High precision methods for online measurement of capacitance in power capacitors

In the development of metallized film power capacitors, accelerated aging experiments are carried out in order to predict the lifetime of a capacitor under nominal operating conditions. For metallized film capacitors the decrease of capacitance with time is very small even in accelerated aging. Consequently, these experiments require extremely precise measurement methods for the capacitance. We have developed two different approaches for the precise calculation of the capacitance from voltage and current data measured during aging. The first method works with calculations in the time domain, whereas the second method works with Fourier transforms of the data. In this paper, we show results of long term measurements on power capacitors during which both measurement methods were applied in parallel. Both methods permit to measure the capacitance with a relative statistical error of only 4ldr10^-5. We show that at this low noise level, even small capacitance changes caused by the temperature dependence of the dielectric constant together with small voltage fluctuations become measurable.