6.2 More detail on the transmission of electricity and explaining the need for step-up and step-down transformers in the National Grid system

and why transmission of electricity is most efficient using a very high voltage power line

•  For a given power increasing the voltage reduces the current required and this reduces the energy losses in the cables.

  • You should know why transformers are an essential part of the National Grid.

  • You also need to know why the long distance power lines use a very high p.d. and relatively low current.

  • So that you can transmit (transfer) the very large quantities of electrical energy per unit time needed, you need to use either a very high current or a very high voltage or both

    • Since P = IV, to deliver lots of power you need to increase current or p.d. or both.

    • The theoretical four possible choices are (i) low current and low voltage, (ii) high current and low voltage, (iii) low current and high voltage or (iv)  high current and high voltage ...

      • (i) obviously couldn't deliver what is needed.

      • (ii) and (iv) increase heat losses, which is a function of the current flowing through a resistor - you always get some conversion of electrical energy into heat - increasing the thermal energy store of the cable and surroundings.

      • but (iii) is the actual choice.

    • So why is 'low current and high voltage' the desired choice for electrical power line transmission?

    • The greater the current flowing through a wire, the greater the heat generated, which in the context of power lines means more waste heat energy the higher the current, which is why (ii) and (iv) are not employed.

      • Since P =  E/t = I²R, the power loss is a function of I² for a fixed resistance - the National Grid cables.

      • This is a good numerical argument for minimising the current I.

    • However, since power = current x voltage, to deliver a particular power rating, you must still increase one of the two variables and decrease the other.

    • Therefore by using a very high voltage (eg 250 000 to 400 000 V, 250 kV to 400 kV) and relatively low current you maximise power transmission for the minimal heat loss of wasted electrical energy.

    • So for a given power transmission increasing the p.d. and reducing the current makes the National Grid system as efficient as it can be with the minimum of electrical energy lost to the thermal energy store of the surroundings.

    • See the equations in the transformer section.

  • However, use of these extremely high voltages (1667 x your domestic voltage of 240 V), means health and safety issues arise and you need lots of big ceramic insulators on pylons and transformers and lots of barbed wire to deter people from climbing up pylons!

• You should know and understand the uses of step-up and step-down transformers in the National Grid.

  • With cables with a p.d. of 400 kV, not only requires transformers, you have to have, rather unsightly, tall pylons to support them and well insulated too! In some places the cables run underground, but this is more expensive (see later comparison discussion).

  • Now, (i) since the national power transmission uses 400 kV, you can hardly use this in the home,

    • and (ii) generators themselves cannot deliver 400 kV, you need a way of increasing (for efficiency), and then decreasing (for safety), the voltage in power lines.

  • A transformer is a means of changing an input voltage in one circuit, into another output voltage in a separate circuit.

    • At the power station end is a step-up transformer to increase the voltage for power line transmission.

    • At the user end is a step-down transformer, to reduce the voltage that is a safe level for factories, domestic homes, street lighting etc.

    • In the home the a.c. voltage is usually 230 to 240 V.