A Real Time Method to Estimate Electrolytic Capacitor Condition in PWM Adjustable Speed Drives and Uninterruptible Power Supplies

Electrolytic capacitors are widely used in various power electronic systems, such as adjustable speed drives (ASD) or uninterruptible power supplies (UPS). Their high energy density (J/cm ³) features make them an attractive candidate for smoothing voltage ripple and pulse discharge circuitry. However, electrolytic capacitors have the shortest life span of components in power electronic circuits, usually due to their wear-out failure. The main wear-out mechanisms in electrolytic capacitors are the loss of the electrolyte by vapor diffusion trough the seals and the deterioration of the electrolyte. Both mechanisms can result in a fluctuation of the capacitor´s internal equivalent series resistance (ESR). In this paper, a real time diagnostic method of the ESR of the electrolytic capacitors in ASDs and UPSs is presented. This method is employed satisfactorily to estimate their deterioration condition. An on board implementation of this method is proposed, which can be very helpful for preventing down time and alerting plant operators to needed maintenance and/or replacement. The approach does not involve removing the capacitor bank from its device and relies on the fact that in steady state, the power in the capacitor is only due to the power losses in the ESR. An analog-DSP based diagnostic system has been implemented and experimental results from a 3-phase 6 kVA/230 V ASD are presented