Sequence Impedance Modeling and Stability Analysis of Grid-Connected LCL-type Voltage Source Converter Considering Control Delay and the Frequency Coupling Effect of the SPWM Modulator

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Abstract
In the modern power system, the penetration level of the LCL-type voltage source converter (VSC) is increasing. Various complicated harmonic oscillation and instability problems are caused by its multi-timescale dynamic characteristic, which is a challenge for the safe and stable operation of the power system. The impedance model based on the traditional state-space average method, which ignores the frequency coupling effect of the SPWM modulator, can only accurately analyze the low-frequency instability. Therefore, to study the wide frequency range oscillation of such system, the frequency coupled impedance model of the LCL-type VSC is established considering control delay and the frequency coupling effect of the SPWM modulator in this paper. And an equivalent decoupled sequence impedance model is derived by the diagonalization method to simplify the stability analysis. On this basis, the grid-connected VSC system stability is investigated. The accuracy of the derived impedance model and the stability analysis results are verified by the time-domain simulation in MATLAB/Simulink.
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