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Protective Relays
Dr.Latif Shaikh
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Protective Relay
Protective relays are the devices that detect abnormal conditions inelectrical circuits by constantly measuring the electrical
quantities which are different under normal and abnormal
conditions .
Under abnormal condition current , voltage , phase angle,frequency may change. The relay operates when it detects the
fault which completes the trip circuit thus opening the circuit
breaker.This disconnects the circuit.
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Protective Relay
A typical relay is shown .When
short circuit occurs at point
F on transmission line CT
current increases .This
increases current in CT coilwhich closes trip coil of
circuit breaker and CB
contacts get opened. Thus
faulty section is isolated.
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Requirements and Qualities of Relays
Selectivity and Discrimination
Speed
Sensitivity
Reliability
Simplicity Economy
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Important Terms Related to Relay Operation
1.Operating Force or torque- The force which tends to close the
contacts of the relay
2.Restraining Force- It is the torque which opposes the operating
torque and tends to prevent the closing of relay contacts.
3.Operating or Pick Up levelIt is defined as the threshold value
of current ,voltage etc.above which the relay will close its
contacts.
4.Drop out or Reset level- It is defined as the value of current,
voltage etc.below which the relay will open and its contacts will
return to normal position.
5.Flag or targetIt is a device which indicates the operation of
relay.
6.Burden- It is power absorbed by the relay expressed in VA.
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Important Terms Related to Relay Operation
7.Operating Time- It is defined as the time which elapses from the
moment when the actuating quantity attains a value equal topick up value until the relay operates its contacts.
8.OverReachThe relay is said to over reach when it operates for
a current which is lower than its setting.
9.Current Setting- Often it is desirable to adjust the pick up currentto any required value .This is known as current setting and it is
usually done by use of tapping on the relay coil. The value
assigned on each tap are expressed in terms of percentage of
full load rating of CT with which the relay is associated and
represents the value above which the relay disc starts to rotate
and finally closes the trip contacts
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Attracted Armature Type Relays
These have a coil or
electromagnet energized by a
coil .The coil is energized by
operating quantity like V or I.
Under normal conditions the coil
cannot attract the plunger
due to spring force. Under
fault condition the fault
current increases so armature
or plunger gets attracted toclose the contacts .
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Attracted Armature Type Relays
These have a coil or
electromagnet energized by a
coil .The coil is energized by
operating quantity like V or I.
Under normal conditions the coil
cannot attract the plunger
due to spring force. Under
fault condition the fault
current increases so armature
or plunger gets attracted toclose the contacts .
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Attracted Armature Type Relays
Applications
1.For over current protection
2.Differential Protection
3.Auxiliary Relays
4.Definite time lag over current
and earth fault protection
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Static Relays
A static relay is the relay which uses solid state components like
transistors ,diodes etc.,for the measurement or comparison of
electrical quantities.
The static network is so designed that it gives an output signal in
the tripping direction whenever a threshold conditions
reached.
The output signal in turn operates a tripping device which may
be electromagnetic or electronic.
Static relays are capable of performing the same function with
the use of electronic control circuit which an electromagnetic
relay performs with the use of moving parts or elements.
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Static Relays
Figure shows the essential components of a static relay
1.The relaying quantity (the output of CT and PT or a
transducer)is first rectified and compared with the
set values.2.The output is actuated when the dynamic
input(relaying quantity)reaches the threshold value.
This valued is amplified and given to the output
device which is usually an electromagnetic.This
energizes the trip coil and the relay operates.
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Static Relays
Advanrages
1.More accurate and fast in operation
2. Most of moving parts and the contacts are eliminated.
3.VA ratings of CT and PT is comparatively lower.
4.Low power consumption.
5.Resetting time and overshoot can be reduced.
6.Very compact.
7.Superior characteristics and accuracy.
8.Several functions can be accommodated in a static relay
9.Simplified testing and servicing is possible.
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Static Relays
Disadvantages
1.Reliability cannot be predicted
2.Construction is not very robust.
3.Easily affected by surrounding interference.
4.Auxiliarty DC supply is required5,.Affected by voltage transients.
Applications
1.Ultra high speed protection of EHV AC transmission lines utilizingdistance protection.
2.In over current and earth fault protection schemes
3.As main element in differential relay
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Microprocessor Controlled Relays
In microprocessor based relays the digital processing unit is the
microprocessor. Microprocessor is now used for protection,fault recording ,fault locating ,data monitoring and several other
functions . Compared to other analog and digital relays ,the
programmable microprocessor based relays have following
superior features
1.Ability to combine a large number of protective and monitoring
functions.
2.Measured values of variables are processed digitally by
microprocessor.
3.High level of flexibilityVarious protective functions can be
freely selected and allocated to various relays by the software.
4.The memory of relay enables the relay to retain values
responsible for tripping ,time taken by relay etc.
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5.Speed- High speed relays are available for line protection. They
use microprocessors.
6.User friendly yet highly capable- Microprocessor based relays
are easy to apply, operate and use. They are highly capable.
7.Increased reliability due to self checking-Microprocessor based
relay checks itself the circuits, power supply memories. This
improves the reliability of the relay.
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Microprocessor Based Static Relays
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Microprocessor Based Static Relays
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Under Voltage Relays
Under voltage protection is provided for AC circuits ,busbars
,motors ,rectifiers, transformer set. Such protection is given by
means of Under Voltage relays. Under voltage relays are
necessary for voltage control and reactive power control of
network buses and load buses.
Under voltage relays have instantaneous characteristics or inversecharacteristics depending upon the construction and design.
Inverse time under voltage relays have inverse characteristics,
their operating time reduces with the reduction in voltage.
Induction disc type of construction is used for under voltagerelays. The relay coil is energized by the voltage to be measured
either directly or through a PT.
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Over Current Protection
Protective Devices used for over current protection are
Fuses
Miniature circuit breakers
Circuit breakers fitted with overloaded coils or tripped by over
current relays
Over current relays along with current transformers
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Applications of Over Current Protection
Motor protection
Transformer protection
Line protection
Protection of utility equipment
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Characteristics of Relay Units for Over Current Protection
Definite Characteristics
Inverse Characteristics
Extremely Inverse
Very inverseInverse
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Connection scheme with three over current relays
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Connection scheme with three over current relays
The figure shows three current transformers and relay coils
connected in star point is earthed. When short circuit occurs in
the protected zone the protected zone the secondary current
of CTs increases. The current flows through the coil and the
relay picks up.
The relay closes and thereby the trip circuit is closed and the
circuit breaker operates.
The over current protection scheme with three over current
relays responds to phase faults including single phase to earthfault. Therefore such schemes are used with solidly earthed
systems where phase to phase and phase to earth faults are
likely to occur.
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Over current relay with time delay relay
In this case an additional relay is used.An additional auxiliary
relay and a definite time relay is used.
When a fault occurs the over current relay operated.
Definite time relay is used in the trip circuit.
The over current relay operates and closes the circuit of definite
time relay.
The definite time relay operates after a defined minimum time.
So the circuit breaker opens only after a over current fault andafter a definite time.
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Differential relays
Differential relay is that type of relay which operates when the
phase difference of two or more similar electrical quantities
exceeds a predetermined value.
Almost any type of relay ,when connected in a certain way can
be made to operate as a differential relay.
The two fundamental systems of differential or balanced
protection are
1.Current Balance protection
2.Voltage balance protection
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Differential Relays (Current Differential Relay)
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Current Differential Relay
Differential relay is that type of relay which operates when the
phase difference of two or more similar electrical quantities
exceeds a predetermined value.
Almost any type of relay ,when connected in a certain way can
be made to operate as a differential relay
The two fundamental systems of differential or balanced
protection are
1.Current balance protection
2.Voltage balance protection
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Current Differential Relay
This system is employed for the protection of feeders ,alternators
and transformers.The CTs of equal ratio are employed when used either at the two
ends of an alternator winding or at the lower ends of a feeder
with no tapping.
When this system is to be used for protection of transformers,correction must be made for different currents determined
approximately by the transformers turns ratio.
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Percentage Differential relay
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Percentage Differential Relay
This relay is called percentage differential relay because the
current required to trip can be expressed as a percentage of
load current. The percentage differential relay is shown in
figure.
The current in the operating coil is (i1-i2) while the current in therestraining coil is (i1+i2)/2 .
Greater the current flowing through the restraining coil, the
higher the value of current required in the operating coil to
trip the relay. Thus when the load is heavy, a greaterdifferential current through operating coil is required for
operation than under light load.
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Voltage Balance Differential Relay
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Voltage Balance Differential Relay
Under normal healthy condition, the two voltages e1 and e2 are
same ,so the difference between them is zero. So the relay
will npot operate under normal condition.
In case of fault, the two voltages will be different either in
magnitude or in phase or on magnitude and phase both.
So under faulty condition, there will be current in the operating
coil and the relay operates.
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Distance Relays
The relays in which the operation is governed by the ratio of the
applied voltage to current in the protected circuit are calledDistance or Impedance relays(impedance is an electrical
measure of distance along the transmission line).A distance or
impedance relay is essentially an ohmmeter and operates when
the impedance of the protected zone falls below apredetermined value.
In impedance relay, the torque is produced by a current element is
opposed by the torque produced by the voltage element,. The
relay operates when the ration V/I is less than a predetermined
value.
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Basic Principle of Operation of Impedance Relay
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Basic Principle of Operation of Impedance Relay
The principle of operation of an impedance relay is shown in
figure.
The voltage element of the relay is excited through a P.T. from the
line to be protected .The current element of the relay is excited
from a C.T. in series with the line. The portion LM of the line is
the protected zone.
Under normal operating conditions, the impedance of the
protected zone is ZL .The relay is so designed that it closes its
contacts whenever the impedance of the protected zone falls
below the predetermined value.(ZL in this case).
On the occurrence of fault at point F1 in the protected zone ,the
impedance Z(V/I,V and I being substation voltage and fault
current respectively) is less than ZL ,the relay operates.
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Basic Principle of Operation of Impedance Relay
However the relay will not operate if the fault occurs beyond the
protected zone( point F2),since Z is greater than ZL at this point.