Terminology
A basic introduction to electricity, EMFs and the terminology
used
Voltages and currents can be pictured as electrical
pressure. The analogy is often used with water in a pipe; voltage
is analogous to the pressure of the water. Voltage is the same thing
as potential difference. This term arises because
the voltage is the potential to do work.
Voltage is, strictly, always measured between two objects; the
potential difference between the two points. However, it is conventional
to define the earth as at zero volts. Then we can talk about the
voltage of a single point or conductor, with the
implied addition “with respect to earth”.
Current is the flow of electricity. A voltage
will always try to drive a current. The size current that is driven
depends on the resistance of the circuit. If the voltage occurs
across an air gap, for instance, a negligible amount of current
will flow until the voltage is so high that the air breaks down.
If the voltage occurs across a conductor, a current will flow.
In metals, the current is carried by electrons,
fundamental particles carrying one negative charge
each. In passing, note electrons move so slowly that, generally,
no one electron will actually flow all the way round a circuit.
A good analogy is a string of ping-pong balls in a pipe. When you
push the end ball, all the balls move (a current flows) but no one
ball moves the whole length.
It is necessary to have a complete circuit for
electricity to flow. If your pipe has a closed end, you can push
the ping-pong balls as hard as you want, and they may squash a bit
but there will be no flow. To have a flow you must make the pipe
into a continuous loop.
Although it so happens that in metals the current
is carried by electrons, this is not fundamental to the nature of
current. Any charged object which can be made to move can carry
a current. When air breaks down under high voltage, the current
is partly carried by ions (molecules of air that
have had electrons stripped off them), and in electrolysis, the
current is carried by ions in solution.
Power is the product of voltage and current. In
the electricity industry, we tend to keep the voltage more-or-less
constant and let the change in power be accommodated by changes
in the current.
The relationship “power = voltage times current”
applies no matter what units you use for measuring the various quantities,
provided the units are consistent with each other. The simplest
units to use are volts, amps and
watts:
- watts (W) = volts (V) x amps (A)
- kilowatts (kW) = kilovolts (kV) x amps (A) = volts (V) x kiloamps
(kA)
- megawatts (MW) = kilovolts (kV) x kiloamps (kA) etc
The power is transmitted through the
transmission and distribution networks
and used by the consumer at the far end. To transmit a given power,
you can have a high voltage and a low current or vice versa.
However, current causes heating. Simplistically,
this is because the electrons, as they move along the wire, keep
banging into the atoms that make up the wire, and these collisions
cause heating. The heating increases as the square
of the current.
Therefore, to transfer a given amount of power, if you use low
volts and high current you will waste a lot more of the power in
heating the wires than if you use high volts and low current. That
is why bulk transfer of power is done at high voltages.
DC and AC
In a direct current (dc) circuit, the voltage and
the current continue in the same direction all the time. Battery-operated
electronics, car electrics, and main-line railways south of the
Thames are all examples of dc circuits.
Most power transmission is, however, done with alternating
currents (ac). The frequency in this country
(and elsewhere including those bits of the world influenced by the
British) is 50 Hertz (Hz). America uses 60 Hz. One Hertz
is one cycle per second. One cycle consists of
the voltage and current starting from zero, rising smoothly to a
maximum in one direction, falling back though zero to the same value
in the opposite direction, and returning to zero. Mains electricity
does this 50 times each second, so each cycle lasts a fiftieth of
a second or twenty milliseconds.
Nowadays, dc is used in power systems only where it is necessary
to transmit power over very long distances indeed, or where you
want to connect two different ac systems but you don’t want
to have to keep them synchronised (eg Britain and
France).
With ac, most of the concepts used to describe dc still apply,
but need small modifications. Voltage and current still mean basically
the same thing. However, because the voltage (or current) are continuously
changing but you want to describe it with a single value, you have
to define which voltage or current you mean. You could define the
voltage as the maximum value reached by the voltage
in either direction. That is called the amplitude.
However, it is more usual to define a different quantity, called
the “rms” voltage or current. Rms stands
for “root mean square”. For practical
purposes in the electricity industry, it is just a constant fraction
of the amplitude: rms = 0.71 x amplitude, amplitude = 1.41 x rms.
(The factor 1.41 is the square root of 2.) Rms is used because an
alternating current usually has the same effect as a direct current
when its rms values is the same as the direct current.
Values in electricity supply are always expressed in rms quantities.
Thus 400 kV is the rms value. The amplitude (that
is, the maximum voltage) is bigger, 566 kV.
Although electricity supply is basically conducted at 50 Hz, in
any practical power system, small amounts of current and voltage
at other frequencies always creep in. These frequencies are usually
exact multiples of the power frequency and are known as
harmonics. Thus second harmonic is 100
Hz, third harmonic is 150 Hz, etc. (Note that musicians
count their harmonics differently to electrical engineers!).
The electricity industry tries to keep
harmonics as low as possible and generally in the transmission system
they are less than 1%. Harmonics tend to be lowest in the transmission
system and get larger in distribution circuits and larger still
in homes. The third harmonic (150 Hz) tends to be the most significant.
The term “power frequencies” is often
used to cover both 50 Hz and the first few harmonics.
Phases
Ideally, in an ac circuit, the voltage and the current are exactly
in phase, that is they pass through zero at the same instant
in time, reach their maximums together, etc. In practice, they are
rarely exactly in phase: there is a phase difference,
expressed in degrees. Another way expressing the
phase difference is as the power factor. A power
factor of unity is equivalent to zero phase difference. Consumers
tend to be charged extra by their supply company if their power
factor gets too far away from unity. Some phase differences, however,
are introduced not by the customer but by the circuits the electricity
is transmitted over.
The fact that the voltage and current may not be perfectly in phase
introduces some subtleties to calculating power. This leads to the
terms “real power” and “reactive
power”, and the quantities “MVA”
and “MVAR”. When we move from dc to
ac we also have to expand the idea of resistance
to include its ac partners, reactance and impedance.
With ac just as with dc, you still need a complete circuit for
current to flow. Many ac circuits are just like dc circuits in having
two wires (“out” and “back”, or “go”
and “return”). However, the power system uses three
wires instead of two. This is known as “three-phase”
electricity, and is more efficient in that it requires only one-and-a-half
times the number of wires (three instead of two) to transmit three
times as much power.
The three phases carry voltages and currents which are nominally
120 degrees out of phase with each other. They
are often called after colours as convenient labels, usually red,
yellow and blue.
When the three phases are not exactly the same voltage and are
not exactly 120 degrees out of phases (which in practice is all
of the time, because of the nature of the loads supplied), it would
be perfectly possible to describe the system by the three individual
voltages and their phases. However, electrical engineers tend to
use a different way to describe the same thing. This is the system
of “positive-phase-sequence voltage”,
“negative-phase-sequence voltage” and
“zero-phase-sequence voltage” (abbreviated
pps, nps and zps.
The “phase” is often left out, hence eg “zero-sequence
voltage”). This has the advantage that the negative
and zero-sequence voltages are usually small, and when the three
phases are at exactly 120 degrees, they disappear altogether.
Three-phase electricity leads to another subtlety in voltages.
The voltage between any two of the three phases is 1.73 (the square
root of three) times larger than the voltage between any one phase
and earth. You therefore have to decide whether to give voltages
between phases or phase-to-earth.
The electricity industry almost always gives phase-to-phase voltages.
Thus 400 kV is 400 kV phase-to-phase and only 231 kV phase-to-earth.
The exception is the final distribution voltage, which can be given
either way. 230 V is phase-to-earth and 400 V is phase-to-phase.
Note that strictly, prior to harmonisation with Europe, these voltages
used to be 240 V and 415 V.
Some orders of magnitude:
- A 400 kV National Grid circuit may carry 1 kA, thus transmitting
a power of 700 MW.
- A 132 kV distribution circuit may carry 300 A, thus transmitting
a power of 70 MW.
- An 11 kV distribution circuit may carry 150 A, thus transmitting
a power of 3 MW.
- A 400 V final distribution circuit may carry 200 A, thus transmitting
a power of 150 kW.
Transformers
Voltages are changed by a transformer. Transformers
are very efficient - in the high nineties of percent - thus power
flows through a transformer with very little being absorbed. A substation
is simply one or more transformers plus their associated
switchgear etc.
For practical purposes, ac electricity cannot be stored in large
quantities. Small amounts of electrical energy are stored in fields,
eg in a transformer and in the region round a transmission line.
With ac, the only way of storing large amounts of electricity over
significant periods of time is to convert the electrical energy
into some other form of energy that can be stored (eg gravitational
potential energy in a pumped storage system, chemical
energy in a battery). Electric power flows through
the transmission and distribution systems, but is not stored anywhere
within them in the conventional sense.
Fields
A field is a very general concept in physics for
a region of space where a quantity exists with a specific value
at each point in the region. You can have a field of almost anything
that varies over space: temperature, for instance,
as well as the more common gravitational and electric
and magnetic fields.
The term “field” is, however, only in common use for
things which are capable of exerting a force. In
formal terms, the field is defined by the force it exerts on an
object placed in it. Thus, formally, the gravitational field is
the force exerted on unit mass, the electric field is the force
exerted on unit electric charge, and the magnetic field can be defined
in terms of the force exerted on unit magnetic charge. (In fact,
magnetic charge is probably a figment of the physicist’s imagination,
but it has its uses as a concept, albeit one which almost certainly
doesn’t actually exist.)
In practice, it is more helpful to think of both electric and magnetic
fields as the regions round electrical conductors in which effects
can be felt or measured. Electric fields can be measured because
they exert a force on charges; magnetic fields can be measured because
they exert a force on moving charges, ie a current.
Electric fields are produced by voltages,
irrespective of how much current is flowing and indeed whether any
current is flowing at all. Magnetic fields are
produced by currents, irrespective of the voltage.
The field at any point is produced by all the sources round it.
If one single source is dominant, the field will have a simple shape.
If there are several significant sources, the field can be quite
complicated.
The fields vary in time in the same way as the voltage or current
which produces them. Thus, dc circuits produce dc fields (in the
same direction all the time) and 50 Hz circuits produce fields which
change direction.
If we have a single ac source or a single-phase
circuit, the field at any point simply oscillates backwards and
forwards along a straight line. This is known as linear
polarisation. If we have more than one source, eg a three-phase
circuit, the field no longer has to oscillate along a straight line.
It actually traces out an ellipse. This is known
as “elliptical polarisation”. The extreme
case is circular polarisation.
More on elliptical
polarisation
The earth has a natural electric and magnetic
field. These are both static or dc fields. Any fields produced by
the power system are superimposed on top of these natural fields.
50 Hz magnetic fields are often (but not always) smaller than the
earth’s field (which is about 50 µT). When the 50 Hz
magnetic field is smaller than the static field, it has no effect
on the average field over time; it simply makes the field slightly
bigger during half the cycle and slightly smaller during the other
half.
Radiation
Although electric fields are produced by voltage and magnetic fields
by currents, once they have been produced, they can interact with
each other. An alternating magnetic field induces
an electric field. The interaction is described by Maxwell’s
equations.
Maxwell’s equations are very simple to write down but harder
to solve. For the present purposes, however, it is enough to know
that at high frequencies, Maxwell’s equations work in such
a way that the electric and the magnetic field always come coupled
together as radiation. They are at right angles
to each other, and propagate at the speed of light.
In principle, this coupling occurs at any frequency. In practice,
it is strongest at high frequencies, and gets progressively weaker
at lower frequencies. At 50 Hz, the coupling is so weak that radiation
is negligible, and effectively the electric and magnetic fields
are separate entities which can be produced independently. Thus
it is incorrect to speak of “radiation” at 50 Hz.
One way of distinguishing high frequencies where radiation does
occur from low frequencies where it does not is to think about the
wavelength. The wavelength is
the distance between two successive cycles of the wave. It is always
related to the frequency by the formula wavelength = speed
of light / frequency. The speed of light
is 3x10 metres
per second. For 50 Hz, the wavelength is very long, 6,000
km. Radio waves have wavelengths eg 1500
m, microwaves eg 12 cm, visible light eg a millionth of a metre,
x-rays eg a billionth of a metre.
The criterion for radiation is whether you are within roughly one
wavelength of the source. If you are less than a wavelength, radiation
will be small. If you are more than a wavelength, radiation will
be significant. These two regimes are called the “near
field” region and the “far field”
region. At 50 Hz we are always within one wavelength, 6000
km, of the source, so we are always in the near-field region and
radiation is always negligible.
An alternative term for fields in the region where radiation is
negligible is “quasi-static fields”.
A physicist will always speak of “electric fields”,
“magnetic fields”, or “electromagnetic
radiation”. When we use the abbreviation “EMFs”,
we mean “electric and magnetic fields”.
The term “electromagnetic fields” is not one that has
a very clear meaning but usually includes both electric fields and
magnetic fields.
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