Analysis, Modelling, and Mitigation of Sub-synchronous Oscillations in a Weak Grid with High Inverter-Based Resources

Australian power system. The area is known for its low system strength, due to its large electrical distance from nearest synchronous generators, a long and sparse transmission network, and large number of IBRs in close vicinity of each other. The SSO events in the area have been intermittent and often without any clear network events or disturbances. The frequency of oscillations has been between 17 Hz and 20 Hz, when measured using root-mean-square quantities, captured through 50 samples/cycle resolution. Although the magnitude of voltage oscillations has mostly been small, at times, it has reached 2% peak-to-peak at some high voltage 220 kV nodes in the area.

In this paper, first the key characteristics observed in the measured oscillations have been analyzed to improve understanding of the phenomenon, perform root cause analysis, and narrow down the potential contributors. Then, the oscillations were accurately replicated, through detailed modelling of what observed in the field. It was identified that the root cause of the oscillations was the DC side voltage instability of a local IBR following a reduction in solar irradiance (e.g. due to cloud coverage), combined with very low system strength at the point of connection (PoC) of the contributing IBR. Following the replication of the oscillations, a number of remedial measures were investigated to mitigate the magnitude of the oscillations. These included: tuning the inverter control system, installing synchronous condensers (Syncons), using grid forming (GFM) inverters, and constraining the number of online inverters of the contributing IBR. While the tuning solution mainly aimed to improve the damping performance and stability of the IBR’s control system, the Syncon and GFM solutions aimed to 1 mitigate the oscillations through improving the system strength at the IBR’s PoC. The efficacy of the proposed solutions was tested using both single-machine infinite-bus and wide-area modelling approaches. Among the investigated solutions, the control system tuning was implemented on the contributing IBR and it was proven to be effective in eliminating the SSO, as is presented in this paper.