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Delivering
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Delivering
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So, the National Grid is part of a gigantic
simple circuit connecting houses, factories,
hospitals and schools to the power stations.
There is something extra...
...the
Transformers!
Why do we need transformers? Why are
the power line cables always in sets of 3?
Why do we need such high voltages for
the power lines cables? Why do we use
alternating current (AC) to deliver electricity
to or homes?
A simple circuit
The houses are the load.
The turbine and generator is the power supply.
The National Grid lines connect the power
supply to the load.
Some facts
Why do the cables heat up?
In the UK there are more than 180 big
power stations. There are millions of homes,
schools and factories. This and the fact that
energy can’t be stored is what makes the
Grid so difficult to operate. It’s like having
lots of batteries and lots of bulbs in the
simple circuit and having to know which
ones will be switched off/on and when!
Whenever an electric current flows through
a wire the wire heats up. This is known as
joule heating. As the negative electrons try
to move past the positive ions in the wire
they attract them and so make the ions
vibrate more strongly. Thin wires heat up
more than thick ones. The thicker the cable
the less the resistance (we could imagine
that a thicker cable represents a wider
roadway) and the smaller the resistance,
the less the heating. The cables of the grid
are already nearly as thick as a rolling pin
and arranged in bundles and are therefore
very heavy. The pylon would have to be a
lot stronger to carry thicker and heavier
cable or more in a bundle.
Demand (load) must be matched by the
supply (generation power supply). This
can be more than 50 000 MW and requires
that people sit down and forecast what the
load will be and how to supply it with the
available power stations. All this is done in
one place in Berkshire, the National Grid
Control Centre.
The Super Grid (voltage greater than 200 kV)
cables are more than 20 000 km long. Like
all conductors, these cables are heated by
the current that flows through them.
In the simple circuit the current is very small
and so the heating is small anyway. The
wires are still thicker than the filament of
the bulb. We have made the wires of the
Grid about as thick as they can be, so if we
want to keep heating losses down the next
way is to reduce the current.
The power P or heat spent (dissipated)
in a resistor (wire or bulb or national grid
conductors, for example) is current x voltage.
P=IxV
But we know from
Ohms Law that
V= I x R, so P=Ix ( I x R )
Or
P=I2 x R
We can see from this that if the current is
halved then the heating, P, is only a quarter
of what it was. This means losses due to
heating are reduced by 75%....a huge
amount. The reason is simply because the
heating effect goes up as the square of the
current. As we cannot reduce R any more
it makes sense to reduce the current. But
if we reduce the current we must increase
the voltage in order to keep the power P
the same.
The current is reduced simply by increasing
the voltage. This ensures that the power
transmitted stays the same but the joule
heating effect is reduced. Easy to say but
not so easy in practice!
What is required is a transformer. Remember
that it is having to transform voltages at very
high power. This means the transformers at
an electricity substation are big – some weigh
more that 270 tonnes before oil is added –
that is bigger than whole diesel locomotives.
The transformer is a neat trick played on
nature. You will know that if a wire is moved
in a magnetic field a voltage is generated
between the ends. The wire “experiences”
a moving magnetic field. Now imagine an
electromagnet with a second coil placed
near it. Moving the second coil will generate
a voltage across the ends of the coil.
Now keep the second coil stationary and
put AC into the electromagnet (AC varies
with time).
The magnetic field of the electromagnet
will increase and decrease and the second
coil will “experience” an increasing and
decreasing magnetic field. The second coil
will “think” it is being moved in a magnetic
field and a voltage will be generated across
the ends of the coil. This is a transformer,
the electromagnet is the primary coil and
the secondary coil is connected to the load.
If the number of coils in the primary and the
secondary, is the same, the voltage across
the secondary is the same as the voltage
across the primary.
With more coils or turns on the secondary
the voltage is increased, step up;
and with fewer turns we have a step
down transformer.
Primary winding
How
ow is the
th
current
reduced?
d
d
Step down transformers are used for
welding, cooking hobs, electric furnaces
for melting metals. In these cases a high
current for the heating effect is desirable.
Step down transformers are used on the
grid for bringing the high voltages down to
a safer 230 V for our domestic use.
Step up transformers are used in fluorescent
tubes, X-ray machines and of course on the
Grid to reduce the current by increasing
the voltage.
VS / VP = NS / NP
If no energy is wasted
in the transformer
Power in = Power out
VP x IP = VS x Is
Of course in reality there are considerable
losses due to heating and these big
transformers are oil cooled. Although the %
of energy lost on National Grid transformers
is small, they have an efficiency of 99.8%
under normal operating conditions. The
fact that they transmit such large amounts
of power means a lot of heat is generated
on the transformer. This heat is removed by
circulating oil around the windings.
Magnetic
Flux ǩ
Secondary winding
Np turns
Ns turns
Primary current Ip
Secondary current Is
+
–
Primary
voltage Vp
Secondary
voltage
–
+
Transformer
Core
Questions
uestio
Answered
Answered
d
Do transformers work with AC and DC?
Transformers will only work with AC. The
first power station was built in London
in 1882 and used DC. At that time
engineers struggled to decide whether
to use DC or AC. AC won because
electricity can be transmitted at high
voltage and lower current so reducing
energy losses. Currents are still high
however and can be more than 5000
Amperes – compare that to the current
of 10 A in your kettle!
Each generator has three coils and so
produces three outputs, each one third
Line 3
of a revolution behind the other. These are
the three phases and each is carried on a
separate cable which is why the conductors
of the power lines come in sets of three. The
neutral or return current is sent through the
ground from your house back to the power
station which is one reason why good
earthing is so important! In some countries
all three phases are used in houses but in
the UK we only use one single phase in
homes. Factories use all three phases for
greater power.
Line 1
Neutral
Line 2
High voltages allow us to transmit at lower
current so why not use even higher voltages?
The power line cables are supported
from the pylon and separated from
them by ceramic insulators. As the
voltage increases, better and heavier
insulation is needed. In any case at
very high voltages the air begins to
break down allowing current to leak
even if a spark is not produced. The
National Grid has arrived at a happy
medium of 400 000 V for the Supergrid.
Some figures
Real power lines may carry currents of more than a thousand
Amperes. At a phase-to-earth voltage of 160 000V this
implies a power of 480 000 000 W or 480 MW shared
across 3 phases. If we use conductors in bundles, rather
than singles, we can substantially increase the current a
line can transmit.
If the resistance of the cable is 0.1 Ohm per kilometre the
voltage drop for each kilometre is 1000 A x 0.11 or 100 V.
The energy dissipated per kilometre is 1000 A x 100 V or 100
kW. So even high voltage lines can dissipate a great deal
of energy through heat losses. This is why it is important
to keep the current and the resistance as low as possible.
GCSE Exam specifications at GCSE
AQA spec A P1.4.2
The National Grid
AQA Physics 3 13.9
How do transformers work?
Edexcel P1 5.12
Generation and
transmission of electricity
OCR Physics B P2b
Find out about the
National Grid
OCR 21st C
Physics A, P 5.4
How is mains
electricity produced?
A simple
simp e
circuitt
The lamp is the load.
The battery is the
power supply.
–
+
The wires connect the
power supply to the load.
Education & Skills
National Grid
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Warwick
CV34 6DA
www.nationalgrideducation.com
Securing our energy supply for future generations