Chapter One: Introduction to Protective Relaying
Protection Basics:
Power system provides continuous generation, transmission, and consumption of
energy, as shown in the figure below.
Relay Requirements
Relaying is the branch of Power Engineering that’s concerned with the principles
of design and operation of relays. It detects abnormal conditions and initiates
corrective action as quickly as possible to return the power system to its normal
state.
There are certain requirements for the relaying system to work effectively:
 The reasons and types of faults in the electrical power system.
 The elements of the protection system, as well as the most famous
Relaying signals used by the protection devices to detect these Faults, and
the names of the most famous types of Protective Relays.
 The most famous types of circuit breakers, and the relationship between
them and the protective devices.
 Types of protection systems - their application areas and the types of
relays used in each application.
 The most important basic concepts in the protection system that it is
necessary for the electrical engineer in general and the protection engineer
in particular to be fully familiar with them.
 The most important requirements that are taken into account when
choosing protection devices in general.
Power system Faults:
Faults in the electrical power system are everything that causes an abnormal
change in the values of current or voltage.
 Faults Cases:
 The most common cause of faults is insulation breakdown in the
conductors due to mechanical or electrical stresses, either due to
aging equipment, or perhaps as a result of weather conditions
(lightning, strong winds), or due to external factors (fires, falling huge
trees, road accidents, etc.)
 The cause of the fault may be a wrong operation (for example, when
the operator closes the line while the Earthing switch is still in a
connecting position), and the fault may be the result of Open Circuit
due to a break in one of the circuit conductors, for example, and the
fault may also be a result of Overload.
In fact, there are many reasons that lead to a breakdown of the basic
insulation values between a line and the ground, or between two lines, and
then there is an abnormal changes in the values of current or voltage or both.
The increase in current will cause the equipment temperature to increase
and it may cause a fire if you do not separate in the specified time. As for the
increase in voltage, it will cause insulation breakdown of the equipment and
consequently the occurrence of a circuit short.
 Faults Classifications:
o Symmetrical Faults
o Asymmetrical Faults
 Permanent Faults
 Transient Faults
 Single Line-to-Ground Fault.
 Double Line-to-Ground Fault.
 Line-to-Line Fault.
 Three-Phase Fault.
 Three Lines-to-Ground Fault
Fault Characteristics:
The type of changes that will occur in the different Relaying Signals in the network
after the fault. It can be detected through tracking the change in either the
magnitude of voltage or current (or both) or the change in phase angles between
them. These changes can be traced by studying the phasor diagram.
Examples:
Phasors in Normal Condition
3L-G Fault
In this case, the current increases by the same value in all phases, and the voltage
decreases by the same value in all phases, but note that the value of the angle
between voltage and current may It rose to about 60 degrees, which is almost a
fixed number in high voltage lines, as each line has an angle that represents the
relationship between R and X for this line, at very high voltages the angle may reach
80 degrees. While in medium voltage lines the angle reaches about 40-50 degrees.
This change is caused by the fault that cancel the load impedance, so only the line
impedance remains, whose angle is fixed and equal to approximately 60 degrees.
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Phasors in Normal Condition
SLG Fault
IB = Ic = 0
Phasors in Normal Condition
2L Fault (Fault between B and C Lines)
There is a large increase in the value of the currents IC and IB with 180 phase shift
between them, and there is a decrease in the values of the voltages VBN and VCN
which are the two faulty phases . Notice the angle between these two voltages has
become less than 120 degrees unlike the normal situation.
In general, this type of Faults causes a significant distortion in the voltage and
current relationships because it is a non-symmetrical fault.
2L-G Fault
Phasors in Normal Condition
Vb = Vc = 0 (if Zf=0)
Open-Circuit Faults:
When one of the three fuses explodes, as in the figure, the current returns to the
source through the stray capacitance of the open phase.
The existence of capacitance and Inductance in series exactly as in the previous
figure, an increase in voltage occurs, which may cause breakdown of the insulators
of the line or the transformer.
The effect of the system grounding on the path of the fault current:
The purpose of the protection system is not to prevent the occurrence of faults, as
this is almost impossible, because many of the faults may have external causes that
the protective devices have no ability to prevent, but the role of the protection
system is to quickly and accurately separate faults. In more detail, we say that the
role of the protection system is:
 Discovering faults and determining their severity and location, and then
sending a trip signal for the circuit breakers required to be opened, or a block
signal for the breakers to be prevented from operating, all of which is done
by the Protective Relay.
 Determine the components affected by the fault to be separated by the CB,
in order to ensure the continuity of current in other circuits that do not have
a fault.
Not every increase in current is caused by a fault. There are many operating cases
that result in a large increase in current, however, they are not classified as faults,
such as the Inrush Currents in transformers that appears at the beginning of the
transformer operation, and may reach 5-7 times the natural current. As well as the
charging current of the capacitors, which also rises strongly at the start of charging.
The system must distinguish between a high current as a result of a real fault and a
current of approximately the same value, but as a result of normal conditions.
Components of the protection system
1.
2.
3.
4.
5.
Protection Relays
Circuit Breakers CB
Current Transformers CT / Voltage Transformers VT.
DC battery.
Communication Channel.
The place of the protection system elements:
There is a separate protection system for each element to be protected
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Transmission Lines
Generators
Transformers
Bus Bars
Each of the above-mentioned elements has its own protection system, but there
will be some kind of coordination between the works of the different protection
systems, as will be seen later.
The protective devices are placed inside the control rooms of the stations, and
the location of the circuit breakers and the CT and VT is either in the yard of the
substation as in the external stations, or it is within the gas-insulated stations
group as in the figure
Gas insulated stations (GIS)
Relaying Signals:
In most cases, there is a noticeable change in the values of voltage and current (one
or both) during the occurrence of a fault so that it can be monitored and detected,
Examples:
 In 3-phase to Ground fault:
 In B-C- to -Ground fault:
 In B-C- to -Ground fault:
Relaying Signals:
The most commonly used signals in detecting faults.
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Current.
Voltage.
Frequency.
Direction of the electric power flowing.
Balance loads in the three phases.
Value of the Circuit Impedance (Z) that seen by the source.
The difference between the value of the incoming current and the value of
the current leaving one of the equipment’s (the large change between
them arises as a result of current leakage inside the equipment.
 Temperature.
 Pressure.
 Speed.
Relay Operating Principles
There are 8 relay operating principles:
Level Detection:
Level detection is the simplest of all relay operating principles:
 A pickup current must be set.
 Pickup current is the current value above which the relay will operate.
 For all currents above the pickup setting, the relay operates; otherwise no
action is taken.
 It is possible to arrange for opposite logic. E.g.: under-voltage relay.
 If the current is I and the pickup value is Ip, then relay will operate either if
I > Ip, or I ≤ Ip.
Magnitude Comparison
The magnitude comparison principle is based on the comparison of one or more
operating quantities with each other.
For example, if there two identical parallel transmission line is as shown:
Differential Comparison
 Differential comparison principle is used to measure the currents at both
ends of a protection zone and act if (I1 – I2 ≥ ε).
 Differential comparison is the most sensitive and effective method. The
relay is capable of detecting very small fault currents.
 Its drawback is that it requires currents from ends of a protection zone.
 Its application is restricted to power apparatus such as transformers,
generators, buses, capacitors, reactors, motors.
Phase Angle Comparison
In this principle, the relay compares the relative phase angle between the two ac
quantities: V and I.
Normally, phase angle is around ± 30º. However, during the fault, it becomes
around ± 90º. Note that the current may reverse its direction as shown in the
upper phasor diagram.
Distance Measurement
A distance relay, as its name implies, has the ability to detect a fault within a preset
distance along a transmission line or power cable from its location.
Every power line has impedance or resistance and reactance per kilometer related
to its design and construction so its total impedance will be a function of its length
or distance.
A distance relay therefore looks at current and voltage and compares these two
quantities on the basis of Ohm’s law as shown:
Pilot Relaying
 The relay action is based on information obtained from remote location.
 Information is usually in the form of contact status – either open or closed.
 Information is sent over a communication channel such as power line
carrier, microwave, or telephone circuits.
Harmonic Content:
 In normal operation, V and I are sinusoidal waveforms with fundamental
frequency.
 Abnormal conditions, such as transformer saturation and transformer
energization, can be detected by sensing the harmonic content through
filters or by calculation in digital relays.
Frequency Sensing:
 Normal frequency is 50 or 60 Hz. Any deviation indicates an abnormal
condition.
 Frequency can be measured by counting of zero crossings in a unit of time.
Types of protective devices and their standard numbers:
ANSI Device Numbers and symbols Table:
The study of protection systems includes:
1. Study the design of the Protective Relays themselves (this includes studying
their specifications, how they work, how to adjust them settings, etc.) such
as:
 Overcurrent Relays
 Distance Relays
 Differential Relays
 Directional Relays
 Overvoltage Relays
 etc.
2. Study the protection of the equipment within the electrical power system,
this type is called the Apparatus Protection:
 Transmission Line Protection
 Transformer Protection
 Generator Protection
 Motor Protection
 Busbar Protection
This type of protection is considered as application of protective relays, i.e.
the practical application of the relays. When studying the apparatus
protection, the term Protective Schemes is frequently used, and it means,
the use of more than one protection device together to protect the
equipment and make coordination between these devices.
3. Study the protection of the general network, it means the study of cases in
which the grid is a part of the protection problem being studied, such as
cases of disconnecting of the Renewable Sources systems from the main
network, which is the so-called systems islanding, or cases of instability and
how to direct the Wide Area Measurement (WAM) control systems to
prevent this from happening.
This type of protection includes studying different protective devices from
those mentioned in the first type, such as:
• Under-frequency relays
• Out-of-step protection
• Rate of change of frequency relays (ROCOF)
• Reverse power flow relays
• Voltage surge relays
For example: studying the problem of islanding, which is intended as a case study
in Figure shown below:
If the public network is disconnected, and only the generators (DG) are left to feed
the loads, It is an operational condition that is rejected for many reasons, and in
this case, the DG, Local Generator must be quickly isolated from the network by
opening the breaker located at the PCC (Point of common coupling), as in the
figure, and then we need a special protection device capable of detecting the
disconnection of the connection to the Grid, and one of these devices, for example,
is the ROCOF Relay, the detection done by tracking the value of the Rate of Change
in frequency at the PCC point.
Circuit Breakers:
When a fault occurs in a region, the Circuit Breakers installed at the beginning and
end of this region is opened based on a signal from the Relay, in order to stop the
flow of the fault current.
The most dangerous thing that these CBs will face when they start working is the
spark (Arc) that will arise between the CB poles, and this spark can cause many
problems, for example, the ignition of fires, and also that if it continues, the current
will pass through the poles of the circuit breaker, Thus, the CB becomes as if it is
still closed.
There are many types of circuit breakers that all agree on one goal: the speed of
extinguishing the spark that arises between the poles of the CB when it is opened.
But they differ among themselves in the method used for this task, and here are
some examples of these CBs.
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Oil CB:
Air CB:
Vacuum CB:
SF6 CB:
Total fault clearing time: it includes
 Relay pickup time: It is calculated from the moment of the fault occurs until
the relay contacts closes (around 20 ms.).
 CB Opening and arcing times:( 50 – 100 ms)
The most noticeable changes that appear on the current and voltage signals from
the moment of the fault until the end of the circuit opening can be seen in the
figure below:
The relationship between the protective device and CBs:
The aim of the protection system is to quickly and accurately separate faults,
through the Protective Relay, which detects the fault and sends a disconnection
signal to the CBs that want to be opened. The process starts from entering the
relaying signals into the Relay by a Voltage/Current Transformer, then the Relay,
based on studying these signals, sends a trip signal to the circuit breaker trip coil.
To open the CB.
What if the Relay detects the fault and the CB fails to open the circuit?
A mechanical failure may occur in the circuit breaker, which fails to respond to the
trip signal that it received from the protection device. To solve this problem, a
special protection device called Breaker Failure (BF) is always used. The idea of its
work is shown in Figure.
When a fault occurs, the protection device R sends a Trip signal to the CB and at
the same time sends a signal to the BF.
 If the CB succeeds in opening the circuit before this time, this means that the
current passing through the current transformer will become zero, and this
signal from the CT is sufficient to stop the work of the BF, and there is
another signal from the Auxiliary Contacts of CB to confirm that the BF has
stopped working.
 But if the CB fails to open the circuit and the Timer finishes it’s counting, then
the BF will send a Trip signal to all the connected lines on the BB on which
this circuit breaker is installed, as it is clear from the diagram, in addition to
sending a trip signal (remote end inter trip) to the devices on the other side.
Protective Zones:
The electrical network is divided into small zones (areas), and the role of the
protective device becomes to protect this small zone only as a main protection,
with the possibility of protecting other areas in a backup protection.
These groups of protective devices must cooperate to ensure the highest levels of
protection.
Note that:
 The protection of each zone is made up of several protection devices, not
one, in order to detect all types of expected faults.
 There must be an overlap between the different protection zones so that
there are no unprotected zones, especially at Busbars .
 Most important elements must be included in at least two zones
Closed and Open Zones:
 Closed protection zones in the figure are:
a) The zone limited by B1 and B2
b) The zone limited by B2, B3, and B4
 Open protection zones in the figure are:
a) The zone limited by B5 from one side while the other side is open
b) The zone limited by B6 from one side while the other side is open
Directional Protection:
The device is disconnected if the fault current is only in the direction out of the
Bus, while operation is prevented if the fault current is directed towards the Bus.
 In normal situation the current lags the voltage in the range of 20 -30
degrees.
 If the current opposite in direction, it becomes leads the voltage.
 Therefore we can discover the change in the direction of the current
by tracing the value of the angle between it and the voltage.
 Forward and Reverse Faults.
 Internal and External Faults.
Example: For a fault F1:
1.
2.
3.
4.
Main Protection: R15 and R16.
For the success of R16, and fail of R15.
R9 and R7 are backup protections with respect to F1.
R8 and R10 are reverse protections with respect to F1, and they doesn’t
response to this fault.
5. For the success of R9, and fail of R7.
6. The R30, R2 and R4 are send trip signals to their CBs to open to prevent flow
of current from the generators G2 and G3 to the fault.
General requirements for protective equipment:
Protection systems must have unified standards and general specifications for
protective devices, which are:
1. Dependability: It is the measure of certainty that the relays will operate for
all faults for which they are designed to operate.
2. Security: It is the measure of certainty that the relays will not operate
incorrectly for any fault.
3. Reliability: It is defined as a measure of degree of certainty that a piece of
equipment will perform as intended.
Relays can be unreliable if they are:
 Undependable - fail to operate when they are expected to.
 Unsecure - operate when they are not expected.
Example: The performance of an overcurrent relay was monitored over a period of
one year. It was found that the relay operated 14 times, out of which 12
were correct trips. If the relay failed to issue trip decision on 3 occasions,
compute dependability ،security and reliability of the relay.
4. Selectivity: It is used to detect and isolate the faulty item only.
The ability to decide whether this Fault falls within the limits of its work and its
zone of operation, or it is outside of it.
5. Sensitivity: It is used to detect even the smallest fault current or system
abnormalities and operate correctly at its setting before the fault causes
irreparable damage.
6. Adequateness: It means trying to design a high-efficiency protection
system at the lowest cost.
If the protection device is designed to detect all types of faults, the cost is
very high, it is often sufficient for the relay to be designed to detect only a
certain type of fault.
The factors that control the suitability of relays are:
 The power rating that the relay will protect.
 The place, importance and cost of this element
 Possible exposure to unusual situations due to internal or external
causes.
 The effect of the downfall of the element on the continuity of the
electrical supply, as some elements are of high importance, such as
the Generator, which if it downfalls, the supply will stop completely.
Adequateness must be taken into account when choosing a protection
system for the electrical network, and this depends on the efficiency of
proper planning. For example for a transformer with a 250kVA capacity it is
enough to use fuses with high cut-off capacity.
7. Speed:
The speed of isolating the fault reduces the possibility of a breakdown of the
equipment to be protected.
For example, if a Circuit Short occurs on the main distribution bars BB with a value
of 40 kA and is isolated in a time of example 80 ms, it is not expected that a
breakdown of these bars will occur despite the large current value, but if this fault
is not detected quickly enough and lasts for example for 5 seconds instead of 80
ms, then a complete breakdown of the BB will occur.
The speed of isolating the fault also helps to improve the stability of the electrical
network, and this is taken into account when developing the protection scheme for
high voltage lines, large generators, large transformers, motors, etc.
Delay time is sometimes desirable for the following reasons:1. To give an opportunity to distinguish between main protection and backup protection.
2. To overcome wrong operation of Relays in the following cases:
 Large currents generated when transformers start up (Inrush Currents)
 Currents resulting from transient faults.
 Change in loads.
Classification:
Based on speed, relays are classified into four types:
 Instantaneous: Operate as soon as a secure decision is made
 Time-delay: An intentional time-delay is inserted between the relay
decision time and the initiation of the trip action.
 High-speed: Operate in less than a specified time (3 cycles = 50 ms)
 Ultra-high-speed: Operate in 4 ms or less
8. Stability: It is used to leave all healthy circuits intact to ensure continuity or
supply.
Generations of Fault Protective Equipment’s:
First Generation: Electromagnetic/Electromechanical Relays
The idea of the relay is based on exploiting the property that the electric current
that passes through a coil always creates an accompanying magnetic field, and is
also accompanied by a magnetic force that can attract and move an iron arm
Sometimes this magnetic force is used to move a rotating iron disk, making it rotate
at a speed proportionate with the intensity of the current. This idea was used, as in
the case of the Induction Disc relays that shown in the figure.
The movement in both types is used to close another electrical circuit (the circuit
of the circuit breaker). Since the force that will move the arm, or turn the disc is
directly proportional to the strength of the current passing through the coil, and
therefore in normal conditions, where the current is small, this force will not be
enough to move the lever or turn the dial to close the circuit, while in the event of
a fault, where the value of the current rises sharply, this force will be sufficient to
make the required movement, and close the CB operating circuit.
This type of device is always stable, and is not affected by the vibrations that may
occur in the network, and the engineers have gained extensive experience in
dealing with these devices for many years, and this explains the reason why they
remain in service despite the emergence of many recent generations after them.
However, it is disadvantaged by the relatively slow response due to the fact that
the moving parts have inertia, so it needs time to start the movement, and one of
its shortcomings is that it needs regular maintenance of the moving parts, and it
needs calibration from time to time to ensure the accuracy of the measurement.
Second Generation: Static Relays
The second generation of methods of manufacturing protective devices appeared
in the early sixties, and it is known as the static relays, and the most important thing
that distinguishes it is that it dispenses with the moving parts used in the previous
generation, which was a source of errors in the work of the devices, as this
generation relied on what is known as the Operational Amplifier, which compares
the value of the current passing through the circuit with certain set limits. If the
current in the circuit exceeds the used setting values, the Op Amp sends a signal to
CB to disconnect it.
One of the most important defects of this generation is that the Op Amp devices
were affected by the change in temperature, and therefore they are unstable, and
therefore this quality did not last long, as in the late sixties and early seventies the
third generation of protection devices appeared, which is known as Digital Relays.
Third Generation: Digital/Numerical Relays:
This type is now widespread in the protective device market. The basic idea of this
new technology is to convert voltage and current signals into digital numbers that
are stored in the computer's memory, with updating them continuously during very
small periods of time, up to 1 ms and less.
It was possible to create new capabilities for the protection device that could not
be implemented using the old technology (Electromechanical or Static), so it was
possible, for example, to change the setting values automatically for the device,
and then the greatest development of these devices occurred after the
development of digital communication systems.
Fourth Generation: Adaptive Digital Relays:
The possibility of adjusting the setting values automatically, especially since the
relay has all the information about the network, and the challenge in this period
was how to distinguish, for example, between the natural increase in the load, and
the malfunction that creates a current close to the load current, so the device
should not be deceived by any increase.
Fifth Generation: Multifunction Digital Relays:
It adds the possibility to store many programs representing different devices within
one Relay.
The most important features of digital relays:
 Additional monitoring
functions.
 Functional flexibility.
 They can implement more
complex and accurate function.
 Self-checking and selfadaptability.
 Able to communicate with
other digital equipment (peer
to peer).
 Less sensitive to temperature
 More Accurate.
 Signal storage is possible.
 Low CT /PT burden.
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Metering.
Fault report.
Fault location.
Event logging.
Oscillography record/fault data
information.
Standard hardware.
Flexibility in operation.
Multifunction.
Communication.
Adaptive relaying.
Connectivity with SCADA.