Advantages of High Voltage A.C. Transmission System:
The following are the advantages of a high voltage a. c. transmission system.
(1) Decrease in the cost of conductor:
The cross-sectional area of the conductor is inversely proportional to the square of the voltage of transmission so there is a decrease in the cost of the conductor in the high voltage a, e, transmission system.
(2) Decrease in the line loss:
The line current decreases due to the increase in the voltage of transmission for the same power transmitted so there is a decrease in the line loss.
(3) Increase in the efficiency of transmission:
There is an increase in the efficiency of transmission as the line losses are decreased.
(4) Increase in the capacity of transmission:
For the same transmission line, more power can be transmitted for higher voltage of transmission.
(5) Flexibility in future development:
If there is a requirement for more power in the future due to the development, the increased power demand can be met with by increasing the system voltage without installing the new line.
Disadvantages/Limitations/Difficulties In high Voltage A.C Transmission System:
The following are the difficulties/disadvantages of high voltage a. c. transmission system.
(1) Corona and radio interference :
When the distance between the conductors is more, compared to their diameters, a violet colour glow is seen around the conductor and the hissing sound is heard when the potential difference is increased beyond some value. This effect is known as the corona effect.
The effect of the corona is increased with the increase in the potential difference.
Due to corona,
(a) there is a power loss and
(b) there is a disturbance to the nearby communication time.
Corona effect is seen in both the a. c. and d. c. systems but its effect is more in a.c.
The corona effect can be decreased by increasing the diameter of conductors by using the bundled conductors.
(2) Skin effect :
In a. c. transmission system, due to the skin effect, the effective resistance of the conductor and the power loss increase.
The majority of current flows through the outer portion of the conductor so the current density in the conductor does not remain uniform and full use of the cross-section of the conductor is not made.
(3) Heavy support:
Due to the increasing mechanical loading and use of the bundle conductors and increased air and ground clearance, strong supports are required. This increases the cost.
(4) Stability :
Power should be transmitted through the line without losing the stability of the system.
From calculations, it is found that the length of the line cannot be kept more than 500 km. so the length of the line is the limiting factor in high voltage a. c. transmission.
(5) Capability of conductor to carry current :
In a. c. system there is the problem of the capacitance of the line. Charging currents flow through the line due to the capacitance of the line.
Capacitance is produced between the conductor and ground. Effect of capacitance is more in underground cables as the capacitance is inversely proportional to the distance and the distance between the conductors is less in underground cables compared to that in overhead lines.
The value of the charging currents increases with the increase in the length of the line. For a 132 kV line, its value is 5 A/km, while for 380 kV cable, its value is 16 A/km.
Heat is produced in the underground cable due to this charging current and this heat should not exceed more than a certain value.
The current at which this temperature reaches is known as the thermal current limit. The length at which
the value of the charging current becomes equal to the thermal current limit is known as the
critical length of the cable.
Hence, if the high voltage a.c. transmission is to be done through the underground cable, its length cannot be kept more than a certain value.
(6) Reactive loss :
Inductance and capacitance increase with the increase in the length of the line. Lagging reactive VA losses increase due to the inductance and leading reactive VA losses increase due to the capacitance.
Both are opposite in nature i.e. one is lagging and the other is leading.
So, for economical transmission, both the reactive losses should become equal. This is possible at the natural impedance Z of the line, but it is not always possible to operate the line at its natural impedance. As a result, there are net reactive losses.
(7) Ferranti effect:
Charging currents flow through the line due to the capacitance of the line.
As a result, the value of the receiving end voltage becomes more than the sending end voltage at no load. This is called the Ferranti effect.
The rise in voltage depends upon the capacitance of the line i.e. depends upon the length of the line.
This increase is 1.5 % for 160 km length, while it is 13% for 500 km and 100% for 960 km length.
This increase cannot be permitted more than a certain value otherwise there will be damage to the equipment due to the overvoltage.
This means that the length of the line cannot be kept more than a certain value