CURRENT CARRYING CAPACITY OF OVERHEAD TRANSMISSION
LINE ACSR CONDUCTOR
1.0 INTRODUCTION
1.0.1 The thermal capabilities of transmission lines in Power System are evaluated based
on the criteria of maximum operating or design temperatures of the transmission
line conductors and the transmission losses.
1.0.2 The thermal rating of conductor is dependent on the following factors :
1) Meteorological/Environmental Conditions
2) Solar radiation
3) Wind velocity
4) Ambient temperature
5) Conductor surface Characteristics
6) Age of the Conductor
7) Maximum design conductor temperature
8) Allowable losses of strength of aluminium metal at design temperature
1.0.3 The maximum value of temperature at which transmission metal at design
temperature operate in India is 650 C for conductors as per provisions of IS: 802
(Part-I) 1973. However, to increases loading capabilities of transmission lines, the
matter is under active consideration by Bureau of Indian Standards, Central
Electricity Authority, State Electricity Boards and other utilities etc. for increasing
the maximum conductor design temperature from 650 C to 750 C as it has been
observed that there is no loss of strength of aluminium metal at 650 C. But it has to
be ensured that the transmission line towers are designed suitably, for that there is
no violation of ground clearance under any circumstances due to safety
considerations. This would significantly help in improving thermal loading
capability of transmission lines, without infringement in ground clearances.
1.1 HEAT BALANCE EQUATION OF ELECTRICAL CONDUCTORS
The current carrying capacity rating is computed by using the following formula under
steady state condition of wind velocity, temperature, solar radiation and electric current:
Q c + Q r = I2 R + Q s
Where, I2 R = Heat generated in the conductor due to flow of current ‘I’ in Amperes, R is
the resistance of the conductor per meter.
Q8 = Solar Heat Gain in Watts per meter of Conductor.
Qc = Convection heat loss in Watts per meter of Conductor.
Qr = Radiated Heat Loss in Watts per meter of Conductor.
From the above equation, current carrying capacity ‘I’ can be determined as:
I = √Qc + Qr - Qs/R Ampere
The effect of Heat Gain due to Magnetic Heat and Corona Heating and Heat Loss due to
Evaporation is usually negligible, hence not considered.
1.2 CURRENT RATING OF VARIOUS CONDUCTORS
The size of the conductor used for transmission lines of various voltage class has now
been standardized, which are as follows:
400kV - 'Moose' ACSR
220kV - 'Zebra' ACSR
132kV - 'Panther' ACSR
TABLE – I
Current Carrying Capacity in Amperes
At maximum designed At maximum designed
Sr. Type of temperature of 650 C temperature of 750 C
No. Conductor
Old Conductor Old Conductor
New Conductor New Conductor
(Beyond 10 (Beyond 10
(Up to one year) (Up to one year)
years) years)
'Dog’
1. 141.12 150.20 229.65 245.06
ACSR
'Panther'
2. 179.89 200.60 340.83 371.42
ACSR
'Zebra'
3. 201.26 249.51 496.46 553.70
ACSR
'Moose'
4. 133.60 218.89 530.51 603.78
ACSR
NOTE:
* Current carrying capacities of conductors indicated above, are for worst condition of
ambient temperature. However, the current carrying capacity of the conductor of the
increases as the ambient temperature decreases.
* The current carrying capacity of the conductor also increases with the age of the
conductor as would be observed from the above TABLE - I.
1.3 POWER TRANSMITTED
1.3.1 The power transmitted for any size of conductor depends on its current carrying
capacity and can be calculated from the following formula:
Power in KW = √3 V I Cos Ф
Power in KW = √3 V I Cos Ф/1000
Where, Cos Ф = Power Factor (P.F.)
V = Voltage in kV
I = Current in Ampere
Assuming Power Factor of 0.8 lagging, power transmitted at various voltages can be
calculated approximately as follows:
At 132kV:
Power in MW = √3 x 132 x I x 0.8/1000
= 0.183 x I
At 220kV:
Power in MW = √3 x 220 x I x 0.8/1000
= 0.305 x I
At 400kV:
Power in MW = √3 x 400 x I x 0.8/1000
= 0.554 x I
Thus value of power transmitted can easily be calculated at 132kV or 220kV or 400kV for
a given value of current.
1.3.2 Normally for continuous operation the transmission lines used on various voltage
are designed to carry or transmit maximum power loads at the designed maximum
conductor temperature of 650 C as follows:
At 132 kV with 'Panther' ACSR = 75 MVA
At 220 kV with 'Zebra' ACSR = 200 MVA
At 400 kV with 'Moose' ACST = 500 MVA
1.3.3 It is worth considering that the maximum permissible conductor temperature for
continuous operation on transmission lines may be introduced as 750 C in order to
raise the current rating of the conductor.
