Open Access

ARTICLE

A Novel FTVSVPWM Strategy for Three-Level NPC Converters in Offshore DFIG Systems

Wenming Zhang¹, Li Zhang^1,*, Feng Zheng², Jianjian Zhao², Shuqian Zhang², Pengxu Song²
1 School of Electrical and Power Engineering, Hohai University, Nanjing, China
2 Huaneng International Power Jiangsu Energy Development Co., Ltd. Clean Energy Branch, Nanjing, China
* Corresponding Author: Li Zhang. Email:

Energy Engineering https://doi.org/10.32604/ee.2026.081252

Received 26 February 2026; Accepted 16 April 2026; Published online 12 May 2026

Abstract

Due to harsh operating conditions, the neutral-point-clamped three-level excitation converters (NPC-TLCs) for offshore doubly-fed induction generators are highly susceptible to open-circuit faults in power devices. Such faults cause the loss of voltage vectors, leading to elevated output harmonics and neutral-point voltage (NPV) imbalance, which ultimately precipitate system instability. To address these critical issues, a fault-tolerant virtual space vector pulse width modulation (FTVSVPWM) strategy is proposed to ensure that the excitation converter maintains both NPV balance and rated power operation under fault conditions. First, by thoroughly analyzing the vector distribution characteristics of the NPC-TLC topology equipped with a redundant phase leg under fault conditions, novel virtual space vectors are constructed to compensate for the missing space vectors. Second, a charge balance criterion is introduced to quantitatively derive the dwell times of the voltage vectors, thereby achieving real-time, autonomous suppression of NPV fluctuations during fault operations. Furthermore, to mitigate the increased switching actions induced by the proposed FTVSVPWM, a switching sequence reconstruction method based on phase duty cycles is employed. This optimization reduces the number of switching actions per carrier cycle by 50% without compromising fault-tolerant performance. Simulation results demonstrate that, under device fault scenarios, the proposed strategy reduces the total harmonic distortion of the current by at least 10% and decreases the torque ripple by 17.58%. This research provides an effective and robust solution for enhancing the reliability of offshore wind energy systems.

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Keywords

DFIG-based wind turbines; three-level excitation converter; fault-tolerant; neutral-point voltage; virtual space voltage vector

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