Power Systems Protection, Power Quality,
Substation Automation
The Engineers Pocket Guide
Electrical
Foreword
FOAX training series specializes in providing high quality state-of-the-art technical
training workshops to engineers, technicians and domain specialists throughout the country.
Many engineers have attended FOAX's workshops over the past 10 years. The tremendous
success of the technical training workshops is based in part on the enormous investment
FOAX puts into constant review and updating of the workshops, an unwavering commitment
to the highest quality standards and most importantly - enthusiastic, experienced FOAX
engineers who present the workshops and keep up-to-date with consultancy work.
The objective of this booklet is to provide today's engineer with useful technical
information and as an aide-memoir when you need to refresh your memory. This edition of
the Pocket Guide Series has been updated to include new information including Power
Systems Protection, Power Quality, Substation Automation. Concepts that are important and
useful to the engineers, technicians and students, independent of discipline, are covered in
this useful booklet.
The IDC Engineers Pocket Guide
Contents
Chapter 1 - Power Quality
Basic Definitions .................................................................................1
Surges and Transients ..........................................................................3
Harmonics and Distortions ..................................................................4
Interruptions ........................................................................................5
Noise Disturbances ..............................................................................6
Notching..............................................................................................7
Noise Definitions.................................................................................7
Recommended Design and Installation Practices .................................8
Zero Signal Reference Grid ...............................................................11
Chapter 2 - Electrical Protection for Power Systems
The Need for Electrical Protection.....................................................14
Protective Relays ...............................................................................14
The Basic Requirements of Protection ...............................................14
Electrical Faults.................................................................................15
Transient and Permanent Faults .........................................................16
Calculation of Short Circuit Currents.................................................16
Fuses .................................................................................................17
The Relay - Circuit Breaker Combination..........................................17
Circuit Breaker Tripping Times .........................................................21
Instrument Transformers ....................................................................21
Current Transformer (CT) Magnetization Curve ................................21
Knee-Point Voltage ............................................................................22
Metering CTs.....................................................................................22
Protection CTs ...................................................................................23
Open Circuiting of CTs......................................................................23
CT Specification ................................................................................23
Special (Class X) Current Transformers.............................................23
Voltage Transformers .........................................................................24
IDMT Relays .....................................................................................25
Why IDMT? ......................................................................................28
Power Systems Protection, Power Quality, Substation Automation
Chapter 3 - Substation Automation
Definition of the Term .......................................................................30
What is Substation Automation..........................................................31
Electrical Protection ..........................................................................32
Control ..............................................................................................32
Measurement .....................................................................................33
Monitoring.........................................................................................33
Data Communication .........................................................................34
Substation Automation Architecture...................................................34
Three Main Divisions ........................................................................35
Communications in Substation Automation .......................................37
Appendices
Appendix A: Glossary of Terms .........................................................39
Appendix B: Units and Abbreviations................................................71
Appendix C: Commonly Used Formulae............................................74
Appendix D: Resistor Color Coding ..................................................82
Who is IDC Technologies
Benefits of Technical Training ...........................................................84
IDC Technologies Approach to Training ............................................84
Technical Training Workshops ...........................................................85
On-site Workshops.............................................................................88
Customized Training..........................................................................89
Locations of Past Workshops .............................................................90
IDC Technologies Worldwide Offices ................................................92
Notes
The IDC Engineers Pocket Guide
Chapter 1
Power Quality
This chapter is broken down into:
• Basic Definitions
• Recommended Design and Installation Practices
• Zero Signal Reference Grid
Basic Definitions
Figure 1.1
Sag
Power Systems Protection, Power Quality, Substation Automation
1
Sag
Momentary:
A temporary RMS reduction in the normal AC voltage, at the power
frequency, for durations from a half-cycle to two seconds.
Sustained:
A RMS reduction in the nominal AC voltage, at the power
frequency, for durations greater than two seconds.
Figure 1.2
Swell
The picture of a momentary and sustained sag (remember that the sagging definition
is a slow, average decrease in voltage) is showing the RMS. This is the reduction
by means of the line to neutral voltage sag as experienced by a BMI type of power
disturbance analyzer. The opposite of sagging, the new terminology now for the
slow, average increase of voltage or temporary increase, is swelling.
The IDC Engineers Pocket Guide
2
Swell
Momentary:
A temporary RMS increase in the normal AC voltage, at the power
frequency, for durations from a half-cycle to two seconds.
Sustained:
A RMS increase in the nominal AC voltage, at the power frequency for
durations greater than two seconds.
Surges and Transients
Figure 1.3
Surge/Transient
What we used to call transients, spikes or impulses are now formally known as
surges. This is the change that took place in the definitions chapter which is
affecting all the IEEE definitions throughout their publications. Figure 1.3 is of a
very abrupt subcycle type of disturbance, a very brief and very steep wave front.
It shows the line to neutral impulse first taking place on a very short millisecond
trace at the bottom of the page and then taking place on a microsecond per division
so that the centre of the bottom trace is now expanded to 128 microseconds from
left to right on the upper trace. Now we get to see what that actual peak is,
something far different from what we see on the bottom where we notice that
there is somewhere in the neighbourhood of 600 volts of impulse now known by
the term surge.
Power Systems Protection, Power Quality, Substation Automation
3
Surge (Transient/Impulse)
A sub-cycle voltage wave in an electric circuit, which is evidenced by a sharp,
brief disturbance of the input-power voltage waveform. The duration is less than
a half-cycle of the normal voltage waveform and generally less than a ms. The term
derives from the appearance of the abrupt disturbance of the normal voltage
waveform. Often it is oscillatory-decaying. Surges are often characterized by
excessive voltage.
The IDC Engineers Pocket Guide
4
Harmonics and Distortions
Figure 1.4
Harmonics/Distortion Factor
Harmonics
The mathematical representation of the distortion of the pure sine wave. Frequency
of these harmonics are obtained by multiplying the harmonic number by the
Fundamental Frequency (50/60 Hz).
Distortion Factor
The ratio of the root-mean-square of the total harmonic content to the root-mean-
square value of the fundamental quantity, expressed as a percentage.
Interruptions
Figure 1.5
Interruption - Momentary/Sustained
Figure 1.5 speaks for itself. Obviously, the momentary loss for something having
a half cycle up to two seconds worth of duration and then the definition of having
a complete loss would be that power interruption that takes place over a two
second long period.
Power Systems Protection, Power Quality, Substation Automation
5
Interruption (Outage)
Momentary:
The complete loss of voltage for a time period between a half-cycle and
two seconds.
Sustained:
The complete loss of voltage for a time period greater than two seconds.
The IDC Engineers Pocket Guide
6
Noise Disturbances
Figure 1.6
Noise
Noise
Electrical noise is unwanted electrical signals, which produce undesirable effects in
the circuits of the sensitive electronic equipment in which they occur.
Power Systems Protection, Power Quality, Substation Automation
7
Electrical noise is shown in a variety of ways by the graphs in Figure 1.6. The
undesirable effect on the circuits that serve sensitive electronic equipment are
shown here. These are neutral to ground impulses, as they used to be called; but
which we now call a transient surge.
Notching
Figure 1.7
Notch
Our next definition is that for an electrical notch, which is a switching or other
type of disturbance in the voltage wave form, having a particular duration and
having a particular polarity and depth. The picture, as we will see, is a look at the
harmonics influence of particularly the influence of the silicon control rectifiers
as they switch from one step to another where two phases appear to go to 0 V and
create a depression of the voltage wave shape; sometimes even the complete loss
for up to a half cycle.
Noise Definitions
Figure 1.8
Noise - Common Mode/Transverse Mode
Notch
A switching (or other) disturbance of the normal power voltage waveform, lasting less
than a half-cycle; which is initially of opposite polarity to the waveform, and is thus
subtractive from the normal waveform in terms of the peak value of the disturbance
voltage. This includes complete loss of voltage for up to a half-cycle. See: Surge.
Noise, Common Mode
The noise voltage which appears equally and in phase from each signal conductor
and ground.
Noise, Transverse Mode
(With reference to a load device input AC power). Noise signals measurable between
or among active circuit conductors feeding the subject load, but not between the
equipment grounding conductor or associated signal reference structure and the
active circuit conductors.
The IDC Engineers Pocket Guide
8
In our electrical noise definitions we have the definition for common mode noise
and the definition for transverse mode noise. The common mode noise is more of
what we might call the mystery noise circulating in the building where the noise
appears between the wires, either the neutral wire or the power phase wire and
ground reference.
This noise is not visible between the phase wires themselves or phase to neutral
conditions as we will see later on in our explanations. But rather noise that is
seeking to find the common conductor, the ground or earthing conductor that we
would have in our wiring. By complement, the transverse noise is the other type
of noise. Noise that is not so much a mystery. Noise that we will see occurring in
the various types of disturbances from line to line or line to neutral as it is wired
in the power circuit. The transverse or the normal mode noise we can see with line
to line measurements. The common mode noise is always going to have to be
examined with respect to the earthing conductor or the earth ground.
Recommended Design and Installation Practices
Figure 1.9
Four Wiring Diagrams
Power Systems Protection, Power Quality, Substation Automation
9
"Worst" Condition
Figure 1.9 takes a look at four conditions of general wiring. On the left hand side,
the picture labelled "The Worst Condition" is where we have taken a single feeder
from a power source. We have taken it to a remote location where there are
sensitive units as well as noise producing units, such as air conditioning
compressors. We have wired it to one single panel board and now we have the
noise producing equipment on the same electrical bus with the noise
sensitive equipment.
"Fair" - Slightly Better
In Figure 1.9 we have put in two panel boards on that same singular feeder separating
the two types of loads by a little bit of wire. It is a fair improvement, but certainly
not the only measure that should be taken.
"Better" - Gets the Job Done
In the picture labelled "Better" what we have done is introduced the use of the
two-winding transformer to separate the sensitive equipment from a primary bus
which has noise producing equipment on it. This transformer has the low internal
impedance with the ability to act as a buffer for noise which is coming on the
primary side. The buffer does not permit it to pass through the flexible coupling
of the primary to the secondary side of the transformer.
"Best"- May not be Available
The picture on the far right is certainly the ultimate in design and application. It
involves having multiple feeds, transfer capability and the transformer
downstream of it all. It may not be possible due to electrical restrictions in a given
area, financial considerations or due to space considerations to engage in the best
of practices. But one thing we can concentrate on is the use of the two-winding
transformer to help us with the wiring and grounding interfacing that we need
between the power source and the load.
The IDC Engineers Pocket Guide
10
Figure 1.10
Poor and Better - Two Transformer Installation Diagrams
Power Systems Protection, Power Quality, Substation Automation
11
Transformer Location
In Figure 1.10, we have the "Poor to Better" comparison of the use of the isolating
transformer and some understanding of why it works so well in the way that it is
laid out. You will notice in the upper example on this drawing the poor application
is where the transformer is located at a considerable distance from the sensitive
equipment. In this particular case the equipment is shown as being a computer
powered by its own circuit breaker panel in a computer room location. The same
application would hold true if it were a computer on a process floor, a computer
that was running a telephone system, a network system of personnel computers or
any other type of digital logic device. The reason why this application is poor is
explained in the paragraph on the figure. The transformer with its grounding point
is at a different location from the grounding point of the sensitive equipment in
the upper picture.
Zero Signal Reference Grid (ZSRG)
Figure 1.11
Data/Communication Signal Referencing
The IDC Engineers Pocket Guide
12
When two of more electronic areas are to be interconnected via metallic signal
paths within the same building, the two areas should be separately constructed on
their own ZSRG structures and the ZSRG structures should then be interfaced to
one another via the use of metallic signal transport ground-plane constructions as
shown typically in Figure 1.11.
The signal transport ground-plane is normally constructed of copper foil of about
0.010" thickness or #22 GA galvanized steel strip, and extends at least several
inches to either side of the cable(s) laid directly upon it. The foil is often laid
into a galvanized steel cable tray and is bonded to it frequently. This form of
construction is most suitable where the cables must transit areas within a building
which are separated in such a way as to make laying the foil directly upon the
floor-slab impractical. This cable tray technique is especially recommended for
cables that must be run either vertically or horizontally in such a manner as to
place strain on the cables themselves. The tray eliminates this problem.
Cables installed upon a signal transport ground-plane are required to be laid as
close to the surface of the ground-plane as is practical, so as to reduce open-loop
coupling areas and to allow the electrical fields between the ground-plane and the
cables to have maximum coupling via short paths. This practice significantly
improves the performance of the resultant installation and provides greater immunity
to externally coupled "noise" current electrical fields.
The popular practice of running metallic cables directly between electronics areas in
buildings is never a good practice as it always involves problems with common-mode
voltages and currents which are always to be expected. Designers should strive to
eliminate the practice of directly interconnected equipment wherever possible in
these cases. The use of fiber optics is clearly suggested as a better method than
using metallic cables between such areas.
In the event that "noise" problems persist in a given cable(s), it is strongly
suggested that a Balun transformer be used at each end of the subject cable as a
means of increasing the common-mode current path's impedance (more
impedance = less current) without affecting the normal-mode signals contained
within the cable. In some cases additional cable shielding may be warranted. In
some cases a different form of cable driver/receiver design may be indicated. In
no case, however, should the grounding system be altered as a means of trying to
reduce "noise" problems if the modification either causes violation of the
Power Systems Protection, Power Quality, Substation Automation
13
applicable National Electrical Code requirements or of the UL listing (safety)
requirements of the subject equipment. Equipment must be either made to operate
on the applicable National Electrical Code of Safety Standard acceptable
wiring/grounding or it should be replaced with equipment that will operate in such
an environment without the creation of safety hazards.
We notice that in data communications cable signal transport between two
processing units, these two areas can be interconnected by means of one form or
another of a continuous ground plane that we will call the zero signal reference
grid (ZSRG). The process described here is to provide as close a coupling
between the signal transport system and this common zero reference grid in order
to avoid and provide immunity to externally coupled noise and electrical fields.
When this is done the open loop coupling areas are made very small and the
electrical fields between the ground plane and the cables will have maximum
coupling via short paths and thus offer the greatest protection to the signal circuit.
The IDC Engineers Pocket Guide
14
Chapter 2
Electrical Protection for Power Systems
The Need for Electrical Protection
It is not economically feasible to design and manufacture electrical equipment
that will never fail in service. Equipment will and does fail, and the only way to
limit further damage to equipment, and to restrict danger to human life, is to provide
fast, reliable electrical protection. The protection of a power system detects
abnormal conditions, localizes faults, and promptly removes the faulty equipment
from service.
Protective Relays
A PROTECTIVE RELAY is the device, which operates to disconnect a faulty part
of the system, thereby protecting the remainder of the system from further damage.
In fact, power protection has the following five main functions as its levels of
discipline and functionality, shown in order of priority.
• To ensure safety of personnel
• To safeguard the entire system
• To ensure continuity of supply
• To minimize damage
• To reduce resultant repair costs
All of these requirements make it necessary to ensure early detection, localization,
and rapid isolation of electrical faults and additionally prompt and safe removal
from service of faulty equipment.
The Basic Requirements of Protection
In order to satisfy the above requirements, protection must therefore have the
following qualities:
RELIABILITY :
To operate in the pre-determined manner when an electrical fault is detected.
Power Systems Protection, Power Quality, Substation Automation
15
SELECTIVITY / DISCRIMINATION:
To detect and safely isolate only the faulty item(s).
STABILITY / SECURITY:
To leave all healthy circuits intact and undisturbed and to ensure
continuity of supply.
SENSITIVITY:
To detect even the smallest values of fault current or system abnormalities
and operate correctly at its pre-set settings.
SPEED:
To operate speedily when it is required thereby minimizing damage and
ensuring safety to personnel.
Electrical Faults
Electrical faults usually occur due to breakdown of the insulating media between
live conductors or between a live conductor and earth. This breakdown may be
caused by any one or more of several factors, e.g. mechanical damage, overheating,
voltage surges (caused by lightning or switching), ingress of a conducting medium,
ionization of air, deterioration of the insulating media due to an unfriendly
environment or old age, or misuse of equipment.
Fault currents release an enormous amount of thermal energy, and if not cleared
quickly, may cause fire hazards, extensive damage to equipment and risk to
human life.
Switchgear needs to be rated to withstand and break the worst possible fault
current, which is a solid three-phase short-circuit close to the switchgear. ('Solid'
meaning that there is no arc resistance. Normally arc resistance will be present,
but this value is unpredictable, as it will depend on where exactly the fault occurs,
the actual arcing distance, the properties of the insulating medium at that exact
instance, which will be changing all the time due to the heating effect of the arc,
etc. Therefore, in fault calculations, the arc resistance is ignored, as it is
undeterminable, with the result that the worst case is calculated. The arc
resistance will tend to decrease the fault current.
The IDC Engineers Pocket Guide
16
Transient & Permanent Faults
Transient faults are faults that do not damage the insulation permanently and as
such allow the circuit to be safely re-energized after a short period of time.
A typical example would be an insulator flashover following a lightning strike,
which would be successfully cleared on opening of the circuit breaker, which
could then be automatically reclosed.
Transient faults occur mainly on outdoor equipment where air is the main
insulating medium.
Permanent faults, as the name implies, are the result of permanent damage to the
insulation of either the transmission medium or the associated equipment attached
to it.
Calculation of Short Circuit Currents
Accurate fault current calculations are normally carried out using an analysis
method called "Symmetrical Components." This method involves the use of higher
mathematics and is based on the principal that any unbalanced set of vectors can
be represented by a set of 3 balanced systems, namely; positive, negative and zero
sequence vectors.
However, for general practical purposes it is possible to achieve a good
approximation of 3 phase short circuit currents using some very simple methods,
which are discussed below.
The short circuit current at the secondary side and close to the transformer, can be
quickly calculated by using the following formula:
short-circuit MVA =
and Short - circuit current
where
P = Transformer rating in MVA
X% = Internal Reactance of Transformer in %
I
kA
= Short-circuit current in kA
kV = Transformer secondary voltage in kV
100P
X%
Power Systems Protection, Power Quality, Substation Automation
17
TYPICAL % reactance values for transformers (X%) are shown in the table below.
Table 2.1
Reactance Values
Normally, the % reactance value of the transformer can be obtained from the
nameplate, or if not, from the transformer data sheets.
If a length of cable (more than 100m) exist between the transformer and the fault,
the impedance of the cable has to be taken into account to arrive at a realistic
value for the worst-case fault current. This is done by calculating the source
impedance and then adding the cable impedance, as follows:
Source Impedance
Fault Current kA = kV
(Z
source
+ Z
cable
)
Z
cable
can be obtained from the manufacturer's cable data sheets.
The above calculation is another approximation, as Z
source
and Z
cable
are not
necessarily in phase, and complex algebra should be used. However, it is accurate
enough for most practical applications.
Fuses
Probably the oldest, simplest, cheapest and most-often used type of protection
device is the fuse. The operation of a fuse is very straightforward: The thermal
energy of the excessive current causes the fuse-element to melt and the current
Primary Voltage Reactance % at MVA rating
MVA Rating Up to 13.8kV 25kV 36kV 69kV 138kV
0.25 3.5 4.0 4.5 5.0 6.5
0.5 4.0 4.5 5.0 5.5 6.5
1.0 5.0 5.5 5.5 6.0 7.0
2.0 5.5 6.0 6.0 6.5 7.5
3.0 6.5 6.5 6.5 7.0 8.0
5.0 7.5 7.5 7.5 8.0 8.0
>=10 10.0 10.0 10.0 10.0 10.0
The IDC Engineers Pocket Guide
18
path is interrupted. Technological developments have served to make fuses more
predictable, faster and safer (not to explode).
A common misconception about a fuse, is that it will blow as soon as the current
exceeds its rated value (i.e. the value stamped on the cartridge). This is far from
the truth. A fuse has a typical inverse time-current characteristic, much the same
as an IDMT relay. The pick-up value only starts at approximately twice the rated
value, and the higher the current, the faster the fuse will blow.
By nature, fuses can only detect faults associated with excess current. Therefore,
a fuse will only blow in earth fault conditions once the current in the faulty phase
has increased beyond the overcurrent value. Therefore, fuses do not offer
adequate earth fault protection. A fuse has only a single time-current characteristic,
and cannot be adjusted. In addition, fuses need to be replaced after every
operation. Finally, fuses cannot be given an external command to trip.
Fuses are very inexpensive. Therefore, they are suitable to use on less critical
circuits and as back-up protection should the main protection fail, offering very
reliable current-limiting features by nature.
Another advantage of fuses is the fact that they can operate totally independently,
i.e. they do not need a relay with instrument transformers to tell them when to
blow. This makes them especially suitable in applications like remote Ring Main
Units, etc.
The Relay - Circuit-Breaker Combination
The most versatile and sophisticated type of protection available today, is
undoubtedly the relay - circuit-breaker combination. The relay receives information
regarding the network mainly from the instrument transformers (voltage and current
transformers), detects an abnormal condition by comparing this information to
pre-set values, and gives a tripping command to the circuit-breaker when such an
abnormal condition has been detected. The relay may also be operated by an
external tripping signal, either from other instruments, from a SCADA master, or
by human intervention.
The most reliable way to provide auxiliary power to the relay is by way of a
Battery Tripping Unit (BTU). The unit basically consists of a set of batteries
which supplies DC power to the relay and trip circuit. The batteries are kept under
Power Systems Protection, Power Quality, Substation Automation
19
charge by a battery charger, which normally is rated to have enough capacity to
supply the standing load of the switchgear panel (relay auxiliary power, indication
lamps, etc.). When a temporary high current is needed, usually to provide a
circuit-breaker tripping supply, the batteries will supply this, and be recharged
after the event. The batteries then also function as a full back up in case of total
power failure.
The AC input to the BTU is usually supplied from the panel VT, or from a
lighting transformer.
What often occurs in practice, and which is very bad engineering practice, is to
power the relay and trip circuit directly from the panel VT. This will function
correctly in most instances, but when a really severe three-phase short-circuit
occurs, the voltage of the substation may drop quite dramatically, causing
malfunction of the tripping circuit.
There are other ways to overcome this, like capacitive tripping circuits, and AC
series tripping schemes, but each has its own disadvantages, and none are as
reliable as the DC shunt tripping arrangement.
The circuit breaker opens its main contacts when the tripping signal has been
received, interrupting the current.
From the protection point of view the important parts of the circuit breaker are the
trip coil, latching mechanism, main contacts and auxiliary contacts.
Circuit breakers are normally fitted with a number of auxiliary contacts, which are
used, as needed, in a variety of ways in control and protection circuits
Initially, circuit breakers used air as the insulating medium, later insulating oil
(the oil also acting as a cooling medium), and nowadays vacuum or SF6 (sulphur-
hexafluoride) gas.
The connection between the relay and the circuit-breaker trip coil is purely
electrical. This used to be one possible weak link in the trip circuit. One popular
method to increase the reliability of the trip circuit for critical substations is to
provide a full back-up trip supply. A back-up trip coil is installed in the circuit
breaker, with back-up protection, powered by a second, independent BTU.
The IDC Engineers Pocket Guide
20
Figure 2.1 illustrates a typical arrangement.
Figure 2.1
Arrangement of D. C. Supplies with Two Trip Coils for Each Circuit Breaker
Often a second set of contacts of the same relay is used for the back-up protection,
which defeats the purpose of having full redundancy somewhat, as the relay itself
then forms the weakest link.
A second method of increasing the reliability of the trip circuit, is to incorporate a
trip circuit supervision relay, which continually monitors the continuity of the trip
circuit, and activates an alarm when an unhealthy trip circuit is detected. One
shortcoming of this method is that it cannot monitor the main protection relay itself.
Modern relays now have advanced self-monitoring and trip circuit supervision
functionality, activating an alarm when it detects a fault within itself or in the trip
circuit, increasing reliability of the complete trip circuit tremendously.
Power Systems Protection, Power Quality, Substation Automation
21
Circuit Breaker Tripping Times
The first characteristic is referred to as the "TRIPPING TIME" and is expressed
in cycles.
Modern high-speed circuit breakers have tripping times between 3 and 8 cycles.
The TRIPPING, TOTAL BREAK TIME is made up as follows:
• (Tripping Time) Opening Time:
This represents the time between the instant of application of tripping power
to the instant of mechanical separation of the main contacts.
• (Tripping Time) Arcing Time:
The time between the instant of mechanical separation of the main circuit
breaker contacts to the instant of arc extinction
The sum of the above: Opening Time + Arcing Time = Breaking Time
Instrument Transformers
The three main tasks of instrument transformers are
• To transform currents or voltages from a usually high value to a value easy to
handle for relays and instruments.
• To insulate the relays, metering and instruments from the primary high
voltage system.
• To provide possibilities of standardizing the relays and instruments etc. to a
few rated currents and voltages.
Current Transformer (CT) Magnetization Curve
This curve is the best method of determining Current Transformer (CT) performance.
It is a graph of the amount of magnetizing current required to generate an
open-circuit voltage at the terminals of the unit. Due to the non-linearity of the
core iron, it follows the B-H loop characteristic and comprises three regions,
namely the initial region, unsaturated region and saturated region.
The IDC Engineers Pocket Guide
22
Figure 2.2
Typical CT Magnetization Curve.
Knee-Point Voltage
The transition from the unsaturated to the saturated region of the open circuit
excitation characteristic is a rather gradual process in most core materials. It is
difficult to define this transition and use is made of the so-called "knee-point"
voltage for this purpose.
It is generally defined as the voltage at which a further 10% increase in volts will
require a 50% increase in excitation current. For most applications, it means that
current transformers can be considered as approximately linear up to this point.
Metering CTs
Instruments and meters are required to work accurately up to full load current, but
above this it is advantageous to saturate to protect the instruments under fault
conditions. They therefore have a very sharp knee-point and a special nickel-alloy
metal is used, having a very low magnetizing current, in order to achieve the accuracy.
Power Systems Protection, Power Quality, Substation Automation
23
Protection CTs
Protective gear, on the other hand, is concerned with a wide range of currents
from fault settings to maximum fault currents many times normal rating. Larger
errors may be permitted and it is important to ensure that saturation is avoided
wherever possible to ensure positive operation of the relays.
Open Circuiting of CTs
Current transformers generally work at a low flux density. The core is usually
made of very good metal to give a small magnetizing current. When it is open circuit
the secondary impedance now becomes infinite and the core deeply saturates.
-*- Flashover will then occur -*-
NEVER OPEN-CIRCUIT A C.T. ON LOAD!
As all of the primary current now becomes magnetizing current.
As the ac wave then moves from positive half cycle to the negative half cycle, the
rate of change of flux
dφ
/
dt
is so great that very high voltages are induced in the
secondary winding.
CT Specification
A current transformer is normally specified in terms of
• A rated burden at rated current.
• An accuracy class.
• An upper limit beyond which accuracy is not guaranteed.
(Known as the Accuracy Limit Factor, ALF).
Special (Class X) Current Transformers
These are normally specified for special purpose applications such as differential
protection, where it is important that CTs have matching characteristics.
The IDC Engineers Pocket Guide
24
For this type of CT an exact point on the Magnetization Curve is specified, e.g.
• Rated primary current
• Turns ratio
• Rated knee point e.m.f. at maximum secondary turns
• Maximum exciting current at rated knee point e.m.f.
• Maximum resistance of secondary winding.
In addition, the error in the turn's ratio shall not exceed +/- 0.25%.
Voltage Transformers
There are two types of voltage transformer used for protection equipment, the
purely electro-magnetic type (commonly referred to as a VT) and the Capacitor
type (referred to as a CVT).
Magnetic Voltage Transformer
The magnetic voltage transformer is similar to a power transformer and differs
only in so far as a different emphasis is placed on cooling, insulating and
mechanical aspects.
The primary winding has a large number of turns and is connected across the line
voltage either line-to-line or line-to-neutral.
The secondary has fewer turns, consequently as the volts per turn remains
constant, then less voltage and higher currents are obtained.
Output burdens of 500 VA per phase are common.
Capacitive Transformer
The capacitor VT is more commonly used on high voltage networks. The capacitor
allows the injection of a high frequency signal onto the power line conductors to
provide end-to-end communications between substations for distance relays
telemetry/supervisory and voice communications.
Power Systems Protection, Power Quality, Substation Automation
25
IDMT Relays
Figure 2.3
IDMT Relays
It can be seen that the operating time of an IDMT relay is inversely proportional to
a function of current, i.e. it has a long operating time at low multiples of setting
current and a relatively short operating time at high multiples of setting current.
Two adjustments are possible on the relay, namely:
Current (Tap) Setting
This setting determines the level of current at which the relay will pick-up or start.
Increasing this value will move the IDMT curve to the right of the graph.
The IDC Engineers Pocket Guide
26
Time Dial Setting
This setting speeds up the tripping time of the relay, and has the effect of moving
the inverse curve down the axis as follows:
Figure 2.4
Time/Current Characteristic.
Power Systems Protection, Power Quality, Substation Automation
27
Other characteristic curves are also available, namely VERY INVERSE and
EXTREMELY INVERSE. The time curves are shown as follows:
Figure 2.5
Example of Different Inverse Curves
The IDC Engineers Pocket Guide
28
Why IDMT?
To achieve selectivity and co-ordination by time grading two philosophies are
available, namely:
• Definite Time Lag (DTL), or
• Inverse Definite Minimum Time (IDMT)
For the first option, the relays are graded using a definite time interval of
approximately 0.5 seconds. The relay A at the extremity of the network is set to
operate in the fastest possible time, whilst its upstream relay B is set 0.5 seconds
higher. Relay operating times increase sequentially at 0.5-second intervals on
each section moving back towards the source as shown in Figure 2.6.
Figure 2.6
Definite Time Philosophy.
The problem with this philosophy is the closer the fault to the source, the higher
the fault current, the slower the clearing time - exactly the opposite to what we
should be trying to achieve!
On the other hand, inverse curves as shown in Figure 2.7 operate faster at higher
fault currents and slower at the lower fault currents, thereby offering us the features
that we desire.
Power Systems Protection, Power Quality, Substation Automation
29
Figure 2.7
Inverse Definite Minimum Time.
This explains why the IDMT philosophy has become standard practice throughout
many countries over the years.
The IDC Engineers Pocket Guide
30
Chapter 3
Substation Automation
Definition of the Term
Substation Automation can be defined as a system for managing, controlling and
protecting a power system. This is accomplished by obtaining real-time information
from the system, having powerful local and remote control applications and
advanced electrical protection. The core ingredients of a Substation Automation
system are local intelligence, data communications and supervisory control
and monitoring.
The term Substation Automation is actually too restrictive and may be misleading.
It is too restrictive in the sense that it refers specifically to a substation only.
However, the concepts encompassed in the definition have a much wider application
than being limited only to substations. It is applicable to electrical power
networks at large, from High Voltage transmission networks, to Medium Voltage
distribution networks, to Low Voltage reticulation networks.
The term may be misleading in that automation usually refers to some type of
process automation, whereas the concepts involved in Substation Automation are
quite unique and far removed from process automation, although there is some
common ground in the underlying principles.
The term Substation Automation evolved due to the fact that most of the equipment
that forms the core of such a system, is located in an electrical substation or
switchroom, and these modern, intelligent devices ensure that the need for human
presence or intervention in a substation is limited. The components of a
Substation Automation system aim to protect, monitor and control a typical
electrical substation.
Therefore the term Substation Automation is probably as descriptive as any other,
and due to the fact that it already became virtually an international accepted term
for the multitude of concepts involved, it is the term that will be used throughout
the text.
Power Systems Protection, Power Quality, Substation Automation
31
(Note: The term "substation" will be used throughout the text to describe mainly
a building housing electrical switchgear, but it may also include switchgear
housed in some sort of enclosure, for example a stand-alone Ring Main Unit, etc.)
What is Substation Automation?
Substation Automation may be best described by referring to Figure 3.1.
Figure 3.1
Functional Structure of Substation Automation
Substation Automation, by definition, consists of the following main components:
• Electrical Protection
• Control
• Measurement
• Monitoring
• Data Communications
The IDC Engineers Pocket Guide
32
Electrical Protection
Electrical Protection is still one of the most important components of any electrical
switchgear panel, in order to protect the equipment and personnel, and to limit
damage in case of an electrical fault.
Electrical protection is a local function, and should be able to function independently
of the Substation Automation system if necessary, although it is an integral part
of Substation Automation under normal conditions. The functions of electrical
protection should never be compromized or restricted in any Substation
Automation system.
Control
Control includes local and remote control. Local control consists of actions the
control device can logically take by itself, for example bay interlocking, switching
sequences and synchronising check. Human intervention is limited and the risk of
human error is greatly reduced.
Local control should also continue to function even without the support of the rest
of the Substation Automation system.
Commands can be given directly to the remote controlled devices, for example
open or close a circuit breaker. Relay settings can be changed via the system, and
requests for certain information can be initiated form the SCADA station(s). This
eliminates the need for personnel to go to the substation to perform switching
operations, and switching actions can be performed much faster, which is a
tremendous advantage in emergency situations.
A safer working environment is created for personnel, and huge production losses
may be prevented. In addition, the operator or engineer at the SCADA terminal
has a holistic overview of what is happening in the power network throughout the
plant or factory, improving the quality of decision-making.
Power Systems Protection, Power Quality, Substation Automation
33
Measurement
A wealth of real-time information about a substation or switchgear panel is
collected, which are typically displayed in a central control room and/or stored in
a central database. Measurement consists of:
• Electrical measurements (including metering) - voltages, currents, power,
power factor, harmonics, etc.
• Other analog measurements, eg. transformer and motor temperatures
• Disturbance recordings for fault analyzes
This makes it unnecessary for personnel to go to a substation to collect information,
again creating a safer work environment and cutting down on personnel workloads.
The huge amount of real-time information collected can assist tremendously in
doing network studies like load flow analyzes, planning ahead and preventing
major disturbances in the power network, causing huge production losses.
Note:
The term 'measurement' is normally used in the electrical environment to refer
to voltage, current and frequency, while 'metering' is used to refer to power,
reactive power, and energy (kWh). The different terms originated due to the
fact that very different instruments were historically used for measurement
and metering. Nowadays the two functions are integrated in modern devices,
with no real distinction between them, hence the terms 'measurement' and
'metering' are used interchangeably in the text. Accurate metering for billing
purposes is still performed by dedicated instruments.
Monitoring
• Sequence-of-Event Recordings
• Status and condition monitoring, including maintenance information, relay
settings, etc.
This information can assist in fault analyzes, determining what happened when,
where and in what sequence. This can be used effectively to improve the efficiency
of the power system and the protection. Preventative maintenance procedures can
be utilized by the condition monitoring information obtained.
The IDC Engineers Pocket Guide
34
Data Communication
Data communication forms the core of any Substation Automation system, and is
virtually the glue that holds the system together. Without communications, the
functions of the electrical protection and local control will continue, and the local
device may store some data, but there can be no complete Substation Automation
system functioning. The form of communications will depend on the
architecture used, and the architecture may, in turn, depend on the form of
communication chosen.
Substation Automation Architecture
Different architectures exist today to implement the components of Substation
Automation in practice. It is important to realize that not one single layout can
exclusively illustrate a Substation Automation system. However, the most
advanced systems today are developing more and more towards a common basic
architecture. This architecture is illustrated in Figure 3.2.
Figure 3.2
Basic Architecture of Substation Automation
Power Systems Protection, Power Quality, Substation Automation
35
The modern system consists of three main divisions:
Object Division
The object division of the Substation Automation system consists of Intelligent
Electronic Devices (IEDs), modern, 3rd generation microprocessor based relays
and/or Remote Terminal Units (RTUs). (PLCs also continue to play an important
role in some systems.) They receive analog inputs from the Current Transformers
(CTs), Voltage Transformers (VTs) and transducers in the various switchgear
panels, as well as digital inputs from auxiliary contacts, other field devices or
IEDs, or the SCADA Master. They are able to perform complex logical and
mathematical calculations and provide an output either to the SCADA Master,
other field instruments or IEDs, or back to the switchgear to perform some
command, for example open a circuit breaker.
The component division consists of the process level (field information from CTs,
VTs, etc) and the bay level (local intelligence in the form of IEDs, RTUs, etc).
The Communications Network
The Communications Network (comms network for short) is virtually the nervous
system of Substation Automation. The comms network ensures that raw data,
processed information and commands are relayed quickly, effectively and error-free
among the various field instruments, IEDs and the SCADA system. The physical
medium will generally be fiber-optic cables in modern networks, although some
copper wiring will still exist between the various devices inside a substation.
The comms network needs to be an 'intelligent' subsystem in its own right to
perform the functions required of it, and is not merely a network of fiber-optic
and copper wiring.
The communication network serves as the interface between the bay level and the
SCADA station level, which might be a SCADA master station in the substation
itself, or remotely in a central control room.
SCADA Master
The SCADA (Supervisory Control And Data Acquisition) master station(s) forms
the virtual brain of the Substation Automation system. The SCADA master
The IDC Engineers Pocket Guide
36
receives data and information from the field, decides what to do with it, stores it
(directly or after some form of processing), and issues requests and/or commands
to the remote devices. Therefore, the SCADA master is effectively in control of
the complete Substation Automation system.
Nowadays, a SCADA master consists simply of an advanced, reliable PC
or workstation (with its peripheral and support hardware) and a SCADA
software package.
A SCADA master station may be installed in each substation of a power
transmission network (station level), with all the substation SCADA stations
forming part of a LAN or WAN (network level); or one SCADA master station
may be directly in control of several substations, eliminating the station level.
Communication Protocols used in Substation Automation
Some of the more popular and widely used communication protocols are listed in
table 3.1, with specific reference to protocols used in Substations at present.
Table 3.1
Protocols used in Substations
Protocol Originally Used by Speed Access Principle OSI Layers
MODBUS Gould-Modicon 19.2 kb/s Cyclic Polling 1,2,7
SPABUS ABB (exclusively) 19.2 kb/s Cyclic Polling 1,2,7
DNP3.0 GE Harris 19.2 kb/s Cyclic Polling (+) 1,2,7 (+)
IEC 60870-5 All 19.2 kb/s Cyclic Polling 1,2,7
MODBUS + Gould-Modicon 1.2 Mb/s Token 1,2,7
PROFIBUS Siemens 0.5 Mb/s Token 1,2,7
MVB ABB 1.5 Mb/s TDM 1,2,7 (+)
FIP Merlin-Gerin 2.5 Mb/s TDM 1,2,7
Ethernet + TCP/IP All 10 Mb/s CSMA/CD 1-7
LON ABB (exclusively) 1.25 Mb/s PCSMA/CD 1-7
UCA 2.0 GE Ï 10 Mb/s CSMA/CD 1-7
Power Systems Protection, Power Quality, Substation Automation
37
Communications in Substation Automation
Configuration
The typical Substation Automation configuration is illustrated in Figure 3.3.
Figure 3.3
Typical Substation Automation Structure
The Process level consists of:
• The equipment providing information to the bay level, e.g. instrument
transformers, temperature sensors, auxiliary contacts of circuit breakers, etc.
The application process is therefore a voltage, current, temperature, breaker
status, etc.
• The equipment executing a command from the bay level, e.g. trip coil of
circuit breaker. The application process is then the command 'open breaker'.
The Bay level consists of four main application processes (APs): Protection,
Control, Measurement/Metering, and Monitoring. These APs can reside in different
devices, or all in one device (the typical IED).
The IDC Engineers Pocket Guide
38
The Station level consists of the Station SCADA (optional) and possibly a gateway
or communications processor. The importance of the Station SCADA will depend
on the specific application. In large transmission substations, this will form the
main SCADA for the specific substation, with several SCADA systems forming
a network. On the other hand, for a distribution substation, the Station SCADA
may be dispensed with, and only a gateway will be required to connect the
substation to the network and to the main SCADA.
The network level may consist of a central SCADA, to which each substation is
connected, and/or a LAN, MAN, WAN or the Internet.
Communication Requirements
The communication requirements for the various applications in substation
automation will be evaluated in this section according to the following attributes:
Table 3.2
Communication Requirements
Performance Low Medium High
Speed / Data < 10 kbps > 10 kbps >1 Mbps
throughput < 1 Mbps
Response time > 1 s
1
< 1 s < 10 ms
> 10 ms
Time Synchronisation 1 s 1 ms
+
0.1 ms 1 μs
+
0.5 μs
Avalanche Handling No data through- Some data All data
put required during through-put through-put
avalanche required required
Data Integrity Some errors Limited errors No errors
allowed allowed allowed
Link Availability All data can Some data No data
wait until link can wait for a can wait
available limited time
Data Priority Can be sent To be sent after To be sent
after all other high priority data; immediately;
data can wait for cannot wait
request for request
Power Systems Protection, Power Quality, Substation Automation
39
Appendix A
Glossary of Terms
10BASE2 IEEE802.3 (or Ethernet) implementation on thin coaxial cable
(RG58/AU).
10BASE5 IEEE802.3 (or Ethernet) implementation on thick coaxial cable.
10BASET IEEE802.3 (or Ethernet) implementation on unshielded
22 AWG twisted pair cable.
A/D Conversion Time This is the length of time a board requires to convert an analog signal
into a digital value. The theoretical maximum speed (conversions/
second) is the inverse of this value. See Speed/Typical Throughput.
A/D Analog to Digital conversion.
Absolute Addressing A mode of addressing containing both the instruction and location
(address) of data.
Accuracy Closeness of indicated or displayed value to the ideal
measured value.
ACK Acknowledge (ASCII - control F).
Acknowledge A handshake line or protocol code which is used by the receiving
device to indicate that it has read the transmitted data.
Active Device Device capable of supplying current for a loop.
Active Filter A combination of active circuit devices (usually amplifiers), with
passive circuit elements (resistors and capacitors), which have
characteristics that more closely match ideal filters than do
passive filters.
Actuator Control element or device used to modulate (or vary) a process
parameter.
Address A normally unique designator for location of data or the identity of
a peripheral device which allows each device on a single
communications line to respond to its own message.
Address Register A register that holds the address of a location containing a data item
called for by an instruction.
AFC Automatic Frequency Control. The circuit in a radio receiver that
automatically keeps the carrier frequency centred in the passband of
the filters and demodulators.
AGC Automatic Gain Control. The circuit in a radio that automatically
keeps the carrier gain at the proper level.
The IDC Engineers Pocket Guide
40
Algorithm Can be used as a basis for writing a computer program. This is a set
of rules with a finite number of steps for solving a problem.
Alias Frequency A false lower frequency component that appears in data reconstructed
from original data acquired at an insufficient sampling rate (less
than two times the maximum frequency of the original data).
ALU see Arithmetic Logic Unit.
Amplitude Modulation A modulation technique (also referred to as AM or ASK) used to
allow data to be transmitted across an analog network, such as a
switched telephone network. The amplitude of a single (carrier)
frequency is varied or modulated between two levels; one for binary
0 and one for binary 1.
Analog A continuous real-time phenomenon in which the information values
are represented in a variable and continuous waveform.
Analog Input Board Printed Circuit Board which converts incoming analog signals to
digital values.
ANSI American National Standards Institute. The principle standards
development body in the USA.
Apogee The point in an elliptical orbit that is furtherest from earth.
Appletalk A proprietary computer networking standard initiated by Apple
Computer for use in connecting the Macintosh range of computers
and peripherals (including Laser Writer printers).
This standard operates at 230 kilobits/second.
Application Program A sequence of instructions written to solve a specific problem facing
organisational management.
These programs are normally written in
a high-level language and draw on resources of the operating system
and the computer hardware in executing its tasks.
Application Layer The highest layer of the seven layer ISO/OSI Reference Model
structure, which contains all user or application programs.
Arithmetic Logic Unit The element(s) in a processing system that perform(s) the mathematical
functions such as addition, subtraction, multiplication, division,
inversion, AND, OR, NAND and NOR.
ARP Address Resolution Protocol. A Transmission Control Protocol/
Internet Protocol (TCP/IP) process that maps an IP address to
Ethernet address, required by TCP/IP for use with Ethernet.
ARQ Automatic Request for Transmission. A request by the receiver for
the transmitter to retransmit a block or a frame because of errors
detected in the originally received message.
AS Australian Standard.
Power Systems Protection, Power Quality, Substation Automation
41
ASCII American Standard Code for Information Interchange. A universal
standard for encoding alphanumeric characters into 7 or 8 binary bits.
Drawn up by ANSI to ensure compatibility between different
computer systems.
ASIC Application Specific Integrated Circuit.
ASK Amplitude Shift Keying. See Amplitude Modulation.
ASN.1 Abstract Syntax Notation One. An abstract syntax used to define
the structure of the protocol data units associated with a particular
protocol entity.
Asynchronous Communications in which characters can be transmitted at an arbitrary,
unsynchronised time, and where the time intervals between transmitted
characters may be of varying lengths.
Communication is controlled by start and stop bits at the beginning
and end of each character.
Attenuation The decrease in signal magnitude or strength between two points.
Attenuator A passive network that decreases the amplitude of a signal (without
introducing any undesirable characteristics to the signals such as
distortion).
AUI CABLE Attachment Unit Interface Cable. Sometimes called the drop cable
to attach terminals to the transceiver unit.
Auto Tracking Antenna A receiving antenna that moves in synchronism with the transmitting
device which is moving (such as a vehicle being telemetered).
Autoranging An autoranging board can be set to monitor the incoming signal and
automatically select an appropriate gain level based on the previous
incoming signals.
AWG American Wire Gauge.
Background Program An application program that can be executed whenever the facilities
of the system are not needed by a higher priority program.
Backplane A panel containing sockets into which circuit boards (such as I/O
cards, memory boards and power supplies) can be plugged.
Balanced Circuit A circuit so arranged that the impressed voltages on each conductor
of the pair are equal in magnitude but opposite in polarity with
respect to a defined reference.
Band Pass Filter A filter that allows only a fixed range of frequencies to pass
through. All other frequencies outside this range (or band) are
sharply reduced in magnitude.
Band Reject A circuit that rejects a defined frequency band of signals while
passing all signals outside this frequency range (both lower than
and higher than).
The IDC Engineers Pocket Guide
42
Bandwidth The range of frequencies available, expressed as the difference
between the highest and lowest frequencies, in hertz (cycles per
second, abbreviated Hz).
Bar Code Symbol An array of rectangular parallel bars and spaces of various widths
designed for the labelling of objects with unique identifications.
A bar code symbol contains a leading quiet zone, a start character,
one or more data characters including, in some cases, a check
character, a stop character, and a trailing quiet zone.
Base Address A memory address that serves as the reference point. All other
points are located by offsetting in relation to the base address.
Base Band Base Band operation is the direct transmission of data over a
transmission medium without the prior modulation on a high
frequency carrier band.
Base Loading An inductance situated near the bottom end of a vertical antenna to
modify the electrical length. This aids in impedance matching.
Baud Unit of signalling speed derived from the number of events per second
(normally bits per second). However, if each event has more than one
bit associated with it, the baud rate and bits per second are not equal.
Baudot Data transmission code in which five bits represent one character.
Sixty-four alphanumeric characters can be represented.
BCC Block Check Character. Error checking scheme with one check
character; a good example being Block Sum Check.
BCD Binary Coded Decimal. A code used for representing decimal digits
in a binary code.
BEL Bell (ASCII for control-G).
BERT/BLERT Bit Error Rate/Block Error Rate Testing. An error checking technique
that compares a received data pattern with a known transmitted data
pattern to determine transmission line quality.
Bifilar Two conducting elements used in parallel (such as two parallel
wires wound on a coil form).
Binary Coded Decimal (BCD) A code used for representing decimal digits in a binary code.
BIOS The basic input/output system for the computer, usually firmware-
based. This program handles the interface with the PC hardware
and isolates the Operating Software (OS) from the low-level activities
of the hardware.
As a result, application software becomes more
independent of the particular specifications of the hardware on which
it runs, and hence more portable.
Power Systems Protection, Power Quality, Substation Automation
43
Bipolar Range / Inputs A signal range that includes both positive and negative values.
Bipolar inputs are designed to accept both positive and negative
voltages. (Example: ±5 V).
Bisynchronous
Transmission See BSC.
Bit Stuffing with A technique used to allow pure binary data to be transmitted on a
Zero Bit Insertion synchronous transmission line. Each message block (frame) is
encapsulated between two flags which are special bit sequences.
Then if the message data contains a possibly similar sequence, an
additional (zero) bit is inserted into the data stream by the sender,
and is subsequently removed by the receiving device. The transmission
method is then said to be data transparent.
BIT (Binary Digit) Derived from "BInary DigiT", a one or zero condition in the
binary system.
Bits & Bytes One bit is one binary digit, either a binary 0 or 1. One byte is the
amount of memory needed to store each character of information
(text or numbers). There are eight bits to one byte (or character),
and there are 1024 bytes to one kilobyte (KB). There are 1024
kilobytes to one megabyte (MB).
Block In block-structured programming languages, a section of programming
languages or a section of program coding treated as a unit.
Block Sum Check This is used for the detection of errors when data is being transmitted.
It comprises a set of binary digits (bits) which are the modulo 2
sum of the individual characters or octets in a frame (block) or
message.
BNC Bayonet type coaxial cable connector.
bps Bits per second. Unit of data transmission rate.
Bridge A device to connect similar sub-networks without its own network
address. Used mostly to reduce the network load.
Broad Band A communications channel that has greater bandwidth than a voice
grade line and is potentially capable of greater transmission rates.
Broadcast A message on a bus intended for all devices which requires
no reply.
BS Backspace (ASCII Control-H).
BS British Standard.
BSC Bisynchronous Transmission. A byte or character oriented
communication protocol that has become the industry standard
(created by IBM). It uses a defined set of control characters for
synchronised transmission of binary coded data between stations in
a data communications system.
The IDC Engineers Pocket Guide
44
Bubble Memory Describes a method of storing data in memory where data is
represented as magnetised spots called magnetic domains that rest
on a thin film of semiconductor material. Normally used in high-
vibration, high-temperature or otherwise harsh industrial environments.
Buffer An intermediate temporary storage device used to compensate for a
difference in data rate and data flow between two device (also
called a spooler for interfacing a computer and a printer).
Burst Mode A high speed data transfer in which the address of the data is sent
followed by back to back data words while a physical signal
is asserted.
Bus A data path shared by many devices, with one or more conductors
for transmitting signals, data or power.
Byte A term referring to eight associated bits of information; sometimes
called a "character".
Cache Memory A fast buffer memory that fits between the CPU and the slower
main memory to speed up CPU requests for data.
Capacitance (mutual) The capacitance between two conductors with all other conductors,
including shield, short circuited to the ground.
Capacitance Storage of electrically separated charges between two plates having
different potentials. The value is proportional to the surface area of
the plates and inversely proportional to the distance between them.
Cascade Two or more electrical circuits in which the output of one is fed
into the input of the next one.
Cassegrain Antenna Parabolic antenna that has a hyperbolic passive reflector situated at
the focus of the parabola.
CCD Charge-Coupled Device (camera).
CCIR Comité Consultatif Internationale des Radiocommunications.
CCITT Consultative Committee International Telegraph and Telephone. An
international association that sets worldwide standards (e.g. V.21,
V.22, V.22bis).
Cellular Polyethylene Expanded or "foam" polyethylene consisting of individual closed
cells suspended in a polyethylene medium.
CGA Color Graphics Adapter. A computer standard utilising digital
signals offering a resolution of 320 by 200 pixels and a palette of
16 colors.
Channel Selector In an FM discriminator the plug-in module which causes the device
to select one of the channels and demodulate the subcarrier to
recover data.
Character Letter, numeral, punctuation, control figure or any other symbol
contained in a message.
Power Systems Protection, Power Quality, Substation Automation
45
Characteristic Impedance The impedance that, when connected to the output terminals of a
transmission line of any length, makes the line appear infinitely
long. The ratio of voltage to current at every point along a transmission
line on which there are no standing waves.
Clock The source of timing signals for sequencing electronic events such
as synchronous data transfer or CPU operation in a PC.
Clock Pulse A rising edge, then a falling edge (in that order) such as applied to
the clock input of an 8254 timer/counter.
Clock The source(s) of timing signals for sequencing electronic events eg
synchronous data transfer.
Closed Loop A signal path that has a forward route for the signal, a feedback
network for the signal and a summing point.
CMRR Common Mode Rejection Ratio - A data acquisition’s board's ability
to measure only the voltage difference between the leads of a
transducer, rejectingwhat the leads have in common. The higher the
CMRR, the better the accuracy.
CMV Common Mode Voltage.
CNR Carrier to Noise Ratio. An indication of the quality of the
modulated signal.
Cold-junction Thermocouple measurements can easily be affected by the interface
Compensation the thermocouples are connected to. Cold-junction compensation
circuitry compensates for inaccuracies introduced in the
conversion process.
Collector The voltage source in a transistor with the base as the control
source and the emitter as the controlled output.
Collision The situation when two or more LAN nodes attempt to transmit at
the same time.
Common Carrier A private data communications utility company that furnishes
communications services to the general public.
Common Mode Signal The common voltage to the two parts of a differential signal applied
to a balanced circuit.
Commutator A device used to effect time-division multiplexing by repetitive
sequential switching.
Compiler A program to convert high-level source code (such as BASIC) to
machine code-executable form, suitable for the CPU.
Composite Link The line or circuit connecting a pair of multiplexers or concentrators;
the circuit carrying multiplexed data.
Composite A video signal that contains all the intensity, color and timing
information necessary for a video product.
The IDC Engineers Pocket Guide
46
Conical Scan Antenna An automatic tracking antenna system in which the beam is steered
in a circular path so that it forms a cone.
Contention The facility provided by the dial network or a data PABX which
allows multiple terminals to compete on a first come, first served
basis for a smaller number of computer ports.
Control System A system in which a series of measured values are used to make a
decision on manipulating various parameters in the system to
achieve a desired value of the original measured values.
Convolution An image enhancement technique in which each pixel is subjected
to a mathematical operation that groups it with its nearest neighbours
and calculates its value accordingly.
Correlator A device which compares two signals and indicates the similarity
between the two signals.
Counter/ Timer Trigger On-board counter/timer circuitry can be set to trigger data acquisition
at a user-selectable rate and for a particular length of time.
Counter Data Register The 8-bit register of an (8254 chip) timer/counter that corresponds
to one of the two bytes in the counter's output latch for read operations
and count register for write operations.
CPU Central Processing Unit.
CR Carriage Return (ASCII control-M).
CRC Cyclic Redundancy Check. An error-checking mechanism using a
polynomial algorithm based on the content of a message frame at
the transmitter and included in a field appended to the frame. At the
receiver, it is then compared with the result of the calculation that is
performed by the receiver. Also referred to as CRC-16.
Cross Talk A situation where a signal from a communications channel interferes
with an associated channel's signals.
Crossed Pinning Wiring configuration that allows two DTE or DCE devices to
communicate. Essentially it involves connecting pin 2 to pin 3 of
the two devices.
Crossover In communications, a conductor which runs through the cable and
connects to a different pin number at each end.
Crosstalk A situation where a signal from a communications channel interferes
with an associated channel's signals.
CSMA/CD Carrier Sense Multiple Access/Collision Detection.
When two devices transmit at the same time on a local area
network,they both cease transmission and signal that a collision has
occurred. Each then tries again after waiting for a random time
period.
Power Systems Protection, Power Quality, Substation Automation
47
Current Sink This is the amount of current the board can supply for digital output
signals. With 10-12 mA or more of current sink capability, a board
can turn relays on and off. Digital I/O boards with less than 10-12 mA
of sink capability are designed for data transfer only, not for hardware
power relay switching.
Current Loop A communication method that allows data to be transmitted over a
longer distance with a higher noise immunity level than with the
standard RS-232C voltage method.
A mark (a binary 1) is represented by current; and a space
(or binary 0) is represented by the absence of current.
Current Inputs A board rated for current inputs can accept and convert analog current
levels directly, without conversion to voltage.
D/A Digital to Analog.
DAS Data Acquisition System.
Data Integrity A performance measure based on the rate of undetected errors.
Data Reduction The process of analysing a large quantity of data in order to extract
some statistical summary of the underlying parameters.
Data Link Layer This corresponds to layer 2 of the ISO Reference Model for open
systems interconnection. It is concerned with the reliable transfer
of data (no residual transmission errors) across the data link
being used.
Data Integrity A performance measure based on the rate of undetected errors.
Datagram A type of service offered on a packet-switched data network. A
datagram is a self contained packet of information that is sent
through the network with minimum protocol overheads.
dBi A unit that is used to represent the gain of an antenna compared to
the gain of an isotropic radiator.
dBm A signal level that is compared to a 1-mW reference.
dBmV A signal amplitude that is compared to a 1-mV reference.
dBW A signal amplitude that is compared to a 1-Watt reference.
DCE Data Communications Equipment. Devices that provide the functions
required to establish, maintain and terminate a data transmission
connection. Normally it refers to a modem.
Decibel A logarithmic measure of the ratio of two signal levels where
dB = 20log
10
V
1
/V
2
. Being a ratio, it has no units of measure.
Decibel (dB) A logarithmic measure of the ratio of two signal levels where
dB = 20log
10
V
1
/V
2
or where dB = 10log
10
P
1
/P
2
and where
V refers to Voltage or P refers to Power. Note that it has no unit
of measure.
The IDC Engineers Pocket Guide
48
Decoder A device that converts a combination of signals into a single signal
representing that combination.
Decommutator Equipment for the demultiplexing of commutated signals.
Default A value or setup condition assigned automatically unless another
is specified.
Delay Distortion Distortion of a signal caused by the frequency components making
up the signal having different propagation velocities across a
transmission medium.
DES Data Encryption Standard.
Deviation A movement away from a required value.
DFB Display Frame Buffer.
Diagnostic Program A utility program used to identify hardware and firmware defects
related to the PC.
Dielectric Constant (E) The ratio of the capacitance using the material in question as the
dielectric, to the capacitance resulting when the material is replaced
by air.
Differential See Number of channels.
Digital A signal which has definite states (normally two).
Digitise The transformation of an analog signal to a digital signal.
DIN Deutsches Institut Fur Normierung.
DIP Acronym for dual in line package referring to integrated circuits
and switches.
Diplexing A device used to allow simultaneous reception or transmission of
two signals on a common antenna.
Direct Memory Access A technique of transferring data between the computer memory and
a device on the computer bus without the intervention of the micro-
processor. Also abbreviated to DMA.
Discriminator Hardware device to demodulate a frequency modulated carrier or
subcarrier to produce analog data.
Dish Antenna An antenna in which a parabolic dish acts a reflector to increase the
gain of the antenna.
Dish Concave antenna reflector for use at VHF or higher frequencies.
Diversity Reception Two or more radio receivers connected to different antennas to
improve signal quality by using two different radio signals to transfer
the information.
DLE Data Link Escape (ASCII character).
DMA Direct Memory Access.
Power Systems Protection, Power Quality, Substation Automation
49
DNA Distributed Network Architecture.
Doppler The change in observed frequency of a signal caused by the emitting
device moving with respect to the observing device.
Downlink The path from a satellite to an earth station.
DPI Dots per Inch.
DPLL Digital Phase Locked Loop.
DR Dynamic Range. The ratio of the full scale range (FSR) of a data
converter to the smallest difference it can resolve. DR = 2
n
where
n is the resolution in bits.
DRAM Dynamic Random Access Memory. See RAM.
Drift A gradual movement away from the defined input/output condition
over a period of time.
Driver Software A program that acts as the interface between a higher level coding
structure and the lower level hardware/firmware component of
a computer.
DSP Digital Signal Processing.
DSR Data Set Ready. An RS-232 modem interface control signal which
indicates that the terminal is ready for transmission.
DTE Data Terminal Equipment. Devices acting as data source, data sink,
or both.
Dual-ported RAM Allows acquired data to be transferred from on-board memory to
the computer's memory while data acquisition is occurring.
Duplex The ability to send and receive data over the same
communications line.
Dynamic Range The difference in decibels between the overload or maximum and
minimum discernible signal level in a system.
EBCDIC Extended Binary Coded Decimal Interchange Code. An 8-bit character
code used primarily in IBM equipment. The code allows for 256
different bit patterns.
EEPROM Electrically Erasable Programmable Read Only Memory. This
memory unit can be erased by applying an electrical signal to the
EEPROM and then reprogrammed.
EGA Enhanced Graphics Adapter. A computer display standard that provides
a resolution of 640 by 350 pixels, a palette of 64 colors, and the
ability to display as many as 16 colors at one time.
EIA Electronic Industries Association. An organisation in the USA
specialising in the electrical and functional characteristics of
interface equipment.
The IDC Engineers Pocket Guide
50
EIA-232-C Interface between DTE and DCE, employing serial binary data
exchange. Typical maximum specifications are 15m at 19200 Baud.
EIA-423 Interface between DTE and DCE, employing the electrical
characteristics of unbalanced voltage digital interface circuits.
EIA-449 General purpose 37 pin and 9 pin interface for DCE and DTE
employing serial binary interchange.
EIA-485 The recommended standard of the EIA that specifies the electrical
characteristics of drivers and receivers for use in balanced digital
multipoint systems.
EIRP Effective Isotropic Radiated Power. The effective power radiated
from a transmitting antenna when an isotropic radiator is used to
determine the gain of the antenna.
EISA Enhanced Industry Standard Architecture.
EMI/RFI Electro-Magnetic Interference or Radio Frequency Interference.
Background 'noise' capable of modifying or destroying
data transmission.
EMS Expanded Memory Specification.
Emulation The imitation of a computer system performed by a combination of
hardware and software that allows programs to run between
incompatible systems.
Enabling The activation of a function of a device by a defined signal.
Encoder A circuit which changes a given signal into a coded combination for
purposes of optimum transmission of the signal.
ENQ Enquiry (ASCII Control-E).
EOT End of Transmission (ASCII Control-D).
EPROM Erasable Programmable Read Only Memory. Non-volatile
semiconductor memory that is erasable in a ultra violet
light and reprogrammable.
Equaliser The device which compensates for the unequal gain characteristic
of the signal received.
Error Rate The ratio of the average number of bits that will be corrupted to the
total number of bits that are transmitted for a data link or system.
Error The difference between the setpoint and the measured value.
ESC Escape (ASCII character).
ESD Electrostatic Discharge.
Ethernet Name of a widely used Local Area Network (LAN), based on the
CSMA/CD bus access method (IEEE 802.3).
ETX End of Text (ASCII control-C).
Power Systems Protection, Power Quality, Substation Automation
51
Even Parity A data verification method normally implemented in hardware in
which each character (and the parity bit) must have an even number
of ON bits.
External Pulse Trigger Many of the A/D boards allow sampling to be triggered by a voltage
pulse from an external source.
Fan In The load placed on a signal line by a logic circuit input.
Fan Out The measure of drive capability of a logic circuit output.
Farad Unit of capacitance whereby a charge of one coulomb produces
a one volt potential difference.
FCC Federal Communications Commission (USA).
FCS Frame Check Sequence. A general term given to the additional bits
appended to a transmitted frame or message by the source to enable
the receiver to detect possible transmission errors.
FDM Frequency Division Multiplexer. A device that divides the available
transmission frequency range in narrower bands, each of which is
used for a separate channel.
Feedback A part of the output signal being fed back to the input of the
amplifier circuit.
Field One half of a video image (frame) consisting of 312.5 lines (for PAL).
There are two fields in a frame. Each is shown alternately every
1/25 of a second (for PAL).
FIFO First in, First Out.
Filled Cable A telephone cable construction in which the cable core is filled with
a material that will prevent moisture from entering or passing along
the cable.
FIP Factory Instrumentation Protocol.
Firmware A computer program or software stored permanently in PROM or
ROM or semi-permanently in EPROM.
Flame Retardancy The ability of a material not to propagate flame once the flame
source is removed.
Floating An electrical circuit that is above the earth potential.
Flow Control The procedure for regulating the flow of data between two devices
preventing the loss of data once a device's buffer has reached
its capacity.
Frame A full video image comprising two fields. A PAL frame has a total
of 625 lines (an NTSC frame has 525 lines).
Frame The unit of information transferred across a data link. Typically,
there are control frames for link management and information
frames for the transfer of message data.
The IDC Engineers Pocket Guide
52
Frame Grabber An image processing peripheral that samples, digitises and stores a
camera frame in computer memory.
Frequency Modulation A modulation technique (abbreviated to FM) used to allow data
to be transmitted across an analog network where the
frequency is varied between two levels - one for binary '0' and
one for binary '1'.
Also known as Frequency Shift Keying (or FSK).
Frequency Refers to the number of cycles per second.
Frequency Domain The displaying of electrical quantities versus frequency.
Fringing The unwanted bordering of an object or character with weak
colors when there should be a clearly delineated edge.
Full Duplex Simultaneous two way independent transmission in both directions
(4 wire). See Duplex.
G Giga (metric system prefix - 10
9
).
Gain of Antenna The difference in signal strengths between a given antenna and a
reference isotropic antenna.
Gain Amplification; applied to an incoming signal, gain acts as a
multiplication factor on the signal, enabling a board to use signals
that would otherwise be too weak.
For example, when set to a gain of 10, a board with a range of +5 V
can use raw input signals as low as +0.5 V (+500 mV); with a gain
of 20, the range extends down to +250 mV.
Gateway A device to connect two different networks which translates the
different protocols.
Genlock This is the process of synchronising one video signal to a master
reference, ensuring that all signals will be compatible or related to
one another.
Geostationary A special earth orbit that allows a satellite to remain in a fixed
position above the equator.
Geosynchronous Any earth orbit in which the time required for one revolution of a
satellite is an integral portion of a sidereal day.
GPIB General Purpose Interface Bus. An interface standard used for parallel
data communication, usually used for controlling electronic
instruments from a computer. Also designated IEEE-488 standard.
Graphics Mode In graphics mode each pixel on a display screen is addressable, and
each pixel has a horizontal (or X) and a vertical (or Y) co-ordinate.
Grey Scale In image processing, the range of available grey levels. In an 8-bit
system, the grey scale contains values from 0 to 255.
Power Systems Protection, Power Quality, Substation Automation
53
Ground An electrically neutral circuit having the same potential as the earth.
A reference point for an electrical system also intended for
safety purposes.
Half Duplex Transmissions in either direction, but not simultaneously.
Half Power Point The point in a Power versus frequency curve which is half the
power level of the peak power (also called the 3dB point).
Hamming Distance A measure of the effectiveness of error checking. The higher the
Hamming Distance (HD) index, the safer is the data transmission.
Handshake Lines Dedicated signals which allow two different devices to exchange
data under asynchronous hardware control.
Handshaking Exchange of predetermined signals between two devices establishing
a connection.
Harmonic An oscillation of a periodic quantity whose frequency is an integral
multiple of the fundamental frequency. The fundamental frequency
and the harmonics together form a Fourier series of the original
wave form.
Harmonic Distortion Distortion caused by the presence of harmonics in the desired signal.
HDLC High Level Data Link Control. The international standard
communication protocol defined by ISO to control the exchange of
data across either a point-to-point data link or a multidrop data link.
Hertz (Hz) A term replacing cycles per second as a unit of frequency.
Hex Hexadecimal.
Hexadecimal Number A base 16 number system commonly used with microprocessor
systems.
HF High Frequency.
High Pass Generally referring to filters which allow signals above a specified
frequency to pass but attenuate signals below this specified frequency.
High-Pass Filter See HPF.
Histogram A graphic representation of a distribution function, such as frequency,
by means of rectangles whose widths represent the intervals into
which the range of observed values is divided and whose heights
represent the number of observations occurring in each interval.
Horn A moderate-gain wide-beamwidth antenna.
Host This is normally a computer belonging to a user that contains
(hosts) the communication hardware and software necessary to
connect the computer to a data communications network.
HPF High-Pass Filter. A filter processing one transmission band that
extends from a cutoff frequency (other than zero) to infinity.
The IDC Engineers Pocket Guide
54
HPIB Hewlett-Packard Interface Bus; trade name used by Hewlett-Packard
for its implementation of the IEEE-488 standard.
I/O Address A method that allows the CPU to distinguish between different
boards in a system. All boards must have different addresses.
IEC International Electrotechnical Commission.
IEE Institution of Electrical Engineers.
IEEE Institute of Electrical and Electronic Engineers. A US-based
international professional society that issues its own standards and,
which is a member of ANSI and ISO.
Illumination Component An amount of source light incident on the object being viewed.
Impedance The total opposition that a circuit offers to the flow of alternating
current or any other varying current at a particular frequency. It is a
combination of resistance R and reactance X, measured in ohms.
Individual Gain A system allowing an individual gain level for each input channel,
per Channel thereby allowing a much wider range of input levels and types without
sacrificing accuracy on low-level signals.
Inductance The property of a circuit or circuit element that opposes a change in
current flow, thus causing current changes to lag behind voltage
changes. It is measured in henrys.
Insulation Resistance (IR) That resistance offered by an insulation to an impressed dc voltage,
tending to produce a leakage current though the insulation.
Interface A shared boundary defined by common physical interconnection
characteristics, signal characteristics and measuring of
interchanged signals.
Interlace This is the display of two fields alternately with one field filling in
the blank lines of the other field so that they interlock. The PAL
standard displays 25 video frames per second.
Interlaced Interlaced - describing the standard television method of raster
scanning, in which the image is the product of two fields, each of
which is a series of successively scanned lines separated by the
equivalent of one line. Thus adjacent lines belong to different fields.
Interrupt An external event indicating that the CPU should suspend its current
task to service a designated activity.
Interrupt Handler The section of the program that performs the necessary operation to
service an interrupt when it occurs.
IP Internet Protocol.
ISA Industry Standard Architecture (for IBM Personal Computers).
ISA Instrument Society of America.
ISB Intrinsically Safe Barrier.
Power Systems Protection, Power Quality, Substation Automation
55
ISDN Integrated Services Digital Network. A fairly recent generation of world-
wide telecommunications networks that utilize digital techniques for
both transmission and switching.
It supports both voice and data communications.
ISO International Standards Organisation.
Isolation Electrical separation of two circuits. For example, optical isolation
allows a high-voltage signal to be transferred to a low-voltage input
without electrical interactions.
Isotropic Antenna A reference antenna that radiates energy in all directions from a
point source.
ISR Interrupt Service Routine. See Interrupt Handler.
ITU International Telecommunications Union.
Jabber Garbage that is transmitted when a LAN node fails and then
continuously transmits.
Jumper A wire connecting one or more pins (on the one end of a cable only,
for example).
k (kilo) Typically multiples of a thousand (e.g. 1 kilometer = 1000 meters)
K In computer terminology, a K is 2
10
=1024. This distinguishes it
from the SI unit k (kilo) which is 1000.
LAN Local Area Network. A data communications system confined to a
limited geographic area typically about 10 kms with moderate to
high data rates (100kbps to 50 Mbps).
Some type of switching technology is used, but common carrier
circuits are not used.
LCD Liquid Crystal Display. A low power display system used on many
laptops and other digital equipment.
LDM Limited Distance Modem. A signal converter which conditions and
boosts a digital signal so that it may be transmitted further than a
standard EIA-232 signal.
Leased (or Private) Line A private telephone line without inter-exchange switching arrangements.
LED Light Emitting Diode. A semi-conductor light source that emits
visible light or infra red radiation.
LF Line Feed (ASCII Control-J).
Line Driver A signal converter that conditions a signal to ensure reliable
transmission over an extended distance.
Line Turnaround The reversal of transmission direction from transmitter to receiver
or vice versa when a half duplex circuit is used.
Linearity A relationship where the output is directly proportional to the input.
The IDC Engineers Pocket Guide
56
Link Layer Layer 2 of the OSI reference model; also known as the
data link layer.
Listener A device on the GPIB bus that receives information from the bus.
LLC Logical Link Control (IEEE 802.2).
Loaded Line A telephone line equipped with loading coils to add inductance in
order to minimise amplitude distortion.
Long Wire A horizontal wire antenna that is one wavelength or greater in size.
Loop Resistance The measured resistance of two conductors forming a circuit.
Loopback Type of diagnostic test in which the transmitted signal is returned to
the sending device after passing through all, or a portion, of a data
communication link or network.
A loopback test permits the comparison of a returned signal with the
transmitted signal.
Low Pass Generally referring to filters which allow signals below a specified
frequency to pass but attenuate a signal above this specified frequency.
Low-Pass Filter See LPF.
LPF Low-Pass Filter. A filter processing one transmission band, extending
from zero to a specific cutoff frequency.
LSB Least Significant Byte or Least Significant Bit.
Luminance The black and white portion of a video signal which supplies
brightness and detail for the picture.
LUT Look-Up Table. This refers to the memory that stores the values for
the point processes. Input pixel values are those for the original
image whilst the output values are those displayed on the monitor
as altered by the chosen point processes.
Lux SI unit of luminous incidence of illuminance, equal to one lumen
per square metre.
Lux-second SI unit of light exposure.
m meter. Metric system unit for length.
M Mega. Metric system prefix for 10
6
.
MAC Media Access Control (IEEE 802).
Manchester Encoding Digital technique (specified for the IEEE-802.3 Ethernet baseband
network standard) in which each bit period is divided into two
complementary halves; a negative to positive voltage transition in
the middle of the bit period designates a binary "1", whilst a positive
to negative transition represents a "0". The encoding technique also
allows the receiving device to recover the transmitted clock from
the incoming data stream (self clocking).
Power Systems Protection, Power Quality, Substation Automation
57
MAP Manufacturing Automation Protocol. A suite of network protocols
originated by General Motors which follow the seven layers of the
OSI model. A reduced implementation is referred to as a mini-MAP.
Mark This is equivalent to a binary 1.
Mask A structure covering certain portions of a photo-sensitive medium
during photographic processing.
Masking Setting portions of an image at a constant value, either black or
white. Also the process of outlining an image and then matching it
to test images.
Master/Slave Bus access method whereby the right to transmit is assigned to one
device only, the Master, and all the other devices, the Slaves may
only transmit when requested.
Master Oscillator The primary oscillator for controlling a transmitter or receiver
frequency. The various types are: Variable Frequency Oscillator
(VFO); Variable Crystal Oscillator (VXO); Permeability Tuned
Oscillator (PTO); Phase Locked Loop (PLL); Linear Master
Oscillator (LMO) or frequency synthesizer.
Media Access Unit Referred to often as MAU. This is the Ethernet transceiver unit
situated on the coaxial cable which then connects to the terminal
with a drop cable.
Microwave AC signals having frequencies of 1 GHz or more.
MIPS Million Instructions per second.
MMS Manufacturing Message Services. A protocol entity forming part of
the application layer. It is intended for use specifically in the
manufacturing or process control industry. It enables a supervisory
computer to control the operation of a distributed community of
computer based devices.
Modem MODulator - DEModulator. A device used to convert serial digital
data from a transmitting terminal to a signal suitable for transmission
over a telephone channel or to reconvert the transmitted signal to
serial digital data for the receiving terminal.
Modem Eliminator A device used to connect a local terminal and a computer port in
lieu of the pair of modems to which they would ordinarily connect,
allow DTE to DTE data and control signal connections otherwise
not easily achieved by standard cables or connections.
Modulation Index The ratio of the frequency deviation of the modulated wave to the
frequency of the modulating signal.
Morphology The study of a structure/form of object in an image.
MOS Metal Oxide Semiconductor.
MOV Metal Oxide Varistor.
The IDC Engineers Pocket Guide
58
MSB Most Significant Byte or Most Significant Bit.
MTBF Mean Time Between Failures.
MTTR Mean Time To Repair.
Multidrop A single communication line or bus used to connect three
or more points.
Multiplexer (MUX) A device used for division of a communication link into two or
more channels, either by using frequency division or time division.
Multiplexer A technique in which multiple signals are combined into one channel.
They can then be demultiplexed back into the original components.
NAK Negative Acknowledge (ASCII Control-U).
Narrowband A device that can only operate over a narrow band of frequencies.
Negative True Logic The inversion of the normal logic where the negative state is
considered to be TRUE (or 1) and the positive voltage state is
considered to be FALSE (or 0).
Network Layer Layer 3 in the OSI model; the logical network entity that services
the transport layer responsible for ensuring that data passed to it
from the transport layer is routed and delivered throughout the
network.
Network Architecture A set of design principles including the organisation of functions
and the description of data formats and procedures used as the basis
for the design and implementation of a network (ISO).
Network An interconnected group of nodes or stations.
Network Topology The physical and logical relationship of nodes in a network; the
schematic arrangement of the links and nodes of a network typically
in the form of a star, ring, tree or bus topology.
NMRR Normal Mode Rejection Ratio - The ability of a board to filter out
noise from external sources, such as AC power lines. NMRR filtering
compensates for transient changes in the incoming signal to provide
greater accuracy. The higher the NMRR, the better the filtering of
incoming data will be.
Node A point of interconnection to a network.
Noise A term given to the extraneous electrical signals that may be generated
or picked up in a transmission line. If the noise signal is large
compared with the data carrying signal, the latter may be corrupted
resulting in transmission errors.
Non-linearity A type of error in which the output from a device does not relate to
the input in a linear manner.
NRZ Non Return to Zero. Pulses in alternating directions for successive
1 bits but no change from existing signal voltage for 0 bits.
Power Systems Protection, Power Quality, Substation Automation
59
NRZI Non Return to Zero Inverted.
NTSC National Television System Committee (USA). A television standard
specifying 525 lines and 60 fields per second.
Null Modem A device that connects two DTE devices directly by emulating the
physical connections of a DCE device.
Number of Channels This is the number of input lines a board can sample. Single-ended
inputs share the same ground connection, while differential inputs
have individual two-wire inputs for each incoming signal, allowing
greater accuracy and signal isolation. See also multiplexer.
Nyquist Sampling In order to recover all the information about a specified signal it
Theorem must be sampled at least at twice the maximum frequency component
of the specified signal.
OCR Optical Character Recognition, optical character reader.
ohm Unit of resistance such that a constant current of one ampere produces
a potential difference of one volt across a conductor.
OLUT Output Look-Up Table.
On-board Memory Incoming data is stored in on-board memory before being dumped
into the PC's memory. On a high-speed board, data is acquired at a
much higher rate than can be written into PC memory, so it is
stored in the on-board buffer memory.
Optical Isolation Two networks with no electrical continuity in their connection
because an optoelectronic transmitter and receiver has been used.
OR Outside Radius.
OSI Open Systems Interconnection. A set of defined protocol layers
with a standardised interface which allows equipment from
different manufacturers to be connected.
Output An analog or digital output control type signal from the PC to the
external 'real world'.
Overlay One video signal superimposed on another, as in the case of
computer-generated text over a video picture.
Packet A group of bits (including data and call control signals) transmitted
as a whole on a packet switching network. Usually smaller than a
transmission block.
PAD Packet Access Device. An interface between a terminal or computer
and a packet switching network.
PAL Phase Alternating Lines. This is the television standard used in
Europe and Australia. The PAL standard is 25 frames per second
with 625 lines.
The IDC Engineers Pocket Guide
60
Parallel Transmission The transmission model where multiple data bits are sent
simultaneously over separate parallel lines. Accurate synchronisation
is achieved by using a timing (strobe) signal. Parallel transmission
is usually unidirectional; an example would be the Centronics
interface to a printer.
Parametric Amplifier An inverting parametric device for amplifying a signal without
frequency translation from input to output.
Parasitic Undesirable electrical parameter in a circuit such as oscillations
or capacitance.
Parity Bit A bit that is set to a "0" or "1" to ensure that the total number of
1 bits in the data and parity fields are even or odd.
Parity Check The addition of non information bits that make up a transmission
block to ensure that the total number of data and parity bits is
always even (even parity) or odd (odd parity).
Used to detect transmission errors but rapidly losing popularity
because of its weakness in detecting errors.
Passive Filter A circuit using only passive electronic components such as
resistors, capacitors and inductors.
Passive Device Device that must draw its power from connected equipment.
Path Loss The signal loss between transmitting and receiving antennas.
PBX Private Branch Exchange.
PCIP Personal Computer Instrument Products.
PCM Pulse Code Modulation. The sampling of a signal and encoding the
amplitude of each sample into a series of uniform pulses.
PDU Protocol Data Unit.
PEP Peak Envelope Power. Maximum amplitude that can be achieved
with any combination of signals.
Perigee The point in an elliptical orbit that is closest to earth.
Peripherals The input/output and data storage devices attached to a computer
e.g. disk drives, printers, keyboards, display, communication boards, etc.
Phase Shift Keying A modulation technique (also referred to as PSK) used to convert
binary data into an analog form comprising a single sinusoidal
frequency signal whose phase varies according to the data
being transmitted.
Phase Modulation The sine wave or carrier has its phase changed in accordance with
the information to be transmitted.
Physical Layer Layer 1 of the ISO/OSI Reference Model, concerned with the
electrical and mechanical specifications of the network
termination equipment.
Power Systems Protection, Power Quality, Substation Automation
61
PIA Peripheral Interface Adapter. Also referred to as PPI (Programmable
Peripheral Interface).
Pixel One element of a digitised image, sometimes called picture
element, or pel.
PLC Programmable Logic Controller.
PLL Phase Locked Loop
Point to Point A connection between only two items of equipment.
Polar Orbit The path followed when the orbital plane includes the north and
south poles.
Polarisation The direction of an electric field radiated from an antenna.
Polling A means of controlling I/O devices on a multipoint line in which the
CPU queries ('polls') the devices at regular intervals to check for
data awaiting transfer (to the CPU).
Slower and less efficient than interrupt driven I/O operations.
Polyethylene A family of insulators derived from the polymerisation of ethylene
gas and characterised by outstanding electrical properties, including
high IR, low dielectric constant, and low dielectric loss across the
frequency spectrum.
Polyvinyl Chloride A general purpose family of insulations whose basic constituent is
(PVC) polyvinyl chloride or its copolymer with vinyl acetate.
Plasticisers, stabilisers, pigments and fillers are added to improve
mechanical and/ or electrical properties of this material.
Port A place of access to a device or network, used for input/output of
digital and analog signals.
PPI See PIA.
Presentation Layer Layer 6 of the ISO/OSI Reference Model, concerned with
negotiation of a suitable transfer syntax for use during an
application. If this is different from the local syntax, the translation
is to/from this syntax.
Pretrigger Boards with 'pretrigger' capability keep a continuous buffer filled
with data, so when the trigger conditions are met, the sample
includes the data leading up to the trigger condition.
Profibus Process Field Bus developed by a consortium of mainly German
companies with the aim of standardisation.
Program I/0 The standard method of memory access, where each piece of data is
assigned to a variable and stored individually by the PC's processor.
The IDC Engineers Pocket Guide
62
Programmable Gain Using an amplifier chip on an A/D board, the incoming analog signal
is increased by the gain multiplication factor. For example; if the
input signal is in the range of -250 mV to +250 mV, the voltage
after the amplifier chip set to a gain of 10 would be -2.5 V to +2.5 V.
PROM Programmable Read Only Memory. This is programmed by the
manufacturer as a fixed data or program which cannot easily be
changed by the user.
Protocol Entity The code that controls the operation of a protocol layer.
Protocol A formal set of conventions governing the formatting, control
procedures and relative timing of message exchange between two
communicating systems.
PSDN Public Switched Data Network. Any switching data communications
system, such as Telex and public telephone networks, which provides
circuit switching to many customers.
PSTN Public Switched Telephone Network. This is the term used to
describe the (analog) public telephone network.
PTT Post, Telephone and Telecommunications Authority.
Public Switched Network Any switching communications system - such as Telex and
public telephone networks - that provides circuit switching to
many customers.
Pulse Input A square wave input from a real world device such as a flow meter,
which sends pulses proportional to the flow rate.
QAM Quadrature Amplitude Modulation.
QPSK Quadrature Phase Shift Keying.
Quagi An antenna consisting of both full wavelength loops (quad) and
Yagi elements.
R/W Read/Write.
RAM Random Access Memory. Semiconductor read/write volatile memory.
Data is lost if the power is turned off.
RAMDAC Random Access Memory Digital-to-Analog Converter.
Range The difference between the upper and lower limits of the
measured value.
Range Select The full-scale range a board uses is selected by one of three methods:
through the appropriate software, by a hardware jumper on the board,
or through the use of an external reference voltage.
Raster The pattern of lines traced by rectilinear scanning in display systems.
Reactance The opposition offered to the flow of alternating current by inductance
or capacitance of a component or circuit.
Power Systems Protection, Power Quality, Substation Automation
63
Real-time A system is capable of operating in real-time when it is fast enough
to react to the real-world events.
Reflectance Component The amount of light reflected by an object in the scene being viewed.
Refresh rate The speed at which information is updated on a computer
display (CRT).
Repeater An amplifier which regenerates the signal and thus expands
the network.
Resistance The ratio of voltage to electrical current for a given circuit measured
in ohms.
Resolution The number of bits in which a digitised value will be stored. This
represents the number of divisions into which the full-scale range
will be divided; for example, a 0-10 V range with a 12-bit resolution
will have 4096(2
12
) divisions of 2.44mV each.
Response Time The elapsed time between the generation of the last character of a
message at a terminal and the receipt of the first character of the
reply. It includes terminal delay and network delay.
RF Radio Frequency.
RFI Radio Frequency Interference.
RGB Red/Green/Blue. An RGB signal has four separate elements;
red/green/ blue and sync. This results in a cleaner image than with
composite signals due to the lower level of distortion and
interference.
Ring Network topology commonly used for interconnection of
communities of digital devices distributed over a localised area, e.g.
a factory or office block. Each device is connected to its nearest
neighbours until all the devices are connected in a closed loop or
ring. Data are transmitted in one direction only.
As each message circulates around the ring, it is read by each device
connected in the ring.
Ringing An undesirable oscillation or pulsating current.
Rise Time The time required for a waveform to reach a specified value from
some smaller value.
RLE Run Length Encoder. A digital image method whereby the first grey
level of each sequential point-by-point sample and its position in the
succession of grey levels is encoded. It is used where there is a
tendency for long runs of repeated digitised grey levels to occur.
RMS Root Mean Square.
ROI Region of Interest.
The IDC Engineers Pocket Guide
64
ROM Read Only Memory. Computer memory in which data can be
routinely read but written to only once using special means when the
ROM is manufactured. A ROM is used for storing data or programs
on a permanent basis.
Router A linking device between network segments which may differ in
Layers 1, 2a and 2b of the ISO/OSI Reference Model.
RS Recommended Standard, for example, RS-232C. More recent
designations use EIA, for example, EIA-232C.
RS-232C Interface between DTE and DCE, employing serial binary data
exchange. Typical maximum specifications are 50 feet (15m) at 19200
baud.
RS-422 Interface between DTE and DCE, employing the electrical
characteristics of balanced voltage interface circuits.
RS-423 Interface between DTE and DCE, employing the electrical
characteristics of unbalanced voltage digital interface circuits.
RS-449 General purpose 37-pin and 9-pin interface for DCE and DTE
employing serial binary interchange.
RS-485 The recommended standard of the EIA that specifies the electrical
characteristics of drivers and receives for use in balanced digital
multipoint systems.
RTU Remote Terminal Unit. Terminal Unit situated remotely from the
main control system.
S-Video The luminance and chrominance elements of a video signal are
isolated from each other, resulting in a far cleaner image with
greater resolution.
SAA Standards Association of Australia.
SAP Service Access Point.
SDLC Synchronous Data Link Control. IBM standard protocol superseding
the bisynchronous standard.
Selectivity A measure of the performance of a circuit in distinguishing the
desired signal from those at other frequencies.
Self-calibrating A self-calibrating board has an extremely stable on-board reference
which is used to calibrate A/D and D/A circuits for higher accuracy.
Self-diagnostics On-board diagnostic routine which tests most, if not all, of a board's
functions at power-up or on request.
Serial Transmission The most common transmission mode in which information bits are
sent sequentially on a single data channel.
Power Systems Protection, Power Quality, Substation Automation
65
Session Layer Layer 5 of the ISO/OSI Reference Model, concerned with the
establishment of a logical connection between two application entities
and with controlling the dialogue (message exchange) between them.
Shielding The process of protecting an instrument or cable from external
noise (or sometimes protecting the surrounding environment of the
cable from signals within the cable.)
Short Haul Modem A signal converter which conditions a digital signal to ensure
reliable transmission over DC continuous private line metallic circuits,
without interfering with adjacent pairs of wires in the same
telephone cables.
Shutter A mechanical or electronic device used to control the amount of
time a light-sensitive material is exposed to radiation.
SI International metric system of units (Système Internationale).
Sidebands The frequency components which are generated when a carrier is
frequency-modulated.
Upconverter A device used to translate a modulated signal to a higher band of
frequencies.
Sidereal Day The period of an earth's rotation with respect to the stars.
Signal to Noise Ratio The ratio of signal strength to the level of noise.
Signal Conditioning Pre-processing of a signal to bring it up to an acceptable quality
level for further processing by a more general purpose analog
input system.
Simplex Transmission Data transmission in one direction only.
Simultaneous Sampling The ability to acquire and store multiple signals at exactly the same
moment. Sample-to-sample inaccuracy is typically measured in
nanoseconds.
Single-ended See number of channels.
Slew Rate This is defined as the rate at which the voltage changes from one
value to another.
Smart Sensors A transducer (or sensor) with an on-board microprocessor to
pre-process input signals to the transducer. It also has the capability
of communicating digitally back to a central control station.
SNA Systems Network Architecture.
SNR Signal to Noise Ratio.
Software Drivers Typically a set of programs or subroutines allowing the user to control
basic board functions, such as setup and data acquisition. These can
be incorporated into user-written programs to create a simple but
functional DAS system. Many boards come with drivers supplied.
The IDC Engineers Pocket Guide
66
Software Trigger Software control of data acquisition triggering. Most boards are
designed for software control.
SOH Start of Header (ASCII Control-A).
Space Absence of signal. This is equivalent to a binary zero.
Spark Test A test designed to locate imperfections (usually pin-holes) in the
insulation of a wire or cable by application of a voltage for a very
short period of time while the wire is being drawn through the
electrode field.
Spatial Resolution A measure of the level of detail a vision system can display. The
value, expressed in mils or inches per pixel, is derived by dividing
the linear dimensions of the field of view (x and y, as measured in
the image plane), by the number of pixels in the x and y dimensions
of the system's imaging array or image digitiser.
Spatial Filtering In image processing, the enhancement of an image by increasing or
decreasing its spatial frequencies.
Spectral Purity The relative quality of a signal measured by the absence of harmonics,
spurious signals and noise.
Speed/Typical The maximum rate at which the board can sample and convert
Throughput incoming samples. The typical throughput is divided by the number
of channels being sampled to arrive at the samples/second on each
channel. To avoid false readings, the samples per second on each
channel need to be greater than twice the frequency of the analog
signal being measured.
Standing Wave Ratio The ratio of the maximum to minimum voltage (or current) on a
transmission line at least a quarter-wavelength long. (VSWR refers
to voltage standing wave ratio)
Star A type of network topology in which there is a central node that
performs all switching (and hence routing) functions.
Statistical Multiplexer Multiplexer in which data loading from multiple devices occurs
randomly throughout time, in contrast to standard multiplexers
where data loading occurs at regular predictable intervals.
STP Shielded Twisted Pair.
Straight Through Pinning EIA-232 and EIA-422 configuration that match DTE to DCE, pin
for pin (pin 1 with pin 1, pin 2 with pin 2,etc).
Strobe A handshaking line used to signal to a receiving device that there is
data to be read.
STX Start of Text (ASCII Control-B).
Subharmonic A frequency that is a integral submultiple of a reference frequency.
Switched Line A communication link for which the physical path may vary with
each use, such as the public telephone network.
Power Systems Protection, Power Quality, Substation Automation
67
Sync A synchronisation, or sync, pulse ensures that the monitor
displaying the information is synchronised at regular intervals with
the device supplying the data, thus displaying the data at the right
location.
For example, a sync pulse would be used between a camera and a
display device to reset the image to the top of the frame for the
beginning of the image.
Synchronisation The co-ordination of the activities of several circuit elements.
Synchronous Transmission in which data bits are sent at a fixed rate, with the
Transmission transmitter and receiver synchronised. Synchronised transmission
eliminates the need for start and stop bits.
Talker A device on the GPIB bus that simply sends information onto the
bus without actually controlling the bus.
Tank A circuit comprising inductance and capacitance which can store
electrical energy over a finite band of frequencies.
TCP/IP Transmission Control Protocol/Internet Protocol. The collective
term for the suite of layered protocols that ensures reliable data
transmission in an internet (a network of packet switching networks
functioning as a single large network).
Originally developed by the US Department of Defense in an effort
to create a network that could withstand an enemy attack.
TDM Time Division Multiplexer. A device that accepts multiple channels
on a single transmission line by connecting terminals, one at a time,
at regular intervals, interleaving bits (bit TDM) or characters
(Character TDM) from each terminal.
TDR Time Domain Reflectometer. This testing device sends pulses down the
cable and enables the user to determine cable quality (distance to defect
and type of defect) by the reflections received back.
Temperature Rating The maximum, and minimum temperature at which an insulating
material may be used in continuous operation without loss of its
basic properties.
Text Mode Signals from the hardware to the display device are only interpreted
as text characters.
Thresholding The process of defining a specific intensity level for determining
which of two values will be assigned to each pixel in binary
processing. If the pixel's brightness is above the threshold level, it
will appear in white in the image; if it is below the threshold level,
it will appear black.
TIA Telecommunications Industry Association.
The IDC Engineers Pocket Guide
68
Time Division The process of transmitting multiple signals over a single channel
by multiplexing taking samples of each signal in a repetitive time
sequenced fashion.
Time Sharing A method of computer operation that allows several interactive
terminals to use one computer.
Time Domain The display of electrical quantities versus time.
Token Ring Collision free, deterministic bus access method as per IEEE 802.2
ring topology.
TOP Technical Office Protocol. A user association in USA which is
primarily concerned with open communications in offices.
Topology Physical configuration of network nodes, e.g. bus, ring, star, tree.
Transceiver A combination of transmitter and receiver.
Transducer Any device that generates an electrical signal from real-world physical
measurements. Examples are LVDTs, strain gauges, thermocouples
and RTDs. A generic term for sensors and their supporting circuitry.
Transient An abrupt change in voltage of short duration.
Transmission Line One or more conductors used to convey electrical energy from one
point to another.
Transport Layer Layer 4 of the ISO/OSI Reference Model, concerned with providing
a network independent reliable message interchange service to the
application oriented layers (layers 5 through 7).
Trigger A rising edge at an 8254 timer/counter's gate input.
Trunk A single circuit between two points, both of which are switching
centres or individual distribution points. A trunk usually handles
many channels simultaneously.
Twisted Pair A data transmission medium, consisting of two insulated copper
wires twisted together. This improves its immunity to interference
from nearby electrical sources that may corrupt the transmitted signal.
UART Universal Asynchronous Receiver/Transmitter. An electronic circuit
that translates the data format between a parallel representation,
within a computer, and the serial method of transmitting data over a
communications line.
UHF Ultra High Frequency.
Unbalanced Circuit A transmission line in which voltages on the two conductors are
unequal with respect to ground e.g. a coaxial cable.
Unipolar Inputs When set to accept a unipolar signal, the channel detects and converts
only positive voltages. (Example: 0 to +10 V).
Unloaded Line A line with no loaded coils that reduce line loss at audio frequencies.
Power Systems Protection, Power Quality, Substation Automation
69
Upconverter A device used to translate a modulated signal to a higher band
of frequencies.
Uplink The path from an earth station to a satellite.
USRT Universal Synchronous Receiver/Transmitter. See UART.
UTP Unshielded Twisted Pair.
V.35 CCITT standard governing the transmission at 48 kbps over 60 to
108 kHz group band circuits.
VCO Voltage controlled oscillator. Uses variable DC applied to tuning
diodes to change their junction capacitances. This results in the output
frequency being dependent on the input voltage.
Velocity of Propagation The speed of an electrical signal down a length of cable compared
to speed in free space expressed as a percentage.
VFD Virtual Field Device. A software image of a field device describing
the objects supplied by it eg measured data, events, status etc which
can be accessed by another node on the network.
VGA Video Graphics Array. This standard utilizes analog signals only
(between 0 and 1 V) offering a resolution of 640 by 480 pixels, a
palette of 256 colors out of 256000 colors and the ability to display
16 colors at the same time.
VHF Very High Frequency.
Vidicon A small television tube originally developed for closed-circuit
television. It is about one inch (2.54 cm) in diameter and five
inches (12.7 cm) long. Its controls are relatively simple and can
be operated by unskilled personnel. The Vidicon is widely used
in broadcast service.
Volatile Memory A storage medium that loses all data when power is removed.
Voltage Rating The highest voltage that may be continuously applied to a wire in
conformance with standards of specifications.
VRAM Volatile Random Access Memory. See RAM.
VSD Variable Speed Drive.
VT Virtual Terminal.
WAN Wide Area Network.
Waveguide A hollow conducting tube used to convey microwave energy.
Wedge Filter An optical filter so constructed that the density increases progressively
from one end to the other, or angularly around a circular disk.
Word The standard number of bits that a processor or memory manipulates
at one time. Typically, a word has 16 bits.
The IDC Engineers Pocket Guide
70
X.21 CCITT standard governing interface between DTE and DCE
devices for synchronous operation on public data networks.
X.25 Pad A device that permits communication between non X.25 devices
and the devices in an X.25 network.
X.25 CCITT standard governing interface between DTE and DCE device
for terminals operating in the packet mode on public data networks.
X.3/X.28/X.29 A set of internationally agreed standard protocols defined to allow a
character oriented device, such as a visual display terminal, to be
connected to a packet switched data network.
X-ON/X-OFF Control characters used for flow control, instructing a terminal to
start transmission (X-ON) and end transmission (X-OFF).
Power Systems Protection, Power Quality, Substation Automation
71
Appendix B
Units and Abbreviations
Table A.1
SI units
Table A.2
Decimal Prefixes
Symbol Prefix Factor by which unit
is multiplied
Ttera10
12
G giga 10
9
M mega 10
6
k kilo 10
3
h hecto 10
2
da deca 10
d deci 10
-1
c centi 10
-2
m milli 10
-3
μ micro 10
-9
n nano 10
-9
p pico 10
-12
Unit Symbol Unit Quantity
m metre length
kg kilogram mass
s second time
A ampere electric current
K kelvin thermodynamic temp
cd candela luminous intensity
The IDC Engineers Pocket Guide
72
Table A.3
Supplementary and Derived Units
Table A.4
Supplementary and Derived Unit (electrical)
Table A.5
Supplementary and Derived Units (magnetic)
Quantity Magnetic unit Symbol Derived unit
magnetic flux weber Wb V s = Nm/A
inductance henry H V s/A = Nm/A
2
magnetic field strength - A/m -
magnetic flux density tesla T Wb/m
2
= (N)/(Am)
Quantity Electrical unit Symbol Derived unit
potential volt V W/A
resistance ohm Ω V/A
charge coulomb C A s
capacitance farad F A s/V
electric field strength - V/m -
electric flux density - C/m
2
-
Quantity Unit Symbol Equivalent
plane angle radian rad -
force newton N kg m/s
2
work, energy heat joule J N m
power watt W J/s
frequency hertz Hz s-1
viscosity: kinematic - m
2
/s 10 c St (Centistoke)
dynamic - Ns/m
2
or Pa s 10
3
cP (Centipoise)
pressure - Pa or N/m
2
pascal, Pa
Power Systems Protection, Power Quality, Substation Automation
73
Table A.6
Physical Constants
Name Symbol Equivalent
Avogadro's number N 6.023 x 10
26
/(kg mol)
Bohr magneton B 9.27 x 10
-24
A m 25
2
Boltzmann's constant k 1.380 x 10
-23
J/k
Stefan-Boltzmann constant d 5.67 x 10
-8
W/(m
2
K
4
)
Characteristic impedance of free space Z
o
(μ
o
/E
o
)
1/2
=120πΩ
Electron volt eV 1.602 x 10
-19
J
Electron charge e 1.602 x 10
-19
C
Electronic rest mass m
e
9.109 x 10
-31
kg
Electronic charge to mass ratio e/m
e
1.759 x 10
11
C/kg
Faraday constant F 9.65 x 10
7
C/(kg mol)
Permeability of free space μ
o
4π x 10
-7
H/m
Permittivity of free space E
o
8.85 x 10
-12
F/m
Planck's constant h 6.626 x 10
-34
J s
Proton mass m
p
1.672 x 10
-27
kg
Proton to electron mass ratio m
p
/m
e
1835.6
Standard gravitational acceleration g 9.80665 m/s
2
9.80665 N/kg
Universal constant of gravitation G 6.67 x 10
-11
N m
2
/kg
2
Universal gas constant R
o
8.314 kJ/(kg mol K)
Velocity of light in vacuo C 2.9979 x 10
8
m/s
Volume of 1 kg mol of ideal gas at
- 22.41 m
3
1 atm & 0
o
C
Temperature
0
C 5/9(
0
F - 32)
Temperature K 5/9(
0
F + 459.67)
5/9
0
R
0
C + 273.15
The IDC Engineers Pocket Guide
74
Appendix C
Commonly used Formulae
Symbols used in formulae
The symbols described in the following table are used in the formulae shown in
the next section
Symbol Description SI Unit
a Velocity of sound ms
-1
a Acceleration ms
-2
A Area m
2
c Velocity of light ms
-1
C Capacitance F
D Diameter m
E Young's modulus Nm
-2
ΔE Energy difference J
f Frequency Hz
F Force N
H Magnetising force magnetic field strength Am
-1
I Current A
I Moment of inertia kgm
2
k Radius of gyration m
kp Pitch factor of winding -
l Length m
l Length of conductor m
L Inductance H
m Mass kg
M Momentum Kg.m.s
-1
n Speed of rotation rpm
N Number of turns -
p Number of pole pairs -
Power Systems Protection, Power Quality, Substation Automation
75
Symbol Description SI Unit
Q Volumetric flow rate m
3
s
-1
Q Charge C
R Resistance Ω
s Fractional slip -
t Time s
T Time Factor -
T Torque Nm
T Temperature (absolute) K
ΔT Temperature difference
o
C
u Velocity ms
-1
v Velocity ms
-1
V Voltage V
V Volume m
3
x Distance (variables as in dx) m
Z Number of armature conductors -
Z Impedance Ω
a Coefficient of volumetric expansion Hm/(mK)
a Resistance coefficient Ω K
-1
b Coefficient of volumetric expansion K
-1
e
o
Permittivity of free space Fm
-1
e
o
Permittivity-relative -
m
o
Permeability of free space Hm
-1
m
r
Permeability-relative -
r
o
Resistivity Ω m
3
r Density kgm
3
s Stefan-Boltzmann constant Wm
-2
K
-4
φ Angle radians
F Magnetic flux, flux per pole Wb
w Angular Velocity rad.s
-1
w
n
Natural frequency rad.s
-1
w
o
Natural frequency rad.s
-1
w
d
Damped natural frequency rad.s
-1
The IDC Engineers Pocket Guide
76
Formulae
Ohm's Law (DC version)
V = IR
Ohm's Law (AC version)
V = I . Z
Kirchhoff's Law
Power
Resistance
Resistors in series:
R = R
1
+ R
2
+ R
3
+ ...
Power Systems Protection, Power Quality, Substation Automation
77
Resistors in parallel:
Inductance
for LR circuit decay, stored energy is calculated as follows:
Capacitance
The IDC Engineers Pocket Guide
78
For n parallel plates:
For RC circuit discharge:
Stored energy:
For capacitors in series:
For capacitors in parallel:
Power Systems Protection, Power Quality, Substation Automation
79
Electrostatics
Electromagnetism
For a solenoid:
The IDC Engineers Pocket Guide
80
Magnetism
For a magnetic circuit:
Stored energy density:
AC Circuits
Power Systems Protection, Power Quality, Substation Automation
81
At resonance the following relationship holds true:
The Q factor can be calculated as follows:
Sound
Note that decibels are not units as such but a ratio of voltages, currents and power,
for example:
where: P
1
,P
2
are the power levels:
For differing input and output impedances the following formula is appropriate:
Where V
1
,V
2
are the voltages
Z
1
,Z
2
are the impedances.
The IDC Engineers Pocket Guide
82
Appendix D
Resistor Color Coding
Resistor values are calculated according to the following color coding:
Table C.1
Color on resistor Value allocation
Black 0
Brown 1
Red 2
Orange 3
Yellow 4
Green 5
Blue 6
Violet/Purple 7
Grey 8
White
9
Power Systems Protection, Power Quality, Substation Automation
83
Common Band Colors
Resistors have the following two major groupings of color coding:
Figure C.1
Color coding for tolerance resistors
