Effect of Balanced and Unbalanced Voltage Fluctuation on DC bus Inductor and Capacitor of Adjustable Speed Drives

Supply voltage with low frequency fluctuations will introduce a low (fluctuation) frequency component in the dc link of Adjustable Speed Drives (ASDs), which may increase the net rms current through the DC choke (inductor) and DC-link capacitor. In order to analyze the impact of such low frequency component on the dc-link components (choke and capacitor), performance parameters are considered such as Capacitor Heating Factor (CHF) and total loss for capacitor and choke respectively. The net rms value of capacitor current estimated using appropriate weights of the frequency components is defined as CHF. This paper (a) analyses the effect of sinusoidal balanced fluctuation on dc bus capacitor with inductor placed either in dc-link or ac side, (b) derives an analytical expression to calculate the magnitude of low frequency component in the capacitor current for a given magnitude and frequency of fluctuation, (c) analyses the impact of unbalanced fluctuation in supply voltage which introduces further low frequency components in the capacitor current resulting in increased CHF and (d) proposes a new index to calculate the fluctuation effect on dc bus capacitor, as the existing indices do not correlate with the effects. As a result, it is also found that, (a) the increase of CHF due to balanced voltage fluctuation is the same with the presence of dc and ac inductor and (b) unbalanced fluctuation effect on CHF is rather more compared to balanced fluctuation, considering the practical level of fluctuation unbalance observed at industrial sites.