Abstract:As a three-phase power factor corrector, the Vienna rectifier has the advantages of high reliability and simple control. It is widely used and plays an important role in power unidirectional flow fields, such as communication power supply, electric vehicle, wind power, and aeronautical power supply. Carrier-based pulse width modulation (CB-PWM) and space vector modulation (SVM) are usually used to achieve unity power factor and neutral point voltage balance. However, these modulation methods have the problem of current zero-crossing point distortion, which is also one of the characteristics of Vienna rectifier topology. Different symbols of modulation voltage and current cause current distortion. Many modulation strategies have been proposed to solve the zero-crossing distortion. However, only some can perfectly applied to the full modulation range or the modulation strategy to overmodulate the modulation voltage. This paper presents a modulation method that can minimize the current zero-crossing distortion while maintaining the neutral point voltage balance on the DC side. Because the Vienna output level is limited by the current symbol, this paper uses the dual modulation wave decomposition method to deal with neutral point voltage balance and zero-crossing distortion. Neutral point voltage balance is an important indicator of the stable operation of the Vienna rectifier. An unbalanced neutral point voltage can cause serious DC side offset, leading to over-voltage of power devices and causing danger. In order to balance the neutral point voltage, this paper calculates the average current to be extracted or injected during a carrier cycle by detecting the difference in capacitance voltage between the upper and lower DC sides obtained from the feedback. Combined with the expression of the duty cycle of each level, the zero-sequence voltage to be injected is calculated to balance the neutral point voltage. In order to deal with the current zero crossing distortion, the modulation wave is divided into fixed modulation and free modulation waves. The fixed modulation wave is affected by the current symbol. In the current zero crossing area, a zero-sequence voltage is injected to clamp its zero-crossing phase voltage to 0. Injecting zero sequence voltage in high regulation will over-modulate, changing the relationship between line voltage and causing current distortion. The current zero crossing distortion cannot be wholly suppressed in high modulation. This paper uses a minimum synthetic error method in high modulation to minimize the change of line voltage. When the current zero crossing, the modulation voltage is obtained by calculation, instead of injecting zero-sequence voltage. Thus, the current zero crossing distortion can be alleviated to the greatest extent when injecting zero sequence voltage over modulation. The steady-state experimental results of SVPWM, ZCC-SVM (Zero-Crossing Clamped SVPWM), and the proposed method are compared. When the modulation index is 0.5, the distortion of the SVPWM modulation strategy in the current zero crossing region is obvious, while the input current of ZCC-SVM and the proposed modulation method has no obvious zero crossing distortion. When the modulation index is low, the suppression effect of current zero crossing distortion of ZCC-SVM and the proposed modulation method is almost the same, because in the region of current zero crossing, the injection of zero sequence voltage will not cause over-modulation. When the modulation index is 0.9, the zero-sequence voltage injected by the ZCC-SVM method to suppress zero crossing distortion will be over-modulated. Compared with ZCC-SVM, since the proposed modulation algorithm adopts the minimum synthesis error method, the low-frequency THD (Total Harmonic Distortion) is reduced from 3.64% to 3.04%, the high-frequency THD is reduced from 5.34% to 4.56%, and the 5th and 7th harmonics of the input current are significantly reduced.
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