A new approach to quantitatively evaluate aging. Case study: Epoxy-mica

Polarization/Depolarization current (PDC) measurements, when recorded for a sufficient long time, provide curves that exhibit a straight portion, usually in the range of a few seconds up to a few tens of seconds. These straight portions fit closely a straight line, when the current data is plotted on Log-Log scales. This applies particularly well for the depolarization current curve. Using the A t
^-n
law, it is possible to evaluate the exponent n with good accuracy. As the data base on both field and lab measurements on epoxy-mica, polyester-mica and asphalt-mica insulations got bigger over the years, some trend began to emerge. This paper shows variations of n as a function of the “chronological” age of the insulation, i.e. the time difference between date of fabrication and the date of the measurement. Concurrent factors such as temperature and other aging stress conditions are discussed in the paper by indicating which way they can modulate the behaviour of n versus time.