Introduction

To meet the demands of the traction industry, NCL filter capacitors are designed with state of the art metallised polypropylene film dielectric systems, in which a very stable, biodegradable liquid impregnant is used to ensure extremely long life and reliable self healing capabilities. NCL has evolved as one of the foremost manufacturers of this product type being the first company to develop this type of capacitor for traction applications.

NCL, with its considerable experience in dielectric technology has developed a range of dielectric systems to satisfy the exacting requirements of the traction market, where long life, total reliability and compact design are an essential prerequisite. These designs provide a unique self-healing feature, so that should a breakdown occur within the dielectric, the special metallised feature of the electrode system will allow the capacitor to continue working as if no fault had occurred. This process of self healing, whereby the energy of the capacitor being discharged into the fault site, causes a low current plasma to form. This in turn evaporates the thin metallised deposit on the film in the immediate vicinity, forming an insulating oxide and thereby restoring the voltage capability of the dielectric system.

By comparison with the more conventional film and foil dielectric systems, where the voltage gradient is lower, a breakdown will cause a short circuit, resulting in a failure of the capacitor often catastrophic in the case of the larger capacitors. Because the voltage gradient is generally higher in metallised film dielectric systems, self-healing will occur, usually during the initial stages of test at the factory, where an over voltage test is applied, or in the early stages of energisation in service. The capacitor should then remain relatively free of self- healing until the approach of "end of life", where because of the change in the polymer structure due to ageing under voltage stress and temperature, the self healing will become more frequent. Metallised construction is accepted as a considerable improvement over the traditional film and foil types, and indeed has become the accepted method of construction at NCL.

There is however a disadvantage of this self-healing system which becomes increasingly significant as a function of the energy contained within a capacitor, or in certain circumstances, a bank of capacitors.

During a self-healing event, the amount of energy which can be fed into the fault site can be uncontrolled and may on occasions be far in excess of the amount of energy required to effect a satisfactory self-healing. In these circumstances the energy developed can cause consequential damage to the layers above and below the original fault site, effectively weakening them and making them more susceptible to a further self-healing action. The resulting input of energy will in turn damage other areas in the locality of the original fault. The effect can become progressive, culminating in an avalanche effect which destroys the capacitor. Small capacitors used in printed circuit board applications do not suffer from this problem as the energy involved is minimal by comparison.

NCL's use of special metallising techniques obviates the possibility of consequential damage but restricts the amount of energy dissipated during a self- healing process, in effect operating more like a fusing action. As a result, the onset of progressive reduction in capacitance value is delayed, the amount of by-products of this fusing operation is reduced, effectively eliminating build-up of internal pressure to which more traditional designs were prone. NCL has declared that "end of life" is considered to have been reached after a >3% reduction in capacitance value from the original measured value at the time of delivery. Under many circumstances, the capacitor can continue to be operated for significant periods after this point has been reached, but please consult us in this eventuality.

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