Rod Earthing

Rod Earthing is an extremely popular form of Earthing as they are easy to install and retrofit.  Rod earthing involves driving metal rods made of copper, copper clad steel or stainless steel.  

The Rods used for earthing are usually about a metre long with a diameter of around 12 mm.  The rods should be driven to a depth of around 2.5 metres.  Usually the rods have threaded sections on one end so that the rods can be driven one on top of another to achieve the required resistance.

The Rods can be hammered into the ground.  This would be possible in sandy soil.  If the soil is hard, a Earthing rod driver which is a tool, usually pneumatic, which drives the earth rods into the soil is used. 

Capacitance Grading

Capacitance Grading is a method of distributing the electrostatic stress across the insulation of cables by using materials with different dielectric strength. When a cable is live, the insulation around the conductor is in a state of electrostatic stress. 

This stress is high near the centre and reduces towards the periphery. This uneven electrostatic stress can cause the failure of insulation. To prevent this kind of catastrophic failure, the insulation is graded. The permittivity of the insulating material is made to vary inversely as the distance from the centre. 

It is not possible to have a single material which has a permittivity which varies depending on the distance. Hence, the insulation is made of a number of layers of insulation made from different materials. 

Each of these materials has a different value of permittivity.   This ensures that the insulation is approximately uniform throughout the cross section of the insulation.

Rated Voltage Factor of Potential Transformers

The Rated Voltage Factor of Potential Transformers is the maximum limit of excess voltage up to which the voltage transformer can maintain its rated characteristics.  It is mentioned in percentage of the nominal voltage.  

For instance a Rated voltage factor of 1.2 would mean that the Potential Transformer can maintain its rated characteristics up to 120% of the nominal voltage.  

The following table lists the common Rated Voltage factors in Potential Transformers for different connections

Rated voltage factor	Rated time	Primary Winding Connection
1.2	Continuous	Between phases in a network Between transformer star-point and the earth in any system
1.2 1.5	Continuous for 30 seconds	Between the phase and earth in a system whose neutral is effectively earthed
1.2 1.9	Continuous for 30 seconds	Between the phase and earth in a  system whose neutral is non-effectively earthed  with automatic fault tripping
1.2 1.9	Continuous for 8 hours	Between the phase and earth in a system whose neutral is isolated without automatic fault tripping or in a system with resonant earthing without automatic fault tripping

Intersheath Grading

Intersheath Grading is a method of ensuring that the voltage gradient across the insulation of a cable does not become so steep as to cause the failure of the insulation.  The insulation of a cable is subjected to constant electrostatic stress.  This electrical stress is dependent on the voltage of the conductor.  The electrostatic stress needs to be uniform across the insulation.  Uneven electrostatic stresses can result in failure of the insulation. 

Intersheath Grading is a method of creating uniform voltage gradient across the insulation by means of separating the insulation into two or more layers by thin conductive strips.  These strips are kept at different voltage levels through the secondary of a transformer.  This ensures that all parts of the insulation are exposed to relatively the same stress.  

Single Circuit and Double Circuit Transmission Lines

Transmission lines which carry three phase power are usually configured as either single circuit or double circuit. A single circuit configuration has three conductors for the three phases.  While a double circuit configuration has six conductors (three phases for each circuit).  

Double Circuits are used where greater reliability is needed.  This method of transmission enables the transfer of more power over a particular distance.  The transmission is thus cheaper and requires less land and is considered ideal from an ecological and aesthetic point of view.  However, running two circuits in close proximity to each other will involve inductive coupling between the conductors.  This needs to be taken into account when calculating the fault level and while designing the protection schemes.

Double circuit transmission lines usually contain bundled conductors with the conductors placed as far as possible to minimize inductance. 

Proximity Effect

Proximity Effect refers to the phenomenon where the resistance of one conductor in a transmission line or a bus bar decreases due to the magnetizing flux from another conductor in close proximity.  

When two conductors are placed close to each other the varying magnetic flux caused by one conductor induces currents in the other conductor.  This increases the current density in the other conductor and results in increased impedance. 

The Proximity Effect increases with frequency.  Conductors are usually spaced with a sufficient distance to minimize the Proximity effect. 

A comparison of Overhead and Underground Transmission

Electric Power needs to be carried over long distances from the point of generation to the point of consumption.  This Transmission is done either through overhead lines or underground cables.  Each of these two methods of transmission has its own advantages and disadvantages. 

Overhead Transmission lines are cheaper as the insulation cost is lesser and the conductor material cost is lesser too.  They also have better heat dissipation.

However, they have significant disadvantages.  Overhead lines are vulnerable to lightning strikes which can cause interruption.  Overhead lines use  bare conductors and can cause damage if they break.  They are considered to be unsightly as they mar the scenery of the landscape.  The maintenance cost of overhead lines is more and the voltage drop in overhead lines is more.

Underground transmission due to cables is costlier than overhead transmission as the ground needs to be excavated.  This can be difficult when passing though geographic obstructions such as hills, marshes and rivers.  Special trenches need to be constructed when passing through loose soil.  Besides, heat dissipation in underground cables is an issue. Hence,  the conductors have to be thicker.  The insulation required for the cables is expensive.  Hence, it is difficult to use underground cables for voltages at HV levels (> 33 kV).  

Underground cables may have to be rerouted to accommodate other underground structures such as pipelines, sewage lines, etc.    It is necessary that the routes of underground lines are clearly marked with sign boards to prevent accidents when excavations are carried out for other reasons at a later date.