As the European grid transitions towards renewable energy targets, new system stability phenomena are being observed through the electrical power system. The share of renewable generation is growing quickly as synchronous machine-based power plants are phased out. Unlike traditional synchronous machine plants, these Inverter-Based Resources (IBRs) do not provide inherent synchronous mechanical inertia. Consequently, the overall system inertia is decreasing, posing a threat to the frequency stability of the system. This trend is observed worldwide and addressed by organisations such as ENTSO-E through the introduction of future minimum inertia thresholds.

Grid-forming (GFM) technologies have emerged as a solution, providing an immediate active power support after a disturbance, via control methods. These technologies enable IBRs to contribute to frequency stability and mitigate the Rate of Change of Frequency (RoCoF).
The share of distributed energy resources has also grown, with utility-scale renewable Power Park Modules (PPMs) connected to the Medium Voltage (MV) network. These PPMs could help unlock further active power frequency support from the MV network. However, whether this support effectively supports the transmission system remains insufficiently explored.

This thesis assesses the effectiveness of synthetic inertia provided to the transmission network by GFM-controlled IBRs connected at the MV level. The study explores the metrics to assess the effectiveness of the synthetic inertia at the MV-HV interface, investigates the technical constraints of the provision from the distribution-connected resources and offers mitigation measures. Additionally, the comparison of the synthetic inertia support from the HV and MV-connected GFM assets is done.

Firstly, using DIgSILENT PowerFactory, a test benchmark is developed, where the response of a storage PPM with grid-forming controls is evaluated under different disturbances, in order to tune the GFM dynamic models and ensure compliance with grid code requirements. To test the synthetic inertia support of PPMs in a realistic model, a combined transmission and distribution system model is developed from an initial model, which has been adjusted to represent the Dutch grid characteristics. Multiple scenarios and disturbances are simulated with the GFM PPM connected at various locations within the MV network to explore the limits of synthetic inertia support.

The results demonstrate that for a frequency disturbance, the RoCoF and the frequency nadir/zenith improve regardless of the PPM connection point. However, the magnitude of the improvement is dependent on the available active power headroom and the current capabilities of the PPM. A critical limitation was observed regarding the parallel provision of active and reactive power when the PPM is connected to the MV grid. For connection points far from the point of interconnection, voltage stability became more important, as MV buses are more sensitive to active/reactive power injection. During large frequency disturbances, the PPM's active power and reactive power demand grew, which led to current limits being hit, as the PPM sustains the active and reactive current. This suggested a need to enhance voltage and reactive power control or alternative voltage stability methods to support the voltage during frequency disturbances for distribution-connected grid-forming units.