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Transmission

“The conveyance of electric power from a power station to consumers premises is known as transmission.”

An electric power supply system consist of three principal components i.e. the power station, the transmission lines and the distribution system. Electric power is produced at the power stations which are located at favorable places, generally quite away from consumers. It is than transmitted over large distances to load centers with the help of conductors known as “transmission lines“. Finally, it is distributed to a large number of small and big consumers through a distribution network.

The electric supply system can be broadly classified into

  • D.C system
  • A.C system

Now a days 3 phase 3 wire A.C system is universally adopted for generation and transmission of electric power as an economical proposition. However, distribution of electric power is done by 3 phase 4 wire A.C system. The underground system is more expensive than the overhead system.

Typical A.C Power Supply Scheme

The large network of conductors between the power station and consumers can be broadly divided into two parts.

  • Transmission system
  • Distribution system

Each part can be further sub divided into

  • Primary transmission
  • Secondary transmission
  • Primary distribution
  • Secondary distribution

The layout of a typical A.C power supply scheme by a single line diagram is as below. It may be noted that it is not necessary that all power schemes include all the stages.

single line diagram of transmission system

Generating Station

Generating station where electric power is produced by 3 phase alternators operating in parallel. The usual generating voltage is 11 KV. For economy in the transmission of electric power, the generation voltage is stepped up to 132 KV at the generating station with the help of 3 phase transformers. The transmission of electric power at high voltages has several advantages including the saving of conductor material and high transmission efficiency. It may appear advisable to use the highest possible voltage for transmission of electric power to save conductor material and have other advantages. But there is a limit to which this voltage can be increased. It is because increase in transmission voltage introduces insulation problems as well as the cost of switch-gear and transformer equipment is increased. Therefore, the choice of proper transmission voltage is essentially a question of economics. Generally, the primary transmission is carried at 66 KV, 132 KV, 220 KV or 400 KV.

Primary Transmission

The electric power at 132 KV is transmitted by 3 phase, 3 wire overhead system to the outskirts of the city. This forms the primary transmission.

Secondary Transmission

The primary transmission lines terminates at the receiving station which usually lies at the outskirts of the city. At the receiving station, the voltage is reduced to 33 KV by step down transformers. From this station, electric power is transmitted at 33 KV by 3 phase 3 wire over head system to various sub stations located at the strategic points in the city. This forms the secondary transmission.

Primary Distribution

The secondary transmission line terminates at the sub station where voltage is reduced from 33 KV to 11 KV, 3 phase, 3 wire. The 11 KV lines run along the important road sides of the city. This forms the primary distribution. It may be noted that big consumers are generally supplied power at 11 KV for further handling with their own substation.

Secondary Distribution

The electric power from primary distribution line (11 KV) is delivered to distribution sub stations. These sub stations are located near the consumers localities and step down the voltage to 400 V, 3 phase, 4 wire for secondary distribution. The voltage between any two phases is 400 V and between any phase and neutral is 230 V. The single phase residential lighting load is connected between any one phase and neutral, whereas 3 phase 400 V motor load is connected across 3 phase lines directly.

It may be worthwhile to mention here that secondary distribution system consists of feeders, distributors and service mains. Feeders radiating from the distribution sub station supply power to the distributors. No consumer is given direct connection from the feeders. Instead, the consumers are connected to the distributors through their service mains.

Comparison Of A.C And D.C Transmission

The electric power can be transmitted either by means of D.C or A.C. Each system has its own merits and demerits. It is, therefore, desirable to discuss the technical advantages and disadvantages of the two systems for transmission of electric power.

D.C Transmission

For some years past, the transmission of electric power by D.C has been receiving the active consideration of engineers due to its numerous advantages.

Advantages

The high voltage D.C transmission has the following advantages over high voltage A.C transmission.

  • It requires only two conductors as compared to three for A.C system.
  • There is no inductance, capacitance, phase displacement and surge problems in D.C system.
  • Due to the absence of inductance, the voltage drop in a D.C transmission line is less than the A.C line for the same load and sending end voltage. For this reason, a D.C transmission line has better voltage regulation.
  • There is no skin effect in a D.C system. Therefore, entire cross section of the line conductor is utilized.
  • For the same working voltage, the potential stress on the insulation is less in case of D.C system than that in A.C system. Therefore, a D.C line requires less insulation.
  • A D.C line has less corona loss and reduced interference with communication circuits.
  • The high voltage D.C transmission , there are no stability problems and synchronizing difficulties.

Disadvantages

  • Electric power cannot be generated at high D.C voltage due to communication problems.
  • The D.C voltage cannot be stepped up for transmission of power at high voltages.
  • The D.C switches and circuit breakers have their own limitations.

A.C Transmission

Electrical energy is almost exclusively generated, transmitted and distributed in the form of A.C

Advantages

  • The power can be generated at high voltages.
  • The maintenance of A.C sub stations is easy and cheaper.
  • The A.C voltage can be stepped up or stepped down by transformers with ease and efficiency. This permits to transmit power at high voltages and distribute it at safe potentials.

Disadvantages

  • An A.C line requires more copper than a D.C line.
  • The construction of A.C transmission line is more complicated than a D.C transmission line.
  • Due to skin effect in the A.C system, the effective resistance of the line is increased.
  • An A.C line has capacitance. Therefore, there is a continuous loss of power due to charging current even when the line is open.

Advantages Of High Transmission Voltage

The transmission of electric power is done at high voltages due to the following reasons.

  • Reduces volume of conductor material. It can be proved that the greater the transmissions voltage, the lesser is the conductor material required.
  • Increases transmission efficiency. It can be shown that transmission efficiency increases when the transmission voltage is increased.
  • Decreases percentage line drop. It can be proved that when transmission voltage is increased, the percentage line drop decreases.

Limitation Of High Transmission Voltage

It is advisable to use high voltage transmission, however, it must be realized that high transmission voltage results in:

  • The increased cost of insulating the conductor.
  • The increased cost of transformers, switch-gear and other terminal apparatus.

Therefore, there is a limit to the higher transmission voltage which can be economically employed. This limit is reached when the saving in cost of conductor material due to higher voltage is offset by the increased cost of insulation, transformer, switch gear etc. Hence, the choice of proper transmission voltage is essentially a question of economics.

Various System Of Power Transmission

It has already been pointed that for transmission of electric power, 3 phase, 3 wire A.C system is universally adopted. However, other systems can also be used for transmission under special circumstances. The different possible systems of transmission are:

D.C System

  • D.C two wire
  • D.C two wire with mid point earthed
  • D.C three wire

Single Phase A.C System

  • Single phase two wire
  • Single phase two wire with mid point earthed
  • Single phase three wire

Two Phase A.C System

  • Two phase four wire
  • Two phase three wire

Three Phase A.C System

  • Three phase three wire
  • Three phase four wire

Now, the cost of conductor material is one of the most important charges in a system. The best system for transmission of power is that for which the volume of conductor material required is minimum. Therefore, the volume of conductor material required forms the basis of comparison among different systems. While comparing the amount of conductor material required in various systems, the proper comparison shall be on the basis of equal maximum stress on the dielectric. There are two cases:

  • When transmission is by overhead system. In the overhead system, the maximum disruptive stress exists between the conductor and the earth. Therefore, the comparison of the systems in this case has to be made on the basis of maximum voltage between conductor and earth.
  • When transmission is by underground system. In the underground  system, the chief stress on the insulation is between conductors. Therefore, the comparison of the systems in this case should be made on the basis of maximum potential difference between conductors.

Economic Choice Of Transmission Voltage

As we know that if transmission voltage is increased, the volume of conductor material required is reduced. This decrease the expenditure on the conductor material. It may appear advisable to use the highest possible transmission voltage in order to reduce the expenditure on conductors to a minimum. However, it may be remembered that as the transmission voltage is increased, the cost of insulating the conductors, cost of transformers, switch gear and other terminal apparatus also increases. Therefore, for every transmission line, there is optimum transmission voltage, beyond which there is nothing to be gained in the matter of economy. The transmission voltage for which the cost of conductors, cost of insulators, transformers, switch gear and other terminal apparatus is minimum is called economical transmission voltages.

The method of finding the economical transmission voltage is as follows. Power to be transmitted, generation voltage and length of transmission are assumed to be known. We choose some standard transmission voltage and work out the following costs:

Transformer

Transformers at the generating and receiving ends of transmission line. For a given power, this cost increases slowly with the increase in transmission voltage.

Switch Gear

This cost also increases rapidly with the increase in transmission voltage.

Lightning Arrestor

This cost increases rapidly with the increase in transmission voltage.

Insulation And Supports

This cost increases sharply with the increase in transmission voltage.

Conductor

This cost decreases with the increase in transmission voltage.

The sum of all above costs gives the total cost of transmission for the voltage considered. Similar calculations are made for other transmission voltages. The method of finding the economical transmission voltage is rarely used in practice as different costs cannot be determined with a fair degree of accuracy.