understanding electrical laws

8/8/2019 Understanding Electrical Laws

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UNDERSTANDING ELECTRICAL

NETWORKS

UNDERSTANDINGUNDERSTANDING

ELECTRICALELECTRICAL

NETWORKSNETWORKS

UNDERSTANDINGUNDERSTANDING

ELECTRICALELECTRICAL

NETWORKSNETWORKS


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CONTENTS

LAWS OFLAWS OF

ELECTROMAGNETISMELECTROMAGNETISM

BASIC VOCABULARYBASIC VOCABULARY

LAWS OFLAWS OF


BASIC VOCABULARYBASIC VOCABULARY


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PART 1: LAWS OF ELECTROMAGNETISM

LAWS OFLAWS OF


LAWS OFLAWS OF



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Electrical Distribution Training - dc.-10 4

Section 1: Variables and units

Variables and units

Physicallaws

Theelectricarc


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- multiples and submultiples of units

Multiples

103 106 109 1012

k M G Tkilo mega giga tera

Submultiples

10-3 10-6 10-9 10-12

m Q n p

milli micro nano pico


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- units ofmeasure 1/2

Basic units

Dimensional equation: examplesSpeed: LT-1 Acceleration: LT-2 Force: MLT-2

Electric charge: IT Voltage: ML2T-3 I-1 ...

L length metre m

M mass kilogramme kg

T time second s

I electriccurrent ampere A

These variables form the basis of all other units


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- units ofmeasure 2/2

Weight:

1 gramme = 15.4 grains

1 kilogramme = 2.2046 pounds1 ton = 0.9842 ton

Length:

1 centimetre = 0.3937 inch

1 metre = 1.094 yards

1 kilometre = 0.6214 mile


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- main variables and related units

force N Newtonenergy J Joule

power W Watt

acceleration m / s2

pressure Pa Pascal

moment of inertia

kg.m2

MECHANICAL

electric charge C Coulomb

voltage V Voltelectric field V / m

impedance ; ohm

resistance ;

reactance ;

inductance H Henry

capacitance F Farad

magnetic induction T Tesla

magnetic field A / m

magnetic flux Wb Weber

permeability Q H / m

permittivity I F / m

conductance S Siemens

resistivity ; . mfrequency Hz Hertz (s-1)

pulsation rd / s

...

ELECTRICAL

Vs

l

U = R I

R =


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- sun-earth power emission

10TW

human activity

180 .103 TW

1400W / m2

received 200TW

photosynthesis

35TW

internal

180 . 103 TWre-emitted

loss of

mass:

4.109 kg /s

390 .1012 TW

radiation

1000nucleargroups = 1 TW


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Section 2: Physical laws

Physical laws

Theelectricarc

Variablesandunits


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- how is electrical energy produced?

A rotating magnet near a circuit containing turns generates alternatingvoltage (Lenz'slaw)

e

= magnetic flux

e =

dt

dJ


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- an alternating variable can be

represented by:

A rotating vector

A sine wave

3T/2

U 0

2T

T/2

T0 T/2 T 3T/2 2T

y = a sin U

y y

r

U

r

UU

rr

U


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- alternating current

0 U[ T[ 2T[

I

t

IdcIrms

i = Idc . sin [t

... Sameapplies forvoltage

Root mean square (rms) value: the value of direct current which

would give off the same energy by Joule effect in a resistor

I rms = Idc / V2


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- Ohm's law applied to direct current

Wire resistance: R = V l / sV = 1.8.10-8 ;.m copper

V = 2.9.10-8 ;.m aluminium

V= 100.10-8 ;.m nickel-chromium

(alloy for resistors)

I in A

U in V

R in ;

U

I R

U = R I

P in W

in . m

I in m

S in m

V ;

P = U I = R I2 = U2/ R


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- Ohm's law applied to alternating current (a.c.)

z = impedance of the a.c. circuit witha frequency f

[ = 2T f current pulsation

RL

Ci

u

u = z . i in complex numbers

z

iu

equivalentcircuit

XZ

R

z is a complex number the real partof which is the resistance R and

imaginary part the reactance X

z = R + j.X z in ;

where X = L[ - 1/ C[ X in ;

L: inductance in Henry f in HzC: capacitance in Farad


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- Ohm's law applied to alternating current (a.c.)

z

iu

z = R + j X

z = (Z , J ) in polar form

where Z2 = R2 + X2

and tgJ= X / R

z = Z e j J

i = I e j [ t

u = z . i = Z.I e j J . e j [ t

= Z.I e j([ t + J) = U e j([ t + J)

z

i

u

JJU = ZI

X

R

u = z . i in complex numbers


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- active power and reactive power

P(t) = U(t) . I(t) = U . I sin([t) sin([t N)

P(t) = U . I cos N (1 cos2 [t) + U . I sin N sin (2[t)

The integral cycle shows:

S = U.I apparent power in VAQ = U.I.sinN reactive power in VAR

P = U.I.cosN active power in W

P Q


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- do not confuse:

"Line loss" and "Voltage drops "

Source Load

line

z = R + j X

V1 V2

I

Joule effect only depends on R

P I Rline= .2

(in Watts)

Depends on R and X

( V = V1 -V2(in Volts)


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- law of impedance combination

z1

z2

Series:

z = z1 + z2

The impedances are added

z1 z2

(Admittance = inverse of impedance)

Parallel:

1 / z = 1 / z1 + 1 / z2

z = z1. z2 / (z1+ z2)

The admittances are added


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- the three-phase diagram

N

Ph3

Ph2

Ph1

2V

V3

V1U23

U12

3

2T

3

2T

3

2T

U31

V1, V2, V3 : single-phase voltages

U12, U23, U31 : phase-to-phase voltages

U12 = V2 - V1 in vectors

in balanced three-phase operating

conditions,

U = V . 3V2 lagging of behind V13

2T

3 windings with phase displacement

of120 (2T/ 3)


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- why three-phase current?

The most economical means of remotely transmitting movement:

minimal number of windings to create a rotating magnetic field

A.c. generator motor


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- three-phase power diagram

Active power

Pa = U.I. cos J in W (= 3 VI cosN)

Reactive power

Pr = U.I. sin J in VAR

Apparent power

S = U.I. in VA (= 3 VI)

3

3

3

J

Pa

Pr

J

RII

jXI


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*Coulomb'slaw

Two charges of the same sign repel each other

Two charges of the opposite sign attract each other

Concreteapplications:static electricity, capacitors...

q M Er4TI r2

q E =

q q' FdF

Electric field on point M created by a punctual load q

Interaction face between two punctual electric loads

q . q' F =

4TI r2

q in C

E in V

r in m

d in m

I in F/mI0 = dielectric constant of vacuum

Ir= relative permittivity of ambient material

Dielectric constant I!I0 . IrI = 10

-7/ 4 T c0 = 8.85 . 10-12 F / m


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*Kirchhof'slaw

Concreteapplications:loop networks protection

protection by open delta earth currentmeasurement

7(u = 0

Around a loop

7 i = 0

On a node

N

A

B

C D

E


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*Ampere'slaw

I

B

r

Concreteapplications: disturbance by proximity of a conductor

Magnetic permeability Q = Q0.Qr

Q0 = 4 T 10-7 = 12.6 . 10-7 0 in H/m

Qr= 1 for vacuum, air, aluminium

Qr= 600- 800 for iron

I . Q0 . c02 = 1

Q0IB =

div B = 0

An infinite rectilinear conductor, through which a current

(I) flows, creates a magnetic induction B in the

surrounding space

B in T

I in Ar in m

2

Tr


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*Laplace'slawof force

Concreteapplications:electromechanical forces motors

F = i . dl 0 B

F = i . dl . B . sin E

I

B

E

F

An induction B exerts a force F on a

conductor through which a current (I)

flows

Right hand

Thumb

Middle finger

Index finger


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Electrical Distribution Training - dc.-1027

*Lenz'slaw

Co

ncrete

applicatio

ns: generator...

A conductive circuit forms a ring around a

surface through which the induction varies;

this circuit is subjected to an electromotive

force E along the circuit

E = - dJ/ dt

B variable(increasing)

E


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*Amperesand Laplaceslaws

Concreteapplications: electrodynamic forces busbar

withstand withdrawable circuit breaker arm

I I

Force of repulsion

---> loop effect

Proportional to the

product of the currents

I I

Force of attraction


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*Lenzsand Laplaceslaws

C

on

crete

application

s: energym

eters electric brakesover-heating of cubicle side panels by Eddy currents

Circular currents induced in the metal frames by a variation in magnetic

flux:

Aluminium

disk


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*Voltage dropcalculation

Concreteapplications: a very useful professional rule

Voltage drop in vectors V1- V2 = z . I = (R + j.X).IAs an absolute value this is very close to AB = AH + HB

Whence (V = R I cos J + X I sin JREMARK:the voltage dropmaybe negative....

Source Load

line

z = R + jX

V2

Z , N

V1

I

( V

N V2

V1

R.I

X.I

I

A H B

$ (V


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Section 3: The electric arc

The electric arc

Variablesandunits

Physicallaws


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- the electric arc

The arc is created when the voltage between two conductors is higher asthe maximum dielectric withstand value of the medium separating them

Irreversible deterioration in solid insulating materials

Ionisation of the medium separating the contacts

air SF6 gas

oil

vacuum: vapourization of the metal of the contacts

The ionised insulating medium becomes temporarily conductive

Presence of an arc voltage depending on the ionised mediumand on the nature of electrodes


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- breaking techniques: the puffer technique

Moving

contact

Fixed

contact

I

Flow ofcurrent

Separationof contacts

& arcing

Lengthening ofthe arc &

blow-out

Extinction ofthe arc as the

current reaches

zero


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- breaking techniques: the vacuum technique

Moving

contact

Fixed

contact

I

Flow of

current

Initial

RMF AMF

Diffuse

U net

Constricted

Arc control Interruption


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PART 2: BASIC VOCABULARY

BASIC VOCABULARYBASIC VOCABULARY BASIC VOCABULARYBASIC VOCABULARY

Backtothebeginningofthepart"Lawsof Electromagnetism"


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Section 1: Electrical definitions

Electrical definitions

Apparatus functions

Typesof networks


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- voltages

Three voltage values define the equipment operating characteristics

rated voltage is the maximum value of the voltage which the

equipment can withstand in normal operation

rated insulation level determines the dielectric withstand to

overvoltage and impulse voltage

- it depends on the rated voltage

service voltage is the voltage applied to the terminals of the equipment


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- voltages


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- currents

Two current values define the equipment operating characteristics:

rated normal current is the value of the current that the equipment can

withstand permanently without exceeding the temperature allowed by the

standard

rated short-time withstand current is the value of the short-circuit

current to be withstood for one second (or sometimes three seconds)

the peak value of t

he rated s

hort-time

withstand current is equal to2.5 times its rms value

Two current values define the network operating characteristics:

service current is the circuit load value calculated from the consumption

of the connected apparatus

short-circuit current is the overcurrent due to a fault or faulty operationon the circuit: Isc


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- frequency

There are two frequencies normally used in the world:

50 Hz in Europe

60 Hz in NorthAmerica.

Some countries use both frequencies:

Saudi Arabia...


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Section 2: Apparatus functions

Apparatus functions

Typesof networks

Electricaldefinitions


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- safety functions: isolating, earthing

DESIGNATION FUNCTION Switching on-off Closing and breaking

AND SYMBOL the service currents the faulted currents

Disconnector

insulate NO NO

Earthing switch

insulate NO NO(to be able toclose on c/c)


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- control functions

NO

NO

NO


AND SYMBOL the service currents the faulted currents

YES

Switch on-off

not insulate

Switch

NO

NO

NO

YESSwitch on-off

insulate

Disconnector switch

NO

NO

NOYESSwitch on-offnot insulate

Contactor


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-protection functions

DESIGNATION FUNCTION Switc ing on-off Closing and breaking

AND SYMBOL theservicec rrents the fa lted c rrents

Protect

not ins late

YES YESFixed circ it-breaker



Switch on-off

Protect - ins latein withdrawn

position

YESYESWithdrawablecirc it-breaker



Protect

not ins late YES(once)

NO

Fuse


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Section 3: Types of networks

Types of networks

Electricaldefinitions

Apparatus functions


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- two types of configuration

MV distribution networks may be either:

overhead or underground

radial or loop configuration


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- radial configuration


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- open loop configuration


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- networks

The networkiseverywherein thelandscape...

andin everybuildingaswell


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-public energy network

LVconsumer

MVconsumer(tertiary,

small industry)

HVconsumer

(heavy industry)

EHVnational

MVlocal

LVlocal

HVregional


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END OF TRAINING

Thanks for the time you have spent on this training course

We hope it will be useful for your job

For any further information, you can contact us by e-mail:

[email protected]

And, of course, the whole Electrical Distribution Training team

remains at your disposal

understanding electrical laws

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