Research On Adaptive Control Strategy Of Three-phase Grid-connected Inverter

with the rapid development of distributed generation technology,the location of access to the power grid is more widely distributed.Whether it is to supply power to remote areas or the increasingly flexible distribution of distributed power sources,these two situations are accompanied by an increase in the number of changes and an increase in transmission lines,which eventually leads to a weakening of the relationship between the “source networks”.At this point,the grid presents the characteristics of a weak grid.This paper will focus on the closed-loop control strategy of the three-phase grid-connected inverter,and the adaptive control strategy of the inverter in the weak grid environment,which ensure it could be reliable and stable operation.The stable operation of the three-phase grid-connected inverter is the basic requirement for its grid-connected operation.Design the system parameters of the three-phase grid-connected inverter based on the output power characteristics of it.Firstly,under strong grid load,build a mathematical model of a three-phase grid-connected inverter in a three-phase stationary coordinate system and a two-phase rotating coordinate system.Among them,the output filter of the grid-connected inverter has three forms: L,LC,LCL.Since the LCL filter has good attenuation characteristics for high frequency harmonics.In order to enable the inverter to feed the required power to the grid,this paper uses an LCL type filter and carries out design parameters according to relevant technical specifications.Analyzing the output power characteristics of the inverter.The system model is simulated and analyzed to verify that the output power of the inverter meets the requirements.Then in the weak grid environment,the control strategy for the stable operation of the inverter is studied.The mathematical model of three-phase grid-connected inverter under weak grid load is established,and analyzing the influence of weak grid impedance on the operation of LCL inverter,and the parameters of closed-loop controller are designed.The power-current control strategies is compared with power-voltage control strategies,which verified that under the weak grid load,the inverter with power-voltage control strategy can operate stably.In addition,when designing the parameters of the controller,considering the resonance problem of the LCL filter,this paper uses the active damping control strategy to suppress the resonance.The real-time detection method of real-time triggering grid impedance based on harmonic current perturbation injection is studied,which is used to achieve accurate measurement of weak grid impedance.Subsequently,analyzing the operating state of the inverter based on the measured weak grid impedance value in real time.On this basis,combined with the above-mentioned grid-connected control strategy,proposing an adaptive control strategy for the stable operation of the inverter,so that the inverter can achieve better stable operation and dynamic response to control signals in the weak grid environment.Analyzing a method for obtaining the reference sine and cosine signals,which's frequency and phase are same with the grid voltage fundamental wave.And establishing the output impedance model of the inverter,which proves that the grid-connected inverter with adaptive control strategy can feed the power grid stably in the case of grid impedance change.Finally,designing a software control system to carry out relevant experimental verification work on the experimental device of the three-phase grid-connected inverter system.The experimental results are analyzed to demonstrate the feasibility and effectiveness of the adaptive power-voltage control strategy,and the correctness of the simulation results.