Module 2 : Equipment and Stability Constraints in System Operation
Lecture 6 : Transmission Line constraints
Objectives
In this lecture you will learn the following
Thermal and dielectric limitations of a transmission line.
Effect of loading on voltages, for long distance ac transmission.
AC line loadability (power transmission capability) versus distance.
Transmission line thermal limits
A large current flow increases the losses in the form of heat. This results in increased conductor temperatures.
Excessive temperature may result in expansion and resultant sag of conductors causing decreased clearance to
ground. Temperature extremes have an "annealing effect" causing reduced mechanical strength of aluminum.
Since thermal time constants are large, it is useful to distinguish between steady state and
transient thermal ratings.
Thermal Capability is a function of 1) ambient temperature 2) Wind conditions 3) Condition of conductor 4)
Conductor type and 5) ground clearance.
Typical steady state thermal specifications of conductors used for 400 kV overhead
transmission:
ACSR (aluminum conductor steel reinforced) Moose Conductor (520 sq mm): For an ambient temperature of 40º
C, and a maximum conductor temperature of 75º C, ampacity is 700 A (approx.). Typically, two moose
conductors form a bundle of 400 kV line so that the total ampacity is 1400A for such a line.
If AAAC (all aluminum alloy conductor) of 520 sq mm is used, higher conductor temperatures as compared
to ACSR are possible. For an ambient temperature of 40º C, and a maximum conductor temperature of 85º C,
ampacity is 850 A (approx.).
The thermal ratings are a strong function of ambient conditions (wind flows, solar radiation) and absorption
and emmisivity of the conductor material ; it makes sense to evaluate the actual ampacity during real time
operation (season to season, day to day or hour to hour).
Dielectric Limits
Exceeding dielectric limits (maximum electric field strength) results in failure of insulation, causing faults. See a
photograph of dielectric breakdown in an insulator here. Electric fields may be excessive (due to overvoltage) under
low loading conditions on long ac transmission lines (Ferranti Effect) or during abnormal conditions like lightning
strokes.
Deviation of voltages beyond certain limits can also be considered to be an unacceptable compromise on the quality
of power being supplied to consumers. Low or high voltages can also damage electrical equipments.
Voltages and reactive power demand of transmission lines are affected by:
Lecture 6 : Transmission Line constraints
Objectives
In this lecture you will learn the following
Thermal and dielectric limitations of a transmission line.
Effect of loading on voltages, for long distance ac transmission.
AC line loadability (power transmission capability) versus distance.
Transmission line thermal limits
A large current flow increases the losses in the form of heat. This results in increased conductor temperatures.
Excessive temperature may result in expansion and resultant sag of conductors causing decreased clearance to
ground. Temperature extremes have an "annealing effect" causing reduced mechanical strength of aluminum.
Since thermal time constants are large, it is useful to distinguish between steady state and
transient thermal ratings.
Thermal Capability is a function of 1) ambient temperature 2) Wind conditions 3) Condition of conductor 4)
Conductor type and 5) ground clearance.
Typical steady state thermal specifications of conductors used for 400 kV overhead
transmission:
ACSR (aluminum conductor steel reinforced) Moose Conductor (520 sq mm): For an ambient temperature of 40º
C, and a maximum conductor temperature of 75º C, ampacity is 700 A (approx.). Typically, two moose
conductors form a bundle of 400 kV line so that the total ampacity is 1400A for such a line.
If AAAC (all aluminum alloy conductor) of 520 sq mm is used, higher conductor temperatures as compared
to ACSR are possible. For an ambient temperature of 40º C, and a maximum conductor temperature of 85º C,
ampacity is 850 A (approx.).
The thermal ratings are a strong function of ambient conditions (wind flows, solar radiation) and absorption
and emmisivity of the conductor material ; it makes sense to evaluate the actual ampacity during real time
operation (season to season, day to day or hour to hour).
Dielectric Limits
Exceeding dielectric limits (maximum electric field strength) results in failure of insulation, causing faults. See a
photograph of dielectric breakdown in an insulator here. Electric fields may be excessive (due to overvoltage) under
low loading conditions on long ac transmission lines (Ferranti Effect) or during abnormal conditions like lightning
strokes.
Deviation of voltages beyond certain limits can also be considered to be an unacceptable compromise on the quality
of power being supplied to consumers. Low or high voltages can also damage electrical equipments.
Voltages and reactive power demand of transmission lines are affected by:
