NEM Inter-Area Modes Dependencies

Inter-area modes in power systems are influenced by a combination of network topology, control systems, generator and load dynamics, and operating conditions. Grid dynamics are currently undergoing changes due to retirement of synchronous generating units and increases in the volume of connected inverter-based resources (IBRs) - solar, wind and batteries.

Managing this change is crucial to maintaining power system oscillatory (small signal) stability and preventing large-scale oscillations that can lead to separations of regions or potentially wide area blackouts.

Displacement of synchronous generators is changing network topology, system inertia and system strength. As a result, the power oscillations dampers (PODs) and the selected power system stabilisers (PSSs) that were tuned to provide damping to the known inter-area modes in the Australian grid, previously comprised primarily of synchronous generators, may not dampen the oscillation modes adequately in the current and future power system. As the

Australian grid evolves further with a larger proportion of IBRs, there is a need for regular assessments of the inter-area modes and the associated damping.

The National Electricity Market (NEM) has three inter-area modes, named I20, I25 and I35.

I20 has historically been the least damped mode, and its frequency is around 1.9 rad/s. I20 mode is unstable without the PODs installed on the three SVCs in southeast Queensland.

As the grid evolves and the generation composition changes, the inter-area modes are subject to change too. With the reduction of system inertia, inter-area modes’ frequencies are expected to rise. However, due to the complexities of damping’s dependencies on many factors, estimation of the damping change is much more difficult to predict.

To assess changes in the inter-area modes, AEMO has analysed the Oscillatory Stability

Monitor (OSM) calculated inter-area modes data for selected years spanning from 2008 to 2024.

The OSM inter-area modes data were then plotted against various variables such as NEM demand, NEM inertia, NEM generation, interconnector flows, regional demands, regional inertia, regional generation etc. Many of the inter-area modes’ dependencies were observed to be as expected; others were contrary to past understanding.

This paper describes the methodology and results of AEMO’s analysis.