Analysis of 0–9 kHz Current Harmonics in a Three-Phase Power Converter Under Unbalanced-Load Conditions

Abstract

The unbalanced condition of typical loads in industrial networks could affect the current harmonics across the dc-link in Adjustable Speed Drives (ASDs). These current harmonics could pass through the dc-link and create new current harmonics at the grid side. Thus, the effect of unbalanced-load condition on the current harmonics across the dc-link needs to be considered in the design process. Moreover, new compatibility levels and emission limits will be developed for the 2–9 kHz current harmonics, so drives need to meet these new requirements in future. These current harmonics could also be significantly affected by the load power factor at the unbalanced-load condition. Due to these reasons, analysing the effect of unbalanced-load and load power factor on the current harmonics between 0 to 9 kHz across the dc-link is vital. Thus, this paper presents an analytical approach for estimating the current harmonics of the inverter at the dc side considering unbalanced-load currents. The proposed mathematical equations are then validated with experimental results. These analyses reveal that the negative-sequence current generates new harmonics across the dc-link. Additionally, the relationship between unbalanced currents at the load side and dc-link are found as a function of positive- and negative-sequence currents.

Keywords

• Power quality
• ASD
• motor drive
• inverter
• PWM
• unbalanced-load