Physics Power Grid
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Transcript of Physics Power Grid
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iNspiriNG learNiNG eNViroNMeNt features
EPN 44/2
beaUty in disGUise the physics behind
the pOWer Gridl Christian OhlerABB Switzerland, Corporate Research, CH-5405 Baden-Dttwil [email protected] DOI: 10.1051/epn/2013204
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Article available at http://www.europhysicsnews.orgor http://dx.doi.org/10.1051/epn/2013204
http://www.europhysicsnews.org/http://dx.doi.org/10.1051/epn/2013204http://dx.doi.org/10.1051/epn/2013204http://www.europhysicsnews.org/ -
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pHysics BeHiNd tHe poWer GridFeatures
28
approximates, with some eort o the imagination, a
hollow cylinder that lets the wind blow through. Such a
cylinder has a higher surace area than a massive conduc-
tor o the same cross-section, and the electric eld at the
conductor suraces is small enough to avoid the ionisa-
tion o the adjacent air - a corona discharge - that wouldcreate losses and make noise. Te suppression o corona
discharges is not perect, as you can tell rom the sizzling
o high voltage lines in humid weather conditions.
Te second reason or bundles is the skin eect. Te cur-
rent density o an alternating current is pushed towards
the outer surace o any conductor. Rather than wasting
aluminium in the centre, it is provided at the outside where
the current preers to ow. Why aluminium and not cop-
per? Sure, copper has a better electrical conductivity by a
actor o 1.6, but it is heavier by a actor o more than 3
and more expensive by a actor o 4. So most high-voltagelines are made o steel-reinorced aluminium conductors.
Te larger eective diameter o the current density re-
duces also the parasitic sel-inductance o the overhead
line. Tis might appear to be a marginal detail, but the
act that lines, transormers, and many o the motor loads
come with parasitic inductances is a severe constraint
o power system operation. Te energy that ows peri-
odically into and out o the magnetic eld o parasitic
inductances is named reactive power, and it has to be
supplied by circuit elements o opposite, capacitive, kind.
Te capability to behave like a capacitor is one o the
merits o synchronous generators (see below).
Next time you pass by a tower o a high voltage
overhead line, please look at it with resh
eyes. Isnt it beautiul in the way its orm
ollows its unction? Do you remember why
it is a tower at all? It is because o the high voltage that
needs to be kept away rom passing people. And why doesthe voltage need to be so high? It is because we need to
save on current. Resistive losses go with the square o the
current. ransmitted power is the product o current and
voltage. Te higher the voltage, the lower the current and
the less power per distance we lose.
A line tower is a miniature Eiel tower. Te lattice struc-
ture minimizes the amount o structural steel or a giv-
en mechanical load and height and lets the wind pass
through without oering resistance.
Another interesting shape is that o the surace o the
cylindrical solid insulators holding the live parts in place(see Fig. 1). Rain water would orm an electrical short to
the tower structure on ground potential. It is prevented
rom orming a continuous path along the insulator sur-
ace by a number o small umbrellas, the sheds. Sheds
orce rain water to drip along the insulator surace and
not to trickle. Sheds are the earmarks o outdoor insula-
tion and by counting the number o sheds per insulator
you can guess the voltage level o an overhead line. Te
only thing you need to know is that a single shed can
stand a voltage dierence o 10 kilovolts or so.
Te conductors o high-voltage lines oen come in bun-
dles o two or our per phase. A bundle o conductors
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7/27/2019 Physics Power Grid
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pHysics BeHiNd tHe poWer Grid features
permeability. It is le as a thought experiment to the
reader how power systems would look like i it was
the other way around.
Tree-phase power transormers use the same trick or
the magnetic ux that three-phase overhead lines use
or the electric current. Te sum o the magnetic return
uxes cancels, and there is no return leg or any o the
phases. A three-phase transormer transorms twice as
rl fom o look lk o xookA child observing high voltage overhead lines during a
car trip will soon notice that the conductor bundles come
in sets o three. Tey herald the three-phase nature o the
power system. Tree is the smallest number o phases
that allows or a constant, time-independent supply o
power rom alternating, time-varying current and volt-
ages. Tree sinusoidal currents o the same magnitude
and displaced in time by one third o the period sum up to
zero at any time. As a consequence, the return conductors
o the three phases can be omitted. Compared to single
phase systems, three-phase systems carry the same power
with hal o the conductor mass.
ransormers work only or alternating current. Tey
step up the voltages rom the power plant so that electric
power can be transmitted over long distances, and they
cascade the voltage down or distribution and consump-
tion o power. ransormers work through the combined
action o Amperes law all electric currents come with
a surrounding magnetic eld and Faradays law time
varying magnetic elds induce electric elds and hence
currents in conductors.
Real transormers dont look like those in physics text-
books. I they would, their leakage inductance would
be terrible. Instead, or each phase, high voltage and
low voltage winding are put coaxially around a stack
o thin laminated layers o core steel (g. 2). Te two
windings should be as close as possible to each other
because the magnetic ux is only imperectly con-tained in the core. Nature has provided us with metals
that have higher electrical conductivity than insulators
by twenty-our orders o magnitude, but she has been
less generous in orders o magnitudes or magnetic
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pHysics BeHiNd tHe poWer GridFeatures
30 EPN 44/2
displaced by 120 degrees along the circumerence o
the cylinder. Tese three coils are connected to the
three-phase power grid, and the magnetic rotor eld
induces periodically a current in each o them. De-
pending on the angle o the rotating rotor relative to
the voltage o the power grid rotating with the samerequency, the machine will either push active power
into the grid acting like a generator, or consume active
power rom the grid acting like a motor.
cu k juokI the synchronous generators are the kings o the power
grids, circuit breakers are their knights, always ready or a
ght, but rarely ghting. Imagine or a moment the vast
network o conductors extending over the continent and
linking all the corners o Europe. Imagine the likelihood
o a short circuit somewhere. I there werent an elementor segmenting the healthy rom the aulty parts o the
network, then good night! indeed.
Circuit breakers are electrical switches designed to pro-
tect a power circuit rom damage caused by short circuits.
Tey are ideal conductors in the closed position, ideal
insulators in the open position, and capable o interrupt-
ing short circuits that may be orders o magnitude larger
than the normal current they carry.
Te outer shape o a simple high-voltage circuit breaker
ollows in a straightorward way rom its unction (g.
6). For each o the three phases, it is a linear, cylindrical
segment the interruption chamber inserted between
the terminals o an incoming and outgoing conductor.
much power as three single-phase transormers o the
same core size would.
Te shape o power transormers is the result o the in-
tention to wind two coils around each other and around
a core cylinder, while maintaining sufcient insulation
distance in between, and joining the three legs o thephases with as little connecting yoke as possible. Te
cross-section o the steel cores and the steel yoke is just
large enough to avoid magnetic saturation. A particularly
illustrative layout o the power transormer is the trian-
gular transormer shown in g. 3.
Wg ol wog wMost o the energy ed into the power grid comes rom
synchronous generators. Tey convert the mechani-
cal power o the power plant into electrical power. Be-
ing made o coils, it is worth noting that synchronousgenerators can provide reactive power, i.e., behave as
a capacitor.
Te shape o a generator ollows rom the need to wind
coils around a cylinder in the wrong way, alongside
the cylinders axis. Tis is to convert a steady mechani-
cal rotation into a rotating magnetic eld that induces
three currents that periodically oscillate. Te winding
o the rotor (g. 4) is ed with a direct current and,
driven rom the mechanical energy o the connected
turbine, creates a rotating magnetic eld just like a
rotating permanent magnet would do. At each rota-
tion, the rotor eld passes by the stator windings (g.
5) that are connected such that they orm three coils
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BoXFigure 7 shows cut through the cylindri-
cl interruption chmber o high-voltge
circuit breker t dierent moments o c-
tion s short circuit is interrupted. Letters
reer to the elements o the circuit breker,
numbers to instnts in time. In closed posi-
tion, both kind o contcts, the min con-
tcts (a) nd the rcing contcts (B), re
closed nd the min contcts crry most o
the current with low losses. Upon the de-
tection o short circuit by current sen-
sor tht is seprte device, both contcts
move, rst seprting the min contcts
nd trnserring the current to the rcing
contcts. When the rcing contcts sep-
rte (1), hot plsm rc is drwn in thegp, nd the short circuit current contin-
ues to ow through it, providing mple
het so tht the plsm remins conduc-
tive. During the high-current phse (2),
some prt o the energy is used to build
up pressure in dedicted volumes (D).
Shortly beore current zero (3) the gs ow
reverses nd cools the rc convectively s
it shrinks with the declining current. at cur-
rent zero (4), with sufcient cooling ow
provided, the current is interrupted, nd
the gp between the contcts (nd lso
the chmber insultor, C) hs to sustin
immeditely the recovery voltge (5). In
successul interruption, the dielectric
strength o the gs recovers ster thn
the recovery voltge o the network rises.
Otherwise, in the cse o iled interrup-
tion, there will be brekdown, nd the
rc will reignite. n
EPN 44/2 31
pHysics BeHiNd tHe poWer Grid features
aou auoChristian Ohler is leading the depart-ment Power products and sensors at
ABBs Corporate Research Centre in Swit-
zerland. He has a master degree and a PhD
in Physics rom RWH Aachen. ABB is a
global company in power and automation
technologies and employs 145,000 people in 100 countries.
refeences
[1] N. Mohan, Electric power systems: a first course, Wiley, New York, 2012.[2] A. Kchler, High voltage engineering: fundamentals, technology,
applications, Springer, Berlin, 2013.
In open position, one o the terminals may be on sys-
tem voltage and the other may be on ground. Tus the
outside o the interruption chamber needs to have an
outdoor insulation with sheds to avoid an outside insu-
lation ailure. In normal operation both conductor sides
are at high voltage; in consequence, the whole interrup-tion chamber is supported by an outdoor insulator o
the same design.
Circuit breakers ght in the way o a judoka. Tey
exploit part o the energy inherent in the short-circuit
to tame the short-circuit. Teir inner lie is complex
and based on plasma physics and hot gas dynamics
(see box).
pl g l owTe power grid was there when we were born. We tend
to orget what a beautiul achievement o our society it is.Te synchronous grid o continental Europe is the largest
in the world by installed power, and the average outage
time o a customer is 15 minutes per year. Te grid is on
99.997% o the time.
With curious eyes you will see transormers and circuit
breakers through the ence o outdoor substations. But
the only components o the grid visible to the general
public are the overhead lines and their towers, the serv-
ants o our power system. A surprising number o things
about the power grid can be learned rom looking at a
line tower. Next time you cross one o these humble serv-
ants, please greet him with the physicists respect that
he deserves.n
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