Degradation from metallized polymer film capacitors with the dielectric polypropylene under the influence of humid heat

Metallized polymer film capacitors are important passive electrical components nowadays, which are currently used in any electronic devices and circuits without hesitation. Amongst other components, they are used in EMC-filters as X- and Y-capacitors, so-called RFI/EMI suppression capacitors. The latest results of studies show that under critical operating conditions, the reliability of those components is called into question. In one case an Inverter supplier was forced to call back a number of inverters, because one of the capacitors used has degraded. "This process results in outgassing within the wiring component of the inverter itself, possibly causing the compartment's cover to pop off with enough force to cause injury or property damage". This circumstance suggests, that the requested quality tests, according to the standard IEC 60384-14:2005 "Fixed capacitors for use in electronic equipment - Part 14: Sectional specification - Fixed capacitors for electromagnetic interference suppression and connection to the supply mains" are not sufficient. There are no sufficient tests, specific to capacitors exposed to the conditions corresponding, when they are used in solar inverters for the outdoor area. The norm only includes a general climate test for the RFI/EMI suppression capacitors in all range of applications. At Fraunhofer IWES an accelerated aging test has been conducted for 1,000 hours, at a defined temperature and relative humidity with different voltage stress conditions (d.c., a.c. and without voltage), after a short time a degradation of some metallized polymer film capacitors was observed. The ageing of RFI/EMI suppression capacitors cause, not only the rise of the electrical disturbances above the permitted limits, but could also affect the leakage currents at the event of a fault in a negative way, so that the isolation ability of the capacitors in case of failure is severely limited and thereby a risk for humans could occur.