As synchronous generators are phased out of modern power systems, grid-forming inverters (GFMs) are increasingly expected to take on the dynamic roles once played by rotating machines, including inertia, voltage control, and damping. This study investigates the extent to which controlled GFMs—supplemented with power oscillation damping (POD) controllers—can replace both synchronous generators and synchronous condensers (syncons) in a high-renewable, low-inertia grid.

This investigation builds on earlier work involving syncons with POD controllers to damp inter-area oscillations in systems dominated by grid-following (GFL) inverters. It employs the publicly available New Zealand South Island transmission system model (provided by the New Zealand Electricity Authority) as a representative weakly interconnected grid for dynamic stability analysis.

In the first phase, syncons that provided fault-level support and damping were replaced with GFMs. These GFMs provided voltage and frequency support, and their control loops (with integrated POD controllers) were tuned to match or exceed the original damping contribution. In the second phase, large synchronous generators were replaced by GFMs using a virtual synchronous machine (VSM) control mode, with a POD controller integrated into their outer control loops to target inter-area oscillatory modes.

Eigenvalue analysis across these phases showed that:

• GFM control improved damping compared to the GFL-only case,
• POD control further improved the damping of inter-area modes,
• The system remained stable even without syncons when GFMs were strategically placed and equipped with tuned POD controllers.

A broadband disturbance approximating white noise was injected at the point of common coupling (PCC) using a Fourier source to support the modal analysis. The frequency response at critical buses was then evaluated using Fast Fourier Transforms (FFTs). This approach allowed direct comparisons between stages, revealing potential resonances and damping effectiveness across various scenarios.

However, implementing POD controllers within GFM control structures introduces challenges, particularly due to interactions among virtual inertia emulation, voltage regulation, and damping control. In this study, the POD controllers were tuned to target inter-area modes using a combination of the GFM terminal frequency and active power flow on remote branches as input signals—similar to the approach used previously with synchronous condensers. This configuration enables the GFMs to respond to wide-area oscillations. The study provides practical insights into GFM–POD coordination and tuning strategies to ensure robust dynamic performance in weak grid environments.

This work demonstrates that GFMs with POD controllers can replace synchronous generators and condensers in weak grids. Dynamic simulations, including eigenvalue and frequency response analyses, form part of a research project on future inverter-based system configurations, supporting this conclusion. The findings are relevant to network planners, developers, and operators managing the transition to high-renewable, low-inertia grids.