3L-NPC Inverter Research Articles

Diode-Free T-Type Three-Level Neutral-Point-Clamped Inverter for Low-Voltage Renewable Energy System

In this paper, the conventional I- and T-type three-level neutral-point-clamped (3L-NPC) inverter for low-voltage renewable energy systems is first investigated. Literature research shows that the T-type inverter improves I-type's insulated-gate bipolar transistor (IGBT) $+$ IGBT current paths. However, the IGBT $+$ diode paths are the same. The calculation in this paper further reveals that the IGBT $+$ diode current paths dominate the conduction losses and even the whole semiconductor losses. Based on the aforementioned recognitions, a novel T-type inverter is presented as an alternative to be applied in the low-voltage renewable energy systems. In the proposed 3L-NPC, four CoolMosfets replace the IGBT $+$ diode middle bidirectional switch. In the proposed topology, there is no diode involved in the current path supposing the unity power factor. In this way, the conduction loss is expected to be reduced compared with that from the conventional T-type 3L-NPC, particularly in the low and medium power ranges.

Novel High-Efficiency Three-Level Stacked-Neutral-Point-Clamped Grid-Tied Inverter

In recent years, high photovoltaic array voltage up to 1000 V and transformerless grid-tied inverter have been increasingly researched and applied to elevate the inverter and the dc power collection efficiency. With the same reasons, a three-level neutral point clamped (3L-NPC) inverter featuring low power device voltage stress and low leakage current becomes increasingly attractive. In this paper, the operation and the features of a novel grid-tied 3L-NPC are presented. The proposed topology is a derivative of the three-level stacked neutral point clamped (3L-SNPC) structure. However, compared with the conventional 3L-SNPC and its pulse width modulation (PWM) strategy, the new topology with novel PWM strategy features completely inactive intrinsic body diodes. Furthermore, only two outer power devices are working with switching frequency, while the other four Insulated Gate Bipolar Transistors (IGBTs) are actually with the grid frequency. Therefore, the relatively high switching frequency is selected to reduce the inverter output filter inductance. A hybrid CoolMosfet and IGBT power module configuration is also proposed based on the new PWM strategy. The calculation shows that the total power device losses are reduced. The experimental results are illustrated in this paper to confirm the operation of the proposed topology and controller.

Novel Three-Phase Three-Level-Stacked Neutral Point Clamped Grid-Tied Solar Inverter With a Split Phase Controller

Characterized by the low leakage current and high efficiency, a three-level neutral point clamped (3L-NPC) inverter becomes more popular for a transformerless photovoltaic grid connected system. The three-level-stacked neutral point clamped (3L-SNPC) structure is a derivative of 3L-NPC providing more advantages such as a double apparent switching frequency and parallel load current paths. In this paper, the power loss distribution and features of 3L-SNPC are analyzed first when applied to solar inverters. Based on the analysis, a novel 3L-SNPC leg structure is proposed for solar applications in order to reduce the power loss particularly for the low power range, given the fact that solar inverters generally operate in the low power range during most of the daytime. Then, a two-stage solar inverter topology, applying the proposed structure to the phase leg, is described. Further, a split phase controller is applied for the two-stage solar inverter, consisting of the maximum power point tracking control, optimized dc-link voltage control, and grid current control. Finally, a new dead-time elimination pulsewidth modulation strategy is proposed and conveniently implemented to each phase based on the split phase controller. Experimental results are illustrated to demonstrate the applicability of the proposed topology and controller.

Analysis and Design of Active NPC (ANPC) Inverters for Fault-Tolerant Operation of High-Power Electrical Drives

Compared with neutral-point-clamped (NPC) inverters, active NPC (ANPC) inverters enable a substantially increased output power and an improved performance at zero speed for high-power electrical drives. This paper analyzes the operation of three-level (3L) ANPC inverters under device failure conditions, and proposes the fault-tolerant strategies to enable continuous operating of the inverters and drive systems under single and multiple device open- and short-failure conditions. Therefore, the reliability and robustness of the electrical drives are greatly improved. Moreover, the proposed solution adds no additional components to standard 3L-ANPC inverters; thus, the cost for robust operation of drives is lower. Simulation and experiment results are provided for verification. Furthermore, a comprehensive comparison for the reliability function of 3L-ANPC and 3L-NPC inverters is presented. The results show that 3L-ANPC inverters have higher reliability than 3L-NPC inverters when a derating is allowed for the drive system under fault-tolerant operation. If a derated operation is not allowed, the two inverters have similar reliability for device open failure, while 3L-NPC inverters have higher reliability than 3L-ANPC inverters for device short failure.