Grid Support Functions and Stability Analysis of Inverter-Based Resources through Real-Time Simulations and Hardware-in-the-Loop Testing

The rapid growth of inverter-based resources (IBRs), such as solar photovoltaic (PV), wind, and battery energy storage systems (BESSs), combined with the ongoing decommissioning of synchronous generators (SGs), is leading to power systems with reduced inertia and a diminished ability to maintain stable voltage waveforms across the network. As a consequence, power systems worldwide are becoming increasingly vulnerable to, for example, steady-state undamped voltage and power oscillations, as well as blackouts triggered by frequency events. Therefore, seeking grid support functions from IBRs has become extremely necessary, particularly from PV systems, given their surging installed capacity in both utility grids and distribution networks. In parallel, it is crucial to implement new stability analysis tools and frameworks to better understand the small-signal interactions between IBRs and grid components which are more likely to occur in future IBR-dominated power systems, complementing existing time-domain analysis methods. These studies are key to ensuring the safe, reliable, and cost-efficient integration of more IBRs into modern power systems. This thesis first investigates advanced grid support functions from large-scale PVPPs without BESSs. A frequency support function that incorporates flexible power point tracking (FPPT) and power reserve control is proposed. This control scheme improves the rate of change of frequency (RoCoF) and frequency nadir during both under-frequency and over-frequency events. While maintaining a constant system-wide power reserve, the proposed approach allocates reserves across PVPPs based on local inertia levels. PVPPs located in low-inertia areas are assigned higher reserve levels and faster response settings, enabling them to deliver more timely and stronger power compensation to help maintain grid frequency within secure operational limits. The effectiveness of the proposed method is validated through extensive real-time simulations on a weak, longitudinal power system model with high penetration of IBRs. A real-time implementation of small-signal stability analysis based on frequency scanning and bus admittance is developed in this thesis. This method allows assessing the small-signal stability impacts of IBRs using black-box models or actual hardware prior to their connection to the grid, while reducing reliance on purely simulation-based studies during early design stages. The developed real-time platform is adaptable to various testing scenarios, including full electromagnetic transient (EMT) simulations using complete black-box IBR models, control-hardware-in-the-loop (CHIL) tests involving actual inverter controllers with simulated plants, and power-hardware-in-the-loop (PHIL) tests with fully integrated commercial inverters. The capability of this real-time and hardware-in-the-loop (HIL) platform is demonstrated through case studies including grid-following inverter models, an actual grid-forming controller, and a commercial power inverter. The stability interactions between an IBR model or hardware and a simulated network can be accurately identified using the bus admittance obtained from frequency scanning. Analysing potential oscillatory modes observed in real-time simulations and HIL tests supports informed decisions in plant sizing and controller tuning, helping to mitigate the risk of similar instabilities occurring in actual plants connected to the grid. Given the increasing number of PV systems connected to the distribution network, this thesis also explores the potential for aggregating these distributed resources to provide frequency support to the utility grid using FPPT and PR control, similar to the proposed method for large-scale PVPPs. The feasibility of this approach is evaluated through both simulations and experiments. By enabling advanced grid support functions in large-scale and distributed IBRs, and studying their interactions with the grid through real-time simulations and HIL platforms, the outcomes of this research contribute to the secure integration of a higher share of IBRs into modern power systems.