• Journal of Power Electronics
• /
• v.18 no.3
• /
• pp.702-713
• /
• 2018

Power loss reduction and total harmonic distortion(THD) minimization are two important goals of improving three-level inverters. In this paper, an optimized pulse width modulation (PWM) strategy that can reduce switching losses and balance the neutral point with an optional THD of three-level neutral-point-clamped inverters is proposed. An analysis of the two-level discontinuous PWM (DPWM) strategy indicates that the optimal goal of the proposed PWM strategy is to reduce switching losses to a minimum without increasing the THD compared to that of traditional SVPWMs. Thus, the analysis of the two-level DPWM strategy is introduced. Through the rational allocation of the zero vector, only two-phase switching devices are active in each sector, and their switching losses can be reduced by one-third compared with those of traditional PWM strategies. A detailed analysis of the impact of small vectors, which correspond to different zero vectors, on the neutral-point potential is conducted, and a hysteresis control method is proposed to balance the neutral point. This method is simple, does not judge the direction of midpoint currents, and can adjust the switching times of devices and the fluctuation of the neutral-point potential by changing the hysteresis loop width. Simulation and experimental results prove the effectiveness and feasibility of the proposed strategy.

• Journal of Power Electronics
• /
• v.13 no.2
• /
• pp.232-242
• /
• 2013

To obtain phase currents information in AC drives, shunt sensing technology is known to show great performance in cost-effectiveness and therefore it is widely used in low cost applications. However, shunt sensing methods are unable to acquire phase currents in certain operation conditions. This paper deals with the derivation of the boundary conditions for phase current reconstruction in three-shunt sensing inverters and proposes a voltage injection method to expand the measurable areas. As the boundary conditions are deeply dependent on the switching patterns, they are typically analyzed on the voltage vector plane for space vector pulse width modulation (SVPWM) and discontinuous pulse width modulation (DPWM). In the proposed method, the voltage injection and its compensation are conducted within one sampling period. This guarantees fast current reconstruction and the injected voltage is decided so as to minimize the current ripple. In addition to the voltage injection method, a sampling point shifting method is also introduced to improve the boundary conditions. Simulation and experimental results are presented to verify the boundary condition derivation and the effectiveness of the proposed voltage injection method.