A mitigation method for non-eliminated harmonics of SHEPWM three-level multipulse three-phase active front end converter

Three-level NPC active-front-end (3LAFE) converters can be applied for high-power medium voltage applications, with 4-quadrant unity-power factor operation, allowing for decentralized energy distribution and utilization, hence making full regenerative systems possible. Switching losses and network interaction limit the operation of 3LAFE high-power converters, especially in the case of GTO-inverters. That is why obtaining a low switching frequency and reduced harmonic distortion are the main goals when using optimum pattern PWM with a selective harmonic elimination method (SHEPWM). For 12-pulse configurations, the lowest switching frequency can be achieved using a 3-angle scheme for SHEPWM where the 11th and 13th harmonics can be eliminated by the correct firing angle application. The remaining non-eliminated harmonics of orders 23, 25, 35 and 37 play a main role on defining the voltage distortion injected into the power network, where the resulting harmonic current injection depends on the series impedance between the network and 3LAFE. This paper presents the experimental study concerning the regenerative unity-power factor operation of a 3LAFE converter, highlighting the possible reduction of non-eliminated harmonics by the introduction of slight variations of modulation index and DC-link voltage regarding a selective harmonic distortion factor (SHD). This work opens new possibilities for avoiding or reducing harmonic filters, especially by operating within weak networks and stringent conditions to be studied in further projects.