Stability Analysis And Impedance Reshaping Strategy Application Of Frequency-coupling Grid-connected System

With the rapid development of distributed power generation systems,more and more renewable generations are connected to the grid through grid-connected inverters,and the interaction stability between grid-connected inverters and the grid is increasingly affected.With the goal of improving the stability and power quality of the grid-connected system,this paper deeply studies the dual-frequency coupling mechanism caused by the nonlinearity of the inverter control system,and the multi-frequency coupling mechanism caused by the unbalanced hardware parameters and grid parameters.And then the frequency-coupling impedance model of the inverter grid-connected system is established.Based on the model,the impedance reshaping method and local harmonic absorption strategy are discussed under the conditions of high short-circuit ratio of the system and harmonic voltage interference.The proposed method improves the stability and power quality of the grid-connected systems.The main contributions are mainly reflected in the following aspects:(1)In the state of balance grid,the dual-frequency coupled impedance models of the voltage-controlled inverter and the power-controlled inverter are established,respectively.The influences of phase-locked loop bandwidth,voltage-loop controller gain and power-loop controller gain on frequency-coupling characteristics of impedance are analyzed.The frequency sweep method of the dual-frequency disturbance injection method is proposed to verify the accuracy of the dual-frequency-coupling impedance model.Further,based on the frequency-coupling model,the paper studies the critical stability conditions of the grid-connected system in a weak grid.And the influence of the stable operating range of the system is analyzed.The proposed model can contribute to the parameter design of inverter control system and impedance-reshaping strategy.(2)Considering the unbalanced grid voltages and the unbalanced filter parameters,the multi-frequency-coupling impedance models of the voltage-controlled and the power-controlled inverter are established,respectively.The multi-frequency coupling models are simplified to the dual-frequency-coupling model of the main harmonic pair,which reduce the order of impedance matrixes.Based on this simplified models,the influences of negative-sequence fundamental voltages and filter parameter imbalance on impedance-coupling characteristics are analyzed.Then the critical stable range of the grid-connected system is analyzed with unbalanced grid voltage and the unbalanced filter parameters,which can evaluate the feasibility of grid-connected inverters when the grid contains single-phase loads or three-phase faults.(3)The mechanism of system instability of the grid-connected system with multiple parallel inverters under weak grids is studied,and the magnitude-reshaping strategy based on the Middlebrook stability criterion is proposed to improve the stabilities of grid-connected system for multiple control types of inverters.The study found that the accumulation of negative impedance of the grid-connected system is the main reason for the decrease of the stability.Based on the frequency-coupled impedance model and Middlebrook stability criterion,the paper proposes a unified impedance magnitude-reshaping strategy,which is suitable for power-controlled and voltage-controlled inverter.The magnitude-reshaping strategy increases the harmonic impedance of the inverter in the subsynchronous,super-synchronous,and mid-to-high frequency bands,so that the impedance ratio between the inverter output impedance and the grid impedance is within the feasible range of the Middlebrook stability criterion.Thereby the stability of the multi-parallel inverter system is guaranteed.(4)Considering the harmonic problems caused by local nonlinear loads and power sources,an impedance magnitude-reshaping strategy based on equivalent impedance analysis of grid-connected systems is proposed to actively absorb these local harmonic currents.Firstly,this article studies the parallel operation of grid-connected inverters and local harmonic sources in weak grids.The analysis points out that local harmonic sources may include integer harmonics caused by nonlinear loads,non-integer high frequency harmonics caused by power-source oscillations,and series compensation equipment or sub-synchronous oscillation of synchronous generator.These harmonics have a greater impact on the power quality of grid.It is found that traditional harmonic control methods are difficult to suppress the above-mentioned harmonics at the same time.Therefore,this paper proposes the magnitude-reshaping strategy,which can simultaneously absorb three local harmonic sources.The comparison experiments beteewn traditional harmonic control strategies and proposed method prove the effectiveness of the proposed method.The magnitude-reshaping strategy can ensuring the power quality of grid-connected current.