Advantage and disadvantage of high voltage line
Advantage of high voltage line
1. Increased in size of generating units
In order to reduce the investment cost per kilowatt, the size of generating units has been increased during the past many years. Transmission of large amount feasible only at voltage in the extra high voltage range.
2. Pit head steam plant and remote hydro plantThe cost of transportation tent to reduce supply of coal to steam plants resulting I fall of generation.
One method to avoid such situation is to have super thermal power plants near coal feeds. These steam plants are known as pit head steam plant. Therefore the trend is towards construction of large size pit head steam plants which has resulted in transmission of bulk power over long distances.
Similarly the hydro plants are situated for away from the load centers because the availabity of water at sufficient head is only at remote location.
3. Right- of – way – problem
The number of circuits and consequent the land requirements for transmission lines decreases with the adoption of higher transmission voltage. For transmitting 600mw over distances of 250KM require 40M right of way at 400KV level using single circuit line and 60m right of way. At 220KV level using double circuit line. Thus the saving in right of way and its cost at EHV level may be decreases.
4. Line cost
The line installation cost per megawatt per kilometer decrease with increase in voltage level. The transmission loss and hence cost of losses decrease with increase in transmission voltage. Therefore the total line cost per megawatt per kilometer decrease with the increase in line voltage.
5. Surge impudence loading ( SIL)
The loading limit of line surge impedance loading. The surge impedance is the load that a line carries load equal to the surge impedance of the line. A short transmission line can be loaded appreciably above its SIL. The load on a long line with series compensation has to be kept at a value less than the SIL due stability factor
The surge impedance loading represents the approximate load carrying capability of the lines and it is equal to SIL = V2/Zc
Disadvantage of high voltage line
1. Radio noise from EHV lines
The radio noise which is also known as radio interference generates from the same source as the corona which means localized breakdown. The radio interference field generated from transmission line varies mostly as the inverse of the radio frequency. The radio interference due to corona assumes importance for lines above 200KV.
2. Use of bundle conductors
For lines up to 220KV the size of a conductor is selected on the basis of its continiues and short term current carrying capacity. For EHV lines the corona and radio interference espects also assume importance the size of the conductors which gives satisfactory corona and radio interference is usually larger than the size compare to current carrying capacity.