Pulse Pattern Optimization for Medium Voltage 3-Level NPC Converter Using Open Source Optimization Tools in Julia

Abstract

This paper presents an optimization approach for the switching angles used in medium voltage 3-level Neutral Point Clamped (NPC) converters with the active front end. Medium voltage (MV) drives use low switching frequencies to reduce the converters' switching losses. This fact limits the number of available switching angles that can be used. Therefore, 5 and 7 pulses (i.e., switching angles) in quarter-wave symmetry are considered in this research. Insulated Gate Bipolar Transistors (IGBTs) or Integrated Gate Commutated Thyristors (IGCTs), which are used in MV drives because of their capability to provide high power density, present obstacles in the optimization. Their high minimum on-time and dead time, which must be considered during the optimization process, are among the challenges associated with the use of IGBTs or IGCTs in MV drives. On-time refers to the duration in which the IGBT is conducting current, while dead-time refers to the interval in which the IGBT is turned off and cannot conduct current. The dead time is introduced to prevent shoot-through currents in the inverter circuit, which can cause damage to the IGBTs and other components. This research aims to improve the switching angles of pulse patterns to minimize low-order harmonics in a wide range of modulation index while meeting all the mentioned requirements and generating the desired fundamental harmonic. To achieve this, a modulation technique called Selective Harmonic Elimination (SHE) Pulse-Width Modulation (PWM) has been employed, which effectively reduces the low-order harmonic distortion caused by converters. The optimization process is performed using open-source optimization tools in the Julia programming language. The research results demonstrate that the proposed approach can effectively optimize the pulse pattern angles of medium voltage 3-level NPC converters, resulting in improved power quality and reduced harmonic distortion while ensuring the continuity of switching angles.