Breakdown and Conduction in Solids
Composite Dielectrics
- It is difficult to imagine complete (perfect) insulation system, in an electric equipment.
- We have combination of dielectric materials. The solids may have voilds, having filled with gases; liquids may have suspected solid impurities, dirt, dust etc.
- Insulation system having combination of more than dielectrics in series and parallel is called as “Composite dielectric”.
- Most commonly encountered composite dielectrics are “solid-liquid combinations or liquid impregnated flexible solid. This type of composite dielectric is used in cables, capacitors, transformer, bushings, oil filled switch gears etc.
- In solid-liquid system, presence of gas bubbles in liquids; prance of cavities in solids will reduce the dielectric strength of the system.
- In practical systems, to reduce such undesirable effects composite insulation is used by combining different dielectrics either in series or in parallel to obtain superior dielectric form from a single material of the same thickness.
Properties of Composite Dielectrics
Generally composite dielectric consists of a large number of layers arranged one over the other. This type is used in transformers, cables, capacitors.
Note the following :
Effect of multiper layers
- Simple composite dielectric consists of two layers of the same material.
- These two sheets have higher diectric strength them a single sheet of the same total thickness.
- The material will have different dielectric strength at different points on its surface.
Effect of layer thickness
- Increase in layer thickness increases break-down voltage.
- The discharge in certain voltage depends on the thickness solid dielectric, dielectric constant.
- Breakdown channels occur at interfaces only and not directly through another layer. The discharge penetrated in one layer can not enter the next until electric field strength at interface is comparable to that of discharge channel.
Effect of Interfaces
- The interface between two dielectric surface is a composite dielectric system plays an important role in determining its pre-breakdown and breakdown strengths.
- Discharge occur at interfaces and magnitude of discharge depends on surface resistance and capacitance with increase in conductivity dielectric damages. In the composite dielectric, dielectric loss should be low to avoid thermal breakdown.