LIQUID DIELECTRICS

Liquids are used in high voltage equipment to serve the dual purpose of insulation and heat condition. They have the advantage that a puncture path is self-healing. Temporary failures due to over voltage are reinsulated quickly by liquid flow to the attacked area. However, the products of the discharges may deposit on solid insulation supports and may lead to surface breakdown over these solid supports.

Highly purified liquids have dielectric strengths as high as 1 MV/cm. Under actual service conditions, the breakdown strength reduces considerably due to the presence of impurities. The breakdown mechanism in the case of very pure liquids is the same as the gas breakdown, but in commercial liquids, the breakdown mechanisms are significantly altered by the presence of the solid impurities and dissolved gases.

Petroleum oils are the commonest insulating liquids. However, fluoro­carbons, silicones, and organic esters including castor oil are used in significant quantities. A number of considerations enter into the selection of any dielectric liquid. The important electrical properties of the liquid include the dielectric strength, conductivity, flash point, gas content, viscosity, dielectric constant, dissipation factor, stability, etc. Because of their low dissipation factor and other excellent characteristics, polybutanes are being increasingly used in the electrical industry. However, in 1970s it was found that Askarels which more extensively used, exhibit health hazards and therefore most countries have legally banned their production and use. Many new liquids have since been developed which have no adverse environmental hazards. These include silicone oils, synthetic and fluorinated hydrocarbons.

In practical applications liquids are normally used at voltage stresses of about 50–60 kV/cm when the equipment is continuously operated. On the other hand, in applications like high voltage bushings, where the liquid only fills up the voids in the solid dielectric, it can be used at stresses as high as 100–200 kV/cm.

 

·      SOLIDS AND COMPOSITES

1. Solid Dielectrics

A good solid dielectric should have some of the properties mentioned earlier for gases and liquids and it should also possess good mechanical and bonding strengths. Many organic and inorganic materials are used for high voltage insulation purposes. Widely used inorganic materials are ceramics and glass. The most widely used organic materials are thermosetting epoxy resins such as polyvinyl chloride (PVC), polyethylene (PE) or cross linked polyethylene (XLPE). Kraft paper, natural rubber, silicon rubber and polypropylene rubber are some of the other materials widely used as insulates in electrical equipment.

If the solid insulating material is truly homogeneous and is free from imperfections, its breakdown stress will be as high as 10 MV/cm. This is the `intrinsic breakdown strength', and can be obtained only under carefully controlled laboratory conditions. However, in practice, the breakdown fields obtained are very much lower than this value. The breakdown occurs due to many mechanisms. In general, the breakdown occurs over the surface than in the solid itself, and the surface insulation failure is the most frequent cause of trouble in practice.

 

2. Composites

In many engineering applications, more than one types of insulation are used together, mainly in parallel, giving rise to composite insulation systems. Examples of such systems are solid/gas insulation (transmission line insulators), solid/vacuum insulation and solid/liquid composite insulation systems (trans-former winding insulation, oil impregnated paper and oil impregnated metallised plastic film etc).

In the application of composites, it is important to make sure that both the components of the composite should be chemically stable and will not react with each other under the application of combined thermal, mechanical and electrical stresses over the expected life of the equipment. They should also have nearly equal dielectric constants. Further, the liquid insulate should not absorb any impurities from the solid, which may adversely affect its resistivity, dielectric strength, loss factor and other properties of the liquid dielectric.

It is the intensity of the electric field that determines the onset of breakdown and the rate of increase of current before breakdown. Therefore, it is very essential that the electric stress should be properly estimated and its distribution known--in a high voltage apparatus. Special care should be exercised in eliminating the stress in the regions where it is expected to be maximum, such as in the presence of sharp points.

 

·                  Estimation and control of electrical

The electric field distribution is usually governed by the Poisson's equation:

 

 

 

Where φ is the potential at a given point,

 Is the space charge density in the region

 Is the electric permittivity of free space (vacuum) However, in most of the high voltage apparatus, space charges are not normally present, and hence the potential distribution is governed by the Laplace's equation:

 


  

 

In Eqs. (2) and (3) the operator  is called the Laplacian and is a vector with properties

 


  

 

 

There are many methods available for determining the potential distribution.
The most commonly used methods are

1.        The electrolytic tank method, and

2.        The numerical methods

3.       The potential distribution can also be calculated directly. However, this is very difficult except
        for   simple geometries. In many practical cases, a good understanding of the problem
        is possible by using some simple rules to plot the field lines and equipotentials.
        The important rules are

4.         The equipotentials cut the field lines at right angles,

5.         When the equipotentials and field lines are drawn to form curvilinear squares, the density of the field lines is an indication of the electric stress in a given region, and in any region, the maximum electric field is given by dv/dx, where dv is the voltage difference between two successive equipotentials, dx apart.

Considerable amount of labour and time can be saved by properly 'choosing the planes of symmetry and shaping the electrodes accordingly. Once the voltage distribution of a given geometry is established, it is easy to refashion or redesign the electrodes to minimize the stresses so that the onset of corona is prevented. This is a case normally encountered in high voltage electrodes of the bushings, standard capacitors, etc. When two dielectrics of widely different permittivity are in series, the electric stress is very much higher in the medium of lower permittivity. Considering a solid insulation in a gas medium, the stress in the gas becomes  times that in the solid dielectric, where  is the relative permittivity of the solid dielectric. This enhanced stress occurs at the electrode edges and one method of overcoming this is to increase the electrode diameter.

Other methods of stress control are shown in Fig. 1


 

Fig. 1 Control of stress at an electrode edge

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