Mechanism Analysis And Suppression Method Of Harmonic Resonance In Multi-Inverter Parallel System

In recent years,under the realistic background of energy crisis and environmental deterioration,the distributed photovoltaic power generation system composed of light energy collection,energy conversion,system protection and power transmission has become the focus and research direction of the power field.However,there are harmonic resonance phenomena among the subsystems of the distributed photovoltaic power generation system that may cause system oscillation and instability.Therefore,this paper takes the LCL multi-inverter parallel system as the research object,and the Linear Auto Disturbance Rejection Control(LADRC)as the core control method.The theoretical derivation and simulation verification are carried out from the establishment of the equivalent model circuit of the parallel system,the generation mechanism of harmonic resonance,the influence of the controller on the system performance,and the performance of the system under different working conditions.A cascade Improved Linear Auto Disturbance Rejection Control(CILADRC)double closed loop control strategy based on first-order Improved Linear Auto Disturbance Rejection Control(ILADRC)and second-order LADRC is proposed.Firstly,the generation mechanism of LCL inverter resonance is analyzed,and based on a single LCL grid connected inverter,the mathematical model of grid connected inverter in dq coordinate system is established;Then,the third-order LADRC strategy is applied to the system,the control block diagram of the closed-loop system is drawn,and the equivalent circuit model of a single inverter is established;Then,the equivalent circuit model of LCL inverter parallel system is established,and the influence of controller parameters on harmonic resonance and system performance is analyzed;Furthermore,in order to improve the reliability of the system and reduce the calculation amount of engineering application,the third-order system is equivalent to the superposition of the first-order and second-order systems.The first-order system uses the first-order LADRC and the second-order system uses the second-order LADRC;Further,in order to reduce the difficulty of setting the controller parameters and enhance the disturbance rejection of the system,the mathematical model of the second-order Linear Extended State Observer(LESO)is rewritten and simplified into a Decoupled Linear Extended State Observer(DESO)to form a first-order ILADRC.Finally,the first-order system adopts the first-order ILADRC,and the second-order system adopts the second-order LADRC to form the CILADRC,and the simulation model is built for simulation verification under different working conditions.The results show that the CILADRC double closed loop control strategy effectively suppresses the harmonic resonance phenomenon,not only ensures the stable and normal operation of the system in the steady state,but also improves the fault ride through ability and disturbance resistance ability of the system in the transient process.It has excellent engineering practical value.