In-Situ Montioring of PWM Converter Electrolytic Capacitors

Within the applied project a novel concept for monitoring and managing of the DC link electrolytic capacitors of PWM converters (used, e.g., for drives in large quantities) shall be developed and verified. The new method is based on an in-situ identification of the capacitor`s loss resistance (equivalent series resistance - ESR) which is ideally suited for detecting the aging status of the capacitor. The permanent ESR determination allows a timely replacement of the capacitor in order to avoid abrupt converter breakdowns with possibly high consequential costs. The new concept proposes a detection of the ESR via determination of the capacitor losses based on voltage/current measurements. The special advantage is, that the principle is insensitive to parameter variations and ideally suited for digital implementation, e.g., utilizing low-cost single-chip microcontrollers. Due to the low realization costs - being almost negligible as compared to the capacitor costs - a very wide application area of the monitoring unit is expected. Furthermore, besides the detection of the aging status the unit also opens a possibility for active voltage balancing in case of series connected capacitors. This commonly is performed by ohmic resistors which, however, cause considerable additional permanent losses. Consequently, the novel monitoring system leads to a substantial improvement of the converter reliability as well as to a significant reduction of the converters stand-by losses.

In the last ten years, the proliferation of power electronic converters in the field of industrial electronics and electrical power generation has increased significantly. In especial, the steadily rising quantity of pulse width modulated (PWM) converters for drive systems shall be mentioned. Central components of these converters, besides the semiconductor switching elements like IGBTs and MOSFETs, are the electrolytic capacitors of the DC voltage link. These capacitors are required to provide switching-frequency harmonics of the IGBT input currents as well as (especially in single-phase systems) line-frequency energy storage. Because of their electro-chemical basic working principle (liquid electrolyte) the lifetime of electrolytic capacitors strongly depends on the component`s temperature. To prevent unexpected, sometimes high costs-causing break- downs to the drive, it is appropriate to enhance the converter with an on-line monitoring for detecting the ageing state of the DC link capacitors. With this a timely replacement of weak capacitors is possible, which increases the reliability of the device significantly. As a good indicator for the aging state of an electrolytic capacitor the equivalent series resistance (ESR) of the component is very suitable since this value is largely determined by the electrolyte (being primarily responsible for capacitor ageing). Aim of this research project therefore was the development and analysis of a monitoring system for inverter DC link capacitors based on a continuous ("in-situ") determination of the ESR value during normal converter operation. An ESR increase to typically 300% of the initial value (novel component) can be interpreted as "end-of-life" for the component. The underlying novel approach of the project is the determination of the ESR based on calculating the capacitor`s losses, determined from voltage and current measurement. This principle allows, in comparison with known methods, an ESR detection being largely insensitive to parameter variations. A substantial additional advantage is that the proposed scheme can be implemented very simple and cost effective utilizing low-cost microcontrollers. The developed control unit was designed as a printed circuit board module, which can be inserted easily between the monitored capacitor and the converter`s DC link bus bars. This gives short signal paths for current and voltage signals and opens the possibility for a simple capacitor temperature sensing which is required to evaluate the detected ESR value. With the developed monitoring unit furthermore also an "active" capacitor voltage balancing for series-connected capacitors can be achieved. This method results in a practically complete avoidance of the balancing losses as they occur with conventional passive voltage dividers. In conclusion, the capacitor monitoring unit developed under this project permits a substantially increase of reliability for PWM converters which enhances the suitability of these systems regarding applications being critically in failure scenarios. By the additionally possible active voltage balancing a very attractive substantial reduction of standby losses of the converter can be achieved.