Robust Control and Stability Analysis of VSC Systems
Time: Fri 2025-09-12 09.00
Location: Kollegiesalen, Brinellvägen 8, Stockholm
Language: English
Subject area: Electrical Engineering
Doctoral student: Feifan Chen , Elkraftteknik
Opponent: Professor Ioannis Lestas, University of Cambridge, Cambridge, England, UK
Supervisor: Professor Xiongfei Wang, Elkraftteknik; Adjunct Professor Lennart Harnefors, Elkraftteknik
QC 20250815
Abstract
The rapid integration of renewable energy sources is transforming traditional power systems into converter-dominated networks, characterized by low inertia and complex dynamic behaviors. This shift introduces fundamental challenges to stable system operation, particularly the risk of low-frequency oscillations, where synchronization, power, and voltage-control dynamics exhibit complex couplings. To this end, the thesis investigates low-frequency oscillation risks from three interconnected dimensions: converter-level control design, decentralized stability assessment, and system-level coordination in multi-converter environments.
The first part of the thesis focuses on control design for grid-connected voltage source converters (VSCs). Starting with grid-following (GFL) converters, which traditionally rely on strong grid conditions for stable operation, the work identifies root causes of instability through block diagram modeling. An active damping control strategy (Q-VID) is proposed, enabling stable operation with a 400-Hz phase-locked loop (PLL) bandwidth even under ultra-weak grids with a short-circuit ratio (SCR) of 1.28. Building on this, a grid-forming (GFM) control scheme is developed using a PLL-synchronized architecture. This approach retains the simplicity and implementation compatibility of conventional GFL methods while providing voltage and frequency support with robust performance under both strong and weak grid conditions.
Effective control design is closely tied to stability assessment methods, which provide essential theoretical support and stability specifications. Moreover, these assessment methods assist in risk localization, elimination, product specification determination, and adherence to grid codes. Hence the second part addresses the need for decentralized stability assessment methods, which become increasingly important in converter-rich power systems. While passivity theory serves as a fundamental tool, its application in the low-frequency range is limited. To overcome these limitations, the thesis introduces a rotated passivity index by integrating passivity and multiplier theory. This extended formulation enables full-frequency-range stability assessment.
The third part of the thesis extends the proposed control and stability analysis framework to multi-converter systems. It begins by modeling the parallel operation of multiple converters. Building on this foundation, the previously developed control strategies and stability assessment methods are integrated into a unified framework that enables risk assessment, instability source identification, and mitigation across multi-converter environments.
In summary, this doctoral research ensures the stable operation of converter-dominated power systems by contributing across three levels: control design (GFL and GFM), theoretical tools for decentralized stability assessment, and system-level coordination in multi-converter networks. All proposed methods are validated through both simulation and experimental results. Together, these contributions form a coherent and practical methodology for next-generation grid integration.