A Novel Method for On-Line Monitoring and Managing of Electrolytic Capacitors of DC Voltage Link PWM Converters

Abstract – The paper presents a novel concept for moni-toring and managing the DC link electrolytic capacitors of PWM converters, especially for drive applications. The unit performs on-line identification of the equivalent series resis-tance (ESR) of the capacitor in order to detect the end-of-life cycle to permit a preventive maintenance of the conver-ter prior to a capacitor failure. The ESR detection scheme is based on the determination of the capacitor AC losses calculated from voltage/current measurements using a simple single-chip microcontroller. The unit is designed as a small printed circuit board located between the capacitors screw terminals and the converter busbars. This also opens the possibility to easily detect the capacitor's operating temperature which is important for the evaluation of the identified ESR values. The intention is, furthermore, that the monitoring unit operates without a separate power supply and draws its supply current out of the power capacitor. The measurement results of the individual con-verter capacitors are gathered by an opto-isolated three-wire bus. Finally, besides the ESR detection the monitoring unit further allows an energy efficient capacitor voltage balan-cing in case a series array of capacitors is used. As opposed to conventional resistor voltage divider networks this method reduces the balancing losses to a very large extent by avoiding high quiescent currents. The paper gives a short introduction into the life cycle problems of electrolytic capacitors, presents a survey of known monitoring concepts and describes a novel concept based on capacitor power loss determination. Subsequently, a laboratory prototype of the proposed monitoring and managing system based on a single chip microprocessor is demonstrated. 1