How to select the conductor size in distribution power system

In power distribution system both aluminium and ACSR are commonly used. Mostly aluminium conductors are used in the distribution system because of cheaper in cost. Some of the factors which decides the size of the conductors designed for distribution system are given below:

-Current carrying capacity of the conductor or distribution line
-Allowable voltage drop or line regulation
-Breakdown strength of the conductor

Current Carrying Capacity of Line:

The current carrying capacity of a conductor decided by the maximum conductor temperature rise or operating temperature. Operating temperature is limited by mechanical aspects such as allowable span, mid-span sag, joints, creep in conductors and long term mechanical efforts. Generally 85℃ (AAAC), 75℃, 70℃, 65℃, or 60℃(ACSR) maximum operating temperature is used. Lower temperature is used for long spot lines particularly in rural distribution system. where jumpers may give trouble at higher loading.

The permissible operating temperature of an overhead conductors depends on the maintenance of adequate clearances and limitations of the loss of strength through annealing. Generally, the maximum current which an over-head conductor size designed to carry must not cause it to be heated such that it may result in the annealing of the metal of the conductor or reduction in the clearances specified. Usually for normal day loa maximum operating temperature 75℃ is permitted which ading, is allowed to reach up to 100℃ for emergency loading.

Voltage drop and Voltage Regulation:

The allowable voltage drop is considered as critical factors in determining the  conductor size for 11kV and Low Tension (LT) distribution line with thermal loading (ampere loading) about 80 percent of the normal thermal rating based on the maximum operating temperature. Large conductor size (cross section) employed in distribution lines reduces the resistance of the line and hence the I2R losses and voltage drop in the line; and hence voltage regulation of the line improves. But using large cross section conductor size will increase the cost as the material required is more.

Hence an optimum value must be chosen in between the cost and improving voltage regulation while designing the conductor size for distribution power system.

Mechanical Characteristics of Conductors:

The choice of conductor size from mechanical view point depends on the :

External Loading: Wind speed, ice loading and ambient temperature.

Internal Characteristics: Stranding, modulus of electricity, thermal expansion of the creep. For example, considering the creep and economics AAC is used in LT distribution lines. The line characteristics includes voltage regulation is influenced by distribution line parameters and system frequency, current carrying capacity is assessed from the heat balance (amount of heat generated and heat dissipated).