Note: West Murray Zone - Torsional mode excitation in synchronous condensers in West

AEMO has been closely monitoring the presence of sub-synchronous oscillations in the West Murray Zone (WMZ) since 2019 and most recently has published the following documents on the AEMO website: • A comprehensive report on WMZ power system oscillations. [...] In 2019, AEMO observed sub-synchronous oscillations in the frequency range of 7-10 Hz in the WMZ. [...] In May 2023, AEMO was made aware of a hypothesis that the mechanical torsional modes of a synchronous condenser could be susceptible to the presence of these sub-synchronous power system oscillations, which may have contributed to mechanical shaft damage. [...] During this phenomenon, the electrical oscillations in the power system induced by the presence of series capacitors or high voltage direct current (HVDC) control modes could interact with the sub-synchronous complement of the turbine-shaft natural mechanical modes and result in damage to the turbine-shaft. [...] As mentioned in AEMO’s reporting, NSPs in the WMZ have installed additional monitoring in the area to improve the monitoring of sub-synchronous oscillations.