Alternative quantities and electricity supply

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Alternative quantities and electricity supply

• 4.1) Generating of alternating
• e.m.f.
• When a loop AB is rotating at a constant speed in
  the uniform magnetic field, an alternating
  voltage is thus induced. Its values can be
  expressed by the following mathematical equation
• v VM sin(?t)

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• If we plot the equation with the time scale, we
  get the following waveform diagram ( Fig. 4.2)

v
Vm
360?
sin(? t )
peak-to-peak
180?
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, ? (?2pf also), is the
  measured angles per unit of traveled (unit in
  radian/sec) and (?t)? 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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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
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So, Ir.m.s. root-mean-square of the current
For a pure sinusoidal wave, Ir.m.s.
0.707 Im Vr.m.s. 0.707 Vm
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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
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.
Fig. 4.6
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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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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.
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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-ph
ase (2-wire system)
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(ii) Three-phase (4-wire system)
Phase voltage Vp VRN VYN VBN 220V
Line voltage VL VRY VRB VBY
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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
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(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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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
(a)
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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 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)