P1aL13 The National Grid

Key Words

Electric Current - the flow of electrons

Mains Electricity - Electricity generated at a central power station and distributed by a network of wires to all our homes.

National Grid - a network of high voltage cables that connect the countries power stations with electricity consumers

Parallel - circuits that branch out.  Each component is in its own branch

Power station - huge industrial plant where electricity is generated in vast quantities.

Step-down transformer - electro-magnetic machine that turns a high voltage into a low voltage

Substation - place where the high voltages in the national grid are reduced to low voltages for distribution.

Voltage - electrical "pressure", or how much energy the electrons carry.

Test Yourself

Homework
Physics GCSE
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Grade E

Electricity is made in huge quantities in power stations.  They are connected to an extensive network of overhead cables, the National Grid, which are carried on pylons which you can see when you go out to the country.  The voltage is very high, up to 415 000 V which would kill you if you were to touch it.

In substations, there are step-down transformers which reduce the voltage from 415 000 V to various voltages, including 230 V for use in our homes.  There may be several step-down transformers between the high voltage power lines and our homes.

Never fly a kite near to high voltage cables; the current will flow down the string and you will be killed.  Never go into a substation to collect a ball that has gone over the fence.  There are lots of high voltage components and you may be killed.

Grade C

Power is given by the equation:

power (W) = voltage (V) × current (A)

Lots of power is generated at a power station.  A big power station might give out 2500 megawatts (2500 000 000 watts).  The electricity is generated at 25 000 V, which would give a current of 100 000 A.  Huge cables would be needed to distribute that level of current.  The wires would also lose a lot of energy as heat.  The picture shows the three really thick cables (looking like pipes) coming out of a power station generator.

The low voltage is converted to a high voltage using step-up transformers, and the current is reduced, so thinner wires can be used and less energy is lost as heat.  Even so, a lot of energy is still lost as heat, about 10 % of all the energy put into the National Grid.

 

Grade A

The current from a battery is direct current (d.c.) that flows one way only round the circuit from positive to negative.

Mains electricity is supplied to homes in Europe at 230 V AC at 50 Hz.  The bit about the voltage is clear enough.   What is meant by AC and 50 Hz?  AC is alternating current. Have a look at the picture below.

 

 

 

The graph shows the way the voltage changes with time.  Note the following:

  • DC voltage does not change at all.  Its direction is constant;

  • The AC voltage is constantly changing from positive to negative.  So its direction is changing all the time.   It does this 50 times a second, so its frequency is 50 Hz.

The time for one complete wave is 1/50th (0.02 seconds or 20 milliseconds [ms]).  A complete wave is from the first peak to the second peak.

 

We can show this using an instrument called a Cathode Ray Oscilloscope (CRO).  It's a bit of a mouthful, but the shorthand CRO is easier to remember.  A CRO looks like this:

 

 

If we show the AC and DC at the same time on the CRO, we get a trace like this:

Notice that the DC line is about 70 % of the height of the wave.  We call this voltage the r.m.s. voltage which is the "DC equivalent" voltage.  The voltage at the crests of the waves (and at the troughs) is the peak voltage.  Its value is Ö2 times the r.m.s. voltage.  So the peak voltage of the mains is 325 V, while the r.m.s. voltage is 230 V.