Current-vector trajectory–based diagnosis of open-circuit faults in NPC inverters for SSSC applications

Abstract This paper presents an enhanced fault diagnosis method for identifying open-circuit faults in the switches of Three-Level Neutral Point Clamped (NPC) inverters integrated into Static Synchronous Series Compensator (SSSC) systems. Although previous studies have mainly focused on open switch faults in two-level converters and have established the principle of vector current path analysis for diagnosing open circuit faults, this work extends this approach to include a three-level NPC converter integrated into an SSSC system operating under Watt–Var control. The proposed method includes single switch faults and 48 potential cases of double switch faults, where the path tilt characteristics for single faults and the distinctive path patterns for double faults are utilized. The proposed approach extends the current-vector trajectory technique to multilevel NPC structures operating in Flexible AC Transmission Systems (FACTS) devices. A theoretical current-vector model is derived using the Concordia transformation, allowing direct classification of typical and atypical faults by analyzing the deformation of the α–β trajectory. A key novelty of this work is the accurate detection of both single-switch and paired-switch open-circuit faults in NPC legs without requiring additional sensors or observers. Simulation results obtained in MATLAB/Simulink show excellent agreement between theoretical patterns and practical trajectories, demonstrating fast detection, high robustness, and suitability for real-time implementation in SSSC applications.