• The electrical power generated in the generating station is
transmitted with the help of transmission lines.
• Transmission line is a conductor or conductors designed to
carry electricity or an electrical signal over large distances with
minimum losses and distortion.
• The parameters associated with these transmission lines are
inductance, capacitance, resistance and conductance.
• These parameters are uniformly distributed along the length of
transmission line .For good electric design of transmission line ,
a sound knowledge of all these parameters are essential.
FACTORS TO BE CONSIDERED WHILE
DECIDING TRANSMISSION LINE
• Type and size of the conductor
• Efficiency of transmission
• Corona loss
• Power flow capability and stability
• Economical aspects
CLASSIFICATION OF TRANSMISSION
1- Overhead transmission lines
• Short transmission lines
• Medium transmission lines
• Long transmission lines
2- Underground cables
Overhead Transmission Lines
1. Short transmission lines-
• Line voltage is less than 20KV
• Length of transmission line is upto about 50Km
• Capacitance effect are small or negligible
2- Medium transmission lines-
Line voltage is moderately high. It is greater than 20KV but
less than 100KV.
Length of lines is about 50 Km to 150Km.
Capacitance is significant.
3- Long transmission lines-
• Line voltage is very high (>100Km).
• Length of an overhead line is more than 150Km.
• Line constants are considered uniformly distributed over the
whole length of the line.
Conductors used to design overhead
• ASCR- Aluminium conductor with steel reinforcement
• ACAR- Aluminium conductor with alloy reinforcement
• Hard drawn copper conductor
• Steel cored copper conductor(SCC)
• Phosphor bronze conductor
• Galvanized steel conductors
• AAC- All Aluminium conductors
Overhead Line Insulators
• It is a ceramic material.
• It is free from cracks, holes etc.
• It’s insulation resistance is very high.
• Dielectric strength of porcelain is about 60kV/cm.
• Cheaper than the porcelain.
• Less stronger than the porcelain.
• Higher compressive strength than porcelain.
• Tensile strength is high.
• Weight is low
• Comparatively cheaper.
Mechanical design of overhead
• Transmission lines are designed primarily on the basis of
economy point of view.
• But due to sag it requires more conductor material.
• For keeping the cost minimum, sag also must be kept minimum
• But if sag is too low then wire gets subjected to an extra
• If sufficient sag is not kept then there is possibility of the
mechanical failure of the transmission lines.so for favorable
condition, the required amount of sag and tension are
calculated considering all the sever conditions like ice coating
and wind pressure.
Advantages of overhead transmission lines-
• Cheaper to install and maintain than underground cables.
• Quicker to fix whenever fault develops.
• They have large transmission capacity.
• Good cooling conditions.
Disadvantages of overhead transmission lines
• Difficult to be erected in the area where the network is complex
• Not very safe to be erected in densely populated area of the
• Also is not beautiful.
• Affected by environmental conditions such as temperature,
wind, rain etc.
• Influenced by thunder and lightening.
• Undergrounding is the replacement of overhead cables
providing electrical power or telecommunications, with
• In thickly populated areas, the use of overhead lines is not
practicable and electrical energy is transmitted and distributes
through underground cables.
• An underground cable is a conductor provided with proper
• As voltage increases, the cost of insulation increases and
hence underground cables are restricted to low and medium
LT cables: Low-tension cables with a maximum capacity of 1000
HT Cables: High-tension cables with a maximum of 11KV
ST cables: Super-tension cables with a rating of between 22 KV
and 33 KV
EHT cables: Extra high-tension cables with a rating of between
33 KV and 66 KV
Extra super voltage cables: with maximum voltage ratings
beyond 132 KV
Belted cables: Maximum voltage of 11KVA
Screened cables: Maximum voltage of 66 KVA
Pressure cables: Maximum voltage of more than 66KVA
Classification of underground cables
• The trench is about 1.5m deep and 45cm wide in dug.
The trench is covered with a layer of fine sand and the cable is
laid over this bed.
2- DRAW IN SYSTEM-
• Duct of cast iron or concrete are laid in the ground with
manholes at suitable positions along the cable route.
• Very less chances of fault occurrence.
3- SOLID SYSTEM-
• The cable is laid in open pipes or trough dug out in earth along
the cable route.
• After the cable is laid, the toughing is filled with the bituminous
compound and covered over.
• It has poor heat dissipation facilities.
Advantages of underground cables
• Low chances of developing faults.
• Low maintenance cost.
• Not influenced by environmental conditions.
• More durable in comparison to overhead transmission lines.
• Underground cables are more safer fore mankind.
• Requires a narrow band of length to install.
Disadvantages of underground cables
• Very expensive. It costs four times the overhead lines.
• Repairing of underground cables is not easy and it takes more
time to repair than overhead lines.
• Maintenance cost is also very high.