Influence of Grid Impedance on the DC Bus Capacitor of an ASD in a Multi-Drive System

Grid impedance is one of the important parameter that alters the performance of an Adjustable Speed Drive (ASD). Design engineers usually consider the grid impedance to validate the performance of a single ASD. However, these performance metrics cannot be easily extrapolated when multiple ASDs are connected. In this latter case (multi-drive systems), there will be an influence of one drive over the other through grid impedance. This paper unfolds the interaction between the drives connected in a multi-drive system. As a result of this study, it is identified that there is a reduction in the ripple in the DC link voltage and RMS value of capacitor current of all the drives that are connected in the system compared to the case when they are connected alone. Further, the reduction depends on the number of drives connected in the system and the value of grid impedance. Interestingly, the percentage reduction in the capacitor current (and also ripple in the DC link voltage) is not the same for all the drives connected in the multi-drive system and the reduction varies based on the power rating of the drive. Moreover, the pattern of reduction is not the same when the supply voltage is unbalanced.