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Failure in metallised polypropylene capacitors

Metallised 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 discharge

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 eventtypically 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 plast

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.

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 closetogether 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 rises high enough to melt th

plastic film.

Figure 2 Optical photograph of a burst

polypropylene capacitor

Then larger breakdowns can occur and significant amounts of gas are generated. The gas can blow out the sid

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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 whi

are shown in Figure 3. There is a sharp peak which spreads out as time progresses. If another breakdown puls

occurs within milliseconds or tens of milliseconds and within tens of microns of the first one, the effects will ad

and the temperature will rise even higher. A sequence of similar pulses will raise the temperature hundreds ofdegrees centigrade. The external damage can be prevented but the additional complications results in extra

cost.

Fig.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 damag

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. To find out more call us on +44

(0)1372 367444 or email.

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