 |
Pylon at Dunstown Wood, carrying 400 kV AC lines from Moneypoint Power Station to Dunstown 400 kV Substation. Photo © Eugene Brennan |
Before reading this, you might like to read my article which explains the difference between AC and DC.
In a previous article I explained why we use AC and very high voltages for
electricity transmission because of the ease with which voltages can be
stepped up or down using a device called a transformer. Also this
reduces power loss. I
also mentioned The War of the Currents in the late 19th century, which
was a series of events surrounding the introduction of Edison's DC and
George Westinghouse's competing AC electric transmission systems.
Edison’s DC was inefficient, requiring generating sources to be located
close to locations to reduce voltage drop. DC also suffered from power
loss in lines, whereas AC could be stepped up or down to raise voltage,
reduce current and significantly reduce this problem. During the War of
the Currents, Edison supporters staged publicity stunts, electrocuting
dogs, calves and a horse to show how AC was more dangerous than DC,
although in truth they’re equally lethal at high voltages.
Capacitive losses
Every
object has a property known as capacitance which allows it to store
electric charge, simply due to the accumulation or removal of electrons.
If you rub a ballpoint pen on your hair, you can pick up small pieces
of kitchen towel with it or bend a narrow stream of water running from a
kitchen tap. Similarly for a balloon. This is due to electrons being
added to or stripped from the plastic. The charge on the material
creates an electric field and a force that can attract other items. If
you take two sheets of aluminium foil and sandwich a sheet of paper
between them, you’ve made a device called a capacitor which can store
charge when connected to a DC voltage source. Capacitors are also used
as components in most electronic gadgets and appliances. Power
transmission lines have capacitance between each other and also the
ground. When AC electricity flows on the lines, it can also flow through
this capacitance to the other lines and also the ground, causing a loss
of power.
Inductive losses
Every
conductor has inductance. Take a piece of wire and wind it into a coil
shape by wrapping it around e.g. a pencil. You’ve created an inductor.
Connecting a voltage source to a wire causes a current to flow. Current
flowing through a wire creates a magnetic field. If the current is
changing, the magnetic field is changing also. However a magnetic field
that changes in magnitude induces a voltage in a conductor (which is how
generators work) which is in the opposite direction to the voltage that
created the current in the first place. This property is called
inductance and inductors are components used in many electronic devices,
e.g. your phone charger. All conductors have inductance, not just ones
where the wire is wrapped into a coil form, but even straight wires.
Power lines that use AC have inductance and this inductance causes power
loss and heating of the wire.
The solution, use DC
If
electricity needs to be sent over a distance of greater than 600 km,
the solution is to use DC for transmission. In the past, converting from
one AC voltage to a higher or lower one was simply done with
transformers, but this was more complicated for DC. It required a
converter system where the input voltage drove a motor which was
coupled by a shaft to a generator that output DC at the required level.
This was costly, more complex than a transformer and the conversion
wasn’t 100% efficient, resulting in a waste of power. Modern power
electronics makes it easier to do DC to DC, DC to AC or AC to DC
conversion.
Is DC transmission used in Ireland?
Yes,
there is one DC interconnector currently in operation between Ireland
and Britain: The East West Interconnector (EWIC) which connects the east
coast to Wales. A second interconnector, the Greenlink Interconnector,
is due to be completed by the end of 2024 and a third interconnector
will connect Ireland to the north coast of France. This is under
construction with a completion date of 2026. A fourth interconnector,
the Moyle Interconnector, connects Northern Ireland to Scotland.
This Eirgrid page explains all the details of existing and under construction interconnectors: