The rapid integration of renewable energy sources presents several challenges, one of which is the decreasing inertia of power systems. As conventional synchronous generators are gradually phased out and replaced with converter-based resources, there is a reduction in overall system inertia. This requires exploring alternative methods to support the grid, with grid-forming converters featuring virtual inertia capability emerging as a potential solution. However, as the number of distributed energy resources increases, the responsibility for providing inertia services expands beyond traditional transmission systems to include generators within the distribution grid.

This paper investigates low-frequency oscillation issues associated with grid-forming converters in distribution networks, specifically focusing on the Virtual Synchronous Machine (VSM) converter control type. An analytical approach is initially employed, using a simplified system consisting of two interconnected VSMs to derive the fundamental equations governing their interactions. This analysis employs linearised equations to examine the system’s eigenvalues through small-signal analysis, revealing that increasing virtual inertia can negatively impact oscillation damping.

To further validate the analysis, time-domain simulations are conducted in a case study that includes both transmission and distribution networks, with multiple VSMs connected at the distribution level. The simulations confirmed the initial results from the analytical study, revealing that as inertia values in the distribution network increase without appropriate damping measures, the system may experience poorly damped oscillation modes and potential instability. It was also shown that suitable damping measures can help mitigate these risks.

The results highlight that distribution networks, like transmission systems, can also experience issues related to low-frequency oscillations. As the level of inertia provision increases, these oscillations may become less damped, potentially worsening the problem. Therefore, the findings emphasise the need for incorporating adequate damping measures and determining the appropriate levels of damping to ensure stable and secure operation in converter-dominated distribution networks.