Research On Control Strategy And Grid-connected Characteristic Of Indirect Current Control Inverter

With the increasing depletion of fossil energy,distributed generation system,as one of the main forms of renewable energy development and utilization,has great significance in improving the operation mode of power grid,improving the flexibility of power supply and the economy of power grid operation.At present,distributed generation system mainly has master-slave control structure and peer-to-peer control structure,which adopt direct current control and droop control grid-connected inverters respectively.Direct current control grid-connected inverters are prone to grid-connected impact due to the change of control loop structure when switching from parallel to off-grid,and the stability of local load power supply is low.Droop control grid-connected inverters can ensure smooth switching from off-grid and have better grid-connected characteristics.However,the frequency and voltage regulation capability provided by the power outer loop needs to be matched with large capacity energy storage link,which is generated when grid-connected.Reactive power increases the capacity and loss of equipment and lines.Indirect current control inverters not only ensure smooth switching of the system and off-grid,but also realize grid-connected per unit power factor.In addition,the system has good grid-connected characteristics,which has very important theoretical value and practical significance.Therefore,this paper focuses on the control strategy and grid-connected characteristics of indirect current control grid-connected inverters.Firstly,the basic principle of indirect current control strategy is introduced,and the parameters of control loop are designed according to different inner loop feedback modes.A phase-locked loop structure suitable for smooth switching of indirect current control is given,which realizes smooth switching of indirect current control inverters parallel to off-grid and grid-connected with unit power factor.Then,a complete positive and negative sequence of indirect current control grid-connected inverters is established.Based on the output impedance model,a notch filter virtual impedance is proposed to improve the grid-connected current waveform distortion caused by dead-time.On this basis,the grid-connected characteristics of indirect current control inverters with different inner-loop feedback modes are analyzed in detail,and compared with the parallel characteristics of direct current control inverters and voltage-source droop control inverters,the line resistance is explained.It has better grid-connected characteristics in the anti-change power grid environment.Finally,the series virtual impedance is proposed to improve the grid-connected stability in the weak power grid environment with lower equivalent capacity.The validity of the proposed indirect current control strategy for smooth switching of operation mode,the correctness of impedance modeling and grid-connected characteristic analysis,and the improvement of grid-connected characteristics of indirect current control inverters by virtual impedance method are verified by simulation and experiment.