Development of a Generalized Multilevel SVM and Capacitor Voltage Balancing Strategy for Multiphase Three-Level NPC Converters

This paper proposes a generalized space vector modulation (SVM) strategy for 3-level neutral-point-clamped (NPC) converters. The proposed strategy, here referred to as multiphase 3-level SVM (MP3L-SVM), is developed as a generic modulation technique applicable for 3-level NPC converters with any number of phases, while for showcasing in this paper the proposed MP3L-SVM is applied to a wind turbine with a direct drive multiphase permanent magnet generator (PMG) whose output is rectified using a 3-level NPC converter before connection to a high-voltage DC (HVDC) grid. A key challenge in 3-level NPC converters is maintaining a balanced voltage across the HVDC-link capacitors, particularly at low speeds. To address this issue, the paper integrates the proposed MP3L-SVM strategy with a voltage balancing algorithm (VBA), which mitigates capacitor voltage imbalances by choosing the three nearest switching states that result in minimal energy deviations across the capacitors. The paper discusses the mathematical modeling of a multiphase PMG interfaced to a 3-level NPC and derives generalized models that govern the proposed strategy based on a visual space vector diagram (SVD). Analytical models are validated through simulations across various 3-level NPC configurations, including 3-phase, 6-phase, and 9-phase NPC converters. Further, the analytical models and simulation results are validated by test results from a scaled-down laboratory prototype 3-level NPC converter that has been prototyped in-house.