INTRODUCTION:

• The transmission lines constitute the arteries of an electric energy system. The availability of a well- developed, high- capacity system of transmission lines makes it technically feasible to move large blocks of electric energy over large distances.
• Electric transmission of bulk power can be accomplished either by alternating current with the line design falling in any one of the following catagories:

1. Aerial lines
2. Underground cables
3. Compressed - gas insulated lines.

• The vast majority of the world's power lines are of three- phase aerial design with bare conductors and with the surrounding air serving as the insulating medium. Because of the dominance of aerial lines they will receive out main attention. Insulation requirements and, lately, possible health hazards set practical limits to the voltage level. An overhead line is characterized by exposed magnetic and electric fields.
• As the voltage level is increased, concern is being expressed that these fields may damaged biological effects. In densely populated areas when aerial lines cannot be used for safety or other reasons, the power is transmitted in underground cables. Due to the proximity in a cable between the phase conductor and the outer grounded sheath the shunt capacitance is of very large magnitud giving raise to very excessive capacitive leakage currents.

DESIGN CONSIDERATIONS:

• Although copper has almost twice the conductivity of aluminium, the latter is almost exclusively used for today's conductors. Not only it is characterized by a weight and price advantage, but perhaps more important is the fact that an aluminium conductor has a larger diameter than a copper conductor of equivalent resistance and weight.

For a conductor considered there would be three electric field around different conductor types. They are as follows:

1. Single copper conductor
2. Single aluminium conductor
3. "Bundled " aluminium conductor

• To make the 3 cases comparable, we have assumed that all are characterized by the same resistance and voltage. The closeness of the flux lines at the conductor surface is a measure of the electric field intensity E. This results in less chance for corona in the latter cases. This phenomenon is a serious problem in high-voltage technology.
• This is caused by a breakdown or ionization of the atmosphere when a certain field intensity (~ 3000kV/m)is reached . Corona is associated with energy loss, will cause communication interference , and is generally not tolerated. For voltages in excess of about 230kV it is in fact not possible to use a round single conductor.
• Instead of going to a hollow conductor, it is more practical to use bundle conductors of two, three or several conductors per phase spaced about 30-40 cm apart. Also the bundling of the conductors also reduces to a considerable extent the line reactance, and therefore gives the added advantage of increased transmission capacity.
• For the conductor sizes in use( diameter often exceeding 30mm) it is not possible to use solid design. Flexible aluminium- conductors steel- reinforced (ACSR) cables, consisting of a central core of stranded steel wires for the mechanical strength, surrounded by the current-carrying layers of Al strands, are the most common design.
• The conductors are suspended from the tower crossarms by flexible insulator strings.
• A systems engineer is interested primarily in the electric performance characteristics of the transmission line. These can be expressed in terms of the following four line parameters, listed in order of importanc:

1. Line inductance
2. Line shunt capacitance
3. Line resistance
4. Line shunt conductance

We shall symbolize these parameters by L,C,R and G, respectively, and shall in most practical cases express them unit length and per phase of the transmission line is unsymmetrical, it is in general not theoretically possible to express some of the parameters on a per-phase basis. However, if we tolerate some slight approximations, it can be done as a practical convenience.

• Written by sindhu.
• In category Engineering.
• 14 years ago.

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