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BREAKDOWN OF COMPOSITE INSULATION

Almost no complete electrical insulation consists of one insulating phase. Usually more than one insulating material will be involved, either in series, parallel or both.

The simplest form of composite insulation system consists of 2 layers of the same material. In this case advantage is taken of the fact that two thin sheets have a higher electric strength than a single sheet of the same total thickness.

In other cases, composite dielectrics occur either due to design considerations (ex: paper with an impregnating liquid) or due to practical difficulties of fabrication (ex: air in parallel with solid insulation).

In certain cases, the behavior of the composite insulation could be predicted from the behavior of the components.

But in most cases, the system as whole has to be considered. The following considerations determine the performance of the system as a whole.

(i) The stress distribution at different parts of the insulation system is distorted due to the component dielectric constants and conductivities,

(ii) The breakdown characteristics at the surface are affected by the insulation boundaries of various components,

(iii) The internal or partial discharge products of one component invariably affect the other components in the system, and

(iv) The chemical ageing products of one component also affect the performance of other components in the system.

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