The Diagnostics of Metallized Film Capacitors under Soft Training Test

A typical approach to minimizing the early failures (“infant mortality”) of the modern metallized film capacitors (MFCs) is to conduct a burn-in test at applied voltage slightly more (at 30 - 50%) than nominal value. During the test, an external capacitor bank is used as a capacitive storage device with an energy significantly exceeding the energy level of the capacitor under the test. Thus, the amount of energy in the self-healing (SH) process is practically unlimited, which may lead to catastrophic failure of the capacitor. Recently, the soft training test for application of such capacitors in overload modes has been proposed. This can significantly improve the MFCs technical characteristics. We present the results of experimental and theoretical study on SH processes of MFCs in soft training test, allowing the capacitors to operate at voltages 3 - 5 times higher than nominal value. The MFC's degradation is associated with the equivalent parallel resistance (EPR) decreasing due to numerous SH events. On the base of the developed physical model, the relationship between critical value of testing voltage and EPR as a diagnostic parameter is offered.