Quantitative analysis of sustained oscillation associated with saturation non‐linearity in a grid‐connected voltage source converter

Abstract

The interaction between the voltage source converter (VSC) and weak grid causes a negatively damped or diverging oscillation (DVO). The diverging oscillation generally evolves into a sustained oscillation (STO) when governed by the nonlinearities in the VSC's control system, such as the saturation of pulse width modulation (PWM). The conventional small-signal impedance/admittance modelling (SSIM/SSAM) and stability analysis methods are not suitable for the characteristic analysis of STO. This paper presents a large-signal impedance/admittance model (LSIM/LSAM)-based quantitative analysis method to better understand the mechanism, characteristics, and occurrence conditions of STO. The LSAM extends the SSAM by incorporating the nonlinearity of PWM saturation using the describing function method. First, a unique time-varying characteristic of the oscillation associated with nonlinearity is investigated. Then, two LSIM/LSAM-based occurrence criteria for STO are established based on an equivalent RLC circuit model of the system. Three critical pieces of information, including the occurrence condition, frequency, and magnitude, associated with STO are revealed by the criteria. Finally, the LSAM-based analysis method is applied to investigate the time-varying admittance of the VSC and to assess the magnitude and frequency of STO. Both the criteria and LSAM-based analysis results have been verified through time-domain simulations on a typical grid-connected VSC.