Abstract：Compared with the two-level inverter, the neutral point clamped three-level inverter (NPC TLI) has the advantages of low electromagnetic interference, low voltage stress of switching devices, and low total harmonic distortion (THD). Therefore, it is widely used in renewable energy generation, the transmission of high voltage and high power, medium/high voltage variable frequency motor drives, and so on. Carrier-based pulse width modulation (CBPWM) method and space vector pulse width modulation (SVPWM) method are usually applied to NPC TLI, which are equivalent to a certain extent. CBPWM generates PWM sequences by comparing double carriers and single modulation waves. In this paper, the carrier of CBPWM is optimized by replacing double carriers with a single carrier. Then, the modulation wave is processed to eliminate the negative value. Accordingly, this paper proposes a modulation wave decomposition method and applies it to virtual space vector pulse width modulation (VSVPWM). The neutral point voltage (NPV) can still be unconditionally balanced under VSVPWM with this method, which proves the feasibility of the method. Compared with CBPWM, the switching loss is increased when VSVPWM is used, even though NPV can be unconditionally balanced, which greatly affects the power conversion efficiency of the inverter. Six modulation waves can be obtained when VSVPWM with the proposed method of modulation wave decomposition is used, and two of them are clamped. Limiting the modulation wave that is not clamped reduces the switching action of a specific phase and the switching loss. Four modes can be obtained after the modulation waves are limited, so this method is named FM_VSVPWM in this paper. Compared with VSVPWM, when the proposed FM_VSVPWM is used, the switching loss can be effectively reduced, and NPV can also be balanced in the full range of modulation index and power factor. The steady-state experimental results show that when the modulation index is 0.9, and the power factor is 0.26. NPV fluctuates with triple fundamental frequency, and the fluctuation amplitude is about 6 % of the DC link voltage when CBPWM is used. When VSVPWM and the proposed FM_VSVPWM are used, NPV can always be balanced in the full range of modulation index and power factor. In addition, the switching losses of the above three methods are compared in this paper. Compared with VSVPWM, when FM_VSVPWM is used, the switching losses are reduced by 13 %, which proves the superiority of the proposed FM_VSVPWM in reducing switching loss. The NPV recovery experimental result shows that even if there is a 10 % offset on the DC link capacitor voltage, NPV can still recover quickly. The dynamic experimental results include the sudden change of modulation index and the sudden change of load. The results show that when FM_VSVPWM is used, NPV can keep balance when external interference is added. Moreover, the current THD of the three methods is compared. The results show that when the modulation index is low, compared with VSVPWM, the current THD of FM_VSVPWM decreases from 11.8 % to 8.5 %.
王金平, 刘斌, 董浩, 姜卫东. 中点钳位型三电平逆变器基于调制波分解的调制策略[J]. 电工技术学报, 2023, 38(12): 3221-3233.
Wang Jinping, Liu Bin, Dong Hao, Jiang Weidong. A Modulation Strategy Based on Modulation Wave Decomposition for Neutral Point Clamped Three-Level Inverter. Transactions of China Electrotechnical Society, 2023, 38(12): 3221-3233.
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2023, Vol. 38 
