Lesson 10: The National Grid

 MUST understand that electricity is transmitted through the National Grid  SHOULD understand why electricity is transmitted at high voltages  COULD interpret a Sankey diagram showing how efficiently energy is transferred and understand the difficulties in assessing risks

 You need to be able to define the following:  National Grid

1)Carries electricity from power stations to users. 2)Does not pollute at the point of use; can be transmitted long distances; can be used in many ways. 3)By transmitting electricity at low current and high voltage. 4)230 V is high enough to provide enough power for most of our needs in the home, without providing an unacceptably high risk of electrocution; 120 V has a lower risk of electrocution, but it does not provide as much power unless the current in the equipment is higher and this increases the fire risk. 5)Some energy is wasted when transmitting the electricity; Sankey diagrams show the average proportions of energy transferred in these types of power station; different types of power station input electricity to the Grid. 6)Useful output is 80 kJ; efficiency is 80% (0.8).

Activity 1 Power station – generates electricity Substation 1 – increases voltages to thousands of volts Power cables – carries electricity from power stations to homes Pylons – supports power cables Substation 2 – reduces voltages to safe levels

Activity 2 Substation 1 – Making the voltage even higher, about 400,000V reduces the current, so reducing the energy loss Substation 2 – The voltage is reduced for the use in the homes to the very much lower level of 240V Power station – Electricity is generated at about 25000V

Activity 3 (High demand) 1)It shows that the power station uses 40 kJ from fuels to produce 15 kJ of electrical energy, so 25 kJ is transferred to the surroundings; the National Grid delivers 13.5 kJ while 1.5 kJ is transferred to the surroundings from the transmission lines; the delivered energy is split between industrial and non-industrial use, 6 kJ and 7.5 kJ respectively. 2)Efficiency of power station = 15/40 = or 37.5% 3)Efficiency of transmission lines = 13.5/15 = 0.9 or 90% 4)Burn the fuel more efficiently, e.g. use gas power stations that use the heat from burning gases as well as steam; allow sensible suggestions such as only generating the electricity that is needed as surplus electricity cannot be stored; in the transmission lines, transmit electricity at even higher voltage to reduce current further.