The electrical energy generates in the power plant. The power plants placed at a far distance from the load. Hence, the HVAC transmission system used to connect the generation system with the load.
There are two types of the transmission system; underground cables and overhead transmission lines.
Click here for the difference between the underground cable and overhead line.
Generally, the overhead transmission lines used for bulk power transmission.
For long-distance and bulk power transmit, high voltage AC transmission line used. The voltage decides by the amount of power and the distance of the line.
For the same power, if the voltage is increased, the current will decrease.
Therefore, the value of voltage and current decide on the economic condition. How to decide economical condition?
If the voltage increased, a higher level of insulation required in the conductor. But the size of the conductor decreased. Hence, for high-level conditions, the insulation cost is high but the material cost of the conductor is less.
If current increased, the size of the conductor increased and a low level of insulation can be implemented. Therefore, in the high current condition, the insulation cost is less but the material cost of the conductor is high.
So, for economic condition decides by adjusting the voltage and current value by focusing on the cost of conductor material and the cost of insulation.
The HVAC transmission system is a conventional system to transmit the power for long-distance. HVDC transmission system developed to overcome the disadvantages of the HVAC system.
Advantages of HVAC transmission system
1) Decrease the cost of conductor
The cross-sectional area of the conductor is inversely proportional to the voltage of the transmission line. For a high voltage transmission line, the cross-sectional area of the conductor is very small. Therefore, the cost of material for the conductor is very less.
2) Increase the efficiency
For high voltage, the current decreased. Therefore, the copper loss of the line reduced. Hence, the overall efficiency of the transmission system increases the high voltage transmission line.
3) Increase the capacity of the transmission line
For a higher voltage of the transmission line, more power can transmit.
4) Flexibility in the future development
The power demand is increasing day by day. To meet this power demand, we can transmit more amount of power by increasing the voltage level. For a certain level, there is no need to install a new transmission system.
Disadvantages of HVAC transmission system
1) Corona loss
When the distance between the conductor is more, compared to the diameter, the corona effect seen around the conductor. The violet color glow and hissing sound heard when the potential difference increased beyond the critical disruptive voltage.
The corona effect increased with the increase in potential difference. Corona effect produces the power loss in the line and it produces the disturbance in the nearby communication line.
The corona effect is seen in both AC and DC transmission lines. But in the AC transmission line, the effect of the corona is more.
It can be decreased by increasing the diameter of the conductor by using the bundled conductors.
2) Skin effect
The effective resistance of the conductor increased due to the skin effect. It increases power loss.
The current density will not remain uniform due to the skin effect. The majority amount of current will flow through the outer region of the conductor and full use of the conductor is not possible.
3) Heavy support
The cost of the transmission tower increases by increasing the mechanical loading and use of bundled conductors.
If the distance between two conductors and the distance between conductor and ground increase, heavy and strong supports required. This increases the cost of the transmission tower.
The power system stability is most important for a reliable power system. If the length of the line increased beyond 500km, it is difficult to make a stable system. Hence, the limiting factor is the length of the line.
5) The capability of a conductor to carry current
For the long transmission system, the capacitance produced between the line and ground. The charging current will flow due to the capacitance.
In an underground system, the conductors placed near each other. Therefore, more capacitance produced in the case of an underground system.
Value of charging current increases with the increase in the length of a line. For the 132-kV line, the charging current is 5 A/km and for 380 kV line, the charging current is 16 A/km.
Due to this charging current, the heat produced. And the heat should not produce beyond a certain level.
The current at which this temperature reaches known as the thermal current limit. The value of charging current becomes equal to the thermal current limit, that length of a line is known as the critical length of a line.
Therefore, the length of the line should not increase than the critical length of the line.
6) Reactive loss
The transmission line impedance has both inductance and capacitance. These both opposite in nature. One is legging and the other is leading.
For the economical operation of the line, these both should be equal. These make reactive loss zero.
This condition can achieve at the natural impedance Z0 of the line. But practically, it is not possible to operate the line at its natural impedance. As a result, there are net reactive losses.
7) Ferranti effect
The charging current will flow through the conductor due to the capacitance. Because of this charging current, the voltage of the receiving end increased than sending end voltage. This effect is known as a Ferranti effect.
The voltage rise depends on the value of capacitance. And the capacitance is depending on the length of a line.
This voltage rise is 1.5% for 160 km line, 13% for 500 km line and 100% for 960 km line. This increase cannot permit beyond a certain level. Otherwise, there is a chance to damage the equipment due to the overvoltage.
Therefore, the length of the line kept under a certain level.