Book Reference : Pages 131-132Book Reference : Pages 131-132

1.1. To understand how electricity is distributed To understand how electricity is distributed in the UK via the National Gridin the UK via the National Grid

2.2. To understand how transformers are used to To understand how transformers are used to minimise energy losses due to heat during minimise energy losses due to heat during electricity distributionelectricity distribution

The UK network of cables (underground and on The UK network of cables (underground and on pylons), transformers and power stations is pylons), transformers and power stations is known as the National Grid.known as the National Grid.

Power stations produce electricity at 25 kV and Power stations produce electricity at 25 kV and at 50Hzat 50Hz

Each power station may well produce in the Each power station may well produce in the order of 50-100 MW of electrical powerorder of 50-100 MW of electrical power

When high currents flow through wires, there is a When high currents flow through wires, there is a heating effect (Iheating effect (I22R).R).

To achieve a given power distribution, there is a To achieve a given power distribution, there is a choice of voltage and current (P=VI).choice of voltage and current (P=VI).

To reduce the energy lost due to heating, the To reduce the energy lost due to heating, the electricity is distributed at a higher voltage and electricity is distributed at a higher voltage and lower currentlower current

In particular step up transformers are used In particular step up transformers are used between the power station and the grid to between the power station and the grid to achieve a transmission voltage of 275-400kVachieve a transmission voltage of 275-400kV

Power Station : 25kVPower Station : 25kV

National grid : 250-400kVNational grid : 250-400kV

Industry 11-33kV (Three phase)Industry 11-33kV (Three phase)

Homes 230V (Single Phase)Homes 230V (Single Phase)

A transformer has a primary coil with 120 turns & A transformer has a primary coil with 120 turns & a secondary with 2400 turns.a secondary with 2400 turns.

Calculate the required primary voltage to achieve Calculate the required primary voltage to achieve a secondary voltage of 230V a secondary voltage of 230V [11.5V][11.5V]

A 230V 60W lamp is connected to the secondary A 230V 60W lamp is connected to the secondary coil.coil.

Calculate the current throughCalculate the current through1.1. The secondary coil The secondary coil [0.26A][0.26A]2.2. The Primary coil The Primary coil [5.2A][5.2A]

Explain why the transmission of electrical power Explain why the transmission of electrical power over long distances is more efficient at higher over long distances is more efficient at higher rather than lower voltagesrather than lower voltages

A power cable has a resistance of 200A power cable has a resistance of 200 is used to is used to deliver 2.0MW of power at 120kV.deliver 2.0MW of power at 120kV.

Calculate :Calculate :

1.1. The current through the cable The current through the cable [16.7A][16.7A]2.2. The power wasted in the cable The power wasted in the cable [56kW][56kW]