Insulation and Grounding

In many power generation and distribution applications, semi-conducting dielectric materials are used to reduce electrical stresses. Here, we will take a look at a simple model which highlights the key components/stages in modelling this. The figure below shows a typical bushing arrangement with an HV conductor at 10 kV (red) emerging from a grounded tank lid (yellow) with a green insulating bushing between them. The permittivity of the bushing is 5.7.

Bushing with lossy dielectric

Bushing with lossy dielectric

Only the dielectric and the surrounding air need to be included in the model. The model has 8-fold symmetry so can be further simplified by including only 45 degrees.

Section modelled with symmetry

Section modelled with symmetry

The electric field just in the air outside the insulator near the lid is close to 40 kV cm-1, as can be seen below. This can lead to breakdown as it exceeds 33 kV cm-1.

Region of high field strength

Region of high field strength

Model with stress relief

To overcome the high electric fields, a coating of stress-grading material can be applied over the insulator. The figure below shows the 45 degree section model with the stress grading material included.

Model with stress relief material added

Model with stress relief material added

The properties of the material are ϵ = 2 , σ = 10−6 S/m. The material connects the high and low voltage boundaries allowing a small current (because of the low conductivity) to flow and thus redistributes the electric field.

Electric field with addition of stress-grading

Electric field with addition of stress-grading

The current density in the lossy dielectric material can also be evaluated, as shown below.

Current density after adding lossy dielectric

Current density after adding lossy dielectric

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