A Carrier-Based Modulation with Planned Zero Sequence Voltage Injection to Control Neutral Point Voltage for Three-Level Inverter

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.