Control Strategy Research And Optimization Of LCL Grid-connected Inverter

In recent years,wind,solar and other clean energy sources have been developed and harnessed on a large scale due to their reproducibility and environmental friendliness.Inverter is the key link of grid connection of clean energy distributed generation system.Among them,the total harmonic distortion of grid-connected current and grid-connected power factor are important indexes to measure the quality of grid-connected power generation.Due to the wide distribution of distributed generation system and the use of a large number of nonlinear loads at the distribution network end,the grid voltage contains a large number of background harmonics and the impedance of the grid can't be ignored,so the grid presents weak grid characteristics.Therefore,the robustness of grid-connected inverter plays a crucial role in the power quality and stable operation of the distribution network.Aiming at the LCL grid-connected inverter,the resonant damping strategy and the method of suppressing the background harmonic of the grid voltage are studied.In engineering,this paper proposes an improved capacitive current compensation control strategy to eliminate the influence of harmonics on the grid-connected current.The improved proportional resonance controller with phase compensation function improves the adaptability to weak power grid impedance.To solve the problem of inherent resonance of LCL filter,the root cause of resonance of LCL filter is analyzed and passive damping resonance suppression technology and active damping resonance suppression technology are discussed.The power control of voltage source type LCL grid-connected inverter is essentially the control of grid-connected current.The physical essence of the ability of PID control and PR control in natural coordinate system based on synchronous rotation coordinate system to track the instruction current signal without static difference is deeply studied.By deducing the closed-loop characteristic equation,it is proved that the inverter side-current-controlled PV inverter system has good stability.By block diagram transformation,it is found that the inverter side-current-controlled system can be equivalent to the inner loop using capacitive current negative feedback to suppress the resonant LCL filter resonant peak and the outer loop using grid-connected current to improve the current quality.The influence of voltage background harmonic on grid-connected current is deduced by block diagram transformation and formula,and the influence of background harmonic is eliminated by voltage feedback correction scheme.But in the case of weak current networks with impedance in the grid,the proportional primary differential The fundamental reason for the failure of the voltage feedforward control strategy is that the voltage feedforward not only introduces the grid voltage to eliminate the influence of the background harmonics on the grid-connected current,but also introduces the positive feedback channel of the grid-connected current,which leads to the large number of harmonics in the grid-connected current.The gain of the current controller at the harmonic frequency can be improved under the weak current network considering the impedance in the grid to eliminate the influence of the background harmonic on the grid-connected current and have a certain adaptability to the impedance in the grid.In engineering,inverter side current is often used to control grid-connected inverter to improve system stability and reduce product cost.But by harmonic impedance analysis found in the resonant frequency,power grid in the background harmonic voltage through the grid impedance,Lg,mesh side inductance L2 and filter capacitor C is harmonic current loop,and with the functions of harmonic compensation ratio resonant controller cannot get harmonic current information so that the grid current contains a lot of harmonic current.The improved capacitive current compensation control strategy can be equivalent to proportional control to control the inverter side current and resonant controller to control the grid-connected side current,so as to maintain the inherent system stability of the inverter side and eliminate the influence of background harmonics on grid-connected current.By applying appropriate phase compensation to the resonant controller at the resonant frequencies of 7,9,11 and 13,the adaptability of the system to the weak power grid impedance and the quality of the grid-connected current are improved.