Introduction

The thermal capabilities of transmission 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.

• The thermal rating of conductor is dependent on the following factors :

1. Meteorological/Environmental Conditions
   1. Solar radiation.
   2. Wind velocity.
   3. Ambient temperature.
2. Conductor surface Characteristics
   1. Age of the Conductor.
   2. Maximum design conductor temperature.
   3. Allowable losses of strength of aluminium metal at design temperature.

• 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 Electy. authority, State Electy 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, fo 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.

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.
Qc + Qr = I2 R + Qs
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 = Ö 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.

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 fallows-
  
  400kv - 'Moose' ACSR
  220kv - 'Zebra' ACSR
  132kv - 'Panther' ACSR

• However, recently in some cases 'moose' conductor has been used on 220kv lines for evacuation of bulk power from generating station, such as Unchahar Power Project, In case of 66kv lines which have been up-graded to 132kv, the conductor used is 'Dog' ACSR. Accordingly, the current capacities of the above conductors are temperature of 47.50 C. for new as well as old conductors for the designed maximum conductor temperature of 650 C as well as 750 C.

TABLE 21-II

Note :

• It is observed that the temperature at some place of UP State goes as high 47.50 C during the months of April, May & June. There fore 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 tabel.

Power Transmited

• 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 = Ö 3 V I Cos Ф
  Power in KW = Ö 3 V I Cos Ф / 1000
  Where-
  Cos Ф = Power Factor (P.F.)
  V = Voltage in KV
  I = Current in Amp.

Assuming Power Factor of 0.8 lagging, power transmitted at various voltages can be calculated approximately as follows-

1. At 132 kv
   Power in MW = Ö 3 132 x I x 0.8 / 1000
   = 0.1828992 x I
   = 0.183 x I
2. At 220 kv
   Power in MW = Ö 3 220 x I x 0.8 / 1000
   = 0.305 x I
3. At 400 kv
   Power in MW = Ö 3 400 x I x 0.8 / 1000
   = 0.554 x I

Thus value of power transmitted can easily be calculated at 132 kv or 200 kv or 400 kv for a given value of current.

• 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
• 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. This issue is under active consideration and IS:802-1973 is under revision.
• It is observed that the current carrying capacity of any conductor at any particular temperature is lowest during 10 to 14 hours of a day. Assuming maximum permissible conductor temperature of 750 C, the value of current carrying capacities of 'Dog', 'Panther', 'Zebra' and 'Moose' ACSR conductor during 10 to 14 hours of a day at different ambient temperatures for various months of a year for the conductor of ages (a) up to one year (b) between 1 to 10 years and (c) Above 10 years given in the Table I, II, III & IV respectively hereinafter.