Research On Resonance Mechanism And Suppression Of Multi-inverter Grid-connected System

With the global shortage of non-renewable energy and serious environmental pollution,the development and utilization of new energy is particularly important.In recent years,the installed capacity of new energy has increased dramatically,forming a high permeability distributed new energy generation unit,which leads to transmission line impedance can not be ignored and constitutes a weak grid environment.Distributed new energy with large capacity and high permeability in weak power grid poses a threat to the safe operation of grid system.With the increase of grid impedance or the number of grid-connected stations,the interaction between output impedance and grid impedance of grid-connected inverters increases,which eventually leads to system resonance.At the same time,the impedance of power grid limits the maximum active power transmission of new energy and reduces the stable region of system operation;while the output phase of traditional phase-locked loop(PLL)is disturbed by voltage background harmonics,and the dynamic response of PLL reduces the amplitude and phase of the output impedance of the inverter,which enhances the interaction between impedance.Based on the above problems,this paper carries out a detailed analysis and gives a solution.The specific research contents are as follows:A hybrid damping control scheme based on phase compensation quasi-proportional complex integral controller(HC-QPCI)is proposed according to the closed-loop structure block diagram and open-loop transfer function of grid-connected inverters in weak grids.The phase margin and amplitude margin are taken as the objectives of detailed parameter design to ensure that the hybrid damping coefficient remains basically constant in weak grid.This method can also effectively suppress resonance while applied to multi-machine grid-connected inverters.To solve the problem of maximum power transmission constraints in weak power grids,this paper draws the conclusion that the introduction of resistance line impedance or reactive power can effectively improve the maximum active power of power transmission.Aiming at the influence of positive and negative sequence voltage on PLL,this paper presents a fundamental complex resonance controller,whichcan extract the fundamental wave voltage of power grid,thus eliminating the influence of harmonics on PLL.In order to reduce the influence of dynamic response of PLL on the amplitude and phase of output impedance of inverters,the output impedance of inverters affected by PLL in αβ coordinate system and dq coordinate system is compared.It is concluded that the output impedance of the inverter is less affected by PLL in the αβcoordinate system,and the control of the system is optimized.Finally,the proposed control strategy of hybrid damped grid-connected inverters for weak grid is simulated and validated on Simulink platform,the experimental platform of DSP+FPGA control board and diode-embedded three-level LCL inverters is built.The experimental results show that the proposed control scheme is effective and feasible.