Long distance power transmission:
Power transmission over long distances using alternating current is complicated by the inductance and capacitance of the line. For satisfactory operation of such lines it is necessary to balance the lagging inductive volt amperes of the line ( I2 ωL) against the leading capacitance volt amperes ( V2 ω C ). Equating the two we get V/I the impedance of the load. √L/C which is also known as characteristic impedance of the line, Z0. The corresponding load is thus V2/Z0 watts per phase or (kV) 2/Z, M W for three phase line where kV is the line voltage in kilo volts. This load is termed as "natural load " on the transmission line. Long distance high power high voltage transmission lines are designed for rated load equal to its natural load.
Natural Loads of transmission lines:
CONDUCTOR MATERIALS of transmission lines:
Voltage kV
|
132
|
220
|
400
|
Z0 (Ω)
|
350
|
320
|
290
|
Natural load, MW
|
50
|
150
|
500
|
Current (A)
|
220
|
385
|
752
|
A material for conducting electric power should have the following properties :
1. High electrical conductivity.
2. Low cost.
3. Low specific gravity.
4. High tensile strength.
Commonly used materials for conductors are:
1. Copper.
2. Aluminium.
3. Aluminium conductor steel Reinforced (ACSR).
4. Galvanized steel.
5. Cadmium copper.
Economic Size of conductor: Kelvin's Law:
The most economical size of conductor is that for which the variable part of the annual charges is equal to the cost of energy losses per year.
Limitations of Kelvin's Law:
1. The law assumes a linear relation between the cost on account of interest and depreciation on the capital outlay which is not necessarily always valid. Moreover, it is difficult to calculate these values.
2. Actual energy loss on a transmission line cannot be estimated without actual load curves. Load curves are not available at the planning stages.
3. The conductor size estimated according to this law may not be the optimum as various aspects of safety etc. have not been taken into account.
4. The law does not take into account some of the aspects like safe current destiny, mechanical strength, corona loss etc.
TRANSMISSION LINES:
Transmission lines are used to transfer electrical power from one place to another. The requirements of transmission lines are :
1. transmission losses should be least
2. power must be delivered at the specified voltage
3. no radio interference
4. high availability
Classification of overhead transmission Lines:
1. Short Transmission lines:
When the length of an overhead transmission line is up to 50 km and the line voltage less than 20 kV, its is known as short transmission line. Due to smaller length and lower voltage, the capacitance effects are small and hence are neglected. Thus resistance and inductance are the major parameters considered for these lines.
2. Medium electric transmission lines:
These lines are 50 km to 150 km and the range of voltage is 20 kV to 100 kV. Due to sufficient length and voltage of the line , the capacitive effects are not neglected.
3. Long electric transmission lines:
The lines are more than 150 km in length and carry voltage higher than 100 kV.
Parameters of a transmission line:
1. Resistance in transmission lines :
Aluminum Conductors Steel Reinforced (ACSR) are used for transmission of power over long distance. The acceptable limits of current density for aluminium is around 95 A in a conductor of 1 cm diameter. In case of copper it is 160 A in a 1 cm diameter conductor. Thus size of a conductor for a transmission line is given by
Diameter of the conductor = (Current to be carried / 95) ½ cm
As aluminum has got low tensile strength therefore steel cored (ACSR) conductor are used.
2. Inductance in transmission lines:
Inductance of a phase single circuit overhead line is given by
L = u0 / 2 π (1/4 + loge S/r) henry/ meter
u0= permeability of air = 4 π x 10 -7 henry/ meter
S = Deq = 3 √(Dab Dbc Dca)
Deq is equivalent equilateral spacing between 3 conductors a, b and c. Dca, Dbc and Dca are distances between conductors a, b and c.
r = radius of the conductor.
3. Capacitance in transmission lines:
Capacitance of a three phase line CA is given by
CA = 2π ε0 / S, Farads per meter, phase to loge S/R neutral
ε0 = permittivity of free air (8.55 x 10-12 Farads/ meter)
where, S and r have same meaning as in the estimation of inductance.
No comments:
Post a Comment