Impedance -based Stability Analysis on IBR Integrated Power System

<p dir="ltr">This thesis examines the small-signal stability of inverter-based resources (IBRs) in a power system that contains grid-forming inverters (GFMs) using impedance-based methods. With the increasing integration of power electronics in modern grids, there is a need to study dynamic stability under the influence of complicated inverter-grid interactions. While the traditional eigen value-based methods have limitations in assessing the stability in converter dominated systems, impedance-based stability analysis is a more suitable alternative. This suitability arises from its ability to analyze interactions without requiring access to the internal state variables of every component; instead, it relies on the terminal frequency response (impedance), which can be measured or scanned from detailed EMT models, thus bypassing the common barrier of proprietary converter control data.<br><br>The study utilises the standard IEEE 9-bus system, where the conventional synchronous generators are gradually replaced with Virtual Synchronous Machine (VSM)-based GFM converters. Electromagnetic Transient simulations were conducted in DigSilent Powerfactory using frequency-domain perturbation methods. A customized simulation setup, based on Powerfactory’s Impedance Scan Template, created by DigSilent Powerfactory is used to extract frequency-dependent impedance characteristics from the inverter terminal. The methodology includes pre-simulation settling runs, automatic frequency scans by measuring the terminal voltage and current to calculate the complex impedance characteristics.<br><br>Stability is determined by accounting for loop gain functions between the inverter and grid impedances and evaluation using Nyquist plot under the Generalized Nyquist Criterion (GNC). This was done in both single IBR and multiple IBR configurations across several operating conditions. The results showed that the inverter’s impedance is relatively insensitive to its operating point when the SCR is high and thereby produces consistent loop gains with stable Nyquist margins. Under reduced grid strength, some sensitivity and potential phase lag was observed leading to reduced phase margins and further proximity to the critical point. These results present some insights into the use of impedance-based methods to identify regions of concern in frequency space to estimate dynamic stability margins in complex converter-dominated systems. The study concludes that impedance-based analysis provides a reliable framework for investigating the stability of grid-forming inverters under different network conditions.<br><br>Quantitatively, the system was found to be stable in both single and multiple IBR configurations, with the Nyquist locus not encircling the critical point (-1,0) in all cases. The analysis revealed that stability is robust in strong grid conditions, confirmed by a calculated Short-Circuit Ratio (SCR) of 8.6. Qualitatively, it was discovered that the inverter's impedance profile is highly dependent on its operating point; stressed, low-power, high reactive-power absorption scenarios produced more resonant impedance characteristics than high-power operation. Despite the presence of sharp mid-frequency grid resonances (300-800 Hz), the low-impedance nature of the GFM inverters provided effective damping. A basic time-domain validation was also conducted, which supports the frequency-domain analysis and identifies the resulting changes to the overall system behaviour when subject to small disturbances. Future work can take this framework further by employing coordinated multi-inverter controls, examining different grid-forming strategies and scaling this work to more complex, realistic transmission networks.</p>