Forced Subsynchronous Oscillations of Turbine-Generators Excited by Interharmonics From Bundled Offshore Wind Farms

There is a potential risk of forced subsynchronous oscillations (FSSO) caused by interactions of turbine-generators and subsynchronous interharmonics (SSIH) from their bundled offshore wind farms (BOSWF). This article investigates the FSSO interactions of turbine-generators and SSIHs from the direct-drive permanent magnet synchronous generators (DPMSG) based BOSWF. Firstly, the switching function method is adopted to analyze the generation of SSIHs from the modulation between the active machine-side converter (MSC) and the grid-side converter (GSC) of DPMSGs. The three conditions where the severe FSSOs of turbine-generators are excited by the SSIHs from DPMSGs are obtained by use of a close-form solution of differential equations with forced terms. The three conditions are the coincidence between the complementary SSIH frequency and the torsional mode frequency of turbine-generators, the relatively weak damping of the torsional mode of turbine-generators, and the relatively large amplitude of SSIHs. It is revealed that the FSSOs are bounded. A severe FSSO of turbine-generators can be caused by the SSIHs from the DPMSG-based BOSWF, but its active power has no obvious subsynchronous oscillations (SSO) except the small-amplitude power disturbance because the mode frequencies of the BOSWFs are far away from the complementary SSIH frequencies. Finally, time-domain simulations verify the proposed theoretical analysis.