Physics Power Grid

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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
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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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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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