The conventional three-level inverter suffers the limitation of voltage buck operation. In order to give both voltage buck and boost operation capability, the quasi-Z-source three-level T-type inverter (3LT $^2$ I) has been proposed. This paper further proposes a space vector modulation (SVM) scheme for the quasi-Z-source 3LT $^2$ I to reduce the magnitude and slew rate of common-mode voltage (CMV). By properly selecting the shoot-through phase, the shoot-through states are inserted within zero vector in order not to affect the active states and output voltage. Doing so, the CMV generated by the quasi-Z-source 3LT $^2$ I is restricted within one-sixth of dc-link voltage, and voltage boosting and CMV reduction can be simultaneously realized. In addition, high dc-link voltage utilization can be maintained. The proposed scheme has been verified in both simulations and experiments. Comparisons are conducted with the conventional SVM method and the phase-shifted sinusoidal PWM method.

A Space Vector Modulation Scheme of the Quasi-Z-Source Three-Level T-Type Inverter for Common-Mode Voltage Reduction

This paper proposes a single-vector-based model predictive control (MPC) for an induction motor drive supplied by a three-level neutral-point-clamped inverter operating at a low switching frequency. Unlike the torque and flux control in the conventional MPC, the proposed MPC tries to minimize the error between the applied voltage vector and a reference voltage vector obtained based on the principle of deadbeat control, thereby reducing the number of weighting factors in the cost function. To reduce the computational burden and restrict the high jumps in both phase and line voltages, two switching tables are proposed and compared for the preselection of candidate voltage vectors. The neutral point potential fluctuation and switching frequency are also included in the cost function to achieve the balance between the upper and lower dc voltages and a relatively low switching frequency. Furthermore, a speed adaptive stator flux observer with a novel gain matrix is proposed to achieve speed-sensorless operation, which has a higher speed and flux estimation accuracy than conventional fixed gains. Finally, the proposed MPC is extended to the field-weakening operation by adjusting the torque and stator flux references online, which significantly widens the speed range and improves the practical value of the MPC. The effectiveness of the proposed method is confirmed by the experimental results obtained at an average switching frequency of less than 600 Hz.

Low Switching Frequency Model Predictive Control of Three-Level Inverter-Fed IM Drives With Speed-Sensorless and Field-Weakening Operations

In this paper, the properties and operation of the virtual-space-vector pulse-width modulation (PWM) for a three-level neutral-point-clamped inverter with unbalanced dc-links are studied. By quantitatively analyzing the characteristics of the revised space-vector diagram (SVD) with unbalanced dc-links, the defined virtual SVD (VSVD) for the unbalanced case is theoretically proven identical to the VSVD of the conventional virtual-space-vector PWM (VSVPWM) strategy with balanced dc-links. The well-developed and fast-processing methods to calculate the durations of the nearest three virtual vectors for the conventional VSVPWM can be directly utilized. In addition, a control strategy to realize the asymmetric control of the dc-link voltages with VSVPWM is also studied, and this method can adjust the unbalancing degree with a high accuracy for any modulation indexes and load power factors. Simulated and experimental results are given to validate the effectiveness.

Virtual-Space-Vector PWM for a Three-Level Neutral-Point-Clamped Inverter With Unbalanced DC-Links

Flying capacitor photovoltaic (PV) inverters have been widely discussed in the literature. However, the relevant leakage current issues have not received much attention. In this paper, the modulation techniques for a transformerless three-phase flying capacitor PV inverter are investigated for the leakage current suppression. First, the theoretical analysis of the system common mode model and leakage current is presented. Second, three kinds of conventional carrier-based modulation techniques are evaluated, and results indicate that they fail to effectively suppress the leakage current. Third, a new carrier modulation and logic function is presented. It is able to maintain the constant common mode voltage, and consequently, the leakage current can be significantly reduced. Finally, the experimental tests are conducted on a transformerless three-phase flying capacitor PV inverter to verify the effectiveness of the proposed method.

Leakage Current Suppression of Three-Phase Flying Capacitor PV Inverter With New Carrier Modulation and Logic Function

The conventional three-level inverter only has voltage buck capability. The quasi-Z-source three-level T-type inverter (QZS 3LT $^2$ I) has been proposed to realize voltage buck–boost operation. In this paper, we further propose a novel modulation scheme for the QZS 3LT $^2$ I to realize voltage boosting, reduce the common-mode voltage (CMV), and control the neutral-point voltage balance simultaneously. The proposed scheme adopts a large vector, a medium vector, a small vector with low CMV magnitude, a zero vector, and a shoot-through vector to generate the output voltage. According to sector number and neutral-point voltage difference, a p-type or n-type small vector with low CMV magnitude is properly selected to balance the neutral-point voltage. Shoot-through states are inserted within zero vector to boost the dc input voltage without affecting the ac output voltage. Dwell times of basic vectors are calculated through the revised volt–second balance equation. Furthermore, a coordinate control strategy between neutral-point voltage balance and voltage boosting is proposed. Doing so, the CMV magnitude can be restricted within one-sixth of dc-link voltage and neutral-point voltage imbalance can be effectively mitigated. The effectiveness of the proposed scheme is verified by simulations and experiments.

Simultaneous Common-Mode Voltage Reduction and Neutral-Point Voltage Balance Scheme for the Quasi-Z-Source Three-Level T-Type Inverter

This paper presents an improved virtual space vector modulation (IVSVM) scheme for a three-level active neutral-point-clamped (3L-ANPC) inverter to balance neutral-point potential (NP), reduce switching loss, and suppress common mode voltage (CMV). In the IVSVM scheme, an improved virtual medium vector (VMV) is synthesized by the original medium vector and the adjacent two pairs of the small vectors since there are always two pairs of the small vectors in any regions of the new space vector diagram. The improved VMV provides the flexibility to control the NP balancing under any modulation indexes and power factors in one switching cycle while achieving SL reduction and CMV suppression. This is achieved by the proper selection of two pairs of small vectors in terms of NP charge and the pulse sequence. The experimental platform based on a 3L-ANPC inverter is established and results obtained to verify the effectiveness of the IVSVM control strategy.

An Improved Virtual Space Vector Modulation Scheme for Three-Level Active Neutral-Point-Clamped Inverter

This study proposes a novel control strategy for a three-level active neutral-point clamped (3L-ANPC) inverter using selective harmonic elimination pulse width modulation (SHEPWM). The proposed control strategy can effectively control the neutral-point potential (NP) balancing and eliminate the selective harmonics simultaneously. The three-phase SHEPWM line-to-line voltages of the inverter are considered as switching-states. Among these switching states, the six pairs of redundant switching-states generate the same line-to-line voltages and thus the same harmonic spectrums with each pair having opposite effects on NP balancing. As a result, the pair of switching-states can be swapped to control the NP while still maintaining the same inverter output. The decision to make such swap will take into account directions of output currents, polarities of NP offsets and exclusion of directly switching between P and N states. The simulation and experimental results on a Matlab/Simulink model and prototype of a three-phase 3L-ANPC inverter are used to verify the effectiveness of the proposed control strategy.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-pel.2017.0229

Neutral-point potential balancing control strategy of three-level active NPC inverter based on SHEPWM

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An Improved Virtual Space Vector Modulation Scheme for Three-Level Active Neutral-Point-Clamped Inverter

Neutral-point potential balancing control strategy of three-level active NPC inverter based on SHEPWM