Protective Relays
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Transcript of Protective Relays
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PROTECTIVE SYSTEM-GENRAL ASPECTS:-
In an electrical power system, as and when a fault occurs or any parameter becomes abnormal, it
becomes necessary to isolate the faulty section instantaneously from the rest of the system. The isolation
should be automatic, such that the supply must get disconnected from the faulty section leaving healthy
remainder in service. This can be achieved by using the protective devices like relays or fuses in the circuit to
isolate the circuit from damage to occur due to short circuit, overhead, earth fault etc.
The use of fuse for isolation or breaking the circuit for protection of the system is limited to low voltagecircuits i.e., upto 11kV. 11kV onwards, the circuit is protected by providing the protective relays.
Main features of good protective system/gear.The main features of a good protective system/gear are:
1. Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity. The
protective system should be so sensitive that it operates even at low values of fault current.
2. Selectivity:It is the ability of the protective system to select correctly that part of the system in trouble
and disconnect the faulty part without disturbing the rest of the system. The protective system should
select correctly the faulty part of the power system and disconnect the same without disturbing the rest
of the system. Thus it is absolutely necessary for a protective system to have the property of
discrimination.3. Reliability: The reliability is one of the most important factors in the system. The component installed
as a part of the protective system should operate definitely under pre-determined condition.
4. Simplicity: The relaying system should be simple so that it can be easily maintained. The simpler the
protection system, the greater will be its reliability.
5. Quickness:The protect system should respond as quickly as possible in order to improve the quality of
service, increase safety of life and equipment and increase stability of operation.
6. Non-interference with future extension: The installation of protective system should be carried out in
such a way that future extension is possible without any interference with the original one.
7. Economy: While selecting a protective gear, due to consideration should be given to its cost. However,
when utmost important and sensitive apparatus (like alternator, transformer etc.) is to be protected thecost factor or economic consideration are ignored.
Types of protection:
The protectionis of the following two types:
1. Main or primary protection: This type of protection is the first line of defence and ensures quick
acting and selective clearing of faults within the boundary of the circuit section or element it produces.
Main protection as a rule, is provided for each section of an electrical installation.
2. Back up or secondary protection: the backup protection is quite important for the proper functioning
of the good system of electrical protection since percent reliability of the of protective system as well as
associated CTs, PTs and circuit breakers cannot be guaranteed. It is the second line of the defence
which function to isolate a faulty section of the system when the main protection fails to function
properly. Back up protection is only provided where the main protection of the adjacent is unable to
back- up the main protection of the given circuit.
PROTECTIVE RELAYS-INTRODUCTION
Relay is a device by means of whi ch an electric cir cuit (tr ip circuit of alarm cir cui t), is controlled by change
in the other cir cui t. Relays are automati c. There are several types and appli cations of r elays. Relays are
essential components of protective systems.
Protective relay is an electr ical relay used for protective of electr ical devices. I t is a device whi ch closed its
contacts, when operating quanti ty reaches certain predetermined magni tude/phase. Closing of relay contacts
in iti ates an alarm cir cuit or tri p cir cuit.
Primary Relays. These are the relays which are connected directly in the circuit to be protected.
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Secondary Relays. These are the relays which are connected in the circuit through current and potential
transformers.
Auxiliary Relays. These are the relays which operate in response to the opening or closing to its operating
circuit to assist another relay in the performance of its function. This relay maybe instantaneous or may have a
time delay.
(example of such a relay is a television power button. When a viewer presses the power button to turn the
television ON, he is opening the relay. A signal is then sent to the main power to turn the television ON. When
he presses the power button again, he is closing the relay and the power will shut off)
Whenever fault occurs on the power system then the relay
detect that fault and closes the tri p coil circuit. This results in
opening of circuit breaker which disconnect the faulty circuit.
Thus this ensures the safety of the circuit equipment being
damaged.
A typical relay circuit is shown in above figure on occurrence
of short circuit at appoint F on the transmission line. There is
enormous increase in the value of current flowing in the line.
This causes a heavy flow of current through the relay coil(R.C), making the relay to operate by closing it contacts. This
in turn close the trip circuit of the breaker, consequently the
circuit breaker opens and the faulty section is isolated from the rest of the system.
Trip circuit: The circuit comprising trip coil, relay contacts, auxiliary switch, battery supply etc., which
controls the circuit breaker for opening operation.
Auxiliary switch: A multipoint switch which operates in conjunction with circuit breaker and
connects/disconnects certain protective, indicating and control circuits in each position.
Auto reclosure: this is the process of automatic re-closing of circuit breaker after opening. When thedisturbance occurs for a short time the relay operates; but it re-closes after sensing it as a temporary fault.
Temporary faults are due to lightning and trees falling on lines.
For EHV line one reclosure in 12 cycles is sufficient to clear the fault.
For lines upto 33kV three re-closures are necessary to clear the fault due to tree falling (with interval of 15-
120secs)
Based on experience it is found that about 80% of faults are cleared after first re closure, 10% after second re-
closure, 2% after third re-closure and 8% not cleared after third re-closure is permanent fault.
Rapid auto reclosure of CBs at both ends of transmission lines is advantageous, because
1. It improves transient stability of the system
2. It improves the power transfer of the system
Essential fundamental elements of the relays: All the relays have the following three essential fundamental
elements:
1. Sensing element: it is also called measuring element .it is the element which responds to change in theactuating quantity, the current in a protected system, in case of over-current relay.
2. Comparing elements: This element compare the action of the actuating quantity on the relay with apredesign relay setting. The relay picks up only if the actuating (operating) quantity is more than the
relay setting.
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3. Control elements: This element accomplishes a sudden change in control quantity such as closing of
the operative current circuit.
Operation of relay: It means either closing or opening of its contacts. Most of the relays are restrained by
spring so that they are at a pre-determined position when correctly de-energized. At this position if the status of
the contact is closed it is said 'NC' (normally closed) contact and if it is open it is said 'NO' (normally open)
contact.
-If a relay operates to open a NC contact or close a NO contact it is said that a relay 'picks up' and the
smallest value of this actuating quantity is called the pick-up value.
-If a relay operates to return back to its original position it is said that the relay rests and the largest value of
operating quantity at which it this occurs is called reset value. Reset value is less than the pick-up value.
Terms Connected with Relays:
Some commonly used terms relating to relays are discussed below:
1. Operating force or torque: It is torque (or force) which tends to close the contacts of the relay.2. Restraining force or torque: It is a torque (or force) which opposes the operating torque and tends to
prevent the closer of relay contacts.
3. Operating or pick up level: It is defined as the threshold value of current, voltage etc., above whichthe relay will close its contacts.
4. Drop out or reset (level): It is defined as the value of the current, voltage etc., below which the relaywill open its contacts and return to normal position.
The ratio of drop out (or reset) value to the operating (or pick up) value is called drop out (or reset)
ratio.
5. Flag or target: It is a device (usually spring or gravity operated) which indicates the operation of arelay.
6. Power consumption (burden): It is power absorbed by circuits of the relay expressed in VA (volt-amperes) in A.C. and in W (watts) in D.C. circuits at the rated current or voltage.
7. Operating time: It is defined as the time which elapses form the moment when the actuating quantityattains a value equal to the pickup value until the relay operates its contacts.
8. Reset time: The time which elapses between the instant when the actuating quantity becomes less thanthe reset value to the instant when the relay contact returns to its normal position.
9. Seal-in-coil: It is the coil which does not permit the relay contacts to open when the current is flowingthrough them.
10.Maximum torque angle: It is defined as the design angle of the relay that will yield maximum torque.It is also known as characteristics angle of relay.
11.Over-reach: A relay is said to over-reach when it operates at a current which is lower than its setting.'Under-reach' is just reverse of over-reach.
12.Pick-Up Current. If the minimum current in relay coil at which the relay starts to operate.13.Current Setting/Plug setting. Often it is desirable to adjust the pickup current to any required value.
This is known as current setting and is usually achieved by the use of tapings on the relay operating
coil. The value assigned to each tap are expressed in terms of percentage full load rating of C.T.
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with which the relay is associated and represents the value above which the disc commences to rotate
and finally closes the trip circuit.
Therefore, Pick-up current (relay pick up current) =Rated secondary current (C.T) x Current setting.
The current setting is sometimes referred as current plug setting. But Plug Setting Multiplier is different
from plug setting.
Plug setting/Current settingbridge is provided with induction disc relays and it provides a wide range
of current settings. The plug setting refers to the magnitude of current at which the relay starts to
operate. The plug setting bridge comprises connections tapped from relay coil. By inserting the plug ina particular gap in the bridge, a certain no of turns of the relay coil are brought into circuit (as we know,
minimum pick up value of the deflecting forceof an electrical relay is constant & the deflecting force
of the coil is proportional to its number of turns and electric current flowing through the coil. Now, if
we can change the number of active turns of any coil, the required current to reach at minimum pick
value of the deflecting force in the coil also changes. That means if active turns of the relay coil is
reduced, then proportionately more current is required to produce desired relay actuating force.
Similarly if active turns of the relay coil is increased, then proportionately reduced current is required to
produce same desired deflecting force)
Suppose, you want that, an over current relay should operate when the system current just crosses 125%
of rated current. If the relay is rated at 2A, the normal pick up current of the relay is 2A and it should be
equal to secondary rated current of current transformer connected to the relay.
That means, the relay should be operated when the electric current of CT secondary becomes more than
or equal 2.5A (which is 125% of rated current).
As per definition,
Current setting = 2.5/2 x 100% = 125%
14.Plug Setting Multiplier (P.S.M).It is the ratio of fault current in relay coil to the pick-up current, thatis
P.S.M=
Pk up urret=
Rated edary urret f C.T x Curret ettg.
PSM will give the severity of the fault, i.e., if PSM
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Time setting.
Fault current.
Current transformer ratio,
The following is the procedure to calculate the actual relay operating time:
By the use of current transformer ratio, convert the fault current into the relay coil current.
Express the relay current as a multiplier of current setting i.e., calculate P.S.M.
Using Time/P.S.M. curve of the relay, read-off the time of the operation for the calculated P.S.M. Determine the actual time of operation by multiplying the above time of the relay by the time setting
multiplier in use.
Classification of Relays:
There are many kind of relays used in the power systems. The relays can be designed and constructed to
operate in response to one or more electrical quantity such as voltage, current, phase angle etc.
The type of relays may be classified as under:
1. According to their construction and principle of operation : Electromagnetic attraction type relays:
o Attracted armature type relaysOperation depends on movement of an armature under the
influence of attractive force due to magnetic field setup by the current flowing through the relay
winding.
o Solenoid type relaysOperation depends on the movement of an iron plunger core along the
solenoid axis.
o Balanced beam type relays
Electromagnetic induction or simply induction relaysOperation depends on movement of a metallic
disc or cylinder free to rotate by the interaction of induced eddy currents and the alternating magnetic
field producing them.o Shaded pole type
o Watt-hour meter type
o Induction cup type
Electro-dynamic relaysIn such a relay, moving member consists of a coil free to rotate in the air gap
of a permanent magnet.
Moving coil type relaysIn such a relay, moving member consists of a coil free to rotate in air gap of a
permanent magnet.
Thermal relaysIn this type of relay, movement depends upon the action of the heat produced by the
current flowing through the element of relay.
Physio-electric relaysBuchholzs relay is an example of this type of relay. Static relaysThis relays employ thermionic valves, transistors or magnetic amplifiers to obtain the
operating characteristics.
2. According to their applications : Under-voltage, Under-current and under-power relaysOperation occurs when the voltage, current or
power falls below a specified value (mostly instantaneous or induction relays).
Over-voltage, over-current and over-power relaysOperation takes place when the voltage, current or
the power rises above a specified value (mostly instantaneous or induction relays).
Directional or reverse current relaysOperation occurs when the applied current assumes a specific
phase displacement with the respect to the applied voltage and the relay is compensated for fall in
voltage (induction current relays). Directional or reverse power relays. This relay is able to sense whether the fault lied in the forward
direction or reverse direction w.r.t the relay location. It can also sense the direction of power flow i.e.,
in the normal direction or in the opposite direction
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Differential relays Operation occurs at some specific phase or magnitude difference between two or
more electrical quantities.
Distance relaysoperation depends upon the ratio of voltage to current.
Polarised relay:the operation of this relay depends on the direction of the current.
Slave relay:it is an output device and does not perform the function of comparison or measurement. It
simply closes the contacts, when an output is received from the static rely. Most of the modern static
relays are using D.C. polarized relay as slave relay because of its low cost and simpler electromagnetic
multi-contact tripping arrangement
3. According to their time of operation or timing characteristics:
1)Instantaneous relaysOperation occurs after a negligibly small interval of time from the incidence of the
current or other quantity which causes operation.
An instantaneous relay is one in which no intentional time delay is provided. In this case, the relay contacts are
closed immediately after current in the relay coil exceeds the minimum calibrated value.
This relay is effective only where the impedance between the relay and source is small compared to the
protected section impedance
Its operating time is sometimes expressed in cycle based on the power system frequency.
e.g., In a 50 cycle system one cycle would be 1/50 sec.
2) Inverse time-lag relayA time of operation is approximately inversely proportional to the magnitude off the
current or other quantity causing operation.
The inverse time delay can be achieved by associating mechanical accessories (e.g., Drag magnet (for
induction type relays), Oil dash pot or time limit fuse).
At values of current less than pick up, the relay never operates.
At higher values the time of operation of relays decreases steadily with the increase of current.
3)Definite time-lag relays...In this type of relay there is a definite time elapse between the instant of pick up
and the closing of relay contacts. This particular time setting is independent of the amount of current through
the relay coil being the same for all values of current in excess of the pickup value.
It may be mentioned here that all inverse time relays are also provided with definite minimum time feature in
order that the relay may never become instantaneous in its action for very long overloads.
4)Inverse-Definite minimum time lag(I.D.M.T) relaysThe time of operation is approximately inversely
proportional to the smaller values of current or other quantities causing operation and tends to a definite
minimum time as the value increases without limit.
The time lag in induction type relays may be achieved by using a permanent magnet which is so
arranged that the relay rotor cuts the flux between the poles of this
magnet. This type of magnet is called Drag magnet.
In other relays it may be achieved in the following 2 ways:
1) When a series connected overload trip coil is used (without any
CT), an oil dash pot can be attached in which a piston is connected
to the lower end of the solenoid plunger of the relay as shown. On
the occurrence of a fault, when the plunger is pulled, the piston,
immersed in the oil, retards plunger motion and thus provides the
necessary drag or delay in the operation of the relays.
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2)If the trip coil is transformer (CT)operated, time delay is
achieved by connecting a time- limit fuse across the trip coil
terminal(as shown).This fuse should have inverse time
current characteristic and will normally carry the C.T
secondary current to by-pass the trip coil. When sufficient
current flows to blow this fuse, the whole current is then
transferred to the trip coil which operates to trip the circuitbreaker.
Most of the relays in service on electric power system today are of electro-mechanical type. They work on the
following two main operating principles:
Electromagnetic attraction
Electromagnetic induction
Electromagnetic Attraction Relays
Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an
electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by d.c. or a.c. quantities.
The force of attraction F=ki2
The important types of electromagnetic attraction relays are:
Attracted armature type relay
Solenoid type relay
Balanced beam type relay
Attracted armature type relay:Figure shows the schematic arrangementof an attracted armature type relay. It consists of a laminated
electromagnet M carrying a coil C and a pivoted laminated armature. The
armature is balanced by a counterweight and carries a pair of spring
contact fingers at its free end. Under normal operating conditions, the
current through the relay coil C is such that counterweight holds the
armature in the position shown. However, when a short-circuit occurs, the
current through the relay coil increases sufficiently and the relay armature
is attracted upwards. The contacts on the relay armature bridge a pair of
stationary contacts attached to the relay frame. This completes the trip circuit which results in the opening of
the circuit breaker and, therefore, in the disconnection of the faulty circuit.
It is a usual practice to provide a number of tappings on the relay coil so that the number of turns in use and
hence the setting value at which the relay operates can be varied
Solenoid type relay: Figure shows the schematic arrangement of a solenoid type
relay. It consists of a solenoid and movable iron plunger arranged as shown. Under
normal operating conditions, the current through the relay coil C is such that it holds
the plunger by gravity or spring in the position shown. However, on the occurrence of
a fault, the current through the relay coil becomes more than the
pickup value, causing the plunger to be attracted to the solenoid. The
upward movement of the plunger closes the trip circuit, thus openingthe circuit breaker and disconnecting the faulty circuit.
Balanced beam type relay:Figure shows the schematic arrangement
of a balanced beam type relay. It consists of an iron armature fastened
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to a balance beam. Under normal operating conditions, the current through the relay coil is such that the beam
is held in the horizontal position by the spring. However, when a fault occurs, the current through the relay coil
becomes greater than the pickup value and the beam is attracted to close the trip circuit. This causes the
opening of the circuit breaker to isolate the faulty circuit.
Induction Relays
Electromagnetic induction relays operate on the principle of induction motor and are widely used for protective
relaying purposes involving a.c. quantities. They are not used with d.c. quantities owing to the principle of
operation.
An induction relay essentially consists of a pivoted aluminium disc placed in two alternating magnetic fields of
the same frequency but displaced in time and space. The torque is produced in the disc by the interaction of one
of the magnetic fields with the currents induced in the disc by the other.
2 sin
The following points may be noted from above expression
The greater the phase angle between the fluxes, the greater is the netforce applied to the disc.
The following three types of structures are commonly used for obtaining the phase difference in the fluxes and
hence the operating torque in induction relays:
Shaded-pole structure
Watt-hour-meter or double winding structure
Induction cup structure
Plug setting and time setting in induction disc relays:In these relays, there is a facility for selecting the plug
setting and time setting such that the same relay can be used for a wide range of current, time characteristics.
Shaded-pole structure:The general arrangement of shaded-pole structure is shown in Figure. It consists of a
pivoted aluminium disc free to rotate in the air-gap of an electromagnet. One half of each pole of the magnet issurrounded by a copper band known as shading ring.
The alternating flux s in the shaded protion of the
poles will, owing to the reaction of the current
induced in thering, lag behind the flux u in the
unshaded portion by an angle . These two a.c. fluxes
differing in phase will produce the necessary torque
to rotate the disc. As proved earlier, the driving
torque T is
sinAssuming the fluxes s and u to be proportional to
the current I in the relay coil then
2 sin
This shows that driving torque is proportional to the square of current in the relay coil.
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Watt-hour meter structure: This structure gets its name from the fact that it is used in watt-hour meters. The
general arrangement of this type of relay is shown in Fig. It
consists of a pivoted aluminium disc arranged to rotate freely
between the poles of two electromagnets. The upper
electromagnet carries two windings; the primary and the
secondary. The primary winding carries the relay current I1
while the secondary winding is connected to the winding of the
lower magnet. The primary current induces e.m.f. in the
secondary and so circulates a current I2 in it. The flux 2
induced in the lower magnet by the current in the secondary
winding of the upper magnet will lag behind 1by
an angle . The two fluxes 1 and 2 differing in phase by will
produce a driving torque on the disc proportional to
2 sin.
An important feature of this type of relay is that its operation can be controlled (ON/OFF of relay) by opening
or closing the secondary winding circuit. That means, If secondary circuit is opened, no flux can be set by the
lower magnet however great the value of current in the primary winding but no torque will be produced (since
from the equation). Therefore, the relay can be made inoperative by opening its secondary winding circuit.
Induction cup structure: Fig shows the general arrangement of an
induction cup structure. It most closely resembles an induction motor,
except that the rotor iron is stationary, only the rotor conductor portion
being free to rotate.
The moving element is a hollow cylindrical rotor which turns on its axis.
The rotating field is produced by two pairs of coils wound on four poles as
shown. The rotating field induces currents in the cup to provide the
necessary driving torque. If 1and 2represent the fluxesproduced by the
respective pairs of poles, then torque produced is proportional to
2 sinwhere is the phasedifference between the two fluxes.
A control spring and the back stopfor closing of the contacts carried on an
arm are attached to the spindle of the cup to prevent the continuous rotation.
Induction cup structures are more efficient torque producers than either the shaded-pole or the watt-hour meter
structures. Therefore, this type of relay has very high speed and may have an operating time less than 01
second. Induction cup relay is very suitable for directional or phase comparison units. This is because, besides
the sensitivity, induction cup relay have steady non vibrating torque and parasitic torques due to current or
voltage alone are small.
Reactance and MHO type Induction Cup Relay
By manipulating the current voltage coil arrangements and the relative phase displacement angles between the
various fluxes, induction cup relay can be made to measure either pure reactance or admittance. Such
characteristics are discussed in greater detail in a session on electromagnetic distance relay.
Induction Type Overcurrent Relay (non-directional):
This type of relay works on the induction principle and initiates corrective measures when current in the circuit
exceeds the predetermined value. The actuating source is a current in the circuit supplied to the relay from acurrent transformer. These relays are used on a.c. circuits only and can operate for fault current flow in either
direction.
Constructional details: Figure shows the important constructional details of a typical non-directional
induction type overcurrent relay. It consists of a metallic (aluminium) disc which is free to rotate in between
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the poles of two electromagnets. The upper electromagnet has a primary and a secondary winding. The primary
is connected to the secondary of a C.T. in the line to be protected and is tapped at intervals. The tappings are
connected to a plug-setting bridge by which the number of active turns on the relay operating coil can be
varied, thereby giving the desired current setting.
The secondary winding is energised by induction from primary and is
connected in series with the winding on the lower magnet. The controlling
torque is provided by a spiral spring. The spindle of the disc carries a moving
contact which bridges two fixed contacts (connected to trip circuit) when thedisc rotates through a pre-set angle. This angle can be adjusted to any value
between 0 and 360. By adjusting this angle, the travel of the moving contact
can be adjusted and hence the relay can be given any desired time setting.
Operation: The driving torque on the aluminium disc is set up due to the
induction principle. This torque is opposed by the restraining torque provided
by the spring. Under normal operating conditions, restraining torque is greater
than the driving torque produced by the relay coil current. Therefore, the
aluminium disc remains stationary. However, if the current in the protected circuit exceeds the pre-set value,
the driving torque becomes greater than the restraining torque. Consequently, the disc rotates and the moving
contact bridges the fixed contacts when the disc has rotated through a pre-set angle. The trip circuit operates the
circuit breaker which isolates the faulty section.
Induction Type Directional Power Relay: This type of relay operates when power in the circuit flows in a
specific direction. Unlike a non-directional overcurrent relay, a directional power relay is so designed that it
obtains its operating torque by the interaction of magnetic fields derived from both voltage and current source
of the circuit it protects. Thus this type of relay is essentially a wattmeter and the direction of the torque set up
in the relay depends upon the direction of the current relative to the voltage with which it is associated.
Constructional details: Fig shows the essential parts of a typical induction type directional power relay. It
consists of an aluminum disc which is free to rotate in between the
poles of two electromagnets. The upper electromagnet carries a
winding (called potential coil) on the central limb which is connected
through a potential transformer (P.T.) to the circuit voltage source. The
lower electromagnet has a separate winding (called current coil)
connected to the secondary of C.T. in the line to be protected. The
current coil is provided with a number of tappings connected to the
plug setting bridge (not shown for clarity, which would be same as
non-directional type). This permits to have any desired current setting.
The restraining torque is provided by a spiral spring.
Operation: The flux 1due to current in the potential coil will benearly 90 lagging behind the applied voltage V. The flux 2due to
current coil will be nearly in phase with the operating current I. The
interaction of fluxes 1 and 2 with the eddy currents induced in the
disc produces a driving torque(energy meter principle) given by:
2 sin
&2 = (90 )
Therefore, 2 sin(90 )
2 cos
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It is clear that the direction of driving torque on the disc depends upon the direction of power flow in
the circuit to which the relay is associated. When the power in the circuit flows in the normal direction, the
driving torque and the restraining torque (due to spring) help each other to turn away the moving contact from
the fixed contacts. Consequently, the relay remains inoperative. However, the reversal of cur rent in the circui t
reverses the direction of dr iving torque on the disc. When the reversed dri ving torque is large enough, the
disc rotates in the reverse dir ection and the moving contact closes the trip cir cui t. Th is causes the operation
of the cir cui t breaker which disconnects the faul ty section.
Induction Type Directional Overcurrent Relay:
The directional power relay discussed above is unsuitable for use as a directional protective relay under
short-circuit conditions. When a short-circuit occurs, the system voltage falls to a low value and there may be
insufficient torque developed in the relay to cause its operation. This difficulty is overcome in the directional
overcurrent relay which is designed to be almost independent of system voltage and power factor.
Constructional details:
Directional element: It is essentially a directional
power relay which operates when power flows in a
specific direction. The potential coil of this element is
connected through a potential transformer (P.T.) to
the system voltage. The current coil of the element is
energised through a C.T. by the circuit current. This
winding is carried over the upper magnet of the non-
directional element. The trip contacts (1 and 2) of the
directional element are connected in series with the
secondary circuit of the overcurrent element.
Therefore, the latter element cannot start to operate
until its secondary circuit is completed. In other
words, the directional element must operate first (i.e.,
contacts 1 and 2 should close) in order to operate the
overcurrent element.
Non-di rectional element:It is an overcurrent element
similar in all respects to a non-directional overcurrent
relay as shown.
It may be noted that plug-setting bridge is also
provided in the relay for current setting but has been
omitted in the figure for clarity and simplicity. The
tappings are provided on the upper magnet of overcurrent element and are connected to the bridge.Operation:Under normal operating conditions, power flows in the normal direction in the circuit protected by
the relay. Therefore, directional power relay (upper element) does not operate, thereby keeping the overcurrent
element (lower element) unenergised. However, when a short-circuit occurs, there is a tendency for the current
or power to flow in the reverse direction. Should this happen, the disc of the upper element rotates to bridge the
fixed contacts 1 and 2. This completes the circuit for overcurrent element. The disc of this element rotates and
the moving contact attached to it closes the trip circuit. This operates the circuit breaker which isolates the
faulty section. The two relay elements are so arranged that final tripping of the current controlled by them is
not made till the following conditions are satisfied:
Current flows in a direction such as to operate the directional element. Current in the reverse direction exceeds the pre-set value.
Excessive current persists for a period corresponding to the time setting of overcurrent element.
Distance or Impedance Relays:
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The operation of the relays discussed so far depended upon the magnitude of current or power in the
protected circuit. However, there is another group of relays in which the operation is governed by the ratio of
applied voltage to current in the protected circuit. Such relays are called distance or impedance relays (since,
Impedance is an electrical measure of distance along a transmission line).
Distance relays are double actuating relays, one coil is energized by voltage and other by current,
voltage coil gives restraining torque and current coil gives operating torque i.e., the torque produced by a
current element is opposed by the torque produced by a voltage element. The relay will operate when the ratio
V/I is less than a predetermined value.
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Differential protection/Pilot Wire Protection
Circulating current differential protection (Merz-Price protection):
Figure shows an arrangement of an overcurrent relay connected to operate as a differential relay. A pair of
identical current transformers are fitted on either end of the section to be protected (alternator winding in this
case). The secondaries of CTs are connected in series in such a way that they carry the induced currents in the
same direction. The operating coil of the overcurrent relay is connected across the CT secondary circuit. This
differential relay compares the current at the two ends of thealternator winding.
Under normal operating conditions, suppose the alternator winding
carries a normal current of1000 A. Then the currents in the two
secondaries of CTs are equal. These currents will merely circulate
between the two CTs and no current will flow through the
differential relay. Therefore, the relay remains inoperative. If a
ground fault occurs on the alternator winding as shown in Figure the
two secondary currents will not be equal and the current flows
through the operating coil of the relay, causing the relay to operate.
Disadvantages:
The impedance of the pilot cables (The two CTs are connected through conductors called pilot cable)
generally causes a slight difference between the currents at the two ends of the section to be protected. If
the relay is very sensitive, then the small differential current flowing through the relay may cause it to
operate even under no fault conditions.
Pilot cable capacitance causes incorrect operation of the relay when a large through-current flows.
Accurate matching of current transformers cannot be achieved due to pilot circuit impedance.
The above disadvantages are overcome to a great extent in biased beam relay.
Biased or Percent differential relay:
The reason for this modification over the circulating current differential relay is to overcome the trouble arising
out of differences in CT ratios for high values of external short circuit currents. The percentage differential
relay has an additional restraining coil connected in the pilot wire as shown.
In this relay the operating coil is connected to the mid-point of the restraining coil. The total no of ampere turns
in the restraining coil becomes the sum of ampere turns in its two halves, i.e.,
2
2which gives the average
restraining current of+
2in N turns. For external faults both I1& I2increase and thereby the restraining torque
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increases which prevents the mal-operation.
The operating characteristic of such a relay is given figure
The ratio of differential operating current to average restraining
current is fixed percentage. Hence the relay is called percentage
differential relay.
The relay is also called biased differential relay because the
restraining coil is also called a biased coil as it provides additional
flux.
Opposed voltage or Balanced voltage differential protection:
Figure shows the arrangement of voltage balance protection. In this scheme of protection, two similar current
transformers are connected at either end of the element to be protected (e.g.an alternator winding) by means of
pilot wires. The secondaries of current transformers are connected in series with a relay in such a way that
under normal conditions, their induced e.m.f.s are in opposition hence the name has come.
Note: The CTs used in such protection are with air gap core so that the core does not get saturated and over-
voltages are not produced during zero secondary current under normal working conditions.
Disadvantages:
As discussed above a multi-gap current transformer construction is required to achieve the accurate
balance between current transformer pairs.
The system is not suitable for protection of long cables, the charging current may be sufficient to operate
the relay even if a perfect balance of current transformers is attained. (It is suitable for cables of relatively
short lengths due to the capacitance of pilot wires)
The above disadvantages have been overcome in Translay (modified balanced voltage) system
Translay System:
This system is the modified form of opposition voltage-balance system. Principle is same but, here opposition
is between voltages induced in the secondary coils wound on the relay magnets and not between the secondary
voltages of the line current transformers. Hence secondary current never be zero so, no problem due to over-
voltages during zero secondary current under normal working conditions. So the current transformers used can
be made of normal design without any air gaps. This permits the scheme to be used for feeders of any voltage.
Constructional details: These relays cover the function of transformer as well as relay. Hence the name
Translay.
Fig shows the simplified diagram illustrating the principle of Translay scheme. It consists of two identical
double winding induction type relays fitted at either end of the feeder to be protected. The primary circuits of
these relays are supplied through a pair of current transformers. The secondary windings of the two relays are
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connected in series by pilot wires in such a way that voltages induced in the former opposes the other.The
compensating devices (cu rings) neutralize the effects of pilot-wire capacitance currents and of inherent lack of
balance between the two current transformers.
Operation: Under healthy conditions, current at the
two ends of the protected feeder is the same and the
primary windings of the relays carry the same
current. These windings induce equal e.m.f.s in the
secondary windings. As these windings are soconnected that their induced voltages are in
opposition, no current will flow through the pilots or
operating coils and hence no torque will be exerted
on the disc of either relay. In the event of fault on
the protected feeder, current leaving the feeder will
differ from the current entering the feeder.
Consequently, unequal voltages will be induced in
the secondary windings of the relays and current
will circulate between the two windings, causing the torque to be exerted on the disc of each relay. As the
direction of secondary current will be opposite in the two relays, therefore, the torque in one relay will tend toclose the trip circuit while in the other relay, the torque will hold the movement in the normal un-operated
position. It may be noted that resulting operating torque depends upon the position and nature of the fault in the
protected zone and at least one element of either relay will operate under any fault condition.
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Split core/Split Conductor protection:
The probability of faults occurrence on the overhead lines is much more due to their greater lengths
and atmospheric conditions. The protective schemes employed for alternators and transformers, with slight
modifications, may also be employed for protection of feeders.
The split-conductor is another method of securing the benefits of a balanced method of protections
without the necessity of using pilot wires. The principle of operation depends on the fact that two conductors of
equal length and impedance, when connected in parallel will share the load equally, provided that insulation of
the system is sound. When a fault develops on any conductor it will carry current than the other, and this
inequality of currents is arranged to operate a relay and thus isolate the faulty line.
In this system of protection each phase of the line is split into two sections having equal impedances. The
two sections are lightly insulated from each other .In this system, as shown in figure a single-turn currenttransformer is inserted at each end of the split conductor. The current transformer consists of laminated iron
rings on which a secondary winding is wound all around the periphery.
Under healthy conditions the current flowing along the two splits are equal and since these are threaded
through the current transformers CT1 and CT2 in the opposite directions hence the voltage across the terminals
of the evenly spread secondary winding is zero. In fault conditions one of the split takes more current than the
other one, thereby giving rise to an unbalance of the primary side of the current transformers .Due to
unbalancing of currents on the primary side of current transformers resultant flux will be set up in the core of
the one of the current coil R will be emergised. The relay contacts will be closed and trip coil will trip the
circuit breaker and isolate the fault
In the best arrangement the splits are carried into the circuit breakers on both sides of the feeders so that
the splits are opened by the breakers. This is explained as follows-
Let the splits be not carried into the circuit breakers and a fault develop at the receiving end of a long
line. Under these conditions, the impedance of the differential current transformer at this end maybe
insufficient to cause unbalance between the currents carried by each split conductor. Hence such a fault will
not will not be cleared by circuit breakers since the relay will not operate. But when the splits are carried into
the circuit breakers the fault current is confined to the faulty split after the sending end circuit breaker has
tripped. In the former case, although the sending end current breaker trips, the fault current is not confined to
the faulty split but it would divide practically equal between the two splits being solidly connected ,so the
receiving end current breaker will not trip. But in the latter case, the fault current is confined to the faulty split
the opening of the receiving end circuit breaker takes place.
From above discussion it is obvious that the split current breakers impart a high sensitivity of operation to the
protection and switchgear.
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DISADVANTAGES:-
The disadvantages of this system is that we have to make use of a special type of cable with the lower
limits for the voltages .In case of overhead feeders, for each phase ,a duplicate set of conductors , insulators,
etc, have to be employed .The lines having step-up or step-down transformers or voltage regulators cannot be
protected by this method.
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Alternator Protection
Different types of fault in generator
External faults.
Thermal overloading.
Unbalanced Loading.
Stator Winding faults .
Field Winding Faults.
Over-voltages.
External Faults:-
During external faults with large short circuit currents, severe mechanical stress will be imposed on the stator
windings. If any mechanical defect already exists in winding, these may be further aggravated. The temperature
rise is however, relatively slow and a dangerous temperature level may be obtained after about 10 seconds with
asymmetrical faults, severe vibration and overheating of the rotor may occur.
The external faults such as fault on bus bar are not covered by generator protection zone. Hence differential
protection of generator does not respond to external faults.
The over current and earth fault protection of generator provides back-up protection to external fault, while the
primary protection as provided by the protective system of respective equipment (e.g. bus-bars, transmission
lines).
Thermal Overloading:-
Continued overloading may increase the winding temperature to such extent that the insulation will be
damaged and its useful life reduced. Temperature rise can also be caused by failure of cooling system. In largemachines thermal element (thermo-couples or resistance thermometers) are embedded in the stator slots and
cooling system. The electrical over current protection (not thermal protection) is generally set at higher value
for responding the excessive overloads. Hence it cannot sense the continuous overloads of less value. Neither
can it sense the failure of cooling system.
Unbalanced loading:-
Continued unbalanced loads, equal to or more than 10% of the rated current cause dangerous heating of the
cylindrical rotor in turbo generator. Salient pole rotor in hydro generators often include damper winding and
are, therefore, much less effected by unbalance loading (i.e., for negative phase sequence current).
Unbalanced loading on generator can be due to
- Unsymmetrical fault in the system near the generating station
- Mal-operation of a circuit breaker near generating station, the 3-phase not being cleared.
Stator Winding Faults:-
Stator winding faults involve armature winding and must therefore be cleared quickly by complete shutdown of
the generator. Only opening the circuit does not help since the e.m.f is induced in the stator winding itself. The
field is opened and de-energized by field suppression.
The stator faults include.1. Stator inter-turn faults.
2. Phase-to-phase faults.
3. Phase-to-earth faults.
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Phase to phase faults and phase inter-turn faults are less common. Inter turn faults are more difficult to detect.
Stator Inter-turn faults:-
Short circuit between the turns of one coil may occur if the stator winding is made up of multi turn coils. Such
faults may develop owing to incoming current surges with a steep wave front which may cause a high voltage
across the turn at the entrance of the stator winding. If however the stator winding is made up of single
turn coil, with only one coil per slot, it is, of course, impossible to have an inter-turn fault. If there are two
coils per slot the insulation between the coil is in such a way, dimensions that an inter-turn fault is not likely tooccur.
Differential protection and over current protection does not sense inter-turn faults. Stator inter turn fault
protection detect the inter turn faults.
Phase to Earth faults:-
These faults normally occur in the armature slots. The damage at the point of fault is directly related to the
selected neutral earthing resistor. With fault current less than 20 A, if the machine is tripped within some
seconds then a negligible burning of the iron core will result. A repair work then amounts to changing the
damaged coil without restacking of core laminations. If, however, the earthing resistor is selected to pass a
much larger earth fault current, severe burning of the stator core will take place, necessitating restacking of
laminations. Even when a high speed earth fault differential protection is used, severe damaged may be caused
owing to the large time constant of the field current and the relatively long time required to completely
suppress the field flux. In the case of high earth fault currents it is therefore normal practice to install a circuit
breaker in the neutral of the circuit breaker in order to reduce the total fault clearance time.
Circulating current biased differential protection provides the earth fault protection. However the sensitivity of
such a protection for earth fault depends upon the resistance in neutral to earth connection and the position of
earth fault in the winding. A separate and sensitive earth fault protection is generally necessary for generator
with resistance earthing.
Phase to Phase Fault:-
Short circuit between the stator windings is very rare because the insulation in a slot between coils of different
phases is at least twice as large as the insulation between one coil and the iron core. However a phase to earth
fault may cause a phase to phase faults within the slots. A phase to phase fault most likely to be located at the
end connection of the armature windings i.e. in the overhanging parts outside the slots. A fault of this nature
causes severe arcing with high temperature, melting of copper and risk of fire if the insulation is not made of
fire resistance, non-flammable material. Since the short circuit current in this case do not pass via the stator
core, the laminations will not be particularly damaged. The repair work may therefore be limited to replacing
the affected coil and mechanical part of the end structure.
Field Winding Faults:-
Rotor faults include rotor inter turn fault and conductor to earth faults these are called by mechanical and
temperature stresses.
The field system is normally not connected to the earth so that a single earth fault does not give rise to any
fault current. A second earth fault will short circuit part of the winding and may thereby produce an
unsymmetrical field system, giving unbalanced force on the rotor. Such a force will cause excessive pressure
on bearing and shaft distortion, if not cleared quickly.
The unbalanced loading on generator gives rise to negative sequence current which cause negative sequence
component of magnetic field. The negative sequence field to rotate in opposite direction of the main field and
induces e.m.f in rotor winding. Thus the unbalanced loading causes rotor heating. Rotor earth fault protection
is provided for large generators.
Over-voltage:-
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Atmospheric surge voltage is caused by direct lighting stroke to the aerial line in the H.V system
induced and capacitive transferred voltage surges can, however, reach the generator via the unit
transformer. To protect generator from severe voltages surges, surge arresters and surge capacitors
and often used. In the case of smaller machines directly connected to a distribution network comprising
overhead line, such protective devices are of prime importance.
Switching Surges: Switching operation may cause relatively high transient voltage if re-striking occurs
across the contacts of the circuit breakers. This transient is similar to those obtained during intermittent
earth fault (arcing grounds) and may be limited by using modern circuit breakers. Arcing Ground:The amplitude of the transient voltage during arcing grounds may theoretically, under
the most unfavorable conditions of arc re-striking, reach a value of five times normal line to neutral
peak voltage by means of the resistance earthing of the generator neutral these over voltages will be
reduced to a maximum value of about 2.5 times of the rated peak voltage.
Biased differential protection:
In this method, the currents at the two ends of the protected section are sensed using current transformers. The
wires connecting relay coils to the current transformer secondaries are called pilot wires.
Under normal conditions, when there is no fault in the windings, the currents in the pilot wires fed fromC.T. secondaries are equal. The differential current i1- i2through the operating coils of the relay is zero. Hence
the relay is inoperative and system is said to be balanced.
When fault occurs inside the protected section of the stator windings, the differential current i1- i2flows
through the operating coils of the relay. Due to this current, the relay operates. This trips the generators circuit
breaker to isolate the faulty section.
Fig. 1 Biased differential protection for star connected alternator Fig. 2 For delta connected alternator
The C.T.s on the delta connected machine winding side are connected in delta while the C.T.s at
outgoing ends are connected in star. The restraining coils are placed in each phase, energized by the secondary
connections of C.T.s while the operating coils are energized from the restraining coil tappings and the C.T.
neutral earthing.
If there is a fault due to a short circuit in the protected zone of the windings, it produces a difference
between the currents in the primary windings of C.T.s on both sides of the generator winding of the same
phase. This results in a difference between the secondary currents of the two currents transformers. Thus, under
fault conditions, a differential current flows through the operating coils which is responsible to trip the relay
and open the circuit breaker. The differential relay operation depends on the relation between the current in the
operating coil and that in the restraining coil.
In addition to the tripping of circuit breaker, the percentage differential relay trip a hand reset multi-contact
auxiliary relay. This auxiliary relay simultaneously initiates the following operations,
1. Tripping of the main circuit breaker of generator
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2. Tripping of the field circuit breaker
3. Tripping of the neutral circuit breaker
4. Shut down of the prime mover
5. Turn on of CO2 gas if provided for safety of generator under faulty conditions.
6. Operation of alarm and / or annunciator to indicate the occurrence of the fault and the operation of
the relay the field must be opened immediately otherwise it starts feeding the fault.
When differential relaying is used for the protection, C.T.s at both the ends of generator must be of equal
accuracy otherwise if the error is excessive, wrong operation of the relay may result. The cause of unequalcurrents on both the sides of C.T.s without any fault are ratio errors, unequal lengths of the leads, unequal
secondary burdens etc.
This scheme provides very fast protection to the stator winding against phase to phase faults and phase to
ground faults. If the neutral is not grounded or grounded through resistance then additional sensitive earth fault
relay should be provided.
The advantages of this scheme are,
1. Very high speed operation with operating time of about 15 msec.
2. It allows low fault setting which ensures maximum protection of machine windings.
3. It ensures complete stability under most severe through and external faults.
4. It does not require current transformers with air gaps or special balancing features.
Restricted earth fault protection
Generally Merz-Price protection based on circulating current principle provides the protection against internal
earth faults. But for large generators, as there are costly, an additional protection scheme called restricted earth
fault protection is provided.
When the neutral is solidly grounded then the generator gets completely protected against earth faults. Butwhen neutral is grounded through earth resistance, then the stator windings gets partly protected against earth
faults. The percentage of windings protected depends on the value of earthing resistance and the relay setting.
In this scheme, the value of earth resistance, relay setting, current rating of earth resistance must be carefully
selected. The earth faults are rare near the neutral point as the voltage of neutral point with respect to earth is
very less. But when earth fault occurs near the neutral point then the insufficient voltage across the fault drivers
very low fault current than the pickup current of relay coil. Hence the relay coil remains unprotected in this
scheme. Hence it is called restricted earth fault protection. It is usual practice to protect 85% of the winding.
Consider that earth fault occurs on phase B due to breakdown of its insulation to earth, as shown in the
Fig. 3. The fault current If will flow through the core, frame of machine to earth and complete the path throughthe earthing resistance. The C.T. secondary current Is flows through the operating coil and the restricted earth
fault relay coil of the differential protection. The setting of restricted earth fault relay and setting of overcurrent
relay are independent of each other. Under this secondary current Is, the relay operates to trip the circuit
breaker. The voltage Vbxis sufficient to drive the enough fault current If when the fault point x is away from the
neutral point.
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Fig. 3 Restricted earth fault protection
If the fault point x is nearer to the neutral point then the voltage Vbxis small and not sufficient to drive
enough fault current If. And for this If, relay cannot operate. Thus part of the winding from the neutral point
remains unprotected. To overcome this, if relay setting is chosen very low to make it sensitive to low faultcurrents, then wrong operation of relay may result. The relay can operate under the conditions of heavy through
faults, inaccurate C.T.s, saturation of C.T.s etc. Hence practically 15% of winding from the neutral point is kept
unprotected, protecting the remaining 85% of the winding against phase to earth faults.
Automatic field suppression and neutral circuit breakers:
When a fault
develops in an alternator
winding even though the
generator circuit breaker is
tripped, the fault continuesto be fed because e.m.f is
induced in the generator
itself. Hence the field circuit
breaker is opened and the
stored energy in the field
winding is discharged
through another resistor.
This method is known as field suppression.
The earthing resistor is selected to pass a much larger earth fault current severe burning of the statorcore will take place, necessitating restacking of laminations. Even when a high speed earth fault differential
protection is used, severe damaged may be caused owing to the large time constant of the field current and the
relatively long time required to completely suppress the field flux. In the case of high earth fault currents it is
therefore normal practice to install a circuit breaker in the neutral of the circuit breaker in order to reduce the
total fault clearance time.
Negative Phase Sequence Protection:
Negative phase sequence relays are used in protection against unbalanced loads. The unbalanced 3-
phase stator currents cause double frequency currents to be induced in rotor. They cause heating of rotor and
damage the rotor. Unbalanced stator currents also cause severe vibrations and heating of stator. From thetheory of symmetrical components, we know that unbalance three-phase currents have a negative sequence
component. This component rotates at synchronous speed in a direction opposite to the direction of rotation of
rotor. Therefore double frequency currents are induced in the rotor.
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Negative sequence current filter with over-current relay provides protection against unbalanced loads
(as shown in figure).
The relative asymmetry of a three-phase generator is defined as the ratio of negative sequence current
(I2to rated current In), i.e.,% =
100
In case of loss of one phase, the
relative asymmetry %S=58%. The time for
which the machine can be allowed to operate
for various amounts of relative asymmetriesdepends on type of machine. The additional
heat caused by negative sequence currents in
rotor is proportional to I22t. The product I22tis
a machine characteristic.
I22t=30 is a generally accepted figure
as per ASA, (I2 in per unit, t in sec.) For
wound rotor machines and 40 for salient pole
machine.
It is generally necessary to install
negative sequence relays that match with theI22t characteristic of the machine.
Fig 1. Protection against unbalanced load using negative sequence filter
Negative sequence filter circuit comprises resistors and inductors connected in the secondary circuit in such a
way that negative sequence component flows through the relay coil, ZL as shown in below figure.
The over-current relay (ZL) of negative phase sequence protection is with inverse characteristics
matching with the I22trating curve of the machine and is arranged to trip the unit.
Negative Phase Sequence Circuit:-
The given figure illustrates the principle of the negative phase sequence circuit. The twin windings of
the two auxiliary current-transformers are so connected to the line current-transformers that under normalbalanced-load condition, currents Ia, Ib and Ic
flow in the direction shown. Impedance Z1
and Z2 are connected across auxiliary current-
transformers T1and T2, and a load impedance
ZLis connected across the terminals XX.
When primary load current flows, the
current through T1 will be (Ib-Ic) and that
through T2will be (Ia-Ib). For a given value of
load impedance ZL, (over-current relay) the
impedance Z1 and Z2 are chosen such thatpoints P and R remain at the same potential,
i.e., the voltages across QR and QP are equal
and opposite. Under balanced conditions,
these voltages differ, and an output is
produced proportional to the negative phase
sequence across XX (voltage E) so as to
operate the relay. The protection remains
stable on symmetrical overloads upto about 3
times rated full load.
As the output is instantaneous inoperation, it is necessary to operate the
equipment in conjunction with a time-lag
relay. These relays are used for protection
against unbalanced loads.
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Transformer Protection
Buchholz relay
Buchholz relay is a type of oil and gas actuated protection relay universally used on all oil immersed
transformers having rating more than 500 KVA. Buchholz relay is not provided in relays having rating below
500 KVA from the point of view of economic considerations.
Buchholz Relay in transformeris an oil container housed the connecting pipe from main tank to conservator
tank. It has mainly two elements (for both the alarm and trip circuits) along with hinged float and mercury
switch assembly. The entire assembly is in an oil proof case which has two glass windows.
The upper element consists of a float. The float is attached to a hinge in such a way that it can move up and
down depending upon the oil level in the Buchholz RelayContainer. One mercury switch is fixed on the float.
The alignment of mercury switch hence depends upon the position of the float.
The lower element consists of a baffle plate and mercury switch. This plate is fitted on a hinge just in front of
the inlet (main tank side) of Buchholz Relay in transformerin such a way that when oil enters in the relay
from that inlet in high pressure the alignment of the baffle plate along with the mercury switch attached to it,
will change.
Operation:
The operation of Buchholz relay is as follows:
In case of incipient faults within the transformer, the heat
due to fault causes the decomposition of some transformer
oil in the main tank. The products of decomposition contain
more than 70% of hydrogen gas. The hydrogen gas being
light tries to go into the conservator and in the process gets
entrapped in the upper part of relay chamber. When a
predetermined amount of gas gets accumulated, it exerts
sufficient pressure on the float to cause it to tilt and close
the contacts of mercury switch attached to it. This
completes the alarm circuit to sound an alarm (because, the
conditions described do not call for the immediate removal
of the faulty transformer. It is because sometimes the air bubbles in the oil circulation system of a healthy
transformer may operate the float. For this reason, float is arranged to sound an alarm upon which steps can
be taken to verify the gas and its composition)
If a serious fault occurs in the transformer, an enormous amount of gas is generated in the main tank. The
oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to
close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling
the transformer.
Advantages
It is the simplest form of transformer protection.
It detects the incipient faults at a stage much earlier than is possible with other forms of protection.
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Disadvantages
It can only be used with oil immersed transformers equipped with conservator tanks.
The device can detect only faults below oil level in the transformer. Therefore, separate protection is
needed for connecting cables.
Mercury Switch Description:
Mercury switches have one or more sets of electrical contacts in a sealed glass envelope which contains a bead of mercury. Theenvelope may also contain air, an inert gas, or a vacuum. Gravity is constantly pulling the drop of mercury to the lowest point in the
envelope. When the switch is tilted in the appropriate direction, the mercury touches a set of contacts, thus completing the electrical
circuit through those contacts. Tilting the switch the opposite direction causes the mercury to move away from that set of contacts,
thus breaking that circuit. The switch may contain multiple sets of contacts, closing different sets at different angles, allowing, for
example, single-pole, double-throw (SPDT) operation.
Advantages
Advantages of the mercury switch over other types are that the contacts are enclosed, so oxidation of the contact points is unlikely. In
hazardous locations, interrupting the circuit will not emit a spark that can ignite flammable gases. Contacts stay clean, and even if an
internal arc is produced, the contact surfaces are renewed on every operation, so the contacts don't wear out. The sensitivity of the
drop to gravity provides a unique sensing function, and lends itself to simple, low-force mechanisms for manual or automatic
operation. The switches are quiet, as there are no contacts that abruptly snap together. The mass of the moving mercury drop can
provide an "over center" effect to avoid chattering as the switch is tilted. Multiple contacts can be included in the envelope for two or
more circuits.
Disadvantages
Mercury switches have disadvantages when compared with other types in certain applications. Mercury switches have a relatively
slow operating rate due to the inertia of the mercury drop, so they are not used when many operating cycles are required per second.
Mercury switches are sensitive to gravity so may be unsuitable in portable or mobile devices that can change orientation or that
vibrate. Mercury compounds are highly toxic and accumulate in any food chain, so mercury is not permitted in many new designs.
Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damaging the envelope. The mercury drop
forms a common electrode, so circuits are not reliably isolated from each other if a multi-pole switch is used.
Restricted Earth Fault Protection:
Earth fault relay connected in residual circuit of
line CTs(as shown) give protection against earth
faults on the delta or unearthed star connected
windings of transformer. Earth fault on secondary
side are not reflected on primary side, when the
primary winding is delta connected or has
unearthed star point. In such cases, an earth fault
relay connected in residual circuit of 3 CTs on
primary side operates on internal earth faults inprimary windings only because earth faults on
secondary side do not produce zero sequence
currents on primary side. Restricted earth fault
protection may then be used for high speed
tripping for faults on star connected earthed
secondary winding transformer.
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In this fig the star connected side is protected by Restricted Earth Fault Protection. An earth fault (F1)
beyond the transformer
causes the currents I2 and I1
in CT secondaries as shown
in Fig. Therefore, the
resultant current in earth fault
relay is negligible. For earth
fault within the transformer
star connected winding (F2),only I2 flows and I1 is
negligible. Hence I2 flows
through the earth fault relay.
Thus restricted earth fault-
relay does not operate for
earth fault beyond the
protected zone of the
transformer.
When fault occurs very near to the neutral point of the transformer, the voltage available for driving earth fault
current is small. Hence fault current would be low. If the relay is to sense such faults, it has to be too sensitive
and would therefore operate for spurious signals, external faults and switching surges. Hence the practice is to
set the relay such that it operates 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.
Harmonic Restraint Differential Relay
When an unloaded transformer is switched on, it draws a large initial magnetizing current which may be
several times the rated current of the transformer. This initial magnetizing current is called the magnetizing
inrush current.
As the current flows only through the primary windings, the differential protection will see this inrush
current as an internal fault. The harmonic contents in the inrush current are different than those in usual faultcurrent. The dc component varies from 40 to 60%, the second harmonic 30 to 70% and the third harmonic 10 to
30 %. The other harmonics are progressively less. The third harmonic and its multiples do not appear in C.T.
terminals as these harmonics circulate in the delta winding of the transformer and the delta connected C.T.s on
the Y side of the transformer. As the second harmonic is more in the inrush current than in the fault current,
this feature can be utilized to distinguish between a fault and magnetizing inrush current.
The operation of the relays because of the magnetizing inrush current can be avoided by using relays
with inverse and definite 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 operation. So, a high speed biased differential relay
incorporating a harmonic restraint feature is immune to the magnetizing inrush current. As 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 components of short-circuit current 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
current. 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 figure below. 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
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when the harmonic current exceeds 15% of the fundamental current. Both the DC and higher harmonics are of
large magnitude during magnetizing inrush.
Fig:- Basic circuit of Harmonic Restraint Differential Relay
The relay may fail to operate due to harmonic restraint feature if an external 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 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 ensured faults.
Fig :- Conceptual representation of Harmonic Restraint Differential Relay
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Generator-Transformer Unit Protection
In a high voltage transmission systems, the bus bars are at very high voltages than the generators. The
generators are directly connected to step up transformer to which it is connected, together from a generator
transformer unit. The protection of such a unit is achieved by differential protection scheme using circulating
current principle. While providing protection to such a unit, it is necessary to consider the phase shift and
current transformation in the step up
transformer.
The figure shows a biased differential
protection scheme used for generator
transformer unit. The zone of such a
scheme includes the stator windings, the
step up transformer and the intervening
connections.
The transformer is delta-star hence the
current transformers on high voltage side
are delta connected while those on
generator side are star connected. Thiscancels the displacement between line
currents introduced by the delta connected primary of the transformer. Where there is no fault, the secondary
currents of the current transformer connected on generator side are equal to the currents in the pilot wires from
the secondaries of the delta connected current transformers on the secondary of main transformer. When a fault
occurs, the pilot wires carry the differential current to operate the percentage differential relay.
For the protection against the earth faults, an earth fault relays is put in the secondary winding of the main
step up transformers as shown. In such a case, differential protection acts as a backup protection to the
restricted earth fault protection. This overall differential protection scheme does not include unit transformer as
a separate differential scheme is provided to it.
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Protection against Over-voltages
Causes of Over-voltages
1. Internal causes
Switching surges
Insulation failure
Arcing ground (The phenomenon of intermittent arc taking place in line-to-ground fault of a 3-
system with consequent production of transients is known as arcing ground, these may exist onungrounded systems)
Resonance
2. External causes (i.e. lightning)
Internal causes do not produce surges of large magnitude. Experience shows that surges due to internal
causes hardly increase the system voltage to twice the normal value. Generally, surges due toi nternal causes
are taken care of by providing proper insulation to the equipment in the power system. However, surges due to
lightning are very severe and may increase the system voltage to several times the normal value. If the
equipment in the power system is not protected against lightning surges, these surges may cause considerable
damage. In fact, in a power system, the protective devices provided against over-voltages mainly take care of
lightning surges.
An electric discharge between cloud and earth, between clouds or between the charge centres of the same
cloud is known as lightning
Introduction: Benjamin Franklin (1706-90) performed his famous experiment (1745) of flying kite in thundercloud. Before his discovery the lighting was considered to be Act of God. Frankling proved that the lighting
stroke is due to the discharge of electricity. Franklin also invented lighting rods to be fixed on tall buildings and
earthed to protect them from lighting strokes.
Static induced charges: An overhead conductor accumulate statically induced charge when a charged
clouds come above the conductor. If the cloud swept away from its place, the charges on conductor is released.The charge travels on either sides giving rise to two travelling waves. The earth wire does not prevent such
surges.
Types of Lightning Strokes
Direct stroke
Indirect stroke
Direct stroke:In the direct stroke, the lightning discharge (i.e., current path) is directly from the cloud to the
subject equipment e.g. an overhead line. From the line, the current path may be over the insulators down the
pole to the ground. The over-voltages set up due to the stroke may be large enough to flashover this path
directly to the ground. The direct strokes can be of two types viz. (i) Stroke A ii) stroke B.
Two points are worth noting about direct strokes. Firstly, direct strokes on the power system are very rare.
Secondly, stroke A will always occur on tall objects and hence protection can be provided against it. However,
stroke B completely ignores the height of the object and can even strike the ground. Therefore, it is not possible
to provide protection against stroke B.
Indirect stroke: Indirect strokes result
from the electrostatically induced charges
on the conductors due to the presence of
charged clouds. This is illustrated in Fig.
24.6. A positively charged cloud is above
the line and induces a negative charge on
the line by electrostatic induction. This
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negative charge, however, will be only on that portion of the line right under the cloud and the portions of the
line away from it will be positively charged as shown in Fig. The induced positive charge leaks slowly to earth
via the insulators. When the cloud discharges to earth or to another cloud, the negative charge on the wire is
isolated as it cannot flow quickly to earth over the insulators. The result is that negative charge rushes along the
line is both directions in the form of travelling waves. It may be worthwhile to mention here that majority of
the surges in a transmission line are caused by indirect lightning strokes.
Protection against Lightning
The most commonly used devices for protection against lightning surges are:
Earthing screen
Overhead ground wires
Lightning arresters or surge diverters
Earthing screen provides protection to power stations and sub-stations against direct strokes whereas overhead
ground wires protect the transmission lines against direct lightning strokes. However, lightning arresters or
surge diverters protect the station apparatus against both direct strokes and the strokes that come into the
apparatus as travelling waves. We shall briefly discuss these methods of protection.
Ground wire/Earth wire:
A ground wire is a form of lighting protection using a conductor or conductors, well-grounded at a regular
intervals, preferably at each support, and attached from support to support above the transmission line. And it is
provided above the overhead transmission lines for protection from lightning strokes. The ground wire shields
the phase or line conductors by attracting itself the lightning strokes which, in its absence would strike the
phase conductors. Besides it, the ground wire reduces the voltage electrostatically or electromagnetically
induced in the conductors by the discharge of a neighboring cloud. It also provides additional protective effect
by causing attenuation of travelling waves set in lines by acting as a short-circuited secondary of the lineconductors.
Limitations:
Earth wire do not provide 100% protection. Weak strokes are not attracted by earth wire. However for the most
dangerous direct strokes earth wire has proved to be a very good solution.
Zone of protection:
Practical experience has shown that
earth wire has a shielding angle. The
conductors coming in the shielded zone are
protected against direct strokes. The shieldingangle () define as follows: A vertical line is
drawn from the earth wire. Angle () is plotted
on each side of this vertical line. The envelope
within angle 2 is called zone of protection.
Shielding angle is between 30 deg. to 40 deg.
An angle of 35 deg. is supposed to be
satisfactory and economical for overhead lines.
Surge absorber
The device, which reduces the steepness of the wave front of a particular surge and thus minimizes the
danger due to over-voltage is called the surge absorber. A condenser when placed between the line and earth
reduces the steepness of the wave front to a considerable extent and hence protects the other apparatus from
damage due to overvoltage. The condenser also provides protection against comparatively low-voltage, high-
Zone of protection
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frequency waves. Since the impedance of a condenser is inversely proportional to the frequency, it is low at
high frequencies and large at low frequencies. The normal frequency voltage produces only a small current in
the condenser, so that negligible loss is caused during normal operation. A condenser is