Study of electrical ageing of polypropylene film capacitors using third harmonics measurement

Capacitors made of metalized polypropylene film are components with high quality and stability. Electrical ageing of these capacitors causes changes in quality of thin film metal electrodes dominantly; changes caused in quality of PP film are low. Method of measurement of current-voltage characteristic nonlinearity was chosen for evaluation of changes caused by electrical ageing of the capacitors. The reason was that this method is especially proper for study of nonlinearity of thin metal films, such like electrodes of the capacitors. Nonlinearity measurement is based on measurement of third harmonics generated on the component powered with the sinusoidal current. Equipment CLT 1 was used for this measurement. Electric ageing was carried out in two stages. The capacitors were loaded by the voltage 450 V for 30 min, measured, and then by the voltage 500 V for following 60 min. It was found that the electric load influences nonlinearity of the capacitors. Due to the self-healing process in these capacitors the high changes were mostly observed after the first stage of the load and small only after the second one.