Generation, Transmission and Distribution

​Electricity Supply

By Michael Coyne

​The Grid

The electricity supply industry comprises:

• generation

• transmission

• distribution.

​Generation

In the UK, power stations are often sited close to the fuel source and other important resources required for generation, eg a large source of cooling water. Most base load power stations produce electricity at around 25,000 volts (25kV).

​Transformers

There are 2 types of transformer:
Step up
Step down.

Step up transformers have an output voltage that is higher than the input
Step down transformers have an output voltage that is lower than the input

Open Ended

What voltage do we generate electricity at?

​Transformers

Transformers use mutual induction, where two coils are placed in close proximity, and an alternating current is passed through one of the coils. The field established around that coil will cut the windings of the second coil, and this alternating field will induce an alternating emf in the second coil.

​Transformers

The practical application of mutual induction is found in the transformer where two separate coils of wire are wrapped onto a common laminated soft iron core.

The field produced by each turn in the primary winding will induce an emf in each turn in the secondary winding, so if there is an equal number of turns on both windings, the output voltage will be the same as the input voltage (assuming no losses).

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​Transformers

However, where there are more turns on the primary winding, the voltage in the secondary winding will be lower because there are insufficient turns in which to induce an emf. But the increased induction in each secondary turn from the ‘excess’ primary turns will result in an increase in the available current in the secondary. In other words, the secondary voltage is stepped down, but the current is stepped up.

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​Transformers

The relationship between the number of primary and secondary turns, the input and output voltages, and the input and output currents may be expressed as…

Where…

Np and Ns = the number of turns on the primary and secondary windings.

Vp and Vs = the primary and secondary voltages.

Ip and Is = the currents in the primary and secondary windings.

Open Ended

If a transformer has an input voltage of 230v, and has 230 turns on the primary winding, what is the voltage output if there are 110 turns on the secondary winding?

Transmission

This electricity needs to be transmitted around the country to the load centres. These transmission lines form the basis of the National Grid. The National Grid is the high-voltage electric power transmission network in Great Britain, connecting power stations and major sub-stations, and ensuring that electricity generated anywhere in England, Scotland and Wales can be used to satisfy demand elsewhere.

By connecting the power stations and load centres in the form of a grid, greater security of supplies can be ensured. Additionally, during periods of light loading, eg in the summer, individual power stations can be shut down to enable maintenance operations to be carried out, whilst maintaining supply to consumers.

Transmission

Open Ended

If electricity is transmitted at 400KV for distribution at 11KV, If there are 12000 turns on the primary side of the transformer, how many turns are on the secondary side?

Transmission

The original Grid that came into operation in 1933 operated at 132kV. In 1949, the British Electricity Authority decided to upgrade the grid by adding 275kV links. From 1965, the Grid was partly upgraded to 400kV to become the supergrid, defined as referring to those parts of the British electricity transmission system that are connected at voltages in excess of 200kV.

Transmission

Most of the grid is formed by overhead power lines, with cables suspended from insulators mounted on metal pylons or – to give them their proper name – transmission towers.

Normally, there are two off three phase circuits on each pylon. One or two earth wires, also called ‘guard’ wires, are placed on top to intercept lightning and harmlessly divert it to ground.

The conductors are generally made of a steel inner core for strength, surrounded by aluminium conductors around the outside; the cable has no insulation applied.

Open Ended

State a transmission voltage

Distribution

​When these transmission lines are in the vicinity of the load centres, using a step down transformer, the voltage is stepped down for secondary transmission (132kV; 66kV). When the load centre is reached, it is stepped down again for local distribution at 33kV and 11kV. Supplies to individual users will see a further step-down to 400V for commercial and industrial users (heavy industry will be supplied at 33kV or 11kV, depending on demand) and 230V for domestic users.
Three distribution voltages are used:
• 33kV
• 11kV
• 400/230V

Distribution

Component parts of the electrical distribution network include:

• sub-stations - contain transformers

• pylons - support transmission cables

• power stations - generate electricity

• cables - carry power across the grid

• insulators - prevent current flowing from cables into the pylons

• transformers - step voltage up for transmission to minimize power loss and step voltage down to useable levels for distribution

Open Ended

State a distribution voltage

Open Ended

If electricity is generated at 25 KV and transformed into 400KV for transmission, If there are 500 turns on the primary side of the transformer, how many turns are on the secondary side?

Categorize

Options (8)

25 KV

400 KV

275 KV

132 KV

33 KV

66KV

11KV

400/230V

Sort the voltages into the correct category

Generation

Transmission

Distribution

Open Ended

What type of transformer converts generation voltage into transmission voltage?

Open Ended

What type of transformer converts transmission voltage into distribution voltage?