Topic 12.3 Transmission of Electrical Power

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Topic 12.3 Transmission of
Electrical Power
1 hour
Power Losses in Transmission Lines
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There are a number of reasons for power
losses in transmission lines such as:
Heating effect of a current
Resistance of the metal used
Dielectric losses
Self-inductance
Power Losses in Transmission Lines
• The main heat loss is due to the heating effect of a current. By
keeping the current as low as possible, the heating effect can be
reduced.
• The resistance in a wire due to the flow of electrons over long
distances also has a heating effect. If the thickness of the copper
wire used in the core of the transmission line is increased, then the
resistance can be decreased. However, there are practical
considerations such as weight and the mechanical and tensional
strength that have to be taken into account. The copper wire is
usually braided (lots of copper wires wound together) and these
individual wires are insulated.
• The insulation material has a dielectric value which can cause some
power loss. Some of the power from the lines goes into changing
the orbits of the electrons in the insulating material.
• Finally, the changing electric and magnetic fields of the electrons
can encircle other electrons and retard their movement on the
outer surface of the wire through self-inductance. This is known as
the ‘skin effect’. The size of the power loss depends on the
magnitude of the transmission voltage, and power losses of the
order of magnitude of 105 watts per kilometre are common.
Power Loss in a Real Transformer
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Power losses in real transformers are due to factors
such as:
Eddy currents
Resistance of the wire used for the windings
Hysteresis
Flux leakage
Physical vibration and noise of the core and windings
Electromagnetic radiation
Dielectric loss in materials used to insulate the core
and windings.
Eddy Currents
• As already mentioned, any conductor that moves in a
magnetic field has emf induced in it, and as such current,
called eddy currents, will also be induced in the conductor.
This current has a heating effect in the soft iron core of the
transformer which causes a power loss termed an iron loss.
There is also a magnetic effect in that the created magnetic
fields will oppose the flux change that produces them
according to Lenz’s Law. This means that eddy currents will
move in the opposite direction to the induced current
causing a braking effect. Eddy currents are considerably
reduced by alloying the iron with 3% silicon that increases
the resistivity of the core. To reduce the heating effect due
to eddy currents, the soft-iron core is made of sheets of
iron called laminations that are insulated from each other
by an oxide layer on each lamination. This insulation
prevents currents from moving from one lamination to the
next.
Copper Loss
• Copper wire is used as the windings on the softiron core because of its low resistivity and good
electrical conductivity. Real transformers used for
power transmission reach temperatures well
above room temperature and are cooled down by
transformer oil. This oil circulates through the
transformer and serves not only as a cooling fluid
but also as a cleaning and anticorrosive agent.
However, power is lost due resistance and
temperature commonly referred to as ‘copper
loss’.
Hysteresis
• Hysteresis is derived from the Greek word that means
“lagging behind” and it becomes an important factor in the
changes in flux density as a magnetic field changes in
ferromagnetic materials. Transformer coils are subject to
many changes in flux density. As the magnetic field strength
increases in the positive direction, the flux density
increases. If the field strength is reduced to zero, the iron
remains strongly magnetized due to the retained flux
density. When the magnetic field is reversed the flux
density is reduced to zero. So in one cycle the
magnetization lags behind the magnetizing field and we
have another iron loss that produces heat. Hysteresis is
reduced again by using silicon iron cores.
• The capacity for the primary coil to carry
current is limited by the insulation and air
gaps between the turnings of the copper wire
and this leads to flux leakage. This can be up
to 50% of the total space in some cases.
• Because the power is being delivered to the
transformer at 60Hz, you can often hear them
making a humming noise. Minimal energy is
lost in the physical vibration and noise of the
core and windings.
• Modern transformers are up to 99% efficient.
Power Transmission
• For economic reasons, there is no ideal value of voltage for
electrical transmission. Electric power is generated at
approximately 11 000 V and then it is stepped-up to the
highest possible voltage for transmission. Alternating
current transmission of up to 765 kV are quite common.
• For voltages higher than this, direct current transmission at
up to 880 kV is used. A.C. can be converted to D.C. using
rectifiers and this is what is done in electric train and tram
operations. D.C. can be converted to a.c. using inverters.
• There a number of D.C. transmision lines such as the
underground cross-channel link between the UK and
France. �The New Zealand high-voltage direct current
scheme has around 610 km of overhead and submarine
transmission lines.
• There are 3 conductors on a transmission line to
maximize the amount of power that can be generated.
Each high voltage circuit has three phases. The
generators at the power station supplying the power
system have their coils connected through terminals at
120° to each other.
• When each generator at the power station rotates
through a full rotation, the voltages and the currents
rise and fall in each terminal in a synchronized manner.
• Once the voltage has been stepped-up, it is transmitted
into a national supergrid system from a range of power
stations. As it nears a city or town it is stepped-down
into a smaller grid. As it approaches heavy industry, it is
stepped down to around 33 – 132 kV in the UK, and
when it arrives at light industry it is stepped-down to
11-33 kV. Finally, cities and farms use a range of values
down to 240V from a range of power stations.
• When the current flows in the cables, some
energy is lost to the surroundings as heat.
Even good conductors such as copper still
have a substantial resistance because of the
significant length of wire needed for the
distribution of power via the transmission
cables. To minimize energy losses the current
must be kept low.
Extra Low Frequency EM Fields
• We have all seen though the media patients being given
shock treatment through 2 electrodes to try and get the
heart beat at its natural frequency. The human body is a
conducting medium so any alternating magnetic field
produced at the extra-low frequency will induce an electric
field which in turn produces a very small induced current in
the body. Using a model calculation in a human of body
radius 0.2 m and a conductivity of 0.2 S/m (sieverts per
metre), it has been shown that a magnetic field of 160 μT
can induce a body surface current density of 1 mA m-2.
• It is currently recommended that current densities to the
head, neck and body trunk should not be greater than 10
mA m-2.
Possible Risks of High Voltage Power
Lines
• Current experimental evidence suggests that
low-frequency fields do not harm genetic
material in adults but there is some evidence
that there could be a link to infant cancer
rates due to low-frequency fields. The risks
attached to the inducing of current in the
body are not fully understood. It is likely that
these risks are dependent on current
(density), frequency and length of exposure.

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