Electric Transmission and Distribution : Concepts
Introduction of Electric power Transmission and Distribution:
For economical generation of power large generating stations are used. Capacities of individual generating sets have gone up recently. Generating sets in the range of 10 MW, 210 MW and 500 MW are being manufactured in many countries. Generating station are now not necessarily located at load centers. In fact other factors like availability of fuel and water play more dominating role in the selection of sites for thermal stations. Hydro stations are obviously located only at the sites where water is available at sufficient head. A vast network of transmission system has been created so that power generated at one station may be fed to grid system and may be distributed over large areas and number of states. The transmission and distribution system comprises a network of three-phase circuits with transforming and or switching substations at the various junctions. The parts of a transmission and distribution network maybe grouped as given below.Electric power TRANSMISSION:
Several generating stations can be inter connected. The main advantages are :
(i) reduction in the number of spare plants required as one station can assist the other at the time of emergency.
(ii) during light loads one station or some generators can be shut off, thus affecting operational economy.
Primary electric power transmission:
High voltages of the order of 66 kV 132 kV 220 kV and 400 kV are used for transmitting power by 3 phase 3 wire overhead system. This is supplied to substations usually at the out skirts of major distribution center or city.Secondary electric power transmission:
The primary voltage is reduced to low values of the order of 3.3 kV, 11 kV or 33 kV for secondary transmission.Primary electric power distribution:
The transmission lines or inner connectors terminate at large main substations from which the power is distributed to small secondary substations scattered throughout the load area. The voltage may range from 11 kV to 132 kV.Secondary electric power distribution:
This consists of the low-voltage network laid along the streets, localities and over the rural areas. From these sources connections to individual customers are provided. The circuit used for this purpose is 3 phase 4 wire, 440 V/220 V from which either 3 phase 440 V or single phase 220 V supply to the consumers may be provided.System layout of electric power transmission and distribution:
From the power stations PS, emanate 3 phase feeder supplying secondary distributions substations located at points throughout the supply area. The normal voltages are 132 kV, 33 kV and 11 kV.
COMPARISON OF AC AND DC power TRANSMISSION:
(A) Advantages of DC electric power transmission:
1. It requires only two conductors.2. There is no problem of inductance, capacitance and phase displacement which is common in ac transmission.
3. For the same load and sending end voltage, the voltage drop in dc transmission lines is less than that in ac transmission.
4. As there is no skin effect on conductors, therefore entire cross-section of conductor is usefully utilized thereby affecting saving in material.
5. For the same value of voltage insulating materials on dc lines experience less stress as compared to those on ac transmission lines.
6. A dc line has less corona loss and reduced interference with communication circuits.
7. There is no problem of system instability so common in ac transmission.
Disadvantages of DC transmission:
1. Generation of power at high dc voltages is difficult due to commutation problems and cannot be usefully utilized at Consumer ends.2. Step up or step-down transformation of dc voltages is not possible in equipment like transformer.
(B) Advantages of AC electric power Transmission:
1. Power can be generated at high voltages as there is no commutation problem.2. Ac voltages can be conveniently stepped up or stepped down.
3. High voltage transmission of ac power reduces losses.
Disadvantages of AC electric power transmission:
1. Problems of inductances and capacitances exist in transmission lines2. Due to skin effect, more copper is required.
3. Construction of AC transmission lines is more complicated as well as costly
4. Effective resistance of ac transmission lines is increased due to skin effect.
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:
| Voltage kV | 132 | 220 | 400 |
| Z0 (Ω) | 350 | 320 | 290 |
| Natural load, MW | 50 | 150 | 500 |
| Current (A) | 220 | 385 | 752 |
CONDUCTOR MATERIALS of transmission lines:
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 byDiameter 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 byL = 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 byCA = 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.
from ur's -- Bellapuri saikumar
Wow what a great blog, i really enjoyed reading this, good luck in your work. Everra
ReplyDelete