Research On Control Of Three-phase Grid-connected Inverter Under Unbalanced Grid Condition

In recent years,with the rapid development of wind power generation,photovoltaic power generation and other clean energy technologies,new energy grid-connected technology has become a research hotspot.Grid-connected inverter is an important interface circuit between new energy generation and power grid.The control strategy adopted by the grid-connected inverter directly affects the safe and stable operation of the grid-connected system.In the most common three-phase voltage unbalance condition,the grid-connected inverter control strategy based on the balanced grid lacks the control of negative sequence voltage,which will lead to the increase of output current harmonic and double frequency fluctuation of DC side voltage and output power.Therefore,it is necessary to adjust the inverter control strategy under unbalanced grid.This thesis takes three-phase grid-connected inverter as the research object,and the control problem of the inverter under unbalanced voltage condition is studied.The research content includes the following parts:As one of the key technologies of inverter grid-connected control,phase-locked loop technology is required to provide voltage/current sequence component information and grid phase and frequency information for inverter control under voltage imbalance condition.Therefore,the three-phase phase-locked loop technology in the case of imbalance,harmonic distortion and DC bias voltage is firstly studied.By introducing adjusting parameters to adjust the natural oscillation frequency,the positive/negative sequence components and phase information can be extracted more accurately without changing the dynamic performance and filtering performance of the phase-locked loop.Secondly,the power constant/current balance coordinated control of inverter under unbalanced grid voltage condition is proposed.According to the instantaneous power theory,the relationship of reference current between the output current balance and power constant is analyzed.It is found that the reference current in the current balance control is the fundamental component of the reference current in the power constant control,and the coordinated control between power constant and current balance is proposed by adjusting the reference current harmonic content.Then the current control loop in which fundamental current and harmonic current are separately controlled is designed,and on the basis of making full use of the frequency selection characteristics of it,the current control loop simplifies the calculation of reference current.Finally,it is verified by simulation.Aiming at the problem that the current control of grid-connected inverter cannot effectively improve the unbalanced voltage of point of common coupling,the voltage compensation control of inverter under unbalanced grid voltage condition is proposed.Firstly,the conventional droop control is analyzed in detail.then the voltage compensation principlebased on positive/negative sequence reactive power is introduced and the principle is used to design negative sequence droop control.Finally,an unbalanced voltage compensation control strategy consisting of positive sequence droop control and negative sequence droop control is obtained.Simulation data confirm that the proposed control strategy can effectively improve the unbalance voltage of point of common coupling.