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| Improved Asynchronized Space Vector Pulse Width Modulation Strategy for High-Power Three-Level Active Neutral Point Clamped Traction Inverter |
| Zhao Mutian1, Ge Qiongxuan1,2, Zhao Lu1,2, Zhu Jinquan1, Chen Weixin1,2 |
1. State Key Laboratory of High Density Electromagnetic Power and Systems Institute of Electrical Engineering Chinese Academy of Sciences Beijing 100190 China;
2. University of Chinese Academy of Sciences Beijing 100049 China |
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Abstract Under high carrier ratio (Cr) conditions, asynchronized modulation is used by the high-power three-level active neutral point clamped (3L-ANPC) traction inverter. In the low modulation ratio region, the output voltage harmonics and amplitude deviations caused by the inverter's nonlinearities are pronounced. As motor speed increases, the inverter modulation ratio gradually increases. To fully utilize the DC bus voltage, the inverter will enter the overmodulation region. To improve harmonic characteristics and dynamic performance, it is necessary to optimize the low-modulation-ratio and overmodulation regions. This paper proposes a three-level phase-voltage spectrum analysis method for nonlinear characteristics and a nonlinear compensation strategy based on an improved extended state observer (ESO) for the low modulation-ratio region. To expand the modulation range, a simplified dual-mode overmodulation strategy is proposed.
First, nonlinear compensation of the inverter is applied in the low modulation ratio region. The ASVM-L strategy with zero-vector initiation is employed to avoid narrow phase-voltage pulses. The nonlinearity of the inverter is equivalent to a dead time. Nonlinearities introduce characteristic harmonics into the phase current using the double Fourier transform. Accordingly, a nonlinear compensation strategy for an improved ESO is proposed to compensate for DC and AC disturbances caused by inverter nonlinearity via the integral and resonance links, respectively. Secondly, to expand the inverter's modulation range, a simplified dual-mode overmodulation strategy is proposed. The vector synthesis method and modulation-ratio interval for each overmodulation region are provided, making implementation straightforward without requiring table lookups or linear fitting. Finally, a smooth switching method for the full modulation-ratio range is presented, and all switching conditions are summarized. The phase-voltage two-level jump can be avoided if the switching occurs during the carrier-countdown period.
Experiments verify the effectiveness of the proposed strategy. In the low modulation ratio region (modulation ratio<0.35), the ASVM-L strategy can broaden the pulse width and reduce the current THD. In the linear modulation region with modulation ratios greater than 0.35, the ASVM-H strategy with small-vector initiation is adopted to improve harmonic performance. Using the ASVM-OV1 and ASVM-OV2 strategies, the fundamental voltage amplitude can be further increased subject to voltage constraints. When the DC bus voltage is 200 V, the fundamental voltage can reach 120.68 V and 127.62 V, respectively. In the double closed-loop experiment of the motor, after adopting the proposed nonlinear compensation strategy, the phase current THD is reduced from 15.58% to 1.32%, the amplitude of the 5th harmonic current is reduced from 6.4 A to 0.31 A, and the amplitude of the 7th harmonic current is reduced from 1.17 A to 0.22 A.
The conclusions are summarized as follows. (1) The effects of nonlinear characteristics, such as inverter dead time, tube voltage drop, and turn-on and turn-off delay, are analyzed, and a nonlinear compensation strategy for improved ESO is proposed. Under variable-frequency conditions, the dynamic response of the motor current and the suppression of current harmonics can be improved. Based on the modulation ratio and the fundamental phase angle, the ASVM-OV1 and ASVM-OV2 overmodulation strategies for one- and two-region cases can be implemented, thereby reducing algorithmic complexity. The actual inverter modulation ratio can be increased by up to 10.26%.
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Received: 20 June 2025
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2026, Vol. 41 
