General pulse width modulation (PWM) model for multi-phase neutral point (NP) clamped (NPC) 3-level inverter (3LI) is described. A novel PWM strategy for N -phase NPC 3LI with unconditional NP voltage balance is proposed as well as its carrier-based realization, in which PWM sequences are generated by comparing a single carrier wave with double modulation waves. The proposed PWM strategy is consistent with the principle of virtual space vector PWM (VSVPWM). However, the mechanism of NP voltage balance under the proposed PWM strategy is based on precisely calculating and executing level 1 action time of each phase, which may be influenced by some non-ideal factors. To avoid that, an active NP voltage control for the proposed PWM strategy is put forward. Then 3-phase and 5-phase NPC 3LIs are built as examples to verify the correctness of the proposed PWM strategy. Finally, the experimental results indicate that the proposed PWM strategy has better NP voltage control abilities.INDEX TERMS 3-level inverter (3LI), N -phase, neutral point (NP) clamped (NPC), carrier-based, neutral point (NP) voltage.

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Aim at reducing switching loss (SL) of neutral-point clamped three-level inverter (NPC TLI) as much as possible, a novel SL minimized discontinuous pulse width modulation (SLMDPWM) strategy is proposed in this paper. The core of SLMDPWM is that the specific maximum or middle phase current is clamped to positive bus, NP or negative bus according to modulation index and power factor angle. The realization of SLMDPWM based on carrier-based PWM (CBPWM) is revealed to simplify the calculation process. The performance characteristics of SLMDPWM and DPWM I-IV in term of the output voltage, SL and NP voltage ripple are compared by simulation and experimental results, and conclusion can be acquired that the SL of SLMDPWM is reduced significantly.INDEX TERMS Discontinuous modulation strategy, neutral-point clamped three-level inverter, neutral-point voltage, switching loss.

Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000.

In recent years, T-type three-level inverter (TLI) has attracted attentions due to its advantages, such as simple structure and higher efficiency. Neutral point voltage (NPV) is very critical for its normal operation, and the necessary conditions of maintaining NPV balance respectively in a switching cycle and in a fundamental cycle are simply reviewed in this paper. With the increment of power switches, the reliability of T-type TLI becomes particularly important. When switch faults occur, fault-tolerant control strategies are usually adopted. However, in existing literatures, it is unknown whether the NPV balance can be still maintained under different fault types. Therefore, the application range of a fault-tolerant control is also unknown. In this paper, based on the necessary conditions of maintaining NPV balance, the application ranges of two representative fault-tolerant control strategies are revealed under different open-circuit (OC) fault types in inner switch. It found that the application ranges are closely related to the modulation index, load power factor (PF) and OC fault type. Finally, the correctness of the theoretical analysis is verified by the experimental results. INDEX TERMST-type three-level inverter, neutral-point balance, open-circuit fault, fault-tolerant control.