Eigenvalue stability analysis of subsynchronous torsional interactions

It is well-known that an LCC HVDC may deteriorate damping of torsional modes of nearby turbine-generators, and for weak AC grid connections the HVDC may even destabilize the system. There are fewer dynamic studies with MMC although recent literature indicates that an MMC HVDC connected in close proximity of an LCC HVDC may generally improve stability. However, these studies do not consider the generator-HVDC torsional interactions in the subsynchronous domain (SSTI). This article investigates the risk of the SSTI in a dual-infeed HVDC system (LCC and MMC) and in comparison to a single HVDC system. The test topology is inspired by the North French system in which a 2 GW LCC HVDC cross-channel link IFA2000 (1986) is connecting the French and the English AC grid. The HVDC is coupled on the French side with a 1.12 GW nuclear generator in the Gravelines plant and experimental studies showed that there may be a substantial risk of SSTI, particularly with respect to the dominant 6.3 Hz torsional mode. This system is now becoming a hybrid dual-infeed HVDC since a 1 GW MMC HVDC Eleclink is being installed in proximity of the existing LCC HVDC, on the French side. This raises concerns of new SSTI stability issues.