IMPACT OF GRID-FORMING INVERTERS ON IMPEDANCE- BASED ANALYSIS METHODS IN INVERTER-DOMINATED POWER SYSTEMS

• Main Author: Herlina
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/86850
• Institution: Institut Teknologi Bandung

The increasing penetration of renewable energy in the past decade has brought significant changes to the landscape of power systems. The large-scale integration of Photovoltaic-Battery Energy Storage Systems (PV-BESS) has become one of the main drivers, with PV serving as a clean renewable energy source and batteries providing flexible energy storage. The declining cost of solar panels, improvements in battery efficiency, and supportive policies from various countries have accelerated the adoption of PV-BESS. However, the dominance of inverters as the interface connecting PV-BESS to the grid introduces new challenges, particularly in small-signal stability. Unlike conventional synchronous generators, which possess high natural inertia, inverters lack inherent inertia, making the system more susceptible to small disturbances and complex, unpredictable dynamics. In this context, the Grid Forming Inverter (GFM) emerges as a key component offering potential solutions. GFMs can independently form voltage and frequency, mimicking the behavior of synchronous generators, thereby providing virtual inertia and enhancing system stability. However, comprehensive understanding of the impact of key GFM parameters—such as inertia, damping, and frequency droop—on small-signal stability remains limited. These challenges become even more complex when systems operate under various dynamic set points that reflect changing load and generation conditions, especially when inverter internal details are unavailable (black box scenarios). Moreover, weak grid conditions, characterized by relatively high grid impedance and low stability margins, further complicate ensuring system stability. This study aims to analyze the impact of GFM on small-signal stability in inverter- dominated systems using impedance-based methods. Three operational set points were chosen to represent varying PV-BESS working conditions: (1) P=0 pu, Q=max pu, emphasizing maximum reactive power supply; (2) P=max pu, Q=0 pu, depicting full active power supply; and (3) P=0.5max pu, Q=0.5max pu, representing a moderate condition with a balanced combination of active and reactive power. These set points were selected to provide a comprehensive v understanding of small-signal stability behavior under diverse, realistic, and even extreme operating scenarios, encompassing ideal and practical conditions. To address the lack of internal inverter information, this study applies an impedance-based approach using admittance scanning with Pseudorandom Binary Sequence (PRBS). This method eliminates the need for detailed internal inverter data, making it suitable for commercial inverters often treated as black boxes. PRBS signals are injected into the system to provoke responses at various frequencies, and through PV-GFM admittance scanning, the system's impedance characteristics can be externally identified. This approach is both efficient and flexible, enabling frequency response evaluation without delving into the complexity of the inverter's internal model. Small-signal stability analysis was conducted in the frequency domain. Bode plots were used to evaluate the system's magnitude and phase response to frequencies, enabling the assessment of stability margins, critical frequencies, and the system's sensitivity to parameter variations. Nyquist plots provided a comprehensive view of closed-loop stability, detecting potential instabilities through the visualization of magnitude-phase interactions in the complex plane. These techniques complement each other: Bode plots facilitate the interpretation of gain and phase margins, while Nyquist plots highlight whether the frequency response trajectory approaches the critical point (-1,0), indicating instability. The impact of system parameters on stability in GFM for the three set points (P=1, Q=0; P=0, Q=1; P=0.5, Q=0.5) was analyzed using Bode plot and Nyquist criteria. Results showed that increasing inertia (H) and damping (D) generally improved stability margins, such as phase margin and gain margin, while gain crossover frequency remained stable within a specific range. Conversely, excessively low droop frequency (FD) tended to reduce stability margins despite increasing gain crossover frequency. Optimal values of H, D, and FD are necessary to maintain system stability, characterized by phase margins above 45° and gain margins near 0 dB. This study provides guidelines for optimal parameter design in inverter-dominated systems to ensure operational stability across various load conditions. Compared to traditional methods, the PRBS-based admittance scan allows for a broader and more efficient identification of frequency responses without requiring access to detailed inverter internals. This approach is highly relevant for grid operators and system designers seeking to understand system stability without developing specific inverter models. The novelty of this research lies in the application of impedance methods to evaluate small-signal stability under diverse operating scenarios. The contributions of this study include practical guidelines for selecting optimal GFM parameters to support the stability of inverter-based power systems. The findings are expected to benefit system designers, grid operators, and researchers in addressing the growing challenges of PV-BESS integration. With appropriate vi GFM parameters, modern power systems—particularly in weak grid conditions— can operate more stably, reliably, and adaptively to dynamic load changes and renewable energy source variations. This study is anticipated to serve as an important reference in the development of next-generation inverter technologies and advanced control strategies to support the clean and sustainable energy transition.