Abstract
In this paper, a carrier-based modulation strategy with planned zero sequence voltage (ZSV) injection for T-type three-level inverter (TLI) is proposed to control neutral point (NP) voltage, which is simple to be implemented. This strategy is analyzed comprehensively based on the relationship between NP current and the injected ZSV in one switching cycle. The planned ZSV is calculated according to the voltage difference between upper and lower capacitors and the injectable ZSV range without over-modulation. Furthermore, in order to reduce switching losses, the closest clamping mode method is applied, in which specific ZSV is injected to form discontinuous pulse width modulation (DPWM), and the NP voltage control ability is sacrificed to some extent. In most applications of the two proposed methods, the NP voltage can be perfectly controlled almost like virtual space vector PWM (VSVPWM). However, the switching numbers of the strategy with planned ZSV injection are a third even half less than that of VSVPWM. And the switching numbers of the closest clamping mode method are even a third less than that of SVPWM. Comprehensive experiments are carried out to verify the feasibility of the proposed two methods.
Highlights
Multilevel inverters have gained more and more attentions since they are suitable for high-power and highand/or medium-voltage industrial products [1], [2]
Case 1: −umax, −umid, −umin ∈/ [−1 − umin, 1 − umax] In this case, the injected uZSV does not change the polarity of the modulation voltages, which indicates that i∗neutral point (NP) has simple linear relation with uZSV
According to the i∗NPref calculated by equation (14), the PZIPWM is proposed by solving the linear equation and selecting the injected zero sequence voltage (ZSV) reference uZSVref to achieve the maximum NP voltage balance capability
Summary
Multilevel inverters have gained more and more attentions since they are suitable for high-power and highand/or medium-voltage industrial products [1], [2]. An improved virtual space vector PWM (VSVPWM) based on the repartitioned space vector sectors is proposed in [15], in which only one small vector instead of redundant small vectors pairs is used to realize NP voltage precise control and balancing over the whole modulation index range In this method, the small vector is selected according to the factor of NP voltage imbalance and direction of phase current. Case 1: −umax, −umid, −umin ∈/ [−1 − umin, 1 − umax] In this case, the injected uZSV does not change the polarity of the modulation voltages, which indicates that i∗NP has simple linear relation with uZSV. It means that i∗NPmax and i∗NPmin can be acquired at the boundary points.
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