Alternative quantities and electricity supply

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Transcript Alternative quantities and electricity supply

Alternative quantities and electricity supply
4.1) Generating of alternating
e.m.f.
N
When a loop AB is rotating at a
constant speed in the uniform
magnetic field, an alternating
Flux
voltage is thus induced. Its values
can be expressed by the following
mathematical equation:
v = VM sin(t)
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B
A
v
Fig.4.1
1
If we plot the equation with the time scale, we get the
following waveform diagram ( Fig. 4.2):
v
Vm
peak-to-peak
180
360 sin( t )
period
Fig. 4.2
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4.2) Amplitude, period, frequency
i) Instantaneous voltage or current ( v or i ) mean
the values of voltage or current at any instant in
time (denotes as t).
ii) Alternating current means sinusoidal current and
normally abbreviated to a.c.
iii) Peak value is the maximum value of the waveform.
This is also called as the amplitude of the waveform,
sometimes it may call the maximum of the waveform
and it always denotes as Vm or Im.
iv) Peak-to-peak value is the vertical distance between
the positive and the negative peaks.
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v) Frequency, f
It is the number of cycles performed in one second
and is measured in Hertz (Hz). i.e. 50 Hz = 50 cycles
per second.
vi) Period, T
It is the time for one complete cycle and is measured
in seconds. T=(1/f ),
for 50Hz, T=(1/50) = 20 ms.
vii) Angular velocity, ω (ω=2πf also), is the measured
angles per unit of traveled (unit in radian/sec) and
(ωt)=θ is the angle between the conductor and the
magnetic field at any instant of time.
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4.3) Average and r.m.s. values
Let us first consider the general case of a
current the waveform of which cannot
be represented by a simple mathematical
expression. For instance, the waveform
shown in Fig. 4.2.
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i
i1
i2
i3
i4
t
i7
in
i8
i9
Fig.4.4
Average value of current
i  i  i    in
= Iav = 1 2 n3
For a pure sinusoidal wave,
Iav = 0.637 Im
Vav = 0.637 Vm
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In a.c. work, however, the average value is of
comparatively little importance. This is due to the fact
that it is the power produced by the current that usually
matters. Thus, if the current represented in Fig.4.4 is
passed through a resistor having resistance R ohms, the
heating effect of i1 is (i1)2R, that of i2 is (i2)2R etc., as
shown in Fig. 4.5.
Therefore, the average heating effect in half-cycle is :
i R  i R    i R
2
 ( I r . m. s . )  R
n
2
1
2
2
2
n
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So, Ir.m.s. = root-mean-square of the current
i  i  i    i

n
2
1
2
2
2
3
2
n
For a pure sinusoidal wave,
Ir.m.s. = 0.707 Im
Vr.m.s. = 0.707 Vm
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Heating effect
r.m.s. value
(i3
(i2
)2
(i8)2
)2
(i4
(i7)2
)2
(i9)2
t
Fig. 4.5
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Electrical power transmission & distribution systems in Hong Kong
Energy
1.Energy in form of coal, gas, fuel etc is applied to
power station.
2.Energy is converted from thermal into electrical
energy by generator.
3.Electrical energy is transmitted from power station
to urban area by high voltage system, 400kV for
CLP and 275 kV for HKE.
Electricity
Transmission
4.The high voltage is stepped down to three phase
132kV by step down transformer at urban area.
5.The 132kV is further stepped down to three phase
11kV by step down transformer when supply to
small industries or commercial buildings.
Electricity
Distribution
Fig. 4.6
6.Further stepped down to three phase 380V and
220 V a.c. single phase supply is transmitted to
household clients by low voltage cables.
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Line voltage and phase voltage:
When supply is obtained from the public electricity supply company, the
voltage falls into one or more of the following:
(i) 220V ±6% single-phase :
Current demand does not exceed 60A single phase;
No 3-phase equipment installed;
(ii) 380V ±6% three-phase 4-wire :
Current demand exceeds 60A single phase;
There is 3-phase equipment installed;
(iii) 11kV +10% or -2.5% three-phase :
The load current is very large, and this voltage is supply by special
approval from electric company;
A special three-phase equipment, such as 11kV motors, is installed.
(iv) Three phase 132kV +10% or –2.5% : this is similar to case (iii).
The standard current is a.c. and the standard frequency is 50Hz ±2%.
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Distribution Network System
Two basic system are adopted for electricity distribution to
household consumers or small industries. They are :
a) Radial System
- The main feature of the system is that the feeders,
distributors and service mains radiate outwards from
the station.
- A fault on any feeder or distributor cuts off the supply
from all consumers who are on the side of the fault
away from the station.
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F1
C
Q
P
F2
Sub-station
C
Fuses or
circuit
breaker
C
C
C
C
C
C = consumer
Fig. 4.7 Radial feed system
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b) Ring System
- Each consumer is supplied via two feeders. If there is
a fault on a feeder at F1, the section between Q and R
can be switched out without interrupting the supply to
the consumers other than between section Q-R.
R
F1
SubC
Q
station
P
F
C
Sub2
C
C
SubC
station
station
Fuses or
circuit breaker
C
C
C = consumer
Fig. 4.8 Ring feed system
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Ring feed system against Radial feed system
Advantages
• Higher security of supply.
• Better voltage regulation.
• Loss minimized.
Disadvantages
• More careful design is necessary in order to avoid overloading
of any section during outage of the order.
• Switching in and out of any section will cause change in the
load distribution of other section thus operational restriction is
imposed.
• More complex protection scheme required.
• Less versatile for further development.
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Types of electricity supply systems
(i)
Single-phase (2-wire system)
220V
Live
From step-down
transformer
Neutral
0V
(Light bulb,
Fan, or TV
set)
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Load
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(ii) Three-phase (4-wire system)
3-phase
step-down
transformer
output
windings
Y
R
R
B
3-phase
load
Y
N
B
Neutral
(3-phase
motor)
Phase voltage: Vp = VRN = VYN = VBN = 220V
Line voltage: VL = VRY = VRB = VBY =
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3VP  380V
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4.5 Electric shock
Electric shock is the electric current that can hurt or kill a
person by flowing through the body.
The main effects that the flow of electric current produces
on the human body may be classified in the sequence of
seriousness as:
(i) Occasionally, a value of 0.5mA current pass through
human body will be felt.
(ii) Current exceeds about 16mA for men and 10mA for
women will cause stiffening of muscles and prevents a
victim of having electrical shock from letting go.
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(iii) When electric shock for a period of 4 minutes, the shock
may affect the nerve center that governs movements of
muscle.
(iv) For shock duration below 0.1 second, with a current over
500mA or 50mA for 1 second or 40mA for 3 seconds may
cause malfunction of heart.
(v) A shock current of 1A may cause heart failure.
(vi) Heat will be generated when current flow through a
resistance. A few amperes current passing through the
human body will cause extensive destruction of tissues,
breakage of the arteries, destruction of nerve center, etc.
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4.6 Importance of protection and earthing
(i) Protection
Electrical equipment shall be mechanically and
electrically protected to prevent danger from
shock, burn, or other injury to person or damage
to property or from fire of an electric origin.
Electric shock protection is a combination of
protection for an electrical installation against both
direct and indirect contacts of live parts. A person
can receive electric shock in two ways:
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(a) by direct contact, i.e. coming into contact with
live parts, at the same time also in contact with
earth potential or alternatively with another
live part of a different potential, and
(b) by indirect contact, i.e. touching metallic parts
that have become live due to an insulation fault.
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Protection against Direct Contact
(a) protection by insulation of live parts;
(b) protection by barriers or enclosures;
(c) protection by obstacles;
(d) protection by placing out of reach.
In practice, an electrical installation involves the use of a
combination of above methods.
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Protection against Indirect Contact
a) protection by earthed equipotential bonding and
automatic disconnection of supply, abbreviated
EEBADS;
b) protection by double or reinforced insulation;
c) protection by non-conducting location;
d) protection by earth-free local equipotential
bonding;
e) protection by electrical separation.
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Overcurrent protection:
Overcurrent protection is essential that the vital relation
between effective overcurrent protection and the safety
of personnel and property must be fully recognized.
Careful and adequacy in selection of overcurrent
protection can both prevent shock and fire hazards and
maintain reliable life of equipment and systems.
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An overcurrent is defined as a current exceeding the
rated value of a circuit or the current-carrying capacity of
a conductor.
Overcurrent may occur in a healthy circuit by connecting
excessive loads to it or due to surges. The resulted current
is called overload current.
Overcurrent may also be caused by a fault of low
impedance between live conductors having a difference
in potential or between live conductors and exposed or
extraneous-conductive parts under normal operating
conditions. The currents thus flow are called short-circuit
current and earth fault current respectively. They are
collectively known as fault currents.
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Every circuit shall be protected against overcurrent by one
or more devices which will operate automatically and timely
to interrupt the supply in the event of an overcurrent to
ensure no danger is caused.
The following devices are suitable for protection against
both overload and fault current provided that they are
capable of breaking and, for a circuit-breaker, making any
overcurrent up to and including the prospective fault current
at the where the device is installed:
• Fuses,
• Miniature Circuit Breakers (MCB);
• Moulded case circuit breakers (MCCB);
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• Circuit breakers incorporating overcurrent release in the
form of integral thermal-magnetic trip device, electronic
trip device, or external overcurrent relays;
• Circuit breakers in conjunction with fuses.
Earthing
The objective of earthing is to provide a low impedance
path for the earth fault current to discharge without
danger when metalwork of electrical equipment other
than current-carrying conductors may become charged
with electricity. The earthing of an installation must be:
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• Continuously effective;
• Capable to carry the earth fault and earth leakage
currents without danger;
 Robust, or having additional mechanical protection;
 Arranged to prevent damage to other metallic parts
through electrolysis.
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Earth-fault current, Ia
L
Touch voltage
Z1
Vt
N
E
Z2
ZE
Earth-fault loop impedance equals to the total
impedance from terminal L to earth.
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Although a proper earthing and bonding are used in
electrical installation, protection against indirect contact
should also be considered. Protective device for automatic
disconnection should be used, such that during an earth fault
the voltage between simultaneously accessible exposed-and
extraneous-conductive-parts are not to cause danger.
The protective device may be either:
 an overcurrent protective device, or
 a residual current device (which is a preferred method
whenever the prospective earth fault current is insufficient
to cause prompt operation of the overcurrent protective
device)
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