Lifetime Estimation of DC-Link Capacitors in Adjustable Speed Drives under Grid Voltage Unbalances

Electrolytic capacitor with a dc-side inductor is a typical dc-link filtering configuration in grid-connected diode rectified adjustable speed drives (ASDs). The criteria to size the dc-link filter are mainly from the aspects of stability and power quality. Nevertheless, the reliability of the dc-link filter is also an essential performance factor to be considered, which depends on both the component inherent capability and the operational conditions (e.g., electrothermal stresses) in the field operation. Nowadays, unbalanced voltage has the most frequent occurrence in many distribution networks. It brings more electrical-thermal stress to the component, affecting the reliability of the capacitors. In order to study the reliability performance of the LC filter in an ASD system quantitatively, this paper proposes a mission profile based reliability evaluation method for capacitors. Different from the conventional lifetime estimation, a nonlinear accumulated damage model is proposed for the long-term estimation, considering the nonlinear process of equivalent series resistor growth and capacitance reduction during the degradation. Based on the proposed lifetime estimation procedure, four case studies are investigated: first, lifetime benchmarking of capacitors in LC filtering and slim capacitor filtering configurations; second, scalability analysis for the lifetime of capacitors in terms of system power rating and grid-unbalanced levels; third, lifetime estimation of capacitors in the dc-link filter with long-term mission profile; and fourth, the impact of the capacitor sizing on the lifetime of the dc-link capacitor under grid-balanced and grid-unbalanced conditions. The results serve as a guideline for proper selection of dc-link configurations and parameters to fulfill a specification in ASDs.