ELECTRICAL TRANSMISSION LINES


High voltage curent transmission can be done  as

•  High Voltage Direct Curent (DC)

or

• High Voltage AC curent

But in Sri Lnka there is only High Voltage AC curent distribution system.
Beacouse AC curent distribution system consiume less cost compared with to DC curent distribution system for short distance . (below 800km)
A transmition line has three parameters

Change of AC  resistance according to the frequency

• Resistance
• Inductance
• Capacitance

These parameters are uniformly distributed through the transmition line.
AC current resistance is greater than the DC curent resistance due to the SKIN EFFCT.
So the current density is larger near the surface of the conductor and decreasing to the center point .
Skin effect increases proportionaly with the frequency and the cross sectional area.

Construction of the transmission line

Transmition line conductors are not straight wires but strand of wire twisted together to form a single conductor to give it a higher tensile strength.

Transmition line types

Aluminium Conductor Steel Reinforced (ACSR)

  The outer strands are Aluminium,chosen for the excelent conductivity,low cost ,and low weight.The center strand is of steel for the strength required to support the weight without streching the aluminium  due to its ductility .
In the conductor there is a less current density in the middle ,since the less conductive steel bars place in the midle of the strand.

All Aluminium Conductors (AAC)

This conductor is made from aluminium-magnesium-silicon alloy of high electrical conductivity containing enough magnesium silicide to give it better mechanical properties after treatment.

AAAC (All Aluminium Alloy Conductors)

they have a better corrosion resistance and better strength to weight ratio and improved electrical conductivity than ACSR CONDUCTOR on equal diameter basis.
ACAR