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6.1 Examples of a National Grid System electricity supply - how the system works from power station to home and industry

The issue of variation of electricity demand - meeting industrial and consumer needs through the day

Introduction - basic description

How do power stations link up with the National Grid? 

What do we use transformers for in the National Grid system?

The power lines and transformers form the first part of the National Grid system, a country's electrical power supply.

An overall 'picture' of a National Grid system of electrical power supply

• Know that electricity is distributed from power stations to consumers along the National Grid

  • You should be able to identify and label the essential parts of the National Grid.

  • The National Grid consists of a vast electricity distribution network of transformers, pylons and suspended cables - insulated power lines running for long distances across the landscape.

    • They are somewhat unsightly, but essential for providing bulk electrical power to towns of homes, shops and factories.

    • All major power stations feed into the National Grid irrespective of their geographical location and many are needed to service millions of users in homes, transport and industry right across the country.

  • You see them stretching for miles and miles across the landscape to supply you, the consumer, very conveniently with a constant (well nearly!) supply of electricity to your city, town or village across the vast majority of the country.

  • Eventually the power is delivered, very conveniently, into your home as a consumer or factory etc.

  • power station: energy resource to drive turbine ==> to drive generator ==> step-up transformer ==> grid system of pylons or underground cables ==> step-down transformer ==> user/consumer

  • For non-renewable energy power stations: chemical/nuclear energy store (fuel) ==> thermal energy store (hot water) ==> kinetic energy store (turbine and generator) ==> electrical energy (National Grid)

    • The largest power stations are usually non-renewable fossil (oil, coal, gas) or nuclear fuelled.

      • The heat generated boils water to power a steam turbine which in turn drives the generator.

      • The generator (a large alternator) consists of a powerful rotating electromagnet that induces a high p.d. alternating current in coils of copper wire.

      • There are several copper coils all joined together in parallel to produce a single output from the generator.

      • Natural gas power stations are the cheapest to build and relatively rapid start-up time.

      • Nuclear power stations are the most costly to build and have the longest start-up time.

    • The National Grid system of electricity supply MUST work off an alternating current (ac) for several reasons, and one important factor is that transformers only work using ac.

      • With alternating current (ac), the current changes direction in a cycle e.g. 5O Hz.

      • With direct current (dc) there is no reversal in current direction, it flows one way with a constant voltage.

      • 

      • Oscilloscope traces comparing ac and dc current signals - showing the alternating + <=> - oscillation of the alternating current p.d. and the constant p.d. of a direct current.

      • Note that some devices in the home work off a dc current - but the output from e.g. the transformer in your computer power supply, is rectified to convert it to a dc supply.

• In the UK the generator output at the power station is 25 kV.

• A step-up transformer increases the p.d. (voltage) to 400 kV in the UK for power line transmission.

  • The main power transition p.d. of 400 k is to minimise electrical energy loss as thermal energy due to the resistance of the electric cables - see 2. Transmission of electricity, need for transformers, use of very high p.d.

• A step-down transformer decreases the p.d. (voltage) of the power line transmission to more suitable and safer levels for home and industry (typically 230 and 11000 V).

  • The transformer sites are referred to as sub-stations.

• A substation will decrease the p.d. even more down to ~230-240 V that is cabled into your house - your domestic electricity supply - which operates on a frequency of 50 Hz (50 cycles per second - the rate of current reversal of the alternating current).

• See Electricity section12. Generator effect, applications e.g. generators generating electricity

• Both the function of generators and transformers depend on the electromagnetic effect.

• The scheme described above is similar for most generation, except that initially for hydroelectric, tidal and wind power generation, the turbine is rotated directly by these renewable energy resources of water or wind - no fuel required.

• For renewable energy power stations: kinetic energy store (water/wind) ==> kinetic energy store (turbine and generator) ==> electrical energy (National Grid)

• The only kind of power generation that does not require a turbine and generator is the solar panel.

• For solar power: nuclear energy store (the Sun) ==> electromagnetic radiation (visible light) ==> electrical energy (National Grid)

• OR if for charging a battery:  nuclear energy store (the Sun) ==> electromagnetic radiation (visible light) ==> electrical energy ==> chemical energy store (battery).

• For more details see ....

• Renewable energy (1) Wind power and solar power, advantages and disadvantages

• Renewable energy (2) Hydroelectric and geothermal power, advantages & disadvantages

• Renewable energy (3) Wave power and tidal barrage power, advantages & disadvantages

• All of these renewable energy sources can contribute to the National Grid system.

A note on variation of electricity demand - meeting industrial and consumer needs

• The demand for electricity varies through the day e.g. there are peak times in the morning and evening and low demand through the night.

• Peak times are associated with cooking and transport needs and demands will increase in the winter when more energy is used for heating.

• Power companies know the demand patterns and can adjust to society's needs.

• Power stations do not run at their maximum output, there must be spare capacity most of the time, so if there is suddenly a huge increase in demand, it can be taken care of.

  • There might be an unplanned shut-down of a power station due to unforeseen circumstances.

  • There are smaller power stations on standby that can be quickly brought into use.

  • There are also pumped-storage systems that are very useful to meet electricity demands at peak times.

    • See Renewable energy (2) including Hydroelectric power

INDEX of notes on National Grid power supply & use of transformers

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Keywords, phrases and learning objectives for National Grid electricity supply

Be able to explain and describe how National Grid System electricity power supply system works from power station to home and industry and ways in which variation in power demands are taken care of.

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