Sequence domain transfer matrix model of three-phase voltage source converters

Abstract

Interactions between three-phase grid-connected voltage source converter (VSC) and ac network have been analyzed using the VSC ac-side impedance models in the dq-domain, and more recently, in the sequence domain. Both dq and sequence domain approaches ignore the effects of a network at the VSC dc-side terminals. Interactions between VSC and dc network have been similarly analyzed by ignoring the effects of the ac network. This paper presents a sequence domain transfer matrix model of a grid-connected VSC, which accurately captures coupling between the ac and dc sides of the VSC and also between the positive and negative sequence components of the three-phase VSC variables. The proposed model is used to analyze stability of a VSC-based HVDC terminal in an offshore wind farm. It is shown that the analysis of resonance and stability problems, at low frequencies on the dc-side and/or around the fundamental frequency on the ac-side, has to consider the complete matrix model of the VSC and the coupling terms cannot be neglected. The transfer matrix model and stability analysis predictions are validated using simulations of an offshore wind farm with the HVDC transmission to grid.