Almost no complete electrical insulation consists of an isolation phase. Typically, more than one insulation material is involve, either in series, in parallel, or both. The composite dielectric advancedly described with composite dielectric breakdown and also solved examples are here with sketches.
The simplest form of a composite insulation system consists of 2 layers of the same material. In this case, the fact that two thin sheets have a higher electrical resistance than a single sheet with the same total thickness is use. other cases, compound dielectrics occur due to design considerations (for example, paper with an impregnating liquid) or due to practical manufacturing difficulties (for example, air parallel to solid insulation). In certain cases, the behavior of the composite insulation could be predict from the behavior of the components. However, in most cases, the entire system must be considered. The following considerations determine the overall system performance.
(i) The voltage distribution in different parts of the insulation system is distorted due to the dielectric constants and the conductivities of the components.
(ii) the decomposition properties of the surface are affect by the insulation limits of various components,
(iii) the products of internal or partial discharge of one component, without exception, affect the other components of the system, and
(iv) Chemical aging products of one component also affect the performance of other components in the system.
Matching dielectric constants
If composite insulation has components with different dielectric constants, the use of materials may be affected. This applies in particular to the dielectric of the oil / transformer plate. This is because the oil has a lower dielectric constant and a lower dielectric strength compared to transformer boards.
Since dielectrics are connect in series,
Example – composite dielectric
A transformer oil with a dielectric constant of 2.2 and a dielectric strength of 25 kV / mm is use as insulation at a distance of 8 mm. Determine the maximum applicable voltage. In this room, a barrier with a thickness of 3 mm made from Transfomerboard with a dielectric strength higher than 50 kV / mm (dielectric constant 4.4) is use to increase the resistance. Does the transformer board fulfill this purpose in this case?
With only transformer oil, the maximum applicable voltage is give by
If a barrier with a thickness of 3 mm is place in the room with the oil, the maximum applicable tension is given by
You can see that the maximum applicable voltage actually drops below that of the oil only. If barriers are to be use, it is important to adjust the permittivities of the component insulation. Therefore, great gains could be make if a transformer plate with a dielectric constant of 2.6 could be used instead of one with 4.4.