed and the mathematical model is established by means of state space averaging method.Aiming at its midpoint voltage unbalance problems,a software balancing control strategy based on the space voltage vector modulation algorithm is proposed.In order to realize the dual mode operation and smooth switching of the inverter,the parallel operation control of the micro-grid multi inverters based on the current droop control is studied in this paper.The adaptive current droop control strategy is proposed,which can accurately share the load in proportion to capacity in parallel operation,at the same time eliminate the influence of system impedance of active current output,and reduce the influence of the inverter equivalent output impedance on bus voltage.In order to make the system impedance in resistance,a voltage and current double closed loop controller with virtual resistance is designed.The stability of the micro-grid system based on current droop control is analyzed,a small signal model of the system is established,and the influence of the changes of the parameters(active current droop coefficient,equivalent system impedance angle and so on)on the stability and response speed of the system is described through mapping root locus.For the leakage current problem in the non-isolated three-phase photovoltaic grid-connected inverter system,firstly the reasons for the arm side current low frequency oscillation and the excessive effective value of the connection line current in the existing leakage current suppression methods are analyzed.Based on the establishment of the circuit equivalent model,a hardware and software integrated leakage current suppression method combining independent split capacitor +LMZVM and LMSVM modulation is proposed.This method changes the modulation mode on the basis of improving the hardware circuit,reduces the common mode voltage change rate,and achieves the purpose of simultaneously suppressing the leakage current and reducing the effective value of the cable current.The effectiveness of the proposed leakage current suppression scheme is proved by experiments.Micro-grid work mode can be divided into grid-connected operation mode and island operation mode,when the switch from island mode to the grid-connected mode,the grid side voltage information need to meet certain standards to achieve the mode switch,otherwise,will have a larger impact current,the stable operation of the system and equipment damage.In this paper,the active synchronous control micro-grid with large power grid is studied,one of the key issues is slip signal detection,a slip signal detection method based on d-q coordinate transformation is proposed,namely the A,B,C three-phase voltage signal conversion to two-phase synchronous rotating coordinate system.By simple and effective calculation,the amplitude difference,phase difference and frequency difference of voltage between micro-grid and the large power grid can be quickly and accurately obtained.On this basis,local synchronous control without static error and the active synchronous control scheme of layered control of micro-grid and large grid are proposed,so as to realize the rapid synchronization and non-impact grid-connection between micro-grid and large power grid.The correctness and effectiveness of the active synchronization control scheme is verified by simulation and experiment.