There are many advantages of transmitting electrical power at high voltage. The volume of conductor material required is reduced by increasing the voltage level. And it results in decreases in expenditure on the conductor material.
To make the expenditure on the conducting material, we need to supply electric power at the highest possible level.
But, an increase in voltage level results in an increase in the cost of insulation, cost of transformer, switchgear and other terminal equipment.
Hence, there is optimum transmission voltage, at which it is economical to supply electrical power. And this voltage level is known as economical transmission voltage.
The economical transmission voltage can be found by the following method. In this method, the length of the transmission line, amount of power to be transmitted, and generation voltage are assumed to be known.
The cost of equipment that needs to be calculated as;
Transformer: It is used at sending and receiving ends of the transmission line. The cost of the transformer increases slowly with an increase in voltage level for the same amount of power.
Lightning arrester: The cost of a lightning arrester is rapidly increased by increasing voltage levels.
Insulation and supports: The cost for insulation and supports increases sharply by increasing supplied voltage.
Conductor: The cost of a conductor decreases with an increase in voltage.
Sum of above cost gives a total cost of transmission line for a particular voltage. The same calculation is made for the other voltage level.
These values draw a curve for the total cost of transmission vs transmission voltage. The shape of this curve will be as shown in the figure below.
The lowest point in the above curve gives the economical transmission voltage. Therefore, in this condition, the optimum voltage level is OA.
This method is rarely used in practice as different costs cannot be determined accurately.
The empirical formula is used in the modern power system networks to find the economical transmission voltage.
According to American practice, the economic voltage between lines in a three-phase AC system is defined as the following equation.
V = line voltage (in kV)
P = maximum kW per phase to be delivered to a single circuit
l = length of transmission line (in km)
From the above equation, it is noted that the power to be transmitted and the distance of the transmission line is considered in the equation.
If the transmission line length increases, the cost of the terminal apparatus decreases, and it results in higher economical transmission voltage.
Also, large generating and transforming equipment can be employed if the power is transmitted in colossal amounts. It decreases the cost per kW of terminal station equipment.
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