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This paper proposes the new clamping angle control (CAC) pulse width modulation (PWM) method to restore its maximum linear modulation range of voltage source inverters (VSIs) reduced by the duty ratio limitation in hardware implementation, which makes the VSIs to operate nonlinearly in the high modulation range. The deadtime and the bootstrap gate driver circuit mainly cause this duty ratio limitation in practice. In particular, the bootstrap gate driver circuit is used in many industrial applications by avoiding the additional power supply to apply the gate-source voltage of power switches, and therefore making the VSIs to be small size with the low cost. First, the proposed CACPWM method is theoretically analyzed. Then, it is applied in the high modulation range to overcome the limitation of the duty ratio due to the use of the bootstrap gate driver circuit. Thereafter, its practical effectiveness is verified by both simulation and experimental tests. Also, the resulting harmonic distortions are compared with those by the conventional PWM methods. [ABSTRACT FROM AUTHOR]

The purpose of neutral-point voltage balancing algorithms in the three-level neutral-point clamped (3L-NPC) topology is to eliminate the voltage unbalance of top- and bottom-side dc-link voltages (neutral-point voltage). These algorithms are divided into dc unbalance and ac unbalance compensation methods. The dc unbalance causes distortion of output currents; therefore, this problem has to be solved. The ac unbalance implies that there is an ac ripple in the top- and bottom-side dc-link voltages. In applications using 3L-NPC topology, a large dc-link capacitor value is required to mitigate this ac unbalance problem. As a result, the size and the cost of applications become large and expensive. In this paper, a compensation method for mitigating the ac unbalance of the neutral-point voltage is proposed, which causes the dc-link capacitor value to be small, when a carrier-based pulse width modulation method is applied. The proposed method compensates the ac unbalance of the neutral-point voltage by adding an optimal compensation value to the reference duty signals. This compensation value is calculated simply by currents and reference duty signals. The calculation process of the optimal compensation value is discussed, and the effectiveness of the proposed method is verified with simulations and experiments. [ABSTRACT FROM PUBLISHER]

Presents a torque ripple reduction technique of direct torque control (DTC) for high power induction motors driven by three-level inverters with the inverter switching frequency limited around 0.5-1 kHz level. Principle of DTC for three-level inverter; Details of torque ripple reduction algorithm; Schematic diagram of the proposed control strategy.

This article proposes a method to eliminate abnormal output voltages generated in three-level hybrid active neutral-point clamped (ANPC) inverters. The hybrid ANPC inverter is a topology in which silicon (Si) devices are replaced with silicon-carbide (SiC) devices in part from the conventional ANPC topology. The hybrid ANPC inverters have the advantages of high switching frequency and low power switching loss due to the SiC devices. In this hybrid ANPC inverter, abnormal output voltages occur when the polarity of the output pole voltage is changed from positive to negative. The cause of the abnormal voltages is the switching time difference between the Si and SiC devices, and it is various from a few nanoseconds to a few microseconds. These abnormal output voltages need to be eliminated as it leads to degraded output power quality in the hybrid ANPC inverters. The proposed switching method effectively eliminates the abnormal output voltages and reduces output current distortion. The performance of the proposed method is verified using a prototype of the new three-level hybrid ANPC inverter type through both simulations and experiments. [ABSTRACT FROM AUTHOR]

This article presents a dc-link capacitor lifetime improvement method for three-level photovoltaic hybrid active neutral-point clamped inverters in full modulation index range. The capacitors are one of the components with vulnerable reliability in a power conversion system. The dc-link capacitor current ripples lead to temperature rise and wear-out failure of the capacitors. The accumulated thermo-mechanical damages from the temperature rise reduce the capacitor lifetime and reliability of the system. The proposed modulation method suppresses effectively the dc-link capacitor current ripple to improve the capacitor lifetime. In this article, the proposed method is discussed via a comparison with conventional space-vector pulsewidth modulation in full modulation index range. The modulation index range is divided into two regions, namely medium-vector modulation and hybrid modulation region. The capacitor lifetime is evaluated by the capacitor lifetime model according to the modulation methods and modulation regions. The effectiveness of the proposed modulation method is verified through simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a novel discontinuous pulse-width modulation (DPWM) method to reduce the current harmonics and improve the system efficiency for a single-phase three-level neutral-point clamped inverter. In single-phase inverters, the unipolar pulse-width modulation (UP-PWM) method is commonly used. However, this method has the disadvantage of power losses due to numerous switching operations. Conventional DPWM methods reduce the power losses and improve efficiency but increase the current total harmonic distortion (THD). To overcome these weaknesses, this paper proposes a hybrid DPWM switching method combining two PWM methods: the UP-PWM method and the conventional DPWM method called one-pole clamped PWM method. Since the proposed DPWM method offers all the advantages of both PWM methods, the optimal performance—with regard to the power losses and current THD—is obtained. The combination of two PWM methods is investigated by analyzing the power losses and current THD. Based on the analysis, the process determining the optimal operating condition is introduced. The effectiveness of the proposed DPWM method is demonstrated through simulations and experiments. [ABSTRACT FROM AUTHOR]

This paper presents an open-circuit fault detection method for a grid-connected neutral-point clamped (NPC) inverter system. Further, a fault-tolerant control method under an open-circuit fault in clamping diodes is proposed. Under the grid-connected condition, it is impossible to identify the location of a faulty switch by the conventional methods which usually use the distortion of outputs because the distortion of the outputs is the same in some fault cases. The proposed fault detection method identifies the location of the faulty switch and the faulty clamping diode of the NPC inverter without any additional hardware or complex calculations. In the case of the clamping diode faults, the NPC inverter can transfer full rated power with sinusoidal currents by the proposed fault-tolerant control. The feasibility of the proposed fault detection and the fault-tolerant control methods for the grid-connected NPC inverter are verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]