The effect of the offshore VSC-HVDC connected wind power plants on the unbalanced faulted behavior of AC transmission systems

This paper studies the effect of the negative sequence current control scheme of a VSC-HVDC system on the positive, the negative and the zero sequence voltage and current components of a 380kV onshore AC transmission line during sustained unbalanced AC faults. It is assumed for this paper that the protection schemes in the AC transmission network fails. Hence, the unbalanced fault is sustained for a longer time period. In this frame the response of the AC transmission system is observed for two different applied negative sequence current control strategies at the onshore converter station. It is shown that the suppression of the negative sequence current, as it is mainly performed by vendors today or required by TSOs, might lead to difficulties in the detection and the isolation of the line-to-line AC faults. On the other hand, the case of negative sequence current injection proportionally to the negative sequence voltage, improves the ability to detect line-to-line faults close to the converter terminals. This paper uses detailed PSCAD/EMTDC time-domain simulations supported by a linear circuit analysis in the positive, the negative and the zero sequence circuits.