Power transmission system:
Power system is divided into three parts;
power generation,
power transmission and
power distribution.
Here we will discuss about power transmission. Power transmission is a chain between power generation and power distribution. Whatever power is generated in power plant, that is transferred to load via transmission line.
Power is generated at 11-15 kV because of economy purpose. In electrical engineering, two quantity makes an important role; voltage and current. Now how to choose voltage level for economy purpose that is depend on application.
The voltage is proportional to insulation level and current is proportional to size of conductor. Now, if you increase voltage, current will decrease (in this case, high insulation cost and low conductor cost) and if you decrease voltage current will increase (in this case, low insulation cost and high conductor cost). So, you have to select voltage level at overall insulation and conductor cost is minimum. For generation, at 11-15 kV cost is minimum.
For transmission, voltage level is depends on length of line and power transfer capacity.
In transmission line, voltage is step up with the help of transformer. Because if voltage is increased, current will reduce. So, cu loss (I2R) loss will be minimum. That will increase overall efficiency of transmission system.
Transmission lines are divided in to three parts according to length; long transmission line, medium transmission line and short transmission lines.
In long transmission line, distance is more than 250 km and generally voltage level is 440 kV, 720 kV. In developed countries, 1200 kV transmission line is also available.
In medium transmission line, distance is between 80 - 250 km. Voltage level is generally, 230 kV to 440 kV.
In short transmission line, distance is less than 80 km. Voltage level is 132 kV or less than that.
Further classification is according to power: HT and LT line
Classification according to installation method:
Underground line, overhead line
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