Do you mean "Subsynchronous Resonance"[SSR] phenomenon?
I saw –in the far past-a synchronous condenser[compensator] of 20/60 MVAR connected in medium voltage system through capacitors in order to delete the 40 MVA transformer reactance and to facilitate rapid compensation of a required reactive power on H.V. system.
A large induction motor connected in the Grid which stator winding was executed with q fractional [q=number of slots per pole and phase] produced a sub-harmonic EMF and a resonance was establish and the synchronous condenser was shut down. However, could be
other effects too. See for instance:
The synchronous frequency is the fundamental frequency (usually 50 or 60 Hz) of your system. Sub-synchronous frequencies are those below the synchronous frequency. I work in a utility environment, the sub-synchronous frequencies that we see are usually due to generator governors or AVRs fighting, or resonances within the transmission system (often excited by faults).