Effect of Input Voltage Unbalanced Harmonics on DC bus Capacitors of Adjustable Speed Drives

Imbalance (unbalance) and harmonics (background distortion) in the supply voltage are major threats to the DC bus capacitor in an Adjustable Speed Drive (ASD). These disturbances create different frequency components other than the six times supply frequency component in the capacitor current which increases the losses in the capacitor and reduces its lifetime. It is identified from the Power Quality (PQ) survey the harmonics in the three phases are not balanced. This paper analyses the impact of these unbalanced harmonics on the DC bus capacitors. The magnitude of extra frequency components apart from six times the supply frequency component in the capacitor current varies based on the magnitude of unbalance in the harmonics present in the supply voltage. As a result of the analysis, the study proposes an index to assess the impact of unbalanced harmonics.