Friday, January 25, 2013

Effect of High Voltage in Transmission System

       Looking at the advantages and disadvantages of the two systems, it can be conclude that high voltage transmission is advantageous. Let us steady the effect of increased voltage level of transmission on
1. Volume of copper used for transmission
2. Efficiency of the line
3. The line voltage drop
1.1 Effect of High Voltage on Volume of Copper
        Let a three phase a.c. system is used for the transmission. The various parameters are,
                P = Power transmitted in KW
               V = Line voltage in volts
               cosΦ = Power factor of load
                = Length of line in meters
               A = Area of cross-section of conductor in square meters
               ρ = Resistivity of conductor material
               R = Resistance per conductor in Ω
               The resistance per conductor is given by,

       The load current I can be obtained as,

       The total copper losses are,

       The volume of copper used is,
       Vol = 3 A 

       It can b seen from the equation (6) that the volume of copper required is inversely proportional to the square of the transmission voltage and the power factor, for given P, W, ρ and   l .
       Thus greater is the transmission voltage level, lesser is the volume of copper required i.e. the weight of copper used for the conductors. The conductor material required is less, for higher transmission voltage.
1.2 Effect of High Voltage on Line Efficiency.
       The power input to the line cab be written as,
       Pin = Pout +  Losses
       Now is Pout  as considered above while the losses are given by the equation (4).

       Let         J = Current density of conductor in A/m2
...                  J = I/A

       Using equation (2) in equation  (9),

       The line efficiency is given by,
        Line efficiency   η = Output/Input

       Mathematically above equation can be approximately written, using Binomial theorem as,

       So for constant values of ρ ,  and J, the equation (12) shows that line efficiency is higher for higher transmission voltages.
1.3 Effect of High Voltage on Line Drop
       Line drop = I x R
       Using equation (1),

        The equation (14) shows that higher is the transmission voltage level, lesser is the percentage line drop.
1.4 Advantages of High Voltage Transmission
       Summarizing the above discussion, the advantages of high voltage transmission can be stated as,
1. The line losses are inversely proportional to the square of voltage and power factor. So line losses are less.
2. For constant losses, the volume of copper required is inversely proportional to the square of the voltage and power factor. Hence the copper required is much less for high voltage transmission.
3. For constant current density, the line efficiency is very for high voltage transmission.
4. The percentage line drop is very small for the high voltage transmission.
       It may be noted that along with the voltage level, the power factor also plays an important role. Higher power factor also gives less losses, reduced volume of copper and increased line efficience. Hence consumers are always recommanded to maintain high power factor values.
1.5 Disadvantages of High Voltage
       Though high voltage transmission offers number of advantages, very high voltage transmission is not practically possible. There is a limit to increase the level of transmission voltage. The high voltage transmission has following limitations.
1. Higher the transmission voltage, higher is the insulation required which can cause problems in connection with conductor supports and clearance between the conductors.
2. Higher insulation means high cost.
3. The cost of transformers, switchgear and other equipments is also high for high voltages.
4. Higher the voltage, sever is the corona effect.
       Thus a compromise is necessary to select a transmission voltage. The insulation and other cost must be compensated by reduction in cost due to copper saving.
1.6 Practical Transmission and distribution Voltage Levels
       Considering the advantages and limitations of high voltage and economical aspects, the following voltage levels are commonly used for the transmission and distribution.
1. For generation : 6.6 KV, 11 KV , 22 KV or 33 KV.
2. For primary transmission : 66 KV, 132 KV, 220 KV upto 400 KV.
3. For secondary transmission : 11 KV , 22 KV or 33 KV
4. For primary distribution : 6.6 KV or 11 KV.
5. For secondary distribution : 230 V and 400 V.

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