Synchronized wide-area communication has become a mature technology, which makes the real-time interaction between the substations and the wide-area protection and control system possible. However, the present protection and control system to handle this real-time data has been recognized to be deficient. This paper begins by reviewing the development history of power system protection, with special attention paid to the recent development in the field of wide-area and integrated protections, in order to look into the future development of protection and control systems. Then the concept of integrated wide area protection and control is introduced, where it can be shown that a hierarchical protection and control system provides the protection and control for wide-area or regional power substations/plants and their associated power networks. The system is mainly divided into three levels: the local, the substation/plant, and the wide-area/regional. The integrated functions at each level are described in detail with an aim to develop an optimal coordination mechanism between each level. The key element in the proposed system is the wide-area real-time protection and control information platform, which not only enables the merger of three lines of defense for power system protection and control but also provides a perfect tool for the application of cloud computing in substations and power networks.
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North China Electric Power University
Final Report of Power System Protection
Title: POWER SYSTEM PROTECTION
Student name: Essam Arifi 🧭🕋 萨姆
Student ID: 120174100033
Department: ELECTRICAL ENGINEERING AND ITS AUTOMATION
Class: Undergraduate 3TH YEAR
Professor: 郑涛
Electronic Signature: Essam Arifi
(15/04/2020)
Abstract:
Synchronized wide area communication has become a mature technology, which
makes the real-time interaction between the substations and the wide area
protection and control system possible. However, the present protection and
control system to handle this real-time data has been recognized to be deficient.
This paper begins by reviewing the development history of power system
protection, with special attention paid to the recent development in the field of
wide-area and integrated protections, in order to look into the future
development of protection and control systems. Then the concept of integrated
wide area protection and control is introduced, where it can be shown that a
hierarchical protection and control system provides the protection and control for
wide area or regional power substations/plants and their associated power
networks.
The system is mainly divided into three levels: the local, the substation/plant,
and the wide area/regional. The integrated functions at each level are described
in details with an aim to develop an optimal coordination mechanism between
each level. The key element in the proposed system is the wide area real-time
protection and control information platform, which not only enables the merger
of three lines of defense for power system protection and control, but also
provides a perfect tool for the application of cloud computing in substations and
power networks.
KEYWORDS:
I. Introduction Power system protection
II. Three phase over current protection
III. Transformers Protection
IV distance protection (Transmission Line Protection)
V. Pilot Protection (Transmission Line Protection)
VI. Generator Protection
I. Introduction: Power system protection:
Power system protection emerged at the beginning of the last century, with the
application of the first electro-mechanical overcurrent relay. The majority of the
protection principles currently employed in protection relays were developed
within the first three decades of the last century, such as overcurrent, directional,
distance and differential protection, as shown in Fig. 1. The development of
modern science and technology, especially electronic and computer technology,
promoted the development of relay technology, such as materials, components
and the manufacturing process of the hardware structure of relay protection
device.
At the same time, great theoretical progress had been made in the relay protection
software, algorithms, etc. As shown in Fig. 1, the progress in modern technology
stimulates the development in power system protection. In the last century from
the emergence of protection to the end of the 1990s, the relay protection had gone
through a number of development stages, migrating from electro-mechanical to
semiconductor, and subsequently to integrated circuit and microprocessor
technologies. Today, microprocessor-based digital and numeric relays are
replacing conventional relays in all areas of power system protection. However,
many of the same relaying principles of protection are still playing a dominant role
to date. In the late 1960s, the application of a centralized substation protection
system based on a centralized computer system was proposed [1].
This constitutes an important milestone in the history of power system protection.
The idea fits well with the concept of an overall integrated protection where the
protection package would not only oversee individual units of a plant but also a
section of the network. However, the idea has not been widely applied until
recently, since there were no available computer hardware/software or
communication technologies to support such an idea. Since then, relay technology
has enjoyed successful developments based on the application of digital
techniques. The introduction of microprocessors into protection in the 1980s
generally followed the conventional approach with the implementation of
distributed processing platforms that concentrated on protecting individual units
of the system. Limited integrated protection was provided in the form of back-up
protection and thus remained a secondary function.
Fig. 1
History of power system protection
❖ What is protection relay?
Protection relay is a smart device that receives data compares them with ref erence
values, and delivers results. Incoming data can be current, voltage, resistance or
temperature. Results can include visual information in the form of indicator lights
and/or an alphanumeric display, communications, control warnings, alarms, and
power on and off. The diagram below answers the question of what is the
protection relay.
❖ What is System Protection?
System protection is the art and science of detecting problems with power system
components and isolating these components. Problems on the power system include:
1. Short circuits
2. Abnormal conditions
3. Equipment failures
✓ Input data
Relay needs information from the system to take a decision. These data can be
collected in various ways. In some cases, the cables in the area can be connected
directly to the relay. In other applications, additional equipment needed to convert
the measured parameters as in a form that can relay process. These accessories can
be carrier current transformers, voltage transformers, RTD (Resistance Temperature
Detector) or other equipment.
✓ Reference values
Many protection relays have adjustable parameters. The user sets the parameters so
that the relay respond to the parameterization limit. Relay compares input values
with the values of these parameters and responds accordingly.
✓ Processes
Once entries (data) are connected and the parameters are set in the relay, the relay
compares these values and takes a decision. Depending on the needs of different
types of relays are available for various functions.
✓ Outputs (Results)
Relay has different ways of communicating. In typical relay, order will act on a
switch (relay contact) to indicate that an input value has exceeded a parameter, or
relay can provide a notification by a visual feedback such as an instrument or LED.
One advantage of the electronic relays is the ability to communicate with a network
or PLC.
❖ Basic objectives of protecting the power system
✓ The main objective of protecting the power system is to provide quick isolation of the power
system's area with fault, in order that shock in the rest of system to be minimized and to be
intact for as long as possible. In this context, there are five basic aspects of the protection relay
application.
✓ Before we talk for them, it should be noted that the use of the term protection does not mean
that protective equipment can prevent problems, such as errors and equipment failures or
electrical shocks due to unintentional human contact. It cannot predict problems. Protection
relays operate only after an abnormal or intolerable situation that has occurred. This protection
does not mean prevention, but rather, minimizing the duration of the problems and limitations
of damages, the time of interruption, and similar problems.
❖ Five basic aspects of protection relay application are:
1. Reliability: security that protection will function correctly.
2. Selectivity: maximal continuity of service with minimum disconnection system.
3. The speed of operation: the minimum duration of the fault, damage to equipment and system
instability.
4. Simplicity: minimum protective equipment and related circuits to achieve the objectives of protection.
5. Economy: maximum protection with minimum cost.
❖ 1. Reliability
Reliability has two aspects, safety and security. Safety is defined as "the security levels which a
relay will function properly". The security has to do with "the level of security that a relay will not
act wrongly." In other words, safety it shows protection system's ability to work properly when
required, while the security is the ability to avoid unnecessary operations during normal operation.
➢ 2. Selectivity Relays have a designated area known as the primary protection zone, but they can
operate properly on responses to conditions outside this zone. In these cases, they provide backup
protection for the area outside the primary area.
Selectivity (coordination of the relay) is the application process and the establishment of relay
protection that surpasses other relays that they can act as quickly as possible within their primary area of
operation but has delays in the reserve area. This is necessary to allow the primary relay to operate in
the reserve area.
➢ 3. Speed
Of course, it is desirable for the protection to isolate the problem area as quickly as possible. In some
applications, it is not difficult, but in some other, especially when selectivity is involved, faster
operation can be achieved by more complex protection and higher costs.
➢ 4. Simplicity
A relay protection system should be simple and direct as much as possible. Each unit or added
components, which may provide improved protection, but it is not necessary under the protection
requirements should be considered very carefully.
➢ 5. Economy
It is essential to have maximum protection with minimum cost, and the cost is always a major factor.
The protection system with the lowest cost cannot be trusted, moreover, it can involve greater
difficulties in the installation and its operation, as well as higher maintenance costs.
❖ Current transformers:
Current transformer (CT) is a type of measuring transformers which is designed to produce
alternative current in the second winding that is proportional to the measured current in the
primary winding.
Current transformers reduce currents of high voltage systems to a much lower value and
provide a convenient way to safely monitor the flow of current in transmission lines using
a standard ampere meter. The working principle of current transformers is the same as an
ordinary transformer.
Unlike ordinary transformer, the current transformer has one or several windings in the
primary winding, while secondary winding, on the contrary, may have a greater number of
windings. In the secondary winding are normally selected to pass currents of 1A or 5A.
Current transformers for protection:
There are two requirements of current transformers for protection: they must have the rated
accuracy limit factor (RALF) and an adequate accuracy class for the application. The rated
accuracy limiting factor (RALF) represents the ratio between the accuracy of limiting current to
the primary nominal current:
𝑅𝐴𝐿𝐹 = 𝐼 1𝑙 / 𝐼1𝑛
where the primary limiting current is the current for which the error is less than 5% for the
accuracy class 5P or 10% for the accuracy class 10P.
Based on the IEC standards, the rated accuracy limit factor has the following values: 5 - 10 - 15
- 20 – 30
Rated output at rated secondary is the value, marked on the rating current plate, of the apparent
power in VA that the transformer is intended to supply to the secondary circuit at the rated
secondary current.
✓ Protection is the art or science of continuously monitoring the power system, detecting the
presence of a fault and initiating the correct tripping of the circuit breaker.
• The objectives of power system protection are to:
— Limit the extent and duration of service interruption whenever equipment failure, human error, or
adverse natural events occur on any portion of the system
— Minimize damage to the system components involved in the failure and Prevention of human injury
• Protection engineering concerned with the design and operation of "protection schemes".
• Protection schemes are specialized control systems that monitor the power system, detecting faults or
abnormal conditions and then initiate correct action.
• In this course the power system is considered as all the plant and equipment necessary to generate,
transmit, distribute and utilize the electric power.
*Typical Short-Circuit Type Distribution
• Single-Phase-Ground: 70 – 80 %
• Phase-Phase-Ground: 17 – 10 %
• Phase-Phase: 10 – 8 %
• Three-Phase: 3 – 2 %
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II. Three phase overcurrent protection
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1. Introduction
✓ Relay development begins with electromechanical relays.
Over the past decades, static relays were used, while nowadays microprocessor relays are used.
Electromechanical relays are the oldest generation of relay and have been in use for a long time, they
started to be used from 1900 until 1963. These relays have earned a reputation for accuracy, safety, and
reliability.
✓ Static relays
represent the second generation of the relays. These relays have started to be used around the early
60's. The term static means that the relay does not have mechanical moving parts in it. Compared to
electromechanical relays, static relays have a higher life expectancy, then they have a reduction in
noise during operation and react faster in case of any failure.
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✓ Microprocessor
relays represent the latest generation of relays, the most advanced generation of relays. Their use began
about two decades ago. Microprocessor technology allows these relays to have features like static relays
and even more. In these relays, the signals of currents and voltages from current and voltage
transformers first are processed as analog signals and then converted to digital signals for further
processing.
❖ Overcurrent protection is used in extra-high voltages and high-voltages networks as
the main protection, and in medium networks it is also used as backup protection
for power transformers.
❖ Time grading for overcurrent relays is used in the radial system.
❖ The minimum possible time delay is selected for the relay which protects the line
farthest from the source. The grading time between the successive relays will be in
the order of 0.4–0.5 seconds, and with modern relays it can be of the order of
0.35 seconds.
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Overcurrent protection:
Transmission and distribution systems are exposed to overcurrent flow into their elements. In an
electric power system, overcurrent or excess current is a situation where a larger than intended
electric current exists through a conductor, leading to excessive generation of heat, and the risk of
fire or damage to equipment.
Possible causes for overcurrent include short circuits, excessive load, transformer inrush current,
motor starting, incorrect design, or a ground fault.
Therefore, for normal system conditions, some tools such as demand - side management, load
shedding, and soft motor starting can be applied to avoid overloads. In addition, distribution
systems are equipped with protective relays that initiate action to enable switching equipment to
respond only to abnormal system conditions. The relay is connected to the circuit to be protected
via CTs and VTs according to the required protection function. In order for the relay to operate, it
needs to be energized. This energy can be provided by battery sets (mostly) or by the monitored
circuit itself.
❖ Overcurrent relays:
The basic element in overcurrent protection is an overcurrent relay. The ANSI device number is 50 for
an instantaneous overcurrent (IOC) or a Definite Time Overcurrent (DTOC) and 51 for the Inverse
Definite Minimum Time. There are three types of operating characteristics of overcurrent
relays:
✓ Definite(Instantaneous)-Current Protection,
✓ Definite-Time Protection and
✓ Inverse-Time Protection.
✓ Definite(instantaneous)-current protection
This relay is referred as definite(instantaneous) overcurrent relay. The relay
operat es as soon as the current gets higher than a preset value. There is no
intentional time delay set. There is always an inher ent time delay of the order
of a few milliseconds. 10 The relay setting is adjusted based on its location in
the network. The relay located furthest from the source, operates for a low
current value. Example, when the overcurrent relay is connected to the end
of distribution feeder it will operate for a current lower than that connected in
beginning of the feeder, especially when the feeder impedance is larger. In
the feeder with small impedance, distinguishing between the fault currents at
both ends is difficult and leads to poor discrimination and little selectivity at
high levels of short-circuit currents. While, when the impedance of feeder is
high, the instantaneous protection has advantages of reducing the relay's
operating time for severe faults and avoiding the loss of selectivity.
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Definite-time protection
In this type, two conditions must be satisfied for operation (tripping), current must exceed the setting
value and the fault must be continuous for at least a time equal to the time setting of the relay. This relay
is created by applying intentional time delay after crossing pick up value of the current. A definite time
overcurrent relay can be adjusted to issue a trip output at definite amount of time after it picks up. Thus,
is has a time setting and pick up adjustment. Modern relays may contain more than one stage of
protection each stage includes each own current and time setting. The settings of this kind of relay at
different locations in the network can be adjusted in such a way that the breaker closest to the fault is
tripped in the shortest time and then the other breakers in the direction toward the upstream network are
tripped successively with longer time delay. The disadvantage of this type of protection is that it's
difficult to coordinate and requires changes with the addition of load and that the short-circuit fault
close to the source may be cleared in a relatively long time in spite of its highest current value. Definite
time overcurrent relay is used as a backup protection of distance relay of transmission line with time
delay, backup protection to differential relay of power transformer with time delay and main protection
to outgoing feeders and bus couplers with adjustable time delay setting.
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Inverse-Time Protection:
In this type of relays, operating time is inversely changed with the current. So, high current will operate
overcurrent relay faster than lower ones. They are available with standard inverse, very inverse and
extremely inverse characteristics. Inverse Time relays are also referred to as Inverse Definite Minimum
Time (IDMT) relay. The operating time of both overcurrent definite-time relays and overcurrent
inverse-time relays must be adjusted in such a way that the relay closer to the fault trips before any
other protection. This is known as time grading. The difference in operating time of these two relays for
the same fault is defined as discrimination margin. The adjustment of definite-time and inverse-time
relays can be carried out by determining two settings: time dial setting and pickup setting. The time dial
setting adjusts the time delay before the relay operates whenever the fault current reaches a value equal
to, or greater than, the relay current setting. The time dial setting is also referred to as the time multiplier
setting. The tripping characteristics for different TMS settings using the IEC 60225 are shown in the
table to the right.
Pickup setting is used to define the pickup current of the relay by which the fault current exceeds its
value. It is determined by:
𝑃𝑖𝑐𝑘𝑢𝑝 𝑠𝑒𝑡𝑡𝑖𝑛𝑔 = 𝐾𝑙𝑑×𝐼𝑛𝑜𝑚/ 𝐶𝑇
where:
𝐾𝑙𝑑 – overload factor,
𝐼𝑛𝑜𝑚 – nominal rated current,
𝐶𝑇 – current transformer ratio
. As we can see from the Fig. 6.4.2 the VI curve is much steeper and therefore the operation increases
much faster for the same reduction in current compared to the SI curve. Very inverse overcurrent relays
are particularly suitable if there is a substantial reduction of fault current as the distance from the power
source increases. With EI characteristic, the operation time is approximately inversely proportional to
the square of the applied current. This makes it suitable for the protection of distribution feeder circuits
in which the feeder is subjected to peak currents on switching in, as would be the case on a power
circuit supplying refrigerators, pumps, water heaters and so on, which remain connected even after a
prolonged interruption of supply.
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Overcurrent protection function
Figure 1 Directional and non-directional protection is a single feeded power system
Current protection
Current protection schemes are integrated into power system designs to protect the power
system components from the excessive withdrawal of current and short-circuit currents.
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Excessive overcurrent could occur on many different power system components such as
motors causing hazards if not cleared within the safe time frame.
Short-circuit currents are undesired currents that occur in electrical circuits and flow along
paths, which have low impedances (almost zero) causing severe hazards.
There are different types of current protection schemes such as overcurrent, undercurrent
and phase-sequence. However, overcurrent protection is also sub-classified into other
categories such as non-directional overcurrent, directional overcurrent and
voltage-dependent overcurrent.
Overcurrent protections follow 19 well-defined time characteristic curves, which define the
time delay before the tripping angle as a function of the current. The curves are divided
according to standard into IEC and ANSI, and the most popular of these curves are the
definite time curve (DT), the extremely long inverse time (ET), the very long inverse time
curve and the normal inverse time curve.
Figure 2 Inverse time characteristics of three different curve groups, the normal inverse, the very inverse and the extremely
inverse
Non-directional overcurrent
Non-directional overcurrent is a protection scheme developed to protect power system
equipment from overcurrent and short-circuit currents regardless of the direction of current
flow.
The overcurrent protection function utilizes different stages for alarming and tripping. It
consists of three stages, the low stage, the high stage and the instantaneous stage. The low
stage is the stage when set provides an alarm signal, the high stage is the stage which is
usually used to trip overcurrent faults and finally the instantaneous stage is the stage used for
tripping short-circuit currents.
In modern numerical protection relays, each stage has its own unique settings including the
time characteristic curve selection.
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Directional overcurrent
Directional overcurrent is a protection scheme developed to operate on a desired overcurrent
value flowing in a predetermined direction. It is commonly used in power systems with ring
configuration and single supply to protect all feeders where the energy flow direction
depends on fault location.
It is also common in power systems with parallel feeding transformers.
Figure 3 Directional protection scheme in a power system with two parallel operating transformers, no current is
allowed to go back to the transformers from the downstream side
Voltage-dependent overcurrent protection
Voltage-dependent overcurrent protection is a protection scheme which is quite similar in
operation to non-directional overcurrent protection, except that the start current depends on
the operating voltage.
In other words, the function will not send any signals even though the set current has been
reached, unless the voltage also reaches a set value. Voltage-dependent overcurrent
protection is commonly used in generator protection.
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Figure 4 The current protection starts only when the voltage reaches a certain value, thereby
voltage-dependent value
Current protection conclusion
There are several current protection functions, however in this article we focused on three
of the most common overcurrent protection functions to acquaint the reader with their
basic operating principles. While they are all bound to the same time characteristic curve
groups, non-directional overcurrent is a protection function which is used to protect power
system element from overcurrent and short-circuit currents flowing in any direction of the
power system.
Directional over current protection functions are developed to protect the power system
from excessive current flowing in an already known direction and voltage-dependent
overcurrent protection is used to protect the equipment at predefined voltage levels.
Overcurrent protection devices
Overcurrent protection is critical to personal safety and protection from a number of hazardous
conditions that can result from materials igniting due to improper overload protection
or short-circuit protection. Additionally, the OCPD guards against explosive ignition and flash
hazards from inadequate voltage-rated or improper interrupting-rated overcurrent protective
devices. Overcurrent protective devices, or OCPDs, are typically used in main service disconnects,
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and in the feeders and branch circuits of electrical systems for residential, commercial, institutional,
and industrial premises (Figure 5.3).
Figure 5.3. Overcurrent protection devices include circuit breakers and fuses.
Overcurrent protection devices are meant to protect against the potentially dangerous effects of
overcurrent, such as an overload current or a short-circuit current, which creates a fault current.
Equipment damage, personal injury, and even death can result from the improper application of a
device's voltage rating, current rating, or interrupting rating. Something as simple as a circuit
breaker can protect against this damage, but if a fuse or circuit breaker doesn't have an adequate
voltage rating, it can rupture or explode while attempting to stop fault currents beyond their
interrupting ratings. Grounding helps to protect against inadequate overcurrent protection or OCPD
failure. The two processes are designed to work together to protect equipment, property, and
people.
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III. Transformers Protection:
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Introduction
.
List of types of protection used above 5MVA transformer
– Overload protection (thermal relays or temperature monitoring systems)
– Gas detector relay (Buchholz relay)
– Overcurrent protection
– Ground fault protection
– Differential protection
– Pressure relay for tap-changer compartment
– Pressure relief device
– Overvoltage protection
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❖ General
When a fault occurs in a transformer, the damage is proportional to the fault time. The
transformer should therefore be disconnected as fast as possible from the network. Fast reliable
protective relays are therefore used for detection of faults.
Monitors can also detect faults and they can sense abnormal conditions which may develop into
a fault.
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The size of the transformer and the voltage level have an influence on the extent and choice of
protective equipment. Monitors prevent faults and protective relays limit the damage in case of
a fault. The Gast for the protective equipment is marginal compared to the total cost and the
cost involved in case of a transformer fault.
There are often different opinions about the extent of transformer protection. However, it is
more or less normal that transformers with an oil conservator are furnished with the following
equipment:
Transformers larger than 5 MVA/:
1. Gas detector relay (Buchholz relay)
2.Overload protection (thermal relays or temperature monitoring systems)
3.Overcurrent protection.
4.Ground fault protection
5.Differential protection
6.Pressure relay for tap-changer compartment.
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7.Oil level monitor
Transformers smaller than 5 MVA.
*Gas detector relay (Buchholz relay)
*Overload protection
*Overcurrent protection
*Ground fault protection
➢ Differential protection for autotransformers figures:
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How to Protect Transformers?
For power transformers, the protection is to be provided usually against dangerous overloads and
excessive temperature rise. Dangerous overloads may be due to external faults or the internal
ones. External faults, however, are cleared by the relay system outside the transformer within the
shortest possible time in order to avoid any danger to the transformer due to these faults. Hence
the protection for internal faults is to be provided in such transformers.
Differential protection is the most important type of protection used for protection against
internal phase-to-phase and phase-to-earth faults. The other protection systems employed for
protection of transformers against internal faults are Buchholz protection, core-balance leakage
protection, combined leakage and overload protection, restricted earth-fault protection.
Buchholz Protection of Transformers:
On the occurrence of internal fault in an oil-immersed power transformer gas is usually
generated, slowly for an incipient fault (such as sparking, small arcing, loose connections in
conducting path etc.,) and violently for heavy faults. Most short circuits caused either by
impulse breakdown between adjacent turns at the end turns of the winding or as a poor initial
point contact which will immediately heat to arcing temperature.
The heat generated by the large local currents causes the transformer oil to decompose and
generate gases, which can be used in detection of winding faults.
The relays based on this principle are:
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(i) Pressure relays and pressure relief devices which act on the measurement of the total
accumulated pressure,
(ii) Rate of pressure rise relay, which acts on the measurement of the rate of formation of the
gas and
(iii) Gas accumulator relay, most commonly known as Buchholz relay, actuated by the gas
formed.
Buchholz protection employing Buchholz relay is the simplest form of protection and is most
commonly used on all oil-immersed transformers provided with conservator.
Core-Balance Leakage Protection of Transformers:
An earth fault usually involves a partial breakdown of winding insulation to ground. The
resulting leakage current is quite small as compared to short-circuit current. The earth fault
may continue for a long time and cause considerable damage before it ultimately develops into
a short circuit and removed from the system. Under such circumstances it is advisable to
provide earth-fault protection in order to ensure that the earth fault or leak is removed in the
early stages. An earth-fault relay used for it is essentially an overcurrent relay of low setting
and operates as soon as earth fault or leak develops. One method of protection against earth
faults in transformers is the core-balance leakage protection.
This system consists of three primary conductors surrounded by the magnetic circuit of a
current transformer. This has a single secondary winding which is connected to the relay
operating coil. Under normal conditions i.e., when there is no earth fault the instantaneous sum
of the currents in the three phases is always zero, and there is no resultant flux in the core of
the CT no matter how much the load is out of balance.
Thus, no current flows through the relay operating coil and trip circuit remains open. When an
earth fault occurs, the sum of the three currents is no longer zero and a current is induced in the
secondary of the CT causing the trip relay to operate and isolate the transformer from the
bus-bars.
Combined Leakage and Overload Protection of Transformers:
The core-balance protection described above suffers from the disadvantage that if the fault
occurs between phases the relay does not operate. This shortcoming is overcome by employing
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three separate CTs. In this system of protection two overload relays and one earth leakage
relay are connected. The overload relays used are high current setting ones and are arranged to
operate against phase-to-phase faults while the earth fault relay has low current setting and
operates under earth or leakage faults only.
The two overload relays are sufficient to protect all the three phases while the earth-fault or
leakage-fault relay is energized by the resultant currents from all the three CTs in case of
leakage fault. The trip contacts of the overload relays and earth-fault or leakage relay are
connected in parallel, as illustrated in the figure. So the circuit breaker will trip in the event of
energization of either overload relay or leakage relay. Thus, the protection against faults and
short circuits either to earth or between phases is achieved.
Biased Differential Protection of Transformers:
In order to avoid undesirable operation on heavy external faults due to CT's errors and ratio
change as a result of tap changing use of biased or percentage differential relay is made,
restraining winding being energized by the through current. Figure 9.4 shows the arrangement
of percentage differential relaying for power transformers.
The power transformer is star connected on one side and delta connected on the other. The CTs
on the star-connected side are delta-connected and those on delta-connected side are
star-connected. The neutrals of CT star and power transformer star connections are grounded.
The restraining coils are connected across the secondary windings of CTs. The operating coils
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are connected between the tapping points on the restraining coils and the star point of the CT
secondary windings.
The operating coils normally carry no current as they are balanced on both sides of the CTs.
On the occurrence of internal fault in the power transformer windings, the balance is disturbed
and the operating coils of the differential relays carry current corresponding to the difference
of the current between the two sides of the power transformers and operate the relays to trip the
main circuit breakers on both sides of the power transformer.
Harmonic Restraint Relay of Transformers:
The operation of the relays because of magnetizing inrush current can be avoided by using kick
fuses across the relay coils or using relays with inverse and definite minimum time (IDMT)
characteristics. However, for EHV transformers, the relay current and time ratings necessary to
ensure stability on the magnetizing inrush current caused by switching-in the transformer are
not adequate for providing high speed protection.
A high speed biased differential relay incorporating a harmonic restraint feature is immune to
the magnetizing inrush current. The magnetizing inrush currents have a high component of
even and odd harmonics (about 63% of second harmonics and 26.8% of third harmonics) while
harmonic component of short-circuit currents is negligible. The use of these facts is made for
restraining the relay from operation during initial current inrush.
The harmonic restraint differential relay is sensitive to fault currents but is immune to the
magnetizing currents. The operating coil of the relay carries only the fundamental component
of current only while the restraining coil carries the sum of the fundamental and harmonic
components.
Basic circuit of a harmonic restraint differential relay is illustrated in Fig. 9.8. The restraining
coil is energized by a direct current proportional to bias winding current as well as the direct
current due to harmonics. Harmonic restraint is had from the tuned circuit (XC – XL) that allows
only the fundamental component of current to enter the operating circuit.
The dc and higher harmonics (mostly second harmonics) are diverted into the rectifier bridge
feeding the restraining coil. The relay is adjusted so that it will not operate when the harmonic
current exceeds 15% of the fundamental current. Both the dc and higher harmonics are of large
magnitude during magnetizing inrush.
The relay may fail to operate due to harmonic restraint feature if an internal fault has
considerable harmonics that may be present in the fault current itself due to an arc, or due to
saturation of CT. Also, if a fault exists at the instant of energization of transformer harmonics
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present in the magnetizing current may prevent the operation of the relay. This problem can be
overcome by providing instantaneous overcurrent relay in the differential circuit which is set
above the maximum inrush current but will operate in less than one cycle on internal faults.
Thus, fast tripping is ensured for all internal faults.
The other method used is harmonic blocking. In this method the harmonic component of
magnetizing inrush current is used for blocking a separate relay, called the blocking relay,
whose contacts are in series with the contacts of the differential relay. The blocking relay
contains a 100 Hz blocking filter in operating coil and 50 Hz blocking filter in the restraining
coil. During inrush currents the second harmonic component is predominant and the blocking
relay is blocked. The blocking relay contacts remain open. During short circuits, fundamental
component is predominant, so blocking relay operates and relay contact circuit is closed.
Self-Balance Protection System of Transformers:
The self-balance protection system for the protection of alternators can also be employed to
power transformers without any modification except that the same type of equipment has to be
employed for both primary and secondary sides. The protective transformers (CTs) can be
located in the oil of the transformer tank. This system of protection of power transformers is
not much used because it cannot provide protection to transformer terminals and the connected
cables up to switchgear.
Differential Magnetic Balance Protection System of Transformers:
This system is necessarily a combination of circulating current and self-balance protection
systems. The main advantages of this protection system are increased stability and sensitivity
and its application to power transformers provided with load tap-changers. Figure 9.9 illustrates
a scheme representing a power transformer having primary (low-voltage) side connected in delta
and secondary (high-voltage) side connected in star.
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The current transformers used are bus type and are connected for circulating current protection.
Their current ratings are so adjusted that they provide equal secondary currents. The relay
operating coil instead of being connected to the equipotential point of the pilot wires is fed from
another winding on hv side CT (CT2). Since the core flux of Iv side CT (CT1) is more because of
its more ampere-turns, the turns of hv side CT are increased so as to make core flux of CT2 zero
under all load conditions resulting in no current flowing in the relay operating coil under normal
operating conditions. The CT2 is made with high permeability alloy core so as to reduce
magnetizing current and provide an accurate balance.
For any through fault CT2 continues to have no flux and so the relay operating coil remains
inoperative. On occurrence of fault in protection zone, say at F, excessive current flows through
CT1 causing flow of current in the relay operating coil. Thus, relay is energized and the circuit
breaker gets tripped.
Self-Stabilizing Magnetic Balance Protection System of Transformers:
For the protection of power transformer having tapings it is necessary that the protective CT
connected on hv side (i.e., CT2) must also be capable of changing its current ratio whenever
power transformer tapping are changed i.e., CT2 windings need some modification.
It is explained as- in Fig. 9.10 (a) plain magnetic balance protection system is illustrated, the
relay connections are not shown for sake of simplicity. CTs used are bus-bar type. In Fig. 9.10
(b) self-stabilizing circuit for magnetic balance protection system is shown. In this circuit the
magnetic core of CT2 is divided into two halves P1 and P2 and the secondary winding is so
wound that the flux developed by the two halves P1 and P2 is equal and opposing each other.
Thus in normal operating conditions no emf is induced in the secondary winding and the relay
operating coil remains inoperative.
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When the transformer is operating under normal operating conditions and carrying full-load
currents, the flux developed by the two halves is equal and relay winding coil is un-energized.
Now when the tapings of the main transformer are changed, mmfs of the two halves are
changed causing the flux developed by them to be different. So, an emf, proportional to the
difference of the two fluxes, will be induced in the relay coil, as shown in Fig. 9.11.
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If under this tap changed condition the load on the transformer is increased, mmf of the two
halves will increase but the difference of fluxes developed will decrease. Thus with the
increase in load on power transformer, the difference in fluxes developed by the two halves of
core of CT2 decreases, as shown in Fig. 9.11. Now if the relay is so designed that its minimum
operating voltage is much more than the induced voltage under any desirable load condition
but with no fault, as illustrated by OD in Fig. 9.11, then stability is ensured. In practice OD is
made twice the minimum ordinate.
Restricted Earth-Fault Protection of Transformers:
Earth fault relays connected in residual circuit of line CTs provide protection against earth
faults on the delta or unearthed star-connected windings of power transformers. The
connections of restricted earth-fault protection for star-connected and delta-connected
windings are shown in Fig. 9.12. A CT is fitted in each connection to the protected and the
secondaries of CTs are connected in parallel to a relay.
Ideally, the output of the CTs is proportional to the sum of zero sequence currents in the line
and the neutral earth connection if the latter is within the protected zone. For external faults
zero sequence currents are either absent or sum to zero in the line and neutral earth connection.
For internal faults, the sum of zero sequence currents is equals twice the total fault current.
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In Fig. 9.13 the star-connected side is protected by restricted earth-fault protection.
When there is an earth fault outside the protective zone, say at F1, it causes the currents I, and
I1 in CT secondaries as illustrated in Fig. 9.13. So the resultant current in earth fault relay is
negligible. For an earth fault within the protected zone, say at F 2, only current I2 exists, being
negligible. Thus current I1 flows through the earth-fault relay. Thus restricted earth-fault relay
does not operate for earth fault beyond the protective zone of the transformer.
For an earth fault near the neutral point of the transformer the voltage available for driving
earth fault current is small. For the relay to sense such fault, it has to be too sensitive and
would, therefore, operate for spurious signals, external faults and switching surges. Hence the
relay is set as per practice, so as to operate for earth fault current of the order of 15% of rated
winding current. Such setting protects restricted portion of the winding, hence the name
restricted earth-fault protection. Stabilizing resistor is connected in series with the relay to
avoid magnetizing inrush current and also saturation of CT core.
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Frame Leakage Protection of Transformers:
The transformer is mounted insulated from the ground, as illustrated in Fig. 9.14. The
transformer tank is connected to earth through a CT to which an instantaneous earth fault relay
is connected. In the case of an earth fault in the transformer (breakdown of insulation in any
winding of the transformer), there is a flow of current to the earth over this connection causing
the relay to operate. Such an arrangement is usually provided where banked transformers are
provided with a single overall differential protection and it is difficult to find as to which
transformer is faulty.
Generator-Transformer Unit Protection:
In hv transmission the bus-bars are operated at higher voltages than that of generation; it is
common practice to connect the generators directly to step-up transformers. In this protection
scheme no circuit breaker is interposed in between the generator and transformer. The main
advantage of such protection is that it simplifies the protection, mainly the differential
protection for both generator and transformer can be combined together by employing CTs on
the neutral side of the generator and on the hv side of the power transformer, as illustrated in
Fig. 9.15.
Because of the occurrence of magnetizing inrush current transients the relay settings in this
protection scheme must be considerably higher than those for protecting a generator only. The
zone of differential protection includes the stator windings of the generator, the step-up
transformer and the intervening connections.
It is necessary to take care of the phase shift within the power transformer and the connections
of CTs. If a unit transformer is tapped off at the generator terminals, this also has to be taken
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care of by suitable connections of the CTs for protection. CTs located on the neutral side of the
generator are star-connected while the CTs on the secondary (hv) side of the main transformer
are delta-connected so as to cancel the 30° displacement between line currents introduced by
delta-connected primary of the main transformer.
The unbalance caused between CT pairs due to load of unit transformer is avoided by
providing another set of star-connected CTs in the primary leads of the latter. In healthy
condition, the sum of secondary currents of these CTs and the secondary currents of the
generator star-point CTs is equal to the currents in the pilot wires from the secondaries of the
delta-co nnected CTs on the secondary side of the main transformer. On occurrence of fault
differential relays are energized. The hv winding of the main transformer is protected against
earth faults by the restricted earth fault protection scheme.
From the schematic diagram of generator-transformer unit protection shown in Fig. 9.15, it is
obvious that the stator winding of the generator and the LV or primary windings of the main
transformer and unit transformer comprise a separate circuit having no electrical connection
with the hv circuits. So an earth fault at any point of this separate circuit will cause a flow of
current through the earth connection and through a PT connected in series with it. An alarm
relay connected across the secondary winding of PT will get energized and give the necessary
signal.
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IV distance protection (Transmission Line Protection):
Transmission lines can vary in length from several hundred feet to several hundred
miles, and in voltage (line-to-line) from 46KV to 750KV. Construction can be
simple, such as a single wood pole with insulators atop a crossarm, with little
spacing between the conductors and from the conductors to ground. At the other end
of the scale are metal lattice structures with bundled conductors (2 or more
conductors per phase) with large spacing between conductors and between
conductors and ground.
Faults
"Faults come uninvited and seldom go away voluntarily." Fault Types:
●Single line-to -ground
● Line-to-line
● Three Phase
● Line-to-line-to -ground
How Do We Protect Transmission Lines?
A. Overcurrent
B. Directional Overcurrent
C. Distance (Impedance)
D. Pilot
1. DCB (Directional Comparison Blocking
2. POTT (Permissive Overreaching Transfer Trip)
E. Line Current Differential
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V. Pilot Protection(TRANSMISSION LINE PROTECTION ):
Background
Electric power systems are most often divided in to three main units:
the generating unit, transmission unit and distribution unit. In such a
system the transmission units have the main responsibility of
supplying the generated electric energy to distribution units, where the
major electric consumers are located. Since delivery of electric energy
to consumers is the aim of electric systems there is a great importance
for operation of transmission unit. The transmission lines are the
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means of transmitting the electric energy over long distances in a
typical transmission unit. Transmission lines can be in form of
overhead lines, cables or combination of them. Among them the
overhead lines are more exposed to faults as a result of their operating
environment. Flashovers between conductors or to ground, or across
insulators can be caused by: lightni ng; breakage of conductors by the
thick ice coatings or violent swings during stormy conditions [1].
These flashovers together with faulty equipment's and aging problems
outstand the unavoidable demand of proper protection scheme for
accurate fault detection on transmission lines.
➢ TRANSMISSION LINE PROTECTION
✓ Introduction
A power system is a complex network with the main responsibility of supplying
reliable electrical energy to consumers within the entire network. Moreover, power
system has dynamic characteristics that acquire the balance between generation and
consumption of electricity in the system. It may experience transient instability
conditions before reaching a new steady state operating condition. Elements in such
system are usually designed to operate in normal operating conditions and transient
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instability conditions with fixed marginal operating boundaries. However, each
disturbance in operation of one or more equipment can lead to abnormal system
operating condition. These abnormal conditions may cause severe equipment
damages. Therefore, to prevent severe damages and maintain the normal operating
condition, monitoring of individual elements' operation and development of
protection schemes for entire system are of great importance. Usually different
protection schemes with responsibility to protect the electrical equipment located in
identified overlapping zones known as protection zones provide protection for an
entire system, as shown in Figure 2.1.
➢ Dependability: the protection scheme must be able to operate
correctively when it is expected to operate. This is the degree of
certainty that the scheme will operate correctly.
➢ Security: the protection scheme must be able to avoid unnecessary
operation in nonfaulty conditions or inception of faults outside their
protection zones. This is the degree of certainty that the scheme will
not operate incorrectly.
➢ Sensitivity: the protection scheme must be able to detect the changes
in the system in order to distinguish between normal and faulted
operation of power system elements.
➢ Selectivity: the protection scheme must be able to correctly locate the
inception of the fault and discriminate between the faults inside its
protection zone and outside the protection zone.
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➢ Fast operation: it is desirable that the protection scheme isolate the
fault affected parts of the system as rapidly as possible in order to
maintain stability of the overall power system and reduce damages to
equipment and property. 4 The protection scheme contains
measurement equipment, protection relays, circuit breakers, etc. The
relays play the most important role in protection scheme because they
detect the fault, determine fault location and also by closing trip coils
send tripping command to circuit breakers. Design of such relays
undertakes considerable technological developments from the first
immerged relays and can be classified as [6]:
➢ Electromechanical relays - in the earliest design of protection relays
moving mechanical parts were utilized. Working principal of
electromechanical relays is based on the electromagnetic interactions.
In these relays by inception of fault the current flow in one or more
windings on a magnetic core or cores results in mechanical forces
required to move the mechanical parts.
➢ Solid-state relays - these relays perform the same functions as
electromagnetic relays by use of analogue electronic devices instead
of coils and magnets. Therefore, they can be considered as an
analogue electronic replacement for electromechanical relays. The
protection functions are acquired by an analogue process of measured
signals. Electronic devices such as transistors and diodes in
combination with resistors, capacitors, inductors, etc., enable the
signal processing and implementation of protection algorithms in
such relay design.
➢ Numerical relays – in numerical relays microprocessors are used for
implementation of the same protection logics as in case of the static
relays. Processing of signals is carried out by conversion of input
analogue signals into a digital representation and processing
according to the appropriate protection algorithms implemented in
the processors.
Nowadays, numerical relays utilizing digital techniques are used to
protect almost all components of power systems. Furthermore, a large
number of functions previously implemented in separate protection
relays can now be integrated in a single numerical relay unit.
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Overview of transmission line protection schemes
In protection of transmission lines, the length of lines together with several factors related
to the power system's requirements and configurations affect the selection of a proper
scheme. In IEEE for protection studies, in spite of using physical length of the line
between both ends, a classification according to Source-to-line Impedance Ratios (SIRs) is
given, [8], where:
➢ Transmission line with 4 SIR defined as short line,
➢ Transmission line with 0.5൏SIR ൏4 defined as medium line,
➢ Transmission line with SIR 0.5 defined as long line.
Obviously, nominal operating voltage level of a line has a significant effect on the SIRs
and therefore this classification will be assigned to different physical lengths in the
following lines:
➢ High Voltage (HV) transmission lines with voltage levels of 69-230kV,
➢ Extra High Voltage (EHV) transmission lines with voltage levels of 230-350kV,
➢ Ultra-High Voltage (UHV) transmission lines with voltage levels over 350kV.
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❖ In this classification the protection schemes for transmission lines are
divided in the following two main categories:
➢ Non-pilot schemes are the schemes in which relays installed in each terminal
of the transmission line do not have any communications with each other an d
the relaying decision is only made by analysis of local measurements at
location of the relay.
➢ Pilot schemes are the schemes in which relays installed in both end of the
transmission line utilize a communication link in order to make a relaying
decision. The following sections present an overview of both categories and
protection schemes belonging to each group.
❖ Non-pilot schemes
Non-pilot protection schemes are usually applicable to short or medium
transmission lines. Directional over current and ste p distance protection
schemes belong to this group of transmission protection scheme.
Directional over current protection scheme
The working principal of the relays used in this scheme is the same as over current
relays widely used for line protection in radial networks. By monitoring current
magnitude of the protected circuit and assigning proper boundaries for deviations of
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current in specific time spans the relaying decision is acquired.
However, to utilize the same principal for interconnected meshed transmission lines,
where the current could flow in both directions, the over current schemes are
enhanced by introducing a reference quantity which can provide the directionality.
These schemes are the simplest and least expensive form of fault protection
schemes and are used widely for protection of transmission lines
Step distance scheme the distance protection scheme
used in non-pilot application is called "step distance" or "zoned distance". Step
distance is inherently a directional protection scheme and essentially measures the
impedance to the fault point from the relay location. Therefore, it is also capable of
identifying the location of the fault on the protected transmission line.
The tripping steps introduced in this scheme are adjusted by time delays in order to
provide higher selectivity. The first step is an instantaneous relaying step and is set
to operate for occurrence of faults located in Zone 1 of the protected line. This is a
distance on transmission line with approximately 80–90% of the line impedance. To
avoid overreaching operation of the relay or unnecessary operation for faults
beyond the remote terminal, the remaining distance plus some margins beyond the
remote end called Zone 2 will be protected after some time delay. Usually a third
step of operation is defined, which can be used as a backup for Zone 2 or even the
relay at remote bus. Consequently, the tripping function for Zone 3 must be time
delayed to coordinate with the Zone 1 and Zone 2 of the remote relay
❖ Pilot schemes
The non-pilot protection schemes discussed in previous section have usually an
acceptable performance on short or medium lines. However, for long lines which
are mostly operating in EHV or UHV levels and transmitting large electric power,
the tripping time delays would cause severe network stability problems due to the
system acceleration. Also, the huge fault currents could cause dramatic damages for
equipment. In such cases, more complex transmission line protection schemes are
required in order to perform a high-speed tripping in both ends of the line.
An alternative protective scheme which has been in use for protection of EHV/UHV
transmission lines, utilizes local information, as well as remote information for a
relaying decision. In this category known as "pilot protection schemes "the relays
installed at terminals, as shown in Figure 2.4, are able to make a common decision
about tripping the line in case of fault inceptions inside the protection zone
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❖ Unit Protection Schemes
Two important unit pilot protection schemes are identified as longitudinal
differential and phase comparison schemes. In such schemes the main
communicated information between the ends of the protected line are either
amplitude and/or phase data of the transmission line components.
In case of an internal fault the result of the compared data will be a differential
value and for specific threshold values the relays in both terminals perform a
relaying operation. Since there is an instantaneous comparison between the analog
values, the information acquired from both relays needs to be time synchronized to
guarantee the comparison of measured data at same time instants from both ends.
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❖ Longitudinal Differential Scheme
Longitudinal pilot systems are based on basic principle scheme proposed by
Charles H. Merz and Bernard Price in 1904. The operation principle of the relay is
expressed by Kirchhoff's first law that says: "the sum of the currents flowing to a
node must be equal with the sum of the currents leaving the same node" [1].
In external faults the same current is entering to protected zone and leaving it from
the second end. But in case of internal fault the current entering the protected zone
is not equal to the current which is leaving the same zone. Therefore, this principal
could be utilized in directional protection schemes for protection of transmission
lines.
❖ Phase Comparison Scheme
In a phase comparison scheme the relay is able to distinguish an internal inception
of the fault on protected transmission line by comparing the current phase angle at
one end with current phase angle at the second end. Where in case of the internal
faults there will be a notable phase difference [10]. However, incorrect op eration of
the relay can happen by changing the system configuration which may affect the
polarity of the quantities used for directional comparison
➢ Non-unit Protection Schemes
Two important non-unit pilot protection schemes are identified as distance and
directional comparison schemes. In such schemes the logical information typically
related to direction of the fault are sent over the communication link for a common
relaying decision. Therefore, there will be less dependency on data synchronization
comparing to unit protection schemes
➢ Distance Scheme
Communication link between relays in pilot distance schemes can eliminate the
time delays for relay decision makings in case of occurrence of faults in second or
even third zones for distance protection schemes. Thus, the local relays can
communicate with the remote relay in order to make sure that the detected fault is
located on protected zone. This provides fast directional fault detection as well as
opportunity of implementing the step distance relays in protection of long
transmission lines
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VI. Generator Protection
Introduction
This course covers generator protection concepts and theory. Protective devices that
are described in this course can be used in multiple generator protection
configurations. There are various protection relays and those are used for protection
against a wide variety of conditions. Protection relays protect the generator, prime
mover, external power system or the processes it supplies. The fundamental
principles that are covered in this course are equally applicable to individual relays
and to multifunction numeric relays. The protection engineer has to balance the
expense of using a particular protection relay against the consequences of losing a
generator. The total loss of a generator may not that bad especially in situations
when it represents a small portion of the investment in an installation. However, the
effect on service reliability and upset to loads has to be considered. Damage and
product loss in continuous processes can represent the major concern rather than the
generator unit. Hence, there is no universal protection solution based on the MW
rating. However, it is expected that a 500kW, 480V, standby reciprocating engine
will have less protection elements and simpler protection arrangement than a
400MW base load steam turbine unit. One typical dividing point is that the extra
CTs required for current differential protection are less commonly encountered on
generators less than 2MVA, generators rated less than 600V, and generators that
never work in parallel with other generation.
This course explains protection relay selection process by detailing how to protect
against each fault type or abnormal condition. Also, recommendations are made for
what is considered to be minimum protection as a baseline. After making the
baseline, extra protection relays, may be introduced. The topics included in this
course are as follows:
- Earth Fault (50/51-G/N, 27/59, 59N, 27-3N, 87N)
- Phase Fault (51, 51V, 87G) - Backup Remote Fault Detection (51V, 21)
- Reverse Power (32)
- Loss of Field (40)
- Thermal (49)
- Fuse Loss (60)
- Overexcitation and Over/Undervoltage (24, 27/59)
- Inadvertent Energization (50IE, 67)
- Negative Sequence (46, 47)
- Off-Frequency Operation (81O/U)
- Sync Check (25) and Auto Synchronizing (25A)
- Out of Step (78) - Selective and Sequential Tripping
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Generators are designed to run at a high load factor for a large number of years
and permit certain incidences of abnormal working conditions. The machine and its
auxiliaries are supervised by monitoring devices to keep the incidences of abnormal
working conditions down to a minimum. Despite the monitoring, electrical and
mechanical faults may occur, and the generators must be provided with protective
relays which, in case of a fault, quickly initiate a disconnection of the machine from
the system and, if necessary, initiate a complete shutdown of the machine. No
international standards exist regarding the extension of the protective schemes for
different types and sizes of generators. The so called "common standard" varies
between different countries and also between power companies within the same
country, depending on their past experience and different ways in which fault
statistics may be interpreted. A relay manufacturer working on the international
market should, therefore, be able to offer a protective system which can be easily
modified to meet different requirements from different users.
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REFERENCES
2. Introduction_to_System_Protection-_Protection_Basics
3. Lennart Söder, KTH School of Electrical Engineering
4. Copyright @ Power Research & Development Consultants Pvt. Ltd
5.https://na.eventscloud.com/file_uploads/8b5452d7f9376912edcba156cd1a5112_WSU_GENPROTOV
ERVIEW_180305.
6. IEEE Std C37.91-2000 IEEE Guide for Protective Relay Applications to Power Transformers
7. RADSB Transformer differential relay 803-5012 E
8. RAKZB Three-phase impedance relay 803-3213 E
9. RAZOA Three-zone, phase and ground distance relay for transmission lines
10. ABB Relays AB, 5-721 71 Västerås, 5weden
11. IEEE Devices used in Transformer Protection
12.
https://na.eventscloud.com/file_uploads/fcdbd21cac1909692839b242e46c9a3c_TransformerProtection_
_180306.pdf
13. Power System Protection Lecture Notes Mohammed T. Lazim Alzuhairi
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14. https://www.sciencedirect.com/topics/engineering/overcurrent-protection
15. https://electrical-engineering-portal.com/res/res4/The-Basics-Of-Overcurrent-Protection.pdf
16. file:///C:/Users/ESSAM%20%20ARIFI/Downloads/25862722%20(1).pdf
17. http://engineering.electrical-equipment.org/
... Elas são caracterizadas pelo desvio do fluxo de corrente elétrica para um caminho indesejado e interrupção brusca no caminho natural -causados por um curto-circuito em determinado ponto da rede. (OZA et al., 2010). ...
... Segundo Oza et al. (2010), as faltas podem ser classificadas, de forma geral, tanto pelo tipo de curto-circuito, quanto pelo tempo de duração -possuindo causas e consequências diferentes. Tratando-se da integridade da rede de distribuição, quanto maiores forem o tempo de duração e a corrente de curto-circuito, maior a probabilidade de dano térmico nos condutores, ruptura térmica de isolação, falha em isoladores e equipamentos elétricos. ...
... Faltas simétricas, ou equilibradas, são curtos-circuitos que envolvem as três fasesgerando um fluxo de potência similar em todas fases. São exemplos os curtos-circuitos trifásico e trifásico-terra, representados na Figura 3. (OZA et al., 2010). ...
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![Vitor Werner de Vargas]()
O sistema de proteção de redes de distribuição de energia elétrica é responsável por manter integridade dos condutores e equipamentos que a compõem quando submetidas a perturbações elétricas. Com o objetivo de operar e planejar expansões às redes, distribuidoras revisam, constantemente, o dimensionamento dos dispositivos de proteção. O processo de dimensionamento desses dispositivos, constituído por métodos iterativos que respeitam critérios e diretrizes da filosofia de proteção da distribuidora, consome recursos da equipe de proteção. Neste sentido, o presente trabalho apresenta um estudo sobre a proteção de sobrecorrente e propõe o desenvolvimento de uma ferramenta computacional para automatização do dimensionamento de elos fusíveis e parametrização de religadores automáticos - baseada em estrutura em árvore. Com o objetivo de aplicá-lo em uma distribuidora, a ferramenta é capaz de modelar alimentadores no simulador OpenDSS, através de redes já modeladas em outro simulador utilizado para cálculo fluxo de potência, e elaborar um relatório completo do dimensionamento. Ao final do trabalho, são explorados a economia de recursos no dimensionamento de três alimentadores reais da distribuidora, os resultados obtidos em um desses alimentadores, bem como os benefícios e a aplicabilidade da ferramenta. Com uma redução expressiva de 99% do tempo de dimensionamento, o desempenho da ferramenta expõe a importância da automatização e valida a metodologia desenvolvida. Os resultados obtidos também indicam a necessidade da adição de novos critérios quantitativos à filosofia de proteção da distribuidora, os quais são propostos pelo autor.
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Source: https://www.researchgate.net/publication/348788183_Title_POWER_SYSTEM_PROTECTION
