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Electric power transmission is the bulk transfer of electrical energy, from generating power plants to electrical substations located near demand centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. Transmission lines, when interconnected with each other, become transmission networks. The combined transmission and distribution network is known as the "power grid" in North America, or just "the grid". In the United Kingdom, the network is known as the "National Grid".

A wide area synchronous grid, also known as an "interconnection" in North America, directly connects a large number of generators delivering AC power with the same relative frequency, to a large number of consumers. For example, there are four major interconnections in North America (the Western Interconnection, the Eastern Interconnection, the Quebec Interconnection and the Electric Reliability Council of Texas (ERCOT) grid), and one large grid for most of continental Europe.

The same relative frequency, but almost never the same relative phase as ac power interchange is a function of the phase difference between any two nodes in the network, and zero degrees difference means no power is interchanged; any phase difference up to 90 degrees is stable by the "equal area criteria"; any phase difference above 90 degrees is absolutely unstable; the interchange partners are responsible for maintaining frequency as close to the utility frequency as is practical, and the phase differences between any two nodes significantly less than 90 degrees; should 90 degrees be exceeded, a system separation is executed, and remains separated until the trouble has been corrected.

Historically, transmission and distribution lines were owned by the same company, but starting in the 1990s, many countries have liberalized the regulation of the electricity market in ways that have led to the separation of the electricity transmission business from the distribution business.

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.



Electric power transmission distribution



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 lines

2. 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.