On resonance instabilities in VSCs connected to weak grids

Abstract

This paper addresses the oscillatory stability phenomena studied in the literature and reported by network operators in grid-connected VSC systems. Negative interactions have been observed at different frequencies in these systems, causing the tripping of power converters (e.g. wind turbines or HVDC applications). The interaction between the controller of power converters and poorly damped AC networks are identified causes of these events. This paper analyses two methods to model grid-connected VSC systems applied in the literature to study stability: the state-space and the impedance-based model. Validation has been carried out between the two methodologies by analyzing eigenvalues and singular values of the system. Furthermore, a time-domain simulation has been done to validate the state-space model with a non-linear MATLAB/Simulink model. The stability is studied in a base case system with a grid-connected VSC to the main AC grid via an LC circuit. A sensibility analysis of the strength of the grid and an analysis of subsynchronous and harmonic oscillatory resonances has been carried out for weak grids.