Mechanism Analysis of Subsynchronous Torsional Interaction With PMSG-Based WTs and LCC-HVDC

In China, the development of wind-thermal-bundled power transmission by line-commutated converter-based high-voltage direct current (LCC-HVdc) is in progress. The dynamics in such a new scenario with three types of devices (three-device scenario), viz., synchronous generators (SGs), wind turbines (WTs), and LCC-HVdc, becomes salient and may cause subsynchronous torsional interaction (SSTI) issue. Such an SSTI issue is still but should not be underestimated. In this article, a path decomposition method is proposed to examine this SSTI issue from a mechanism perspective. With this method, the external electrical system is modeled as four explicit paths viewed from the SG rotor. This gives insight into the dominant paths and contributing devices of the SSTI damping and builds bridges among single-/two-/three-device equivalent scenarios that are used for SSTI studies. With these explicit paths, the impact of the interaction between WT and LCC-HVdc on the SSTI damping of SG is quantified. It is exposed from a specific case that the SSTI instability phenomenon in a three-device scenario does not appear in single-/two-device scenarios. The findings not only reveal that the interaction between WT and LCC-HVdc imposes a significant damping effect on the SSTI issue but also demonstrate that it is inadvisable to study the practical SSTI issue by simplifying such a three-device scenario to single-/two-device scenarios. The path decomposition method can be extended to power systems with more than three devices.