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UNDERSTAND THE BASICS OF ELECTRICITY
Understand the basic laws andvocabulary of electricity
Duration: 29 mn
Expert: P. GIVORD / D. FULCHIRONPedagogy: F. FINCHELSTEINProduction: M. ALLAGNAT / E. RODRIGUES
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Variables and units
Electrical definitions
Breaking techniques
Types of networks
Device functions
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VARIABLES AND UNITS
Variables and units
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- multiples and sub-multiples of units
Multiples
103 106 109 1012k M G T
kilo mega giga tera
Sub-multiples
10-3 10-6 10-9 10-12m n p
milli micro nano pico
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- international system of measurement units
Base units
Dimensional equation: examples
Speed: LT-1 Acceleration: LT-2
Force: MLT-2 Electric charge:IT Voltage:ML2T-3 I-1
L length meter mM mass kilogram kg
T time second sI electric current ampere A
These variables form the base of all other units
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- particular measurement units
Length:
1 centimeter = 0.3937 inch
1 meter = 1.094 yards
1 kilometer = 0.6214 mile
Weight:
1 gram = 15.4 grains
1 kilogram = 2.2046 pounds
1 metric ton = 0.9842 ton
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- main variables and related units
rs
l
U = R I
R =
force N Newtonenergy J Joulepower W Wattacceleration m / s2pressure Pa Pascalmoment of inertia kg.m2
electric charge C Coulombvoltage V Voltelectric field V / mimpedance W ohmresistance Wreactance W
inductance H Henrycapacitance F Faradmagnetic induction T Teslamagnetic field A / mmagnetic flux Wb Weberpermeability m H / mpermittivity e F / m
conductance S Siemensresistivity W . mfrequency Hz Hertz (s-1)angular frequency rd / s
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- sun - earth power emission
10 TWhuman activity
180.103 TW1400 W / m received
200 TWphotosynthesis
35 TW
internal
180.103TWre-emitted
loss of
mass:
4.109kg /s
390.1012TWradiation
1000 nuclear reactors = 1 TW
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Electrical definitions
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Device functions
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ELECTRICAL DEFINITIONS
Electrical definitions
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- voltages
Equipment operating characteristics are defined by three voltagevalues:
rated voltage
service voltage
rated insulation level
overvoltage withstand at power frequency for one minute
standardized impulse withstand voltage
l
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- voltages
rated voltage
rated insulation level rated impulse withstandvoltage
STANDARD IEC VOLTAGES
t
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- currents
Equipment
rated normal current
rated short-time withstand current
The peak value of rated short-time withstand current is equal to2.5 times the rms value
Network
service current
short-circuit current (or Isc)
f
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- frequency
Two frequencies are commonly used in the world:
50 Hz in Europe
60 Hz in North America
Some countries use both frequencies:
Japan, Saudi Arabia...
PHYSICAL LAWS
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Electrical definitions
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PHYSICAL LAWS
Physical laws
how is electrical energy produced?
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- how is electrical energy produced?
A magnet rotating near a circuit containing turns generatesalternating voltage (Lenz's law)
e
= magnetic flux
e =dt
d
representation of an alternating variable
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- representation of an alternating variable
An alternating variable may be represented by a rotating vector anda sine wave
3p/2
q 02p
p/2
p0 p/2 p 3p/2 2p
y = a sin qy y
r
q
r
rr
q
alternating current
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I rms = Isc / 2
- alternating current
Root mean square (rms) value: the value of direct current thatwould give off the same energy by the Joule effect in a resistor
0 q/w p/w 2p/w
I
t
IscI rms
i = Isc . sin wt
... The same thing applies to voltage
Ohm's law applied to direct current
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P in W
in . m
I in m
S in m
r
- Ohm s law applied to direct current
U = RI
Wire resistance: R = l / s
r = 1.8.10- .m copper
r = 2.9.10- .m aluminumr = 100.10- .m nickel - chromium (alloy for resistors)
I inA
U inV
R in
U
I R
P = U I = R I = U / R
Ohm's law applied to alternating current (AC)
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- Ohm s law applied to alternating current (AC)
z =impedanceof the AC curcuit with afrequency fw = 2 p f current angular frequency
RL
Ci
u
u = z . i in complex numbers
equivalent circuit
u
z
i
X Z
R
z is a complex number,the real part of which is resistance Rand the imaginary part reactance X
z = R + j.X z in Wwhere X = Lw - 1/ Cw X in WL: inductance in Henry f in HzC: capacitance in Farad
Ohm's law applied to alternating current (AC)
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- Ohm s law applied to alternating current (AC)
z
iu
z = R + j X
z = (Z , ) in polar formwhere Z2 = R2 + X2and tg = X / Rz = Z e j
i = I e j w t
u = z . i = Z.I ej
. ej w t
= Z.I e j(w t + ) = U e j(w t + )
z
i
u
U = ZIX
R
u = z . iin complex numbers
- active power and reactive power
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active power and reactive power
P(t) = U(t) . I(t) = U . I sin(wt) sin(wt)
P(t) = U . I cos (1 cos2 wt) + U . I sin sin (2 wt)
The integral cycle shows:
S = U.I apparent power in VA
Q = U.I.sin reactive power in VARP = U.I.cos active power in W
P Q
- line losses and voltage drops
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line losses and voltage drops
Source Load
Line
z = R + j X
V1 V2
I
The Joule effect depends only on R
P I Rline= .2
(in Watts)
Depend on R and X
D V = V1 - V2(in Volts)
Line losses Voltage drops
- law of impedance combination
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law of impedance combination
z1
z2
z = z1 + z2
The impedances add up
z1 z2
1 / z = 1 / z1 + 1 / z2
z = z1 . z2 / (z1 + z2)
The admittances add up
Series:
Parallel:
Admittance = reciprocal of impedance
- three-phase diagram
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V2 lagging behind V1
3
2p
three phase diagram
N
Ph3
Ph 2
Ph1
2V
V3
V1
U23
U12
3
2p
3
2p
3
p2
U31
V1, V2, V3: phase voltages
U12, U23, U31: phase-to-phase voltages
U12 = V2 - V1 in vectors
In balanced three-phase operatingconditions,U = V . 3
3 coils with phase
displacement of 120 (2 p / 3)
- why three-phase current?
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why three phase current?
The most economical way to transmit movement: minimum numberof coils to create a rotating magnetic field
AC generator motor
- three-phase power diagram
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Apparent power
S = U.I. in VA (= 3 VI)3
Active power
Pr = U.I. cos in W (= 3VI cos)3
Pa
Pr
RII
jXI
p p g
Reactive power
Pr = U.I. sin in VAR3
- Coulomb's law
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q M Er4p e r2
q
E =
Electric field created on M by a point charge q
Interaction between two electric point charges
q q' FdFq . q'
F =
4p e r2q in CE in Vr in md in me in F/m e0 = dielectric constant in a vacuum
er
= relative permittivity of ambient material
Dielectric constant e = e0 . e re0 = 10
-7 / 4 p c0 = 8.85 . 10-12 F / m
Two charges of the same sign repel each otherTwo charges of the opposite sign attract each other
- Kirchhoff's law
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On a node
S i = 0N
A
B
C D
E
Around a loop
S D u = 0
- Ampre's law
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p
A continuous straight conductor, through which a current (I) flows,creates magnetic induction B in the surrounding space
I
B
rMagnetic permeability m = m0.mrm0 = 4 p 10
-7 = 12.6 .10-7 0 in H/m
mr = 1 for vacuum, air, aluminum
mr= 600 800 for iron
e0. m0 . c02 = 1
B in T
I in A
R in m
m0 I
div B = 0
B =2pr
-Laplace's law of force
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Induction B exerts force F on a conductor through which a current(I) is flowing
F = i . dl . B . sin aF = i . dl L B
I
B
a
F
Right hand
Thumb
Middle finger
Index finger
- Lenz's law
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A conductive circuit forms a ring around a surface through whichthe induction varies. The circuit is subjected to an electromotiveforce E along the circuit.
B variable(increasing)
E
E = - d / dt
- Ampre's and Laplace's laws
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II
Force of repulsion---> loop effect
Proportional tothe product
of the currentsI I
Force of attraction
- Lenz's and Laplace's laws
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Circular currents induced in metal frames by a variation in magneticflux
Aluminumdisk
- calculation of voltage drop
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Voltage drop in vectors V1 - V2 = z . I = (R + j.X).I
As an absolute value, this is very close to AB = AH + HB
Hence DV = R I cos + X I sin PLEASE NOTE: voltage drop may be negative....
Source Load
line
z = R + jX
V2
Z , V1
I
D V
V2 R.I
A H B
V1
X.I
I DV
TYPES OF NETWORKS
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Variables and units
Electrical definitions
Breaking techniques
Types of networks
Device functions
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Types of networks
- different types of MV networks
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Overhead or underground
Two types of configuration
Radial
Loop
- radial configuration
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Substation
Clusteredconnection
Direct connection
Substation
- open loop configuration
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substation substation substation
- networks
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- public network
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LVconsumer
MVconsumer(service sector,small industry)
HVconsumer(heavy industry)
EHVnational
MVlocal
LVlocal
HVregional
DEVICE FUNCTIONS
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Device functions
- safety functions: isolation, earthing
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DESIGNATION FUNCTION Switching on/off Closing/breakingAND SYMBOL of service currents of fault current
DisconnectorIsolate NO NO
Earthing switch
Isolate NO NO(capable ofclosing on
short-circuit)
- control functions
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NO
DESIGNATION FUNCTION Switching Closing and breakingAND SYMBOL on/off of of fault currents
service currents
YESSwitch on / off
not isolate
SwitchNO
NOYESSwitch on / offnot isolate
DisconnectorNO
NOYESSwitch on / offnot isolate
NOContactor
- protection functions
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Protectnot isolate YES
(once)
NOFuse
Switch on / off
Protect - isolatein "disconnected"
position
DESIGNATION FUNCTION Switching on-off Closing and breaking
AND SYMBOL the service currents the faulted currents
Protectnot isolate
YES YESFixed circuit breaker
DESIGNATION FUNCTION Switching on/off Closing and breakingAND SYMBOL of service currents of fault currents
YESYESWithdrawable circuit-breaker
BREAKING TECHNIQUES
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Breaking techniques
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Breaking techniques
- the electric arc
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An arc is created when the voltage between two conductors isgreater than the maximum dielectric withstand of the mediumbetween the conductors
Irreversible deterioration in solid insulating materials
Ionization of the medium between the contacts air
SF6 gas
oil vacuum: vaporization of metal on the contacts
The ionized insulating medium becomes temporarily conductive
Presence of an arc voltage according to the ionized medium andthe type of electrodes
- the "puffer" technique
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Movingcontact
Fixed
contact
I
Flow ofcurrent
Separationof contacts &
arcing
Lengtheningof the arc &
blow-out
Extinction of the arcwhen the current
reaches zero
- the vacuum technique
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Movingcontact
Fixedcontact
I
Flow of
currentInitial
AMF
Diffuse
U net
RMF
Constricted
Control of the arc Interruption
- end of the module
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