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| Active Neutral-Point Voltage Balance Control Strategy for Three-Level Three-Phase Inverter Under SHEPWM |
| Dong Zhiqiang, Wang Chenchen, Zhou Minglei, Liu Mingyue, Li Kai |
| School of Electrical Engineering Beijing Jiaotong University Beijing 100044 China |
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Abstract In the high-power system, the three-level neutral-point clamped inverter needs to adopt SHEPWM to reduce the switching power loss. The traditional neutral-point potential active balance control strategy adjusts the duty cycle of the zero-level of phase voltage by superimposing a slight angle based on the switching angle of SHEPWM from the polarity of each phase load current and neutral-point potential shift. However, the small number of SHEPWM switching angles limits the neutral-point potential balance ability. The fewer the number of SHEPWM switching angles, the weaker the neutral-point potential balance ability. Therefore, this paper analyzes the influence of the fundamental wave and the third harmonic of the output phase voltage under SHEPWM on the neutral-point potential in a different range of voltage phase angles, designs two switching angle combination schemes with opposite effects on the neutral-point potential in the fundamental wave period, and proposes a method to control the neutral-point potential by selecting different switching angle combination scheme to modulate according to the offset value and the hysteresis of the neutral-point potential.
The influence analysis of the fundamental wave and the third harmonic wave on the neutral-point current shows that the neutral-point current components generated by the fundamental wave and the third harmonic wave have triple frequency fluctuation. Moreover, when the amplitude of the third harmonic is 0.2636 times that of the fundamental wave, the integral of the neutral-point current components generated by the fundamental wave and the third harmonic cancels each other in the interval of the fundamental wave phase angle every 60 degrees. The neutral-point current integral in the whole fundamental wave cycle can be adjusted by setting the third harmonic content at every 60 degrees of the fundamental wave phase angle to control the neutral-point potential. Therefore, two switching angle combination schemes are proposed, with opposite effects on the neutral-point potential and no effect on the fundamental voltage. However, the two switching angle combination schemes may increase the number of switching times of the system because it is sometimes necessary to add extra switching angles to ensure the correct switching of the switching angles with different third harmonic content. The analysis of the switch angle combination schemes with the different numbers of switch angles shows that the proposed schemes can only increase four switching actions in the whole fundamental wave cycle at most, and will not affect the low switching frequency characteristics of SHEPWM. However, the neutral-point voltage hysteresis control is needed to determine the switching angle used to modulate. When the neutral-point voltage deviation is within the acceptable range of the system, the SHEPWM switching angle without modification is used to ensure the switching power loss and the harmonic performance of the system.
The experimental results show that compared with the traditional neutral-point potential active balancing method based on SHEPWM, the proposed method has a better dynamic balancing effect under different working conditions.
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Received: 06 November 2022
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2024, Vol. 39 
