Download Electrical Systems and Construction Student`s Book

BookBook
Title
Student’s
Student’s
Book
FET FIRST
Level 3
FET FIRST
ELECTRICAL Author
SYSTEMS
AND
Level 3
CONSTRUCTION
Author
NQF Level 4
Student's Book
T. Neeuwfan, B. Els
FET FIRST Electrical Systems and Construction
NQF Level 4 Student’s Book
© T. Neeuwfan; B. Els 2010
All rights reserved. No part of this publication may be reproduced,
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Any person who does any unauthorised act in relation to this
publication may be liable for criminal prosecution and civil
claims for damages.
First published 2010 by
Troupant Publishers (Pty) Ltd
P O Box 4532
Northcliff
2115
Distributed by Macmillan South Africa (Pty) Ltd
Cover design by René de Wet
Edited by Jeannie van den Heever and Roy Atkins
Typeset by Golden Pear Desktop Publishing
Printed by
ISBN: 978-1-920334-40-6
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Contents
Topic 1: Electrical infrastructure and construction. . . . . . . . . . . . . . . . . . . . . . . 1
Module 1 Electrical infrastructure and construction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Unit 1.1: High-, medium- and low-voltage networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Unit 1.2: The ratings on switchgear, transformers, control gear and instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Unit 1.3: How alternators can be switched into or out of the grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Unit 1.4: The main components of a coal-fired power station. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Unit 1.5: The main components of a small town power grid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Unit 1.6: Radial and ring feeds and the effects of faulty transmission lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Unit 1.7: Medium-voltage overhead networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Summative assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Topic 2: Three-phase circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Module 2 Designing and constructing a three-phase circuit . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Unit 2.1: Electrical symbols and components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Unit 2.2: Interpreting a task and formulating a plan of action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Unit 2.3: Designing a three-phase circuit diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Unit 2.4: The components list. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Unit 2.5: Constructing a three-phase circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
Unit 2.6: Evaluating the circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Unit 2.7: Testing the design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Summative assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Topic 3: Three-phase medium-voltage overhead supply . . . . . . . . . . . . . . . . . . 75
Module 3 Constructing a three-phase medium-voltage overhead supply to
domestic houses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Unit 3.1: Statutory requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Unit 3.2: Worksite procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Unit 3.3: The components and equipment list, terrain assessment and marking out the route. . . . . . . . . . . . . . . . . . . . . . . . . 85
Unit 3.4: Construction of the three-phase medium-voltage overhead supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Unit 3.5: Completing the project. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
Summative assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Topic 4: Three-phase industrial/commercial installations. . . . . . . . . . . . . . . . . 95
Module 4 Testing and inspecting three-phase industrial/commercial installations. . . . . . 96
Unit 4.1: Understanding building plans and electric schematic and wiring diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Unit 4.2: Planning, selecting tools and safety rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Unit 4.3: Identifying environmental hazards and safety risks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Unit 4.4: Inspecting the installation for compliance with statutory requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Unit 4.5: Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
Summative assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Topic 5: Fault-finding, repair and maintenance of three-phase
electric circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Module 5 Fault-finding on three-phase electric circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
Unit 5.1: Principles and procedures of fault-finding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
Unit 5.2: Planning and preparing for fault-finding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Unit 5.3: Finding faults on faulty three-phase AC systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Module 6 Repairing three-phase voltage electric circuits. . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Unit 6.1: Principles and procedures for repairing three-phase AC systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Unit 6.2: Planning and preparing for repairing three-phase AC systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
Unit 6.3: Repairing faulty three-phase AC systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
Unit 6.4: Testing and commissioning the repaired three-phase AC system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
Module 7 Maintaining three-phase voltage electric circuits . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Unit 7.1: Principles and procedures for maintaining three-phase AC systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Unit 7.2: Planning and preparing for maintenance on three-phase AC systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Unit 7.3: Maintaining three-phase AC systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Unit 7.4: Recording data and maintenance scheduling on the three-phase AC system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
Summative assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
Glossary
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
PoE guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
Topic 1
Electrical
infrastructure and
construction
Module 1
Electrical infrastructure
and construction
Overview
At the end of this module, you will be able to:
• understand the concepts of high-voltage networks, mediumvoltage networks and low-voltage networks
• understand the ratings on switchgear, transformers, control gear
and instruments
• understand how alternators can be switched into or out of the
grid
• explain with the aid of diagrams the main components of a
coal-fired power station
• explain with the aid of diagrams the main components of a
typical small town power grid
• explain radial and ring feeds and the effects of faulty
transmission lines (short circuit and open circuit)
• list and explain component parts and equipment required to
install medium-voltage overhead networks.
Range: Includes, but is not limited to, knowledge of the layout of
the South African power grid, operating principles of coal-fired
power stations, layout of a typical small town power grid and how
to install and terminate medium-voltage overhead networks.
Units in this module
Unit 1.1: High-, medium- and low-voltage networks
Unit 1.2: The ratings on switchgear, transformers, control gear and
instruments
Unit 1.3: How alternators can be switched into or out of the grid
Unit 1.4: The main components of a coal-fired power station
Unit 1.5: The main components of a small town power grid
Unit 1.6: Radial and ring feeds and the effects of faulty
transmission lines
Unit 1.7: Medium-voltage overhead networks
2
Module 1: Electrical infrastructure and construction
Unit 1.1: High-, medium- and lowvoltage networks
Unit outcomes
At the end of this unit, you will be able to:
• understand the concepts of high-voltage networks, medium-voltage
networks and low-voltage networks.
Introduction
Electric power transmission is the bulk transfer of electrical power
over long distances. A power transmission network connects power
stations to substations located near residential, industrial and
commercial areas. The substations are connected to a distribution
network that delivers electricity to the consumer.
Electric power transmission allows distant energy sources such as coalfired or hydroelectric power stations to be connected to consumers
in population centres. It also allows low-grade fuel resources such as
coal to be exploited that would otherwise be too costly to transport to
generating facilities.
The power grid
A power transmission network is referred to as a grid. A power
grid is a complex interconnection of power lines that require
careful maintenance to remain operational. If the network is poorly
managed, it can result in the collapse of the power grid which has
a negative impact on the economy and essential service providers
such as hospitals. Multiple lines between points on the network are
provided so that power can be routed from any power plant to any
load centre, through a variety of routes, based on the economics of the
transmission path and the cost of power.
Much analysis is done to determine the maximum reliable capacity of
each line which, due to system stability considerations, may be less
than the physical electricity-carrying capacity of the line. Deregulation
of electricity companies in many countries has led to a renewed
interest in the reliable, economical design of transmission networks.
Fig. 1.1 illustrates a power grid.
Module 1: Electrical infrastructure and construction
3
Fig. 1.1: A power grid
1.
The power generation station
2. The transmission network starts at the power station
3 and 4. The transmission network ends at the substation where the
distribution network starts
5. The distribution network ends at the consumer
Voltage networks
The power grid supplies electricity to residential, commercial and
industrial areas throughout the country. It can be divided into three
sections, namely generation, transmission and distribution. Power is
generated at the power stations. It is transmitted over long distances
and then distributed to consumers.
The different sections of the power grid are classed according to
voltage levels. Electric power is generated at a power station at a
different voltage to that used for transmission and distribution.
The reason for this is to minimise power loss between the point of
generation and the delivery point.
Transmission lines have a specific electrical resistance. If the voltage
at which a certain quantity of power is transmitted is increased, the
current decreases by the same amount. Voltage and current are both
4
Module 1: Electrical infrastructure and construction
directly related to a set amount of generated power – if one quantity
increases, the other must decrease to maintain the fixed amount of
power. A reduced transmission current results in lower resistive losses
and therefore an efficient power grid.
The sections of the power grid that carry different voltages are called
the low-, medium- and high-voltage networks. Electricity is generated
at a medium-voltage level, transmitted at high-, extra high- or ultra
high-voltage levels and then distributed at medium voltages to
industrial customers and low voltages to residential customers.
The voltage classifications of the different sections are given in
Table 1.1.
Classification
Low voltage
Medium voltage
High voltage
Extra high voltage
Ultra high voltage
Voltage range (V)
Below 1 000 V
Between 1 000 and 69 kV
Between 69 kV and 230 kV
Between 230 kV and 800 kV
Above 800 kV
Table 1.1 Voltage classifications
Electric power is transmitted over very long distances so the resistance
of the conductors carrying the current is substantial. Transmitting
power at low currents and high voltages through conductors with
a significant resistance minimises conductor losses. When current
flowing through a conductor encounters resistance, some electrical
energy is converted to heat, and the conductor heats up. When a
transmission cable heats up, its sag increases and its clearance from
the ground decreases. So additional benefits of limiting conductor
losses are that the conductor’s thermal limit is not exceeded and the
conductor does not sag to dangerously low levels.
Fig. 1.2 shows an example of a high-voltage
overhead transmission line while Fig. 1.3
shows a medium-voltage distribution line.
Note the difference in size between the highvoltage transmission line and the mediumvoltage distribution line. The higher the
voltage is across the conductors, the larger the
structure used to carry the cables. The reasons
for this include the following:
• The higher the current is in a conductor, the
more concerns there are around ensuring
the safety of the network.
• Larger transmission lines minimise losses.
• The higher the voltage is, the larger the
distance between the conductors must be.
Because high-voltage conductors are larger,
they require larger mechanical structures to
support them.
Fig. 1.2: High-voltage overhead transmission line
Module 1: Electrical infrastructure and construction
5
The low-voltage network is the 240 V network installed
in your house by a qualified electrician. The wiring
from the 380 V grid is terminated in the distribution
box, also called a trip switch box. From the distribution
box 220 V – 240 V is distributed throughout the
building according to SABS standards. This is a singlephase supply with three wires, namely live, neutral and
ground. Electricity is distributed from the distribution
panel through conduit pipes installed during the
construction of the building in accordance with
building regulations.
Three-phase networks
Three-phase networks are used to transmit electricity.
This means that three conductors that have the same
voltage across them with equal currents flowing
through them transmit power from the generation to
the load side. The current through and voltage across
each conductor are displaced from each other by 120º
in time. Fig. 1.4 illustrates the superimposed voltages
across (or currents through) each conductor. Each sine
wave is displaced from the next by 120º. Three-phase
theory is beyond the scope of this text and will not be
covered any further here. You are, however, strongly
advised to research three-phase electricity
generation and transmission.
A
V
W
Fig. 1.3: Medium-voltage distribution line
Did you know?
B
360˚
0
120˚
240˚
Fig. 1.4: Three-phase waveforms
According to Eskom’s annual report for
the year April 2006 to March 2007, the
total electricity sold by Eskom was
218 120 GWh (gigawatt hour).
• Electricity sold to other countries
including Botswana, Mozambique,
Namibia, Zimbabwe, Lesotho,
Swaziland and Zambia amounted to
13 589 GWh.
• Six per cent of Eskom’s total
electricity sales were to other
countries in Southern Africa. This
was higher than the total residential
electricity sales in South Africa for
this period.
• The export of electricity to other
countries exceeded the total output
from South Africa’s only nuclear
power station Koeberg which
produced 11 780 GWh during the
same period.
In South Africa the output from an
Eskom power station is usually 22 kV.
For transmission over long distances it is
stepped up to between 220 kV and 765 kV.
6
Module 1: Electrical infrastructure and construction
Assessment activity 1.1
1. What is a power grid?
2. For power grids, what range of voltages is classified as:
a) low voltage
b) medium voltage
c) high voltage
d) ultra high voltage?
3.Why is it necessary to transmit electricity at high to ultra high
voltages?
Unit 1.2: The ratings on switchgear,
transformers, control gear and
instruments
Unit outcomes
At the end of this unit, you will be able to:
• understand the ratings on switchgear, transformers, control gear
and instruments.
Switchgear ratings
When a fault occurs in one section of the power grid or when
maintenance needs to be carried out on a section, this section of power
lines must be isolated from other sections. Switchgear is also used to
reroute the flow of power to optimise its use. Switchgear includes the
following:
1.Switches: Switches are used to de-energise sections when
maintenance work needs to be done on a section of power lines.
2.Fuses and circuit breakers: When a fault occurs, fuses or circuit
breakers disconnect the affected power lines from the rest of the
power grid to minimise damage.
Switchgear is
normally located
in a substation.
The function of
a substation is
to connect two
sections of the
transmission
network, for
example the
generation and
the transmission
network or the
transmission and
Did you know?
Dangers of high voltages to
humans
When a person’s body
provides a path for current
flow due to accidental
contact with high voltages,
the result is severe injury
or death. Note that the
contact must be such that
high currents can flow. This
depends on the person
making contact with two
points of the circuit or
providing a path for current
to flow through to the
ground.
Tissue damage results
from the heating effect of
the high current and heart
failure may result from
the voltage. Other injuries
may include burns from
the arc generated by the
accidental contact. If the
victim’s airways are affected
by tissue damage it can
be especially dangerous.
Injuries may also be suffered
as a result of the physical
forces exerted on people
by muscle contraction as
people may fall from a
height or be thrown onto
equipment.
A high voltage is not
necessarily dangerous.
The common static electric
sparks generated under
low-humidity conditions
during winter create
voltages well above 700 V.
The typical spark you feel
when you touch an object
(after dragging your feet
over a carpeted floor, for
example) can be due to
voltages in the kilovolt
range. These sparks have
a limited amount of stored
energy so the current
produced is low and usually
for a very short period.
Fig. 1.5: A substation
Module 1: Electrical infrastructure and construction
7
the distribution network. Power lines enter the substation and connect
to an arrangement of transformers and switchgear. Power lines then
exit the substation. A substation is shown in Fig. 1.5.
Switches, fuses and circuit breakers are rated for a specific area of
application, i.e. they are selected to handle a certain level of voltage
and current. Many other ratings are also considered when selecting
switchgear, including the following:
1.Rated voltage: This is the voltage at which the switchgear is
designed to operate.
2.Busbar circuit ratings: A busbar is a system of electrical
conductors in a generating or receiving station into which power
is fed for distribution. They usually carry large currents and are
therefore rated for the normal operating or rated current but must
also be able to withstand much higher short-circuit currents (fault
currents) for a certain period of time.
3.Rated frequency: This is the frequency at which the switchgear is
designed to operate.
4.Rated power frequency withstand voltage: This is the rms
value of sinusoidal power frequency voltage that the switchgear
can withstand during tests made under set conditions and for a
set time. It is the required withstand voltage of the equipment
between phase and earth and between phases.
5.Rated impulse withstand voltage: This is the maximum voltage
spike that the equipment can withstand.
6.Symmetrical breaking current: This is the rms value of the AC
component of the current in the pole at the time contacts are
separated.
7.Rated short-time current: This is the maximum current the
equipment can handle over a specified short time.
8.Rated peak making current: This is the maximum current the
circuit breaker can handle immediately after closing.
9.Degree of protection: This is the amount of protection that
the enclosure provides for the switchgear inside. This refers to
protection against the entry of water, dust and other foreign
bodies.
10.Also of importance when considering switchgear are the
dimensions of its enclosure, its volume and its
weight.
Transformer ratings
Transformers are devices used to change the
voltage level at which electricity is transmitted.
Step-up transformers increase the voltage level
while step-down transformers decrease the
voltage level.
In the national power grid, transformers step up
the voltage from, for example, 22 kV to 220 kV,
275 kV, 400 kV or 765 kV and feed the electricity
into the transmission network. This voltage is
stepped down at the substations to a voltage
8
Module 1: Electrical infrastructure and construction
Fig. 1.6: A transformer (reduction from 33 kV to 11 kV)
appropriate for the consumer. This could be 11 kV for large factories or
380/220 V for shops and homes. A transformer is shown in Fig. 1.6.
The main ratings of a transformer are its rated voltage and current
values and the apparent power it can handle. The rated values differ
for the primary and secondary side. Exceeding the rated values can
result in insulation failure, damage to the structure of the transformer
and overheating which can lead to the failure of the transformer.
The thermal rating of the transformer is also an important limit. This is
the maximum temperature at which the transformer can operate safely
without failing.
The frequency at which a transformer operates is also an important
consideration. Frequency is directly related to the transformer’s
reactance which means the higher the frequency is, the higher the
reactance is. A high reactance will result in voltage drops inside the
transformer which will reduce the voltage across the load.
The following information is likely to appear on a transformer
nameplate:
1. Name of manufacturer
2. Type of transformer – single or three phase
3. Load rating – secondary coil apparent power
4. Primary and secondary voltage ratings
5.A phasor diagram that relates the voltages in the different phases
to each other and also relates the currents and the voltages to the
currents
6. Weights and volumes
7. Dimensions of its enclosure
8. Winding material.
The details will vary from manufacturer to manufacturer.
Fig. 1.7 shows an example of a transformer and its nameplate. The
nameplate contains the ratings of the transformer:
1.This is a three-phase transformer that operates at a frequency of
60 Hz.
2.The load apparent power ratings (kVA) are given at different
temperatures.
3.The phasor diagram shows that the high-voltage side of the
transformer is connected in a delta configuration and that the lowvoltage side is connected in a wye configuration. Wye and delta
relate to the shape of the phasor diagram. A delta configuration is
a triangular configuration and a wye configuration looks like a Y.
4.The nameplate also indicates the high-voltage (primary side in this
case) and the low-voltage (secondary side in this case) ratings and
the type of winding material.
5.The physical dimensions are also given.
6.Other details that are stated on the nameplate include the type of
cooling mechanism and the voltage impulse that the transformer
can withstand.
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Fig. 1.7: A transformer and its nameplate
Control gear ratings
Control gear consists of an arrangement of electronic circuitry and
controllers that control or limit a process to within set specifications.
In the context of a power grid, control gear can be used to control
the operation of a motor. In this instance the control gear does the
following:
1. Starts and stops a motor
2. Limits the starting current of a motor
3. Controls the speed and direction of the motor.
In a substation, for instance, motors can be used to open and close
contactors. The ratings of the control gear will depend on the
application and the design chosen for the particular process or
operation. Typical ratings include current and voltage. The control gear
will often be required to monitor variations in a process variable and
then effect some sort of corrective action or even respond to changes
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in its objectives. The speed at which it responds to all these changes is
also an important parameter.
Instruments
Various instruments are used in a power grid for many applications,
for example to measure voltage, current and power at different points
in the grid. These instruments include multimeters, voltmeters,
ammeters, voltage or current probes used with an oscilloscope,
wattmeters and varmeters. At the point of power generation, various
instruments are used to measure the speed at which the generator
turns. It is important to highlight that these are not the only types of
instruments that are used in a power grid.
Measuring instruments are also used to monitor a variable. The most
important consideration is whether the instrument is rated to operate
in the complete range of values of the variable being measured. For
example, an instrument that measures a voltage that varies between
0 V and 1 000 V must be able to read values up to and even beyond
1 000 V. The instrument must be rated to operate in the environment in
which it is used and be able to measure all values of voltage, current,
power or speed.
Assessment activity 1.2
1. Name and give a brief description of switchgear ratings.
2. What are the factors to be considered in rating a transformer?
3. What information typically appears on a transformer nameplate?
Unit 1.3: How alternators can be
switched into or out of the grid
Unit outcomes
At the end of this unit, you will be able to:
• understand how alternators can be switched into or out of the grid.
Switching alternators into or out of the grid
An alternator is a dynamo that generates alternating current.
Alternators need to be shut down for maintenance from time to
time. The alternator cannot just be disconnected from the grid. A set
procedure must be followed to avoid damage that may be caused by
the alternator rotor spinning out of control or by an excessive torque
being applied to the alternator shaft. The alternator should also rotate
at a rate that maintains the frequency of the generated current and
voltage at a value equal to that of the grid voltage and frequency
(usually 50 Hz). The voltage and current generated must also be in
phase with grid quantities, i.e. they must be synchronised.
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The following procedure must be followed to switch an alternator into
and out of the grid:
• The energiser voltage to the electromagnetic core (field windings) is
reduced while the speed of the generator is kept constant to hold it
in phase with the grid power.
• At a pre-determined point the generator’s electrical connection is
switched out of the grid.
• Its shaft is connected to a mechanical load.
• The steam feed to the turbine is then removed so that the load can
slow down the generator.
• With the turbine/alternator slowly reducing its speed, the whole
system starts to cool down from the previous superheated steam
and load-carrying temperatures. It is, however, still very hot. It is
finally engaged to a gear drive that will keep it rotating until the
huge metal parts of the alternator and turbine have cooled down
sufficiently for maintenance to start.
Reversing this procedure is also a slow process. The procedure
outlined below is followed:
• The turbine/alternator combination is slowly sped up.
• When close to working speed, the electromagnetic core (field
windings) is carefully energised with the shaft of the alternator still
connected to the mechanical load.
• Phase monitoring is then used and the speed adjusted until
the generator is in phase with the grid power so that it can be
connected to the grid.
• At this stage the current is increased carefully until the system is
delivering full power.
Assessment activity 1.3
Describe the procedure for switching an alternator into and out of the power grid.
Unit 1.4: The main components of a
coal-fired power station
Unit outcomes
At the end of this unit, you will be able to:
• explain with the aid of diagrams the main components of a coalfired power station.
Coal-fired power stations
Most of the power stations in operation in South Africa are coal-fired
power stations. This is partly a consequence of the availability of coal
deposits. Coal-fired power stations are usually built near a coal mine.
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Electricity is generated in a coal-fired power station as follows:
Conveyers are used to transport the coal from the mine to the power
station. The coal is then ground to finer parts and fed into a furnace.
A fire burns inside the furnace in which pipes containing water are
installed. The heat in the furnace boils the water inside the pipes into
superheated steam which is then used to drive a steam turbine. The
steam turbine is connected on the same shaft as a generator and serves
as a prime mover for the generator. The basic operating principle
outlined above is illustrated in Fig. 1.8.
Boiler
(furnace)
Transmission
lines
Turbine
Steam
Coal
Water
River
Generator
Condenser
cooling water
Condenser
Transformer
Fig. 1.8: The basic operating principle of a coal-fired power station
A coal-fired power station can be compared to other power stations in
which a source of energy in the form of heat is used to convert water
to superheated steam that drives a turbine. Other power stations
generate the heat by burning oil, natural gas and nuclear fuels instead
of coal. The water is converted to steam at high temperatures and
pressures.
Coal-fired units are the most economical type of power station in
South Africa. The superheated steam, at tremendous pressure and
temperatures of between 500 °C and 535 °C, is used to turn large
turbines, which in turn drive the generators to generate electricity at
22 kV. The steam is cooled, condensed back into water and returned to
the boiler or reactor to start the process over. Power stations therefore
operate on the principle of converting energy from one form to
another.
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The main components of a coal-fired power station
Fig. 1.9 shows the different components of a coal-fired power
station.
Fig. 1.9: The main components of a coal-fired power station
1 – Cooling tower
12 – Deaerator
21 – Reheater
2 – Cooling water pump
13 – Feedwater heater
22 – Air intake for combustion
3 – Transmission line
14 – Conveyor
23 – Economiser
4 – Step-up transformer
15 – Coal hopper
24 – Air preheater
5 – Generator
16 – Coal pulveriser
25 – Precipitator
6, 9, 11 – Turbines
17 – Boiler steam drum
26 – Fan
7 – Boiler feedback pump
18 – Bottom ash hopper
27 – Fuel gas stack
8 – Surface condenser
19 – Superheater
10 – Steam control valve
20 – Fan
Coal
Coal is mined from the earth at mining sites. Coal hoppers crush the
coal into pieces of approximately 50 mm in size. The coal is processed
and then transported to the power station on a conveyor belt. Most of
the power generated in South Africa is from power stations located
near the coal fields.
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Coal pulveriser
Next, the coal is sent through a pulveriser that crushes the coal to
a fine powder. The fine powder is used as an energy source and is
blasted into the coal furnace or boiler with streams of air.
Boiler
The coal powder that reaches the furnace burns instantly when it
comes into contact with the flames in the furnace. Purified water is
pumped through tubes that form part of the boiler walls. The high
temperatures caused by the burning coal turn the water to steam
under high pressure.
Superheated steam boiler
Most boilers only heat water until it boils and then the steam is used
at saturation temperature. Superheated steam boilers boil the water
and then further heat the steam in a superheater. See Fig. 1.10. This
provides steam at a much higher temperature. This can decrease the
overall thermal efficiency of the steam plant because the higher steam
temperature requires a higher fuel gas exhaust temperature. However,
there are advantages to superheated steam. For example, useful heat
can be extracted from the steam without causing condensation. This
results in less damage to piping and turbine blades caused by steam
condensing on the turbine and piping and corroding the equipment.
Clayton
feedwater pump
Saturated steam
to superheater
section
Helical coil heat
exchanger with integral
superheater
Condensate to
feedwater receiver
from receiver
Feedwater
Superheated
steam to process
Mechanical
separator
Fig. 1.10: A superheated steam boiler
Superheated steam presents unique safety concerns. If there is a leak in
the steam piping, steam at such a high pressure and temperature can
cause serious harm to anyone in its path. Since the escaping steam is
superheated vapour, it is not easy to see the leak, although the intense
heat and sound will clearly indicate its presence.
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The superheater works like coils on an air-conditioning unit, although
to a different end. The steam piping with steam flowing through it is
directed through the flue gas path in the boiler furnace. This area is
typically between 1 300 ºC – 1 600 ºC. Some superheaters are radiant
types and absorb heat by radiation, others are convection types and
absorb heat via a fluid or gas while some are a combination of the two.
The extreme heat in the boiler furnace or flue gas path heats the
steam in the superheater piping. It is important to note that while the
temperature of the steam in the superheater is raised, the pressure of
the steam is not. The turbine or moving pistons offer a continuously
expanding space and the pressure remains the same as that of the
boiler. The process of superheating steam is most importantly designed
to remove all moisture content from the steam to prevent damage to
the turbine blades and associated piping.
Precipitator
An electrostatic precipitator (ESP), or electrostatic air cleaner, is a
device that removes particles from a flowing gas such as air using
the force of an induced electrostatic charge. See Fig. 1.11. Electrostatic
precipitators are highly efficient filters that use electrostatic forces to
extract solids such as dust and smoke from an air stream. The filter is
designed to impede the flow of gases through the device as little as
possible.
Burning coal produces carbon dioxide (CO2), sulphur dioxide (SO2),
nitrogen oxide (NOx) and fly ash as by-products. These pollutants
need to be removed from the gases before exhausting the gases into
the atmosphere.
The electrostatic precipitator removes the fly ash by electrostatic
attraction to electrostatically charged plates. The plates are brushed
off, the fly ash collected in large hoppers or bins and washed into ash
lagoons. It can then be used to make cement.
Waste gases without
smoke particles
Smoke particles
are attracted to the
collecting plates
Positively charged
collecting plate
Collecting plates are
knocked to remove
the smoke particles
Smoke particles
pick up a negative
charge
Negatively charged
metal grid
Waste gases containing
smoke particles
Fig. 1.11: An electrostatic precipitator
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Words &
Terms
flue gas path: th
e path
taken by exhaust
fumes.
Steam turbine
Steam turbines are used for the generation of electricity in thermal
power plants. The steam is passed to the turbine – a massive shaft
with thousands of propeller blades. The blades are designed to use
the expansion energy from steam jet streams to cause the turbine
shaft to spin. Turbines are expensive to make and require precision
manufacturing and high-quality materials.
Turbines need to be run up to speed slowly to prevent damage.
Additionally, the generation of electricity requires precise speed control
so the turbine is controlled by means of a governor. Uncontrolled
acceleration of the turbine rotor can cause it to run so fast that a
trip is activated. The nozzle valves that control the flow of steam to
the turbine are then closed. If this fails, the turbine may continue
accelerating until it breaks apart.
The turbines used for electric power generation are usually directly
coupled to their generators. The most common speed is 3 000
revolutions per minute (rpm) for 50 Hz systems as used in South
Africa. Fig. 1.12 shows the size of these machines that can deliver as
much as 1 500 000 kW of mechanical energy.
Fig. 1.12: A turbine opened up for maintenance
Generator
In electricity generation, an electrical generator is a device that
converts the mechanical kinetic energy from the spinning shaft of the
steam turbine into electrical energy using electromagnetic induction.
The generators must rotate at constant synchronous speeds according
to the frequency of the electric power system. Some sets rotate at
1 500 rpm and have a four-pole generator. Modern generators in
thermal power plants typically produce 500 MW – 600 MW of power.
See Fig. 1.13.
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Did you know?
The Medupi power station
in Limpopo province will
have a capacity of between
4 200 MW and 4 500 MW
by the time its last unit is
commissioned in January
2015. The Medupi power
station uses supercritical
boilers operating at higher
temperatures and pressures
than the older-generation
boilers. It also employs
direct dry-cooling, making it
Eskom’s fourth dry-cooled
base-load station after
the Kendal, Majuba and
Matimba power stations.
Fig. 1.13: Generators used in a power station
Condensers and the cooling system
The condensers and cooling systems condense the exhaust steam from
the steam turbine and transfer it away from the power station.
Condensers
The function of the condenser is to condense exhaust steam from the
steam turbine. The condenser operates on the principle of heat transfer
where the heat in the steam is transferred to the cold water in the tubes
over which the steam is passed. Alternatively, the steam can be inside the
tubes and the cooling water is passed over the tubes. When the steam
loses its heat, it turns into water droplets. These fall to the bottom of the
condenser where water collects and is passed on to the next process.
The cooling water, on the other hand, exits the condenser at a higher
temperature. Fig. 1.14 illustrates the basic operation of a condenser.
Steam in
Cold water in from
cooling tower
Hot water out
(to cooling tower)
Water (condensate) out
Fig. 1.14: The basic operation of a condenser
Condensers in which the cooling water flows inside tubes and the
steam flows over the tubes are called shell-and-tube heat exchangers.
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