FAILURE IN METALLIZED POLYPROPYLENE CAPACITORS

Metalized polypropylene capacitors are used extensively in fluorescent light fittings and on electric motors. The vast majority last for years without problem, but there are occasional incidents where they burst or even catch fire.

These types of capacitor possess a safety mechanism called “self-healing”. If an electrical breakdown occurs in the film, the charges on the capacitor electrodes in the vicinity of the fault flow to that point and are discharged. Considerable energy can be dissipated in the discharge.

This is usually limited by using a metal film that is so thin that the metal evaporates due to the high local current densities. The evaporation disconnects the fault from the rest of the film, so that the effects are limited. The event typically lasts for microseconds. Part of the film is lost and the capacitance is reduced, but the capacitor continues to be usable. A piece of film with a self-healing spot is shown Figure 1.

Figure 1: SEM image of a group of breakdown points in a polypropylene plastic film at high magnification

Part of the design process for the capacitors consists of balancing the need for film thin enough for self-healing against the need to keep the series resistance low. After completing the manufacturing steps a selection of the capacitors are put on life tests to ensure that the weakest spots in the film are disconnected before the capacitors are put into service. The metal electrodes are sometimes composed of small islands of metal connected by narrow links. The links are easily broken by an excess current at a breakdown without disturbing the neighbouring islands.

Figure 2: Optical photograph of a burst polypropylene capacitor

The energy dissipated in the breakdown appears as heat and the temperature rises locally. The plastic film at the breakdown point is carbonised with the release of gas. If a series of breakdowns occur close together within the capacitor or closely spaced in time, the capacitor may not be able to dissipate the heat generated in the breakdown events. The temperature then rises locally with each breakdown and the gas accumulates. If sufficient breakdowns occur, the temperature raises high enough to melt the plastic film.

Then larger breakdowns can occur and significant amounts of gas are generated. The gas can blow out the side of the case or blow the case off entirely. Under extreme conditions the gas mixture can ignite. An example is shown in Figure 2. These blow-outs can damage neighbouring components.

A simplified thermal model has been used to estimate the temperature rise around a breakdown results of which are shown in Figure 3. There is a sharp peak which spreads out as time progresses. If another breakdown pulse occurs within milliseconds or tens of milliseconds and within tens of microns of the first one, the effects will add and the temperature will rise even higher. A sequence of similar pulses will raise the temperature hundreds of degrees centigrade. The external damage can be prevented but the additional complications results in extra cost.

Figure 3: A set of plots of the temperature as a function of distance from a breakdown in the form of a disc for a series of times after the event using a simplified thermal model.

FURTHER INFORMATION

Other common forms of failure in metallised film capacitors are partial discharges and ion migration to damage sites. ERA has examined many failed capacitors and batches of capacitors before and after use to assess the quality of construction. The pattern of the damage on the film can used to assess the processes involved in the failure. Sometimes it is found that there are batches of suspect capacitors.