Partial discharge is characterised by the fact that initially only part of the insulation length is penetrated, not the whole of it. However, these partial discharges in part of the insulation system lead to ever greater damage over time. After hours, days, weeks, months or years, the insulation system is weakened to such an extent by the continuous partial discharge that a full disruptive discharge occurs.
PD or partial discharge resistance therefore mainly describes the resistance of the insulation material to continuous stress loading from high (alternating) voltages. At the same time, it is an (albeit vague) measure of the long-term reliability of the insulation film.
Partial discharge tests in equipment manufacturing are used to check the quality of a complete insulation system and of the electrical assembly or to detect quality defects in windings. The test can be carried out in conjunction with both a high voltage test and a surge voltage test
Experience with customers and measurement series have shown that fluoropolymer films are particularly suitable. They depend only to a small extent on the εr of the temperature and have very high chemical and electrical resistance to corona discharges. They also have good loss factor behaviour at high frequencies of the sort that can occur in switching power units.
In addition, the CMC range includes a variant that uses the well-known Kapton® HN film, which is distinguished by its very good corona resistance (glow discharge, partial discharge): Kapton® CR.
This makes use of the fact that inorganic substances are significantly less susceptible to corona discharges than polymers (typical example: the ceramic upright insulators on high-voltage pylons).
The polyimide matrix of the Kapton® CR is therefore mixed with an inorganic filler. The Kapton material largely retains its outstanding physical and chemical properties, although the dielectric strength falls slightly. On the other hand, the corona resistance improves dramatically.
For applications in which particularly high reliability and a long service life are important (frequency converters in wind power and solar systems), it may be that cheap insulation materials such as polyester or PEN are not adequate. In particular in damp and warm conditions (e.g. in a control cabinet/box in an outdoor area), additional degradation processes can occur in the insulation material (hydrolysis). Dirt, condensation and contamination on the surface of the insulation resulting from defective installation reduce the insulated section. Partial discharges can then lead more easily and quickly to destruction of the insulation (carbonisation, increased susceptibility to dirt because of a rough surface).
The information above and our technical advice on applications, whether verbal, written or as the result of experiments, is given to the best of our knowledge but is non-binding. Our advice does not exempt you from checking our latest information – in particular our datasheets and technical information – and our products in relation to their suitability for the intended processes and purposes.
Overall, adhesive tape products such as CMC 75730 (FEP film), CMC 75738 (PTFE film) and CMC 77700 (ETFE film, very low tracking tendency) achieve excellent service life ratings, as these types of polymers are inherently extremely resistant to ageing and have good PD performance (high PD extinction voltage or low sensitivity to tracking path formation). All variants are slightly elastic, so that they provide safe separation of primary and secondary windings if the windings and the coil core are carefully designed (as an alternative to the feathering of polyester film, for example).
Kapton® CR film from CMC has long been used in high-voltage motor and generator construction. Extensive tests by ABB and Siemens have shown that the glow discharge resistance is significantly higher than “normal” Kapton® HN or polyimide film made by other manufacturers.
Resistance to corona discharge (IEC 60343)
(service life standardised as 1 x for Kapton HN)
Operation at 750VAC, 1kHz 1000VAC, 360Hz
Kapton HN 1 x 1 x
Kapton CR 600 x 60 x
Unlike fluoropolymer film, Kapton® film does not have a tendency to yield under pressure. At the same time, the film is also stiffer, which can lead to problems in the margins between the winding and coil carrier (reversing).
Insulation composites in which Nomex T418 is used are similar (e.g. Nomex T418 - Kapton Cr - Nomex T418; Nm-Kc-Nm). This Nomex aramid paper contains approximately 50% mica, which resists attack by partial discharge very well as an inorganic product. However, the material alone has only low dielectric strength and is usually used in impregnated form.
If several types of stress apply simultaneously – high voltage and environmental factors – and a long service life is required, using these relatively expensive insulation films may make sense.
With other, cheaper insulation materials, which have already been in use for many years with good results, the service life of the device in which the transformer or coil is installed is usually shorter than the service life of the insulation. That may change with today’s long-term requirements (15-25 years) – conclusive evidence is only available to a limited extent.