This document discusses several types of protective relays used in electrical engineering. It describes overcurrent relays, distance/impedance relays, differential relays, directional overcurrent relays, and synchronism check relays. The key aspects covered include how each type of relay works, what electrical quantities they measure, and their applications in protecting electrical equipment from faults.

1. [Type here]
Protective Relay
In electrical engineering,aprotective relayisa relaydevice designedtotripa circuitbreakerwhena
faultisdetected.
Types of protective relay
 1 Overcurrentrelay
 2 Inverse definite minimumtime
 3 Distance relay/Impedance relay
 4 Currentdifferentialprotectionscheme
 5 Directional relay
 6 Synchronismcheck

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Overcurrentrelay
An overcurrentrelayisatype of protective relaywhichoperateswhenthe loadcurrentexceedsapickup
value.The ANSIdevice numberis50 for an instantaneousovercurrent(IOC) oraDefinite Time
Overcurrent(DTOC).Ina typical applicationthe overcurrentrelayisconnectedtoa current transformer
and calibratedtooperate ator above a specificcurrentlevel.Whenthe relayoperates,one ormore
contacts will operate andenergize totrip(open) acircuitbreaker.
Distance relay/Impedance relay
Distance relays differ in principle from other forms of protection in that their performance is not
governed by the magnitude of the current or voltage in the protected circuit but rather on the
ratio of these two quantities. Distance relays are actually double actuating quantity relays with
one coil energized by voltage and other coil by current. The current element produces a positive
or pick up torque while the voltage element produces a negative or reset torque. The relay
operates only when the V/I ratio falls below a predetermined value (or set value). During a fault
on the transmission line the fault current increases and the voltage at the fault point decreases.
The V/I ratio is measured at the location of CTs and PTs. The voltage at the PT location depends
on the distance between the PT and the fault. If the measured voltage is lesser, that means the
fault is nearer and vice versa. Hence the protection called Distance relay. The load flowing
through the line appears as an impedance to the relay and sufficiently large loads (as impedance
is inversely proportional to the load) can lead to a trip of the relay even in the absence of a fault.

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Differential relay
A differential relayisdefinedasthe relaythat operateswhenthe phase difference of twoormore
identical electrical quantitiesexceedsapredeterminedamount.The differential relayworksonthe
principle of comparisonbetweenthe phase angle andmagnitudeof twoormore similarelectrical
quantities.
Differential relay :- A two-winding relay that operates when the difference between the currents
/ Voltage in the two windings reaches a predetermined value is called differential relays.
There are three fundamental systems of differential or balanced protection:
I.current differential relay
II.voltage differential relay
III.Biased beam relay or percentage differential relay .
Uses:
 Transformer Protection.
 Generator Protection.
 Bus-bar Protection.
 Motor Protection.

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A differential relay is defined as the relay that operates when the phase difference of two or more
identical electrical quantities exceeds a predetermined amount. The differential relay works on
the principle of comparison between the phase angle and magnitude of two or more similar
electrical quantities. Comparing two electrical quantities in a circuit using differential relays is
simple in application and positive in action.
For example, consider the comparison of the current entering a protected line and the current
leaving it. If the current enters the protected line is more than the current leaves it, then the extra
current must flow in the fault. The difference between the two electrical quantities can operate a
relay to isolate the circuit.
For the operation of the differential relay, it should have two or more electrical quantities, and
these quantities should have a phase displacement (normally approximately 180). Any types of
the relay can operate as a differential relay depends on upon the way it is connected in a circuit.
In other words, it doesn’t depend on the construction of the relay it depends on the way it is
connected to the circuit.
Differential protection provides unit protection. The protected zone is exactly known by the
location of current and potential transformers. The phase difference is achieved by suitable
connections of secondaries of CTs and PTs.
The differential protection principle is employed for the protection of generator, generator-
transformer units, transformers, feeders, large motors, and bus-bars. The differential protection
relay is mainly classified into four categories. These are
1.Current Differential Relay
2.Voltage Differential Relay
3.Biased or Percentage Differential Relay
4.Voltage Balance Differential Relay
Current Differential Relay
A relay which senses and operates the phase difference between the current entering into the
electrical system and the current leaving the electrical system is called a current differential
relay. An arrangement of overcurrent relay connected to operate as a differential relay is shown
in the figure below.

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The dotted line represents the element of the system that is to be protected by the differential
relay. The system element might be a length of the circuit, a portion of the bus or a winding of a
generator or that of a transformer. A pair of current transformers is fitted on the either ends of the
section to be protected. The secondaries of current transformers are connected in series with the
help of the pilot wires in such a way that they carry the induced current in the same direction.
The operating coil of an overcurrent relay is connected across the current transformer secondary
circuit shown in the figure below.

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When there is no fault current or there is an external fault, then the current in the secondaries of
the current transformers are equal, and the relay operating coil, therefore, does not carry any
current. When the short circuit developed anywhere between the two current transformers, then
the currents flow to the fault of both sides, and the sum of the current transformer secondary
current will flow through the differential relay.
In other words, the differential relay current will be proportional to the phase difference between
the currents entering and leaving the protected element. If the differential current exceeds the
relay’s pick up value, then the relay will operate.
Biasedor Percentage Differential Coil
This is the most used form of differential relay. Their arrangement is same as that of the current
differential relay; the only difference is that this system consists an additional restraining coil
connected in the pilot wires as shown in the figure below and current flows in both CTs flows
through it.

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The operating coil is connected to the midpoint of the restraining coil. The reasons for this
modification in circulating current differential relay is to overcome the difficulty arising out of
differences in current transformers ratio for high values of short circuit current.
Induction Type Biased Differential Relay
This relay consists of a pivoted disc free to rotate in the air gaps of two electromagnets. The
portion of each pole of the electromagnet magnet is fitted with a copper shading ring. The ring
can be moved further in, or out of the pole. The disc experiences two torques one due to
operating element and other due to restraining element.

8. [Type here]
If the shading rings were in the position on each element, then the resulting torque experienced
by the disc would be zero. But if the shading rings of restraining element were moved further
into the iron core, the torque exerted by the restraining element will exceed than that of the
operating element.
Voltage Balance Differential Relay
The current differential relay is not suitable for the protection of the feeders. For the protections
of the feeders, the voltage balance differential relays are used. In this arrangement, the two
similar current transformers are connected at either end of the system element under protection
using pilot wires.

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The relays are connected in series with the pilot wires, one at each end. The relative polarity of
the current transformers is such that there is no current through the relay under normal operating
conditions and under fault conditions. The CTs used in such protections should be such that they
should induce voltages in the secondary linearly with respect to the current. Since the magnitude
of the fault current is very large, so that the voltage should be a linear function of such large
currents, the CTs should be aired cored.
When the fault occurs in the protected zone, the currents in the two primaries will differ from
one another, and so voltage induced in the secondaries of the CTs will differ and circulating
current will flow through the operating coils of the relays. Thus the trip circuit will be closed,
and the circuit breaker will be open.

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IDMT relays
IDMT relaysare protection relays.Theyare usedon transmissionlinestosee tothatthe line current
doesn'texceedsafe valuesandif itdoes,triggersthe circuitbreaker. IDMTmeansinverse definite
minimumtime.So as the current keepsincreases,the relaytakesminimumtimetotripthe circuit.
Phase Overcurrent Protection [50/51]
The phase overcurrent protection consists oflow-set stage and high-set stage for
all the three phases namely, IL1, IL2, and IL3. The low-set element setting and
the high-set element setting are common to all the three phases.
7SR10
Overcurrent and Earth Fault Relay

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Value of α and K determines the degree of inverse in the IDMT curve
Type of Curve α K
Normal Inverse 0.02 0.14
Very Inverse 1.0 13.5
Extremely Inverse 2.0 80
Long-Time Inverse 1.0 120
Required Tms 0.03
Fault Current 5.0
Settings Current 0.8
Required Time 0.329
CalculatedTMS 0.087729
CalculatedTime 0.112505
I= Actual SecondaryCurrent
Is= RelaySettingCurrent
Ir = is the ratio of the fault current to the relay setting current. TD is the Time Dial setting.
The above equations result in a "family" of curves as a result of using different time multiplier setting
(TMS) settings. It is evident from the relay characteristic equations that a larger TMS will result in a
slower clearance time for a given PMS (Ir) value.

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Directional overcurrent relays
Directional overcurrentrelays(67) respondtoexcessive currentflowinaparticulardirectioninthe
powersystem.The relaytypicallyconsistsof twoelements.One isa directional element,which
determinesthe directionof currentflow withrespecttoa voltage reference.
Directional overcurrentrelays

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Abstract:
Directional overcurrent relaying (67) refers to relaying that can use the phase relationship of
voltage and current to determine direction to a fault. There are a variety of concepts by which
this task is done. This paper will review the mainstream methods by which 67 type directional
decisions are made by protective relays. The paper focuses on how a numeric directional relay
uses the phase relationship of sequence components such as positive sequence (V1 vs. I1 ),
negative sequence (V2 vs. I2), and zero sequence (V0 vs. I0) to sense fault direction, but other
concepts such as using quadrature voltage (e.g., VAB vs Ic) are included .
Synchronism check
A synchronismcheckingrelayprovidesacontact closure whenthe frequencyandphase of twosources
are similartowithinsome tolerancemargin.A "synchcheck"relayisoftenappliedwhere twopower
systemsare interconnected,suchasat a switchyardconnectingtwopowergrids,orat a generator
circuitbreakerto ensure the generatorissynchronizedtothe systembefore connectingit.