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Grading of Cables

CABLES GRADING


It has been observed that the electrostatic stress in a single core cable is maximum at the Conductor surface and decrease towards the Sheath.

In a Cable having homogeneous insulation the maximum voltage which can safety applied to a Cable is limited by the electrostatic stress at the Conductor surface.

Grading of Cables
Grading of Cables

If a Dielectric of high strength is used in a Cable, then no doubt it is useful just over the Conductor where the stress is Maximum, but as we away from the Conductor surface, the value of Dielectric stress decrease, so the Dielectric will be unnecessary strong and expensive.

If, however, by means the stress can be so Distributed that it's value in the outer layers of the Dielectric in increased without increasing the Stress near the Conductor, Insulation of test thikness will be required for any given Working Voltage, and an important economy effected.

The process of achieving uniformity in Dielectric stress is Known as Grading of Cables.

There are two methods of Grading viz.

1) Capacitance Grading and

2) Inter sheath grading.

(1) Capacitance Grading


In this method of cable grading uniformity in Dielectric stress is achieved by using various layers of Different Dielectric in such a manner that the permittivity, ∈r of any layer is inversely perpotional to its radius or distance from the centre.

The advantage of grading is that it the over all diameter is same for a non - graded and graded Cable, then the permissible same potential of the cable is increased i.e., for the same permissible safe potential, the size of the graded Cable will be smaller than that of the non - graded Cable.

(2) Inter sheath grading


In this method of cable grading homogeneous Dielectric is used which is divided into various layers by suitably placing the metallic Inter Sheath, These is thus a definite potential Difference Between the inner and outer Layers of each Sheath, so that each Sheath can be treated like a homogeneous single core cable.

Grading in Practice


Grading is only useful very high voltage cables for which the ratio D/d is large.

If D/d is 2, grading will increase the permissible safe working Voltage only 30%

            D
But if ----- = 5 , grading will increase the
             d
safe working Voltage by 100%.

Note :- while Calculating the safe potential, the peak values of Voltage are to be taken and not the RMS Values.

            

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