The impact of grid unbalances on the reliability of DC-link capacitors in a motor drive

DC-link capacitor is one of the reliability-critical components in motor drive applications, which contributes to a considerable cost, size and failure. Its reliability performance depends on both inherent physical strength and external loading. The grid unbalances could alter the electro-thermal stresses of key components in a motor drive. Therefore, this paper investigates the impact of grid voltage amplitude and phase unbalances on the lifetime of DC-link capacitors used in a standard three-phase motor drive. The theoretical stress models and experimental measurements of the capacitor voltages and ripple currents are presented. The relationship between the DC-link capacitor lifetime and the level of unbalances and loads are discussed based on a 7.5 kW motor drive system. The results serve as a guideline to size the DC-link capacitors to be robust enough at the presence of grid unbalance conditions.