TopicEffect of earth on transmission Effect of earth on transmission

line, Bundle conductor & Method line, Bundle conductor & Method of GMDof GMD

EFFECTS OF EARTH ON THE CAPACITANCE OF THREE PHASE TRANSMISSION LINESThe capacitance of transmission line is affected by the presence of earth. Because of earth, electric field of a line is reduced. If we assume that the earth is a perfect conductor in the form of a horizontal plane of infinite extent, we realize that the electric field of charged conductors above the earth is not the same as it would be if the equipotential surface of earth were not present.       The method of images is used while considering this type of problems. For this consider a single phase line having 2 conductors as shown in the Fig.       

 

A fictitious conductor is placed below each conductor of the same size and shape as the overhead conductor lying directly below the original conductor at a distance equal to twice the distance of the conductor above the plane of ground. If the earth is removed and a charge equal and opposite to that an overhead conductor is assumed on the fictitious conductor, the plane midway between conductor and its image is an equipotential surface and occupies the same position as the equipotential surface of earth. This fictitious conductor is called image conductor having the charge opposite to that of overhead conductor.

oComparing above equation with expression for capacitance of single phase line without considering the effect of earth, we can see that earth tries to increase the capacitance of line by small amount.

oBut the effect is negligible if the conductors are high above ground compared to distances between them.

Bundle Conductorso Bundled Conductors are used in transmission lines where the voltage exceeds 230 kV.

o At such high voltages, ordinary conductors will result in excessive corona and noise which may affect communication lines.  

o The increased corona will result in significant power loss.  Bundle conductors consist of three or four conductors for each phase.

o The conductors are separated from each other by means of spacers at regular intervals.  Thus, they do not touch each other.

How Bundle Conductor Work.ofor extra high voltage lines corona causes a large problem if the conductor has only one conductor per phase. Corona occurs when the surface potential gradient of a conductor exceeds the dielectric strength of the surrounding air.

oThis causes ionization of the area near the conductor. Corona produces power loss. It also causes interference with communication channels. Corona manifests itself with a hissing sound and ozone discharge. Since most long distance power lines in India are either 220 kV or 400 kV, avoidance of the occurrence of corona is desirable. 

oThe high voltage surface gradient is reduced considerably by having two or more conductors per phase in close proximity. This is called conductor bundling .

oThe conductors are bundled in groups of two, three or four . The conductors of a bundle are separated at regular intervals with spacer dampers that prevent clashing of the conductors and prevent them from swaying in the wind. They also connect the conductors in parallel. 

Advantage Of Bundle Conductoro Bundled conductors are primarily employed to reduce the corona loss and radio interference. However they have several advantages:

o Improvement in the transmission efficiency as loss due to corona effect is countered.o Bundled conductor lines will have higher capacitance to neutral in comparison with single lines. Thus they will have higher charging currents which helps in improving the power factor.

Method of GMD

In this expression, the term 2 × 10-7 × (1/4) is the inductance due to flux within the solid conductor. For many purposes, it is desirable to eliminate this term by the introduction of a concept called self-GMD or GMR. If we replace the original solid conductor by an equivalent hollow cylinder with extremely thin walls, the current is confined to the conductor surface and internal conductor flux linkage would be almost zero. 

Consequently, inductance due to internal flux would be zero and the term 2 × 10-7 × (1/4) shall be eliminated. The radius of this equivalent hollow cylinder must be sufficiently smaller than the physical radius of the conductor to allow room for enough additional flux to compensate for the absence of internal flux linkage.

Self - GMD.  

Mutual-GMD. 

• The value of Ds is the same for all the phases as each conductor has the same radius Mutual-GMD between phases A and B is 

• DAB = (Dab × Dab × Da b × Da b ) 1/4 ′ ′ ′ ′• Mutual-GMD between phases B and C is 

• DBC = (Dbc × Dbc × Db c × Db c ) 1/4 ′ ′ ′ ′• Mutual-GMD between phases C and A is 

• DCA = (Dca × Dca × Dc a × Dc a ) 1/4 ′ ′ ′ ′• Equivalent mutual-GMD, Dm = (DAB × DBC × DCA) 1/3 

• It is worthwhile to note that mutual GMD depends only upon the spacing and is substantially independent of the exact size, shape and orientation of the conductor.