| Stress Control |
- More the uniformity in field, better is the utilisation of the dielectric.
- An ideal utilisation is accomplished only where η is equal to one, which is not possible in practice.
- More nonuniform field represents higher electric stress in the dielectric. It could be at only a particular location. Insulation design in an
equipment is made with due consideration to the value of estimated maximum electric field intensity.
- It is possible to achieve a higher degree of uniformity of fields by giving suitable shapes and sizes to various electrodes in an equipment.
For example, abrupt interruption of electrodes, both anode or cathode, in high voltage equipment leads to concentration of electric field at the brim, resulting in a tremendous enhancement of electric stress on the dielectric. The dielectric in the vicinity thus becomes highly vulnerable to breakdown.
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- The electrodes must be given a suitable shape at the brim to control the stress.
- For stress control, in principle the electrodes are extended and formed in such a way that higher field intensity than in the main field region does not appear anywhere in the dielectric.
- Rogowski suggested in 1923, a shape by which the electrodes could be extended, known as 'Rogowski Profile', Fig. 4.1(a).
- One can see in this figure that the field intensity continuously reduces beyond the main field region.
- Another shape of the electrode credited to Borda known as 'Borda Profile' , Fig 4.1(b), was actually worked out by him in as early as 1766 in France, more than 200 years ago.
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| Fig 4.1 Equipotential and Field (current flow) lines between plane and brim field |
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