protec cio introd 1
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ProtectionsPrinciples
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PRINCIPLES
Introduction
General concepts of protection systems
Elements of a protection system
Relay types
Relay design and construction
Relay operating principles
Applying protective relays
LINES PROTECTION TRANSFORMERS PROTECTION
STATION BUS PROTECTION
AGENDA
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PRINCIPLES
Introduction
General concepts of protection systems
Elements of a protection system
Relay types
Relay design and construction
Relay operating principles
Applying protective relays
LINES PROTECTION TRANSFORMERS PROTECTION
STATION BUS PROTECTION
AGENDA
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Introduction
Electric energy is one of the fundamental resources of the modern
industrial society
Electrical power is available to the user instantly, at the correct voltage
and frequency, at exactly the amount needed.
Yet the power system is subject to constant disturbances:
Random load changes
Faults by natural causes
Equipment or operator failure
The power system maintains its steady state mainly because of the
correct and quick remedial action taken by the protective relaying
equipment.
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Introduction
The response of the protection system must be automatic, quick, and
should cause a minimum amount of disruption to the power system.
To accomplish this is necessary:
Examine all possible types of faults
Analyze the required response and design the protective equipment
necessary
Provide for a back-up protective function to prevent failure of the protection
itself
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Possible types of faults
Overcurrent
Short - circuits Not wanted contact between phases or between phase and ground
Electrodynamic stress
Thermal stress
Overloads
Thermal stress
Ground faults
Fire hazard
Personal hazard
Overvoltage Switching
Temporary
Lightning strikes
Provoke isolation damage which could develop into short circuits
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Function of protective relaying
DETECT THE ELEMENT THAT STARTS TO OPERATE IN AN
ABNORMAL MANNER
REMOVE THIS ELEMENT FROM THE POWER SYSTEM AS
QUICKLY AS POSSIBLE
SIGNALING, LOGGING AND REPORTING
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PRINCIPLES
Introduction
General concepts of protection systems
Elements of a protection system
Relay types
Relay design and construction
Relay operating principles
Applying protective relays
LINES PROTECTION TRANSFORMERS PROTECTION
STATION BUS PROTECTION
AGENDA
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Protective relays attributes
SENSITIVITY - Ability to detect deviations of the parametersinside the zone or element to protect
SELECTIVITY - Ability to discern when it must actuate, wait orblock, to remove the least number of elements
QUICKNESS - Minimum time in the process Detect- Select-Trip
RELIABILITY - Degree of certainty that an element will performas intended
SECURITY - Certainty that the relay will not operate incorrectly for anyfault
DEPENDABILITY - Certainty that the relay will operate correctly for
all the faults it is designed to operate
ROBUSTNESS (STRENGTH) - Ability to withstand over yearsthe adverse conditions at which they are submitted
BURDEN - minimum, so as not to oversize the instrumenttransformers
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Selectivity and zones of protection
Selectivity is defined in terms of regions of a power system (zones of
protection) for which a given relay is responsible.
The relay will be considered secure if it responds only to faults within its
zone of protection
A zone boundary is usually defined by a CT and a CB.
The CT provides the ability to detect a fault inside the zone
The CBs provide the ability to isolate the fault
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All power system
elements must be
encompassed by atleast one zone.
The more important
elements must be
included in at least two
zones
Zones must overlap to
prevent any element
from being unprotected.
The overlap must be
finite but small to
minimize the likelihoodof a fault inside this
region.
Such a fault will cause
both protections to
operate removing a
larger segment of thesystem from service
Selectivity and zones of protection
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PROTECTION REACH
ZONE OF PROTECTION
PROTECTION REACH
ZONE OF PROTECTION
87
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DEFINED REACH
CLOSED ZONE
PROTECTION
UNDEFINED REACH
OPEN ZONE
PROTECTION
Zones of protection
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Primary and back up protection
It is essential that provision be made to clear the fault by some alternate
protection system in case of the primary protection could fail.
These are referred to as back up protection systems
On EHV is common to use duplicate primary protection systems
Back up relaying may be installed locally, in the same substation, or remotelly
Remote back up are completely independent of the relays, CT, breakers,
etc.
Remote back up may remove more sources that can be allowed
Local back up use common elements an can thus fail to operate as the
primary protection
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REACH OF PROTECTION 21P
REACH OF PROTECTION 21B
21P
Back up protection locally at the same position
21B
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50/51
REACH OF PROTECTION 21
21
50/51
REACH OF PROTECTION 50/51 OF THE TRANSFORMER
REACH OF PROTECTION 50/51 OF BUS TIE BREAKER
A B
Back up protection locally at different positions
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REACH OF PROTECTION 21B
REACH OF PROTECTION 21A
21
Remote back up protection
21
SUBSTATION ASUBSTATION B
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87
Teletrip
TELETRIP
TELETRIP
86
1
2
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PRINCIPLES
Introduction
General concepts of protection systems
Elements of a protection system
Relay types
Relay design and construction
Relay operating principles
Applying protective relays
LINES PROTECTION
TRANSFORMERS PROTECTION
STATION BUS PROTECTION
AGENDA
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3
Elements of a Protection System
1
5
2.1
2.4
F.A.
A D
2.2P
2.3
2
4
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Elements of a Protection System
1
The function of transducers (usually CT and VT) is to provide current and
voltage signals to the relays, to detect deviations of the parameters watched
over.
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Elements of a Protection System
1
2.1
2.4
F.A.
A D
2.2P
2.3
2
Relays are the logic
elements which initiate
the tripping and closing
operations.
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Elements of a Protection System
1
2
4
Circuit breakers
isolate the fault by
interrupting the
current.
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3
Elements of a Protection System
1
5
2.1
2.4
F.A.
A D
2.2P
2.3
2
4
Tripping power, as
well as power
required by therelays, is usually
provided by the
station battery
because is safer than
the ac faulted
system.
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PRINCIPLES
Introduction General concepts of protection systems
Elements of a protection system
Relay types
Relay design and construction
Relay operating principles
Applying protective relays
LINES PROTECTION
TRANSFORMERS PROTECTION
STATION BUS PROTECTION
AGENDA
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Classification of relays
Relays can be divided into six functional categories:
Protective relays. Detect defective lines, defective apparatus, or
other dangerous or intolerable conditions. These relays generally tripone or more circuit breakers, but may also be used to sound an
alarm.
Monitoring relays. Verify conditions on the power system or in the
protection system. These relays include fault detectors, alarm units,
channel-monitoring relays, synchronism verification, and networkphasing. Power system conditions that do not involve opening circuit
breakers during faults can be monitored by verification relays.
Reclosing relays. Establish a closing sequence for a circuit breaker
following tripping by protective relays.
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Classification of relays
Relays can be divided into six functional categories:
Regulating relays. Are activated when an operating parameterdeviates from predetermined limits. Regulating relays function
through supplementary equipment to restore the quantity to the
prescribed limits.
Auxiliary relays. Operate in response to the opening or closing of
the operating circuit to supplement another relay or device. Theseinclude timers, contact-multiplier relays, sealing units, isolating
relays, lock-out relays, closing relays, and trip relays.
Synchronizing (or synchronism check) relays. Assure that proper
conditions exist for interconnecting two sections of a power system.
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Classification of relays Performance Characteristics
Differential
Distance
Directional overcurrent
Inverse time
Definite time
Undervoltage
Overvoltage
Ground or phase
High or low speed Pilot
Phase comparison
Directional comparison
Current differential
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By design mode:
Electromechanical
Plunger type
Induction type
Thermal
Solid state
Computer type
By parameter controlled:
Current
Voltage
Power
Impedance (distance)
Direction
Frequency
By mode of Detection of
faults :
Level detection
Magnitude comparison
Differential comparison
Phase angle comparison
Pilot relaying
Harmonic content
Frequency sensing
By operating time:
Instantaneous
Time delay
Independent delay
Dependent delay
Classification of relays
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Relay speed
Relays are generally classified by their speed of operation as follows:
Instantaneous These relays operate as soon as a secure decision is made.
No intentional time delay is introduced to slow down the relay response
Time delay
An intentional time delay is inserted between the relay decision time andthe initiation of the trip action
This time delay can be dependent on some parameter (usually inverse time
dependent) or independent
High speed
A relay that operates in less than a specified time (usually 3 cycles)
Ultra high speed
This term is not included in the Relay Standards but is commonly
considered to be operation in 4 milliseconds or less
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Analog/Digital/Numerical
Analog relays are those in which the measured quantities are converted into
lower voltage but similar signals, which are then combined or compared directly
to reference values in level detectors to produce the desired output.
Digital relays are those in which the measured ac quantities are manipulated in
analog form and subsequently converted into square-wave (binary) voltages.Logic circuits or microprocessors compare the phase relation-ships of the
square waves to make a trip decision.
Numerical relays are those in which the measured ac quantities are sequentially
sampled and converted into numeric data form. A microprocessor performs
mathematical and/or logical operations on the data to make trip decisions.
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PRINCIPLES
Introduction General concepts of protection systems
Elements of a protection system
Relay types
Relay design and construction
Relay operating principles
Applying protective relays
LINES PROTECTION
TRANSFORMERS PROTECTION
STATION BUS PROTECTION
AGENDA
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Protective relays for power systems are made up of one or more fault-
detecting or decision units, along with any necessary logic networks and
auxiliary units.
Because a number of these fault-detecting or decision units are used in a
variety of relays, they are called basic units.
Basic units fall into several categories: electromechanical units, solid-
state units integrated circuits, and microprocessor architecture.
Combinations of units are then used to form basic logic circuits
applicable to protective relays.
Relay design and construction
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Four types of electromechanical units are widely used: magnetic
attraction, magnetic induction, D'Arsonval, and thermal units.
Plunger units have cylindrical coils with an external magnetic structure
and a center plunger.
When the current or voltage applied to the coil exceeds the pickup value,
the plunger moves upward to operate a set of contacts.
The force F required to move the plunger is proportional to the square ofthe current in the coil.
The plunger unit's operating characteristics are largely determined by the
plunger shape, internal core, magnetic structure, coil design, and
magnetic shunts.
Plunger units are instantaneous in that no delay is purposely introduced.
Typical operating times are 5 to 50 msec, with the longer times occurring
near the threshold values of pickup.
Electromechanical units. Plunger units
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The unit shown is used as a high-dropout instantaneous overcurrent unit.
The steel plunger floats in an air gap provided by a nonmagnetic ring in
the center of the magnetic core.
When the coil is energized, the plunger assembly moves upward,
carrying a silver disk that bridges three stationary contacts (only two are
shown).
A helical spring absorbs the ac plunger vibrations, producing goodcontact action.
Electromechanical units. Plunger units
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The more complex plunger unit shown is used as an instantaneous
overcurrent or voltage unit.
An adjustable flux shunt permits more precise settings over the nominal
four-to-one pickup range.
This unit is relatively independent of frequency, operating on dc, 25-Hz,
or nominal 60-Hz frequency. It is available in high- and low-dropout
versions.
Electromechanical units. Plunger units
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Clapper units have a U-shaped magnetic frame with a movable
armature across the open end.
The armature is hinged at one side and spring-restrained at the other.
When the associated electrical coil is energized, the armature moves
toward the magnetic core, opening or closing a set of contacts with a
torque proportional to the square of the coil current.
The pickup and dropout values of clapper units are less accurate thanthose of plunger units.
Clapper units are primarily applied as auxiliary or go/no-go units.
Electromechanical units. Clapper units
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Upward movement of the armature releases a target, which drops to
provide a visual indication of operation (the target must be reset
manually).
The ac unit operates as an instantaneous overcurrent or instantaneous
trip unit. It is equipped with a lag-loop to smooth the force variations due to
the alternating current input. Its adjustable core provides pickup
adjustment over a nominal four-to-one range.
Electromechanical units. Clapper units
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Polar units operate from direct current applied to a coil wound around the
hinged armature in the center of the magnetic structure.
A permanent magnet across the structure polarizes the armature-gappoles, as shown.
The nonmagnetic spacers, located at the rear of the magnetic frame, are
bridged by two adjustable magnetic shunts.
This arrangement enables the magnetic flux paths to be adjusted forpickup and contact action.
Electromechanical units. Polar units
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With balanced air gaps, the flux paths are as shown and the armature will
float in the center with the coil deenergized.
With the gaps unbalanced, some of the flux is shunted through thearmature.
The resulting polarization holds the armature against one pole with the coil
deenergized. The coil is arranged so that its magnetic axis is in line with the
armature, and at a right angle to the permanent magnet axis.
Electromechanical units. Polar units
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Current in the coil magnetizes the armature either north or south,
increasing or decreasing any prior polarization of the armature, if as
shown in Figure b, the magnetic shunt adjustment normally makes thearmature a north pole, it will move to the right.
Direct current in the operating coil, which tends to make the contact end
a south pole, will overcome this tendency and the contact will move to
the left.
Depending on design and adjustments, this polarizing action can begradual or quick.
The left-gap adjustment controls the pickup value, the right-gap
adjustment the reset value.
Some units use both an operating and a restraining coil on the armature. The polarity of the restraint coil tends to maintain the contacts in their
initial position. Current of sufficient magnitude applied to the operating
coil will provide a force to overcome the restraint, causing the contacts to
change position.
Electromechanical units. Polar units
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A combination of normally open or normally closed contacts is available.
These polar units operate on alternating current through a full-wave
rectifier and provide very sensitive, high-speed operation on very lowenergy levels.
The operating equation of the polar unit is
where K1 and K2 are adjusted by the magnetic shunts; K3 is a designconstant; is the permanent magnetic flux; Iop is the operating current;
and Ir is the restraint current in milliamperes
Electromechanical units. Polar units
3
21
KIKIK rop
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There are two general types of
magnetic induction units:
induction disc
cylinder units.
Originally, induction disc units
were based on the watt-hour
meter design. Modern units,
however, although using thesame operating principles are
quite different. All operate by
torque derived from the
interaction of fluxes produced by
an electromagnet with those frominduced currents in the plane of a
rotable aluminum disc.
Magnetic induction units
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The E unit, in Fig, has three poles on one side of the disc and a common
magnetic member orkeeper on the opposite side.
The main coil is on the center leg. Current I in the main coil produces flux,which passes through the air gap and disc to the keeper. (A small portion
of the flux is shunted off through the side air gap.)
The flux T, returns as L, through the left-hand leg and R through the
right-hand leg, where T = L + R. A short-circuited lagging coil on the left
leg causes L to lag both R and T, producing a split-phase motor action.
Magnetic induction units. Induction disk
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Flux L induces voltage Vs, and current Is flows, essentially in phase, in
the shorted lag coil. Flux T, is the total flux produced by main coil
current, I. The three fluxes cross the disc air gap and induce eddycurrents in the disc. These eddy currents react with the pole fluxes and
produce the torque that rotates the disc. With the same reference
direction for the three fluxes as shown, the flux shifts from left to right and
rotates the disc clockwise, as viewed from the top.
Magnetic induction units. Induction disk
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There are many alternate versions of the induction disc unit.
The unit shown, for example, may have a single current or voltage input.
The disc always moves in the same direction, regardless of the direction
of the input. If the lag coil is open, no torque will exist.
Other units can thus control torque in the induction disc unit.
Most unusually, a directional unit is connected in the lag coil circuit.
When the directional unit's contact is closed, the induction disc unit hastorque; when the contact is open, the unit has no torque.
Magnetic induction units. Induction disk
M i i d i i I d i di k
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Induction disc units are used in power or directional applications by
substituting an additional input coil for the lag coil in the E unit
The phase relation between the two inputs determines the direction ofthe operating torque.
A spiral spring on the disc shaft conducts current to the moving contact.
This spring, together with the shape of the disc (an Archimedes spiral)
and design of the electromagnet, provides a constant minimum operating
current over the contact travel range.
A permanent magnet with adjustable keeper (shunt) dampens the disc,
and magnetic plugs in the electromagnet control the degree of saturation.
The spring tension, damping magnet, and magnetic plugs allow separate
and relatively independent adjustment of the unit's inverse-time currentcharacteristics.
Magnetic induction units. Induction disk
M ti i d ti it C li d it
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The operation of a cylinder unit is similar to
that of an induction motor with salient poles
for the stator windings. The basic unit used for relays has an inner
steel core at the center of the square
electromagnet, with a thin-walled aluminum
cylinder rotating in the air gap.
Cylinder travel is limited to a few degrees bythe moving contact attached to the top of
the cylinder and the stationary contacts.
A spiral spring provides reset torque.
Magnetic induction units. Cylinder unit
M ti i d ti it C li d it
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Operating torque is a function of the product of the two operating
quantities applied to the coils wound on the four poles of the
electromagnet, and the cosine of the angle between them. The torqueequation is
where K and are design constants; I1 and I2 are the currents through
the two coils; 12 is the angle between I1 and I2; and Ks is the
restraining spring torque. Different combinations of input quantities canbe used for different applications, system voltages or currents, or network
voltages.
sKIKIT 1221 cos
Magnetic induction units. Cylinder unit
DA l it
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In the D'Arsonval unit, a magnetic structure and
an inner permanent magnet form a two-pole
cylindrical core. A moving coil loop in the air gap is energized by
direct current, which reacts with the air gap flux
to create rotational torque.
The D'Arsonval unit operates on very low energy
input, such as that available from dc shunts,bridge networks, or rectified ac.
The unit can also be used as a dc contact-
making milliammeter or millivoltmeter.
DArsonval unit
Th l it
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Thermal units consist of bimetallic strips or coils that have one end fixed
and the other end free.
As the temperature changes, the different coefficients of thermal
expansion of the two metals cause the free end of the coil or strip to
move.
A contact attached to the free end will then operate based on
temperature change.
Thermal units
AGENDA
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PRINCIPLES
Introduction General concepts of protection systems
Elements of a protection system
Relay types
Relay design and construction
Relay operating principles
Applying protective relays
LINES PROTECTION
TRANSFORMERS PROTECTION
STATION BUS PROTECTION
AGENDA
L l d t ti O t l
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This is the simplest of all relay operating principles.
The relay operates for values of the parameter above or under what iscalled pick up setting.
Examples of this type are over-current relays and under-voltage relays
Level detection. Over-current relays
L l d t ti O t l
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The operating characteristic of an overcurrent relay can be presented
as a plot of the operating time vs. the current.
Level detection. Over-current relays
This figure represents the operating
time for an independent delay time
overcurrent relay.
It will operate always at the same time
for currents over the pick up setting
This relays are defined by the pick up
current, as number of times the normal
current, and the operating time
Coordination of different protections of
this type is achieved by time delaying
and pick up setting
It must be a minimum of 0,3 sec. to
permit operating of the first breaker
t
iIn n*In
t 0
Rel tiempo independ.
50 (ANSI)
O t l D d t ti d l
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This type of relay will have an operating time depending on the value
of the current, generally with an inverse characteristic, that is to say ,
the bigger the current, the shorter the time.
Over-current relays. Dependent time delay
This characteristic permits a
reasonable coordination between
protections just changing the pick
up setting.
These relays will be defined by the
pick up setting and the type of
tripping curve, which can be
adjusted
There are three or five types ofcurves, Normal (NI), Very inverse
(VI) and Extremely inverse (EI)
t
iIn n*In
Curva trafo
Rel tiempoinverso
t 0
Rel tiempo independ.
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O t t ti
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The working principle of an inverse time overcurrent relay is depicted in
this figure.
Overcurrent protection
The current to be controlledfeeds a coil with multiple taps
which allow the pick up current
setting.
The generated magnetic field
makes the disc rotate with aspeed proportional to the
current.
A timing dial allows the
adjustment between contacts
and hence sets the op. time.
The braking magnet lessens the
rotating speed and acts as an
opposing force to the rotation.
Varying the magnetization,
different tripping curves can be
achieved.
Current
taps
Induction
disk
Lagging
coil
Timing
dial
Braking
2 4 6
1
2
Directional o erc rrent protection
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It is triggered when the current exceeds the reference value and also
the energy or power flow has the determined direction.
An overcurrent element controls the current magnitude
A directional element controls the direction of the power flow
Directional overcurrent protection
V
I
Cylinder
Magnetic
core
IV
IIII
IV
I
V
Directional overcurrent protection
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One coil is fed with voltage and
the other one with the current.
The rotating torque will be
proportional to the product of
both magnitudes (hence to
power) and will invert if any of
the aforementioned magnitudesalso inverts.
The torque will peak when the
phase angle between both fields
is 90.
The rotation will only be allowedin one direction (corresponding
to the selected one) closing a
contact which allows the
functioning of the overcurrent
unit.
V
I
Cylinder
Magnetic
core
IV
IIII
IV
I
V
The working principle of a directional unit is shown in the figure.
Directional overcurrent protection
Directional overcurrent protection
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Given that the maximum rotating torque is reached when the fields
owing to the current and the voltage have a phase angle of 90, it is
necessary to ensure that, in case of fault, the measured values have a
phase angle close to the aforementioned value.
An internal phase angle is usually introduced to ensure the previously
stated.
When a fault occurs, the voltage of the affected phase can be
significantly reduced, so it is recommended to measure the line to line
voltage of the other phases, in order to avoid an incorrect performance
of the protection.
I
Abrir
V
VI
Cerrar
IV
L.P.N
L.P.M (+)L.P.M (-)
L.P.M. : Lnea Par Mximo
L.P.N. : Lnea Par Nulo
= 90
L.P.M (-)
I
V
V
I
IV
L.P.N
L.P.M (+)
Directional overcurrent protection
Magnitude comparison
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Magnitude comparison
This operating principle is based upon the comparison of one or more
operating quantities with each other.
A current balance relay may compare the current in one circuit with the
current in another circuit, which should have equal or proportional
magnitudes under normal operating conditions.
The relay will operate when the current division in the two circuits varies by
a given tolerance
Differential relaying for transformer protection
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87With internal fault Id > 0 Trip
With external fault Id = 0 No trip
It compares the current entering the transformer with the current
leaving the element.
If they are equal there is no fault inside the zone of protection
If they are not equal it means that a fault occurs between the two ends
Differential relaying for transformer protection
Differential comparison
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Alternatively one could form an algebraic sum of the two currents
entering the protected element, which could be termed as differentialcurrent (Id), and use a level detector relay to detect the presence of a
fault.
In general this principle is capable of detecting very small
magnitudes of fault.
Its only drawback is that it requires currents from the extremitiesof a zone of protection
Differential comparison
Phase angle comparison
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A B
IA IB
Internal fault = IA e IB are in phase reversal = Trip
External fault = IA e IB are in phase = No trip
Phase angle comparison
Circuit breaker failure protection
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Circuit breaker failure protection
The breaker may have a mechanical failure if it is not able to open any
of the poles when it is ordered to do so, or even an electrical failure if
although open, is not capable of breaking the current, which will keepon flowing as an arc.
This implies a current flow that keeps on feeding the fault which
can be used to detect the breaker failure itself.
In those applications which even though the mechanical failureexist, the current could not be high enough to be detected, the
opening must also be verified by means of breaker auxiliary
contacts.
Circuit breaker failure protection
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87B+FI
Circuit breaker failure protection
21I falta
A tripping order for thecircuit breaker initiates
the time delay count
down for the protection.
Circuit breaker failure protection
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87B+FI
TELEDISPARO
21
I falta
T 250 ms
Once the time delay is
over , if the breaker is
not yet open, theprotection sends a
tripping order to all the
adjacent breakers,
including those at the
end of the lines if
necessary.
Sometimes two time
delays are used, the
first one to repeat the
tripping order for the
breaker itself, and thesecond for the other
breakers.
Circuit breaker failure protection
AGENDA
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PRINCIPLES
Introduction General concepts of protection systems
Elements of a protection system
Relay types
Relay design and construction
Relay operating principles
Applying protective relays
LINES PROTECTION
TRANSFORMERS PROTECTION
STATION BUS PROTECTION
AGENDA
Factors Influencing Relay Performance
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Factors Influencing Relay Performance
Relay performance is generally classed as
(1) correct,
(2) no conclusion
(3) incorrect.
Incorrect operation may be either failure to trip or false tripping.
The cause of incorrect operation may be (1) poor application, (2)
incorrect settings, (3) personnel error, or (4) equipment malfunction.
Equipment that can cause an incorrect operation includes current
transformers, voltage transformers, breakers, cable and wiring, relays,
channels, or station batteries.
Incorrect tripping of circuit breakers not associated with the trouble area
is often as disastrous as a failure to trip. Hence, special care must be
taken in both application and installation to ensure against this.
Noconclusion is the last resort when no evidence is available for a
correct or incorrect operation. Quite often this is a personnel
involvement.
Protective relaying systems and their design
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Protective relays or systems are not required to function during normal
power system operation, but must be immediately available to handle
intolerable system conditions and avoid serious outages and damage. Thus, the true operating life of these relays can be on the order of a few
seconds, even though they are connected in a system for many years.
In practice, the relays operate far more during testing and maintenance than
in response to adverse service conditions.
In theory, a relay system should be able to respond to an infinite number
of abnormalities that can possibly occur within the power system.
Protective relaying systems and their design
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Applying protective relays
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The first step in applying protective relays is to state the protection problem
accurately.
Although developing a clear, accurate statement of the problem can often bethe most difficult part, the time spent will pay dividends particularly when
assistance from others is desired.
Information on the following associated or supporting areas is necessary:
System configuration
Existing system protection and any known deficiencies
Existing operating procedures and practices, possible future expansions
Degree of protection required
Fault study
Maximum load, current transformer locations and ratios
Voltage transformer locations, connections, and ratios Impedance of lines,
transformers, and generators
Applying protective relays
Applying protective relays
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System configuration is represented by a single-line diagram showing the
area of the system involved in the protection application.
This diagram should show in detail the location of the breakers, busarrangements, taps on lines and their capacity, location and size of the
generation, location, size, and connections of the power transformers
and capacitors, location and ratio of ct's and vt's, and system frequency.
Transformer connections are particularly important. For ground relaying,
the location of all ground sources must also be known
Applying protective relays
Applying protective relays
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The existing protective equipment and reasons for the desired change(s)
should be outlined.
Deficiencies in the present relaying system are a valuable guide toimprovements.
New installations should be so specified.
As new relay systems will often be required to operate with or utilize parts of
the existing relaying, details on these existing systems are important.
Whenever possible, changes in system protection should conform with
existing operating procedures and practices.
Exceptions to standard procedures tend to increase the risk of personnel error
and may disrupt the efficient operation of the system.
Anticipated system expansions can also greatly influence the choice ofprotection.
Applying protective relays
Applying protective relays
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An adequate fault study is necessary in almost all relay applications.
Three-phase faults, line-to-ground faults, and line-end faults should all be
included in the study.
Line-end fault (fault on the line-side of an open breaker) data are
important in cases where one breaker may operate before another.
For ground-relaying, the fault study should include zero sequence
currents and voltages and negative sequence currents and voltages. These quantities are easily obtained during the course of a fault study
and are often extremely useful in solving a difficult relaying problem.
Applying protective relays
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