Understanding Intersheath Grading in Power Cables

Power cables designed to transmit high voltages experience significant electric stress. This stress is not uniformly distributed across the insulation layer. It is highest at the surface of the inner conductor and decreases as you move outwards towards the sheath. This non-uniform stress distribution can lead to insulation breakdown, especially at higher voltages.

What is Cable Grading?

Cable grading is a technique used in high-voltage cables to improve the electric field distribution across the insulation. The goal is to make the electric stress more uniform throughout the dielectric material, thereby preventing localized overstressing that could cause insulation failure.

There are primarily two methods of cable grading:

• Capacitance grading
• Intersheath grading

Focus on Intersheath Grading

Intersheath grading is a method specifically employed to address the non-uniform electric stress distribution in cables. This technique involves using layers of insulation material with different dielectric constants or by introducing conducting layers (intersheaths) within the main insulation.

In the intersheath grading method, conducting layers are placed coaxially within the dielectric material. These intersheaths divide the main insulation into several layers. The intersheaths are connected to taps from the main voltage supply through a suitable impedance (like resistances or reactances) or maintained at specific intermediate potentials by other means.

How Intersheath Grading Manages Stress

By introducing these intersheaths at calculated radial positions, the total potential difference across the insulation is divided into steps. Each layer of insulation between the conductor and the first intersheath, between successive intersheaths, and between the last intersheath and the outer sheath, experiences a reduced voltage difference compared to the total voltage. This stepped voltage distribution helps in controlling the electric stress within each layer.

The potential of each intersheath is controlled such that the voltage gradient (electric stress) across each section of insulation is approximately the same or maintained within permissible limits. This leads to a more uniform electric stress distribution throughout the entire insulation thickness.

Purpose of Intersheath Grading

Based on this understanding, the primary purpose of intersheath grading in cables is to effectively control and redistribute the electric field within the insulation. This process ensures that the electric stress is distributed more evenly, particularly alleviating the high stress concentration near the conductor surface.

Therefore, intersheath grading is specifically used to:

• Improve the distribution of electric stress across the cable insulation.
• Reduce the maximum stress near the conductor.
• Allow the cable to withstand higher operating voltages without immediate breakdown.
• Effectively utilize the insulation material more uniformly.

Considering the options provided, the function of intersheath grading directly relates to managing the electric stress within the cable insulation.

Analysis of Options:

• Minimize the stress: While it reduces the maximum stress, the primary goal is 'distribution' and making it uniform rather than just minimizing overall stress which might imply using less stress.
• Avoid the requirement of good insulation: This is incorrect. Grading methods require good insulation; they just help the existing insulation perform better under stress.
• Provided proper stress distribution: This precisely describes the mechanism and purpose of intersheath grading – to make the electric stress distribution within the insulation uniform or more manageable.
• None of the above: This is incorrect as one of the options accurately describes the purpose.

Thus, the main application of intersheath grading is to achieve a proper or more uniform distribution of electric stress throughout the cable insulation.

Revision Table: Cable Grading Concepts

Additional Information: Stress Distribution Principles

The electric stress (E) at any point in a cable's insulation is given by the formula:

$$E = \frac{V}{x \ln\left(\frac{R}{r}\right)}$$

Where:

• $V$ is the voltage between conductor and sheath.
• $x$ is the radial distance from the cable center.
• $r$ is the radius of the conductor.
• $R$ is the inner radius of the sheath.

This formula shows that stress ($E$) is inversely proportional to the radial distance ($x$). This means the stress is maximum at the conductor surface ($x=r$) and minimum at the sheath surface ($x=R$). Grading techniques, like intersheath grading, aim to modify this natural stress profile to make it more uniform across the insulation thickness, allowing cables to operate safely at higher voltages.