Search

This paper proposes an improved offset selection method for discontinuous-pulse-width-modulation (DPWM)-based back-to-back converters to reduce dc-link current ripple. DPWM is introduced to power converters to diminish the stress on power transistors and prolong their lifespan. However, when using the DPWM method, the dc-link current ripple is increased in nonswitching regions of the power transistors. Moreover, in DPWM-based back-to-back converters, the dc-link current ripple reaches its maximum when the two transistors of both inverters are clamped in opposite directions. Therefore, the dc-link capacitors endure more stress, resulting in decreased life duration. To overcome this issue, the switching method should consider the clamping periods, when the current ripple increases. This can be achieved by modifying the DPWM offset, so that the clamping states of both converters are matched. The effectiveness of the proposed method is confirmed by both simulation and experimental results. [ABSTRACT FROM PUBLISHER]

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]

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]

A new pulsewidth modulation (PWM) strategy which is an alternative approach of the discontinuous PWM (DPWM) for a three-level neutral-point-clamped (NPC) inverter is proposed in this paper. A three-level NPC inverter is completely mature and very well-established topology in high-power applications. However, the three-level NPC inverter has an inheritance problem of the neutral-point voltage unbalancing due to the split dc-link capacitors. This structure can cause large neutral-point voltage ripple. Furthermore, output currents of the three-level NPC inverter are distorted by the neutral-point voltage ripple. Therefore, the neutral-point voltage must be controlled. In this paper, a new DPWM method using two different offsets for the neutral-point ripple reduction is proposed and the effectiveness of the proposed DPWM method is verified by using the simulation and the experimental results. [ABSTRACT FROM PUBLISHER]

This paper proposes a neutral-point voltage balancing method for a three-level inverter that uses discontinuous pulse width modulation (DPWM). The performance of a three-level inverter system is affected by the neutral-point voltage unbalancing. Therefore, the voltages of series connected dc-link capacitors should be equal. Generally, the offset voltage is added to reference voltages in the conventional voltage balancing method. However, when using DPWM, the performance of conventional balancing method is limitary because turn-on and turn-off switching occurs at regular intervals. The proposed method is implemented by adjusting the discontinuous pulsewidth of positive and negative cycle. The proposed method maintains low switching losses and achieves the effective voltage balancing, without additional hardware and complex calculations. Simulation and experimental results confirm the feasibility and effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper analyzes the combination in a cascaded H-bridge multilevel inverter (CHBI) based on transformers with the different turn ratios for increasing the voltage level and proposes the switching method for achieving the output voltage distribution among H-bridge cells (HBCs). The transformers used in this paper are connected to the output of the respective HBCs, and the secondary sides of all the transformers are connected in series for generating the final output voltage. Only one of the transformers, in particular, has a different turn ratio for increasing the output voltage level. In this paper, the possible turn ratio of the special transformer with a different turn ratio is discussed in detail, and a switching method based on the level-shifted switching method for the topology used in this paper is proposed. To verify the effectiveness of the proposed method, a three-phase 21-level CHBI is experimentally tested. [ABSTRACT FROM PUBLISHER]