relays
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Types of Relays
Types of protection relays are mainly based on their characteristic, logic, on actuating parameter and operation mechanism.
Based on operation mechanism protection relay can be categorized as electro magnetic relay, static relay and mechanical relay.
Actually relay is nothing but a combination of one or more open or closed contacts. These all or some specific contacts the relay
change their state when actuating parameters are applied to the relay. That means open contacts become closed and closed
contacts become open. In electromagnetic relay these closing and opening of relay contacts are done by electromagnetic action
of a solenoid.
In mechanical relay these closing and opening of relay contacts are done by mechanical displacement of different gear level
system.
In static relay it is mainly done by semiconductor switches like thyristor.
In digital relay on and off state can be referred as 1 and 0 state.
Based on Characteristic the protection relay can be categorized as-
1. Definite time relays
Definite Time Over Current Relay
This relay is created by applying intentional time delay after crossing pick up value of the current. A definite time over current
relay can be adjusted to issue a trip output at definite amount of time after it picks up. Thus, it has a time setting adjustment
and pick up adjustment.
1. Inverse time relays with definite minimum time(IDMT)
Ideal inverse time characteristics can not be achieved, in an over current relay. As the current in the system increases, the
secondary current of the current transformer is increased proportionally. The secondary current is fed to the relay current coil.
But when the CT becomes saturated, there would not be further proportional increase of CT secondary current with increased
system current.
From this phenomenon it is clear that from trick value to certain range of faulty level, an inverse time relay shows exact inverse
characteristic. But after this level of fault, the CT becomes saturated and relay current does not increase further with increasing
faulty level of the system. As the relay current is not increased further, there would not be any further reduction in time of
operation in the relay. This time is referred as minimum time of operation.
Hence, the characteristic is inverse in the initial part, which tends to a definite minimum operating time as the electric current
becomes very high. That is why the relay is referred as inverse definite minimum time over current relay or simply IDMT
relay.
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1. Instantaneous relays.
Instantaneous over current relay, which operates very fast with no intentional time delay and the operating time of these relay can
be as low as 0.01sec . These relay operates only when the impedance between the relay and the source zs is very small
compared to the impedance to the impedance of the protected section zl .
7. Voltage restraint over current relay.
The voltage restrained over current protection function(51V) is used to protect generators. The operation set point is adjusted
according to the voltage to take into account cases of faults close to the generator which cause voltage dips and short circuit
current. This protection function is three-pole. It picks up if one ,two or three phase currents reach the voltage-adjusted operation
set point.
Based on of logic the protection relay can be categorized as-
1. Differential.
Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares
between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary
currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image001.gif[/IMG]
2. Unbalance.
3. Neutral displacement.
4. Directional.
The over-current protection can be given directional feature by adding directional element in the protection system.
Directional over-current protection responds to over-currents for a particular direction flow. If power flow is in the
opposite direction, the directional over-current protection remains un-operative.
Directional over-current protection comprises over-current relay and power directional relay- in a single relay casing.
The power directional relay does not measure the power but is arranged to respond to the direction of power flow.
Directional operation of relay is used where the selectivity can be achieved by directional relaying. The directional
relay recognizes the direction in which fault occurs, relative to the location of the relay. It is set such that it actuates
for faults occurring in one direction only. It does not act for faults occurring in the other direction. Consider a feeder
AC (Fig. 9) passing through sub-section B. The circuit breaker CB3 is provided with a directional
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image002.emz[/IMG](Fig. 9) Principle of directional
protection
Relay `R' which will trip the breaker CB3 if fault power flow in direction C alone. Therefore for faults in feeder AB,
the circuit breaker CB3 does not trip unnecessarily. However for faults in feeder BC the circuit-breaker CB3 trips
Because it's protective relaying is set with a directional feature to act in
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direction AC
Another interesting example of directional protection is that of reverse power protection of generator (Fig. 10). If the
prime mover fails, the generator continues to run as a motor and takes power from bus-bars.
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image003.emz[/IMG](Fig. 10) Reverse powers protection
against motoring action of a generator
Directional power protection operates in accordance with the direction of power flow.
Reverse power protection operates when the power direction is reversed in relation to the normal working direction.
Reverse power relay is different in construction than directional over-current relay.
In directional over-current relay, the directional element does not measure the magnitude of power. It senses only
direction of power flow. However, in Reverse Power Relays, the directional element measures magnitude and
direction of power flow.
1. Restricted earth fault.
An external fault in the star side will result in current flowing in the line current transformer of the affected phase and at the same
time a balancing current flows in the neutral current transformer, hence the resultant electric current in the relay is therefore zero.
So this REF relay will not be actuated for external earth fault. But during internal fault the neutral current transformer only carries
the unbalance fault current and operation of Restricted Earth Fault Relay takes place. This scheme of restricted earth fault
protection is very sensitive for internal earth fault of electrical power transformer. The protection scheme is comparatively
cheaper than differential protection scheme
6. Over fluxing.
7. Distance schemes.
8. Bus bar protection.
9. Reverse power relays.
10.Loss of excitation.
11.Negative phase sequence relays etc.
Based on actuating parameter the protection relay can be categorized as-
1.Current relays.
2. Voltage relays.
3. Frequency relays.
4. Power relays etc.
Based on application the protection relay can be categorized as-
1.Primary relay.
2. Backup relay.
Primary relay or primary protection relay is the first line of power system protection whereas backup relay is operated only when
primary relay fails to be operated during fault. Hence backup relay is slower in action than primary relay. Any relay may fail to be
operated due to any of the following reasons,
1) The protective relay itself is defective.
2) DC Trip voltage supply to the relay is unavailable.
3) Trip lead from relay panel to circuit breaker is disconnected.
4) Trip coil in the circuit breaker is disconnected or defective.
5) Current or voltage signals from CT or PT respectively is unavailable.
As because backup relay operates only when primary relay fails, backup protection relay should not have anything common with
primary protection relay.
Some examples of Mechanical Relay are-
1. Thermal
(a) OT trip (Oil Temperature Trip)
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(b) WT trip (Winding Temperature Trip)
(C) Bearing temp trip etc.
2. Float type
(a) Buchholz
(b) OSR
(c) PRV
(d) Water level Controls etc.
3. Pressure Switches.
4. Mechanical Interlocks.
5. Pole discrepancy Relay.
WORKING OF SINGLE-POLE DOUBLE-THROW RELAY
SPDT relay is an electromagnetic switch consisting of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed
terminal (87a) and a normally open terminal (87).
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image004.jpg[/IMG]
Fig1. Poles of a SPDT Relay Switch
When the coil of the relay is at rest (not energized), the common terminal (30) and the normally closed terminal (87a) have
continuity.
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image005.jpg[/IMG]
Fig2. Un-energized Relay Position
When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity.
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image006.jpg[/IMG]
Fig3. Energized Relay Position
Basically, the coil is an electromagnet that causes the arm that is always connected to the common (30) to pivot when
energized thereby breaking contact with the normally closed terminal (87a) and making it with the normally open terminal (87).
Below is an example of how the connections are made to a load in a circuit using a SPDT relay switch?
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image007.jpg[/IMG]
Fig4. Connection of an SPDT Relay to Load
When the relay coil is energized, contact is established between the common (COM) and normally-open (NO) terminal thereby
completing the supply connections to the load. Hence, the load is switched on when the relay is energized. Also check this
article to understand the working of relays with the help of pictures, here - How Relay works
Different Types of Relay
1. Single Pole Single Throw (SPST)- these types of relay comprise of 4 terminals. Two terminals are used as coil points and
other two can be used to connect or disconnect the circuit (A and B).
2. Single Pole Double Throw (SPDT)- these types of relay comprise of 5 terminals two for coil one for common terminal(C)
and rest two can be connected to the common terminal.
3. Double Pole Single Throw (DPST)- these types of relay comprise of 6 terminal two for coil and other four for connecting and
disconnecting two device. In other words it contains two SPST relay in one package.
4.DoublePole Double Throw(SPDT)- these types of relay comprise of 8 terminal two for coil and another two as common point
and rest for connecting and disconnecting devices. In another words in this two SPDT relay are connected in one package.
[IMG]file:///C:\Users\arptst\AppData\Local\Temp\msohtmlclip1\01\clip_image008.png[/IMG]
Comparison of Different Type Relay:
Characteristic Electro Mechanical
Relay
Static Relay Digital Relay Numerical Relay
Technology
Standard
1st generation
Relays.
2nd generation
Relays.
Present generation Relays. Present generation Relays.
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Operating
Principle
They use principle of
electromagnetic
principle.
In this relays
transistors and ICs
r been used
They use Microprocessor. Within built
software with predefined values
They use Microprocessor. Within built
software with predefined values
Measuring
elements/
Hardware
Induction
disc,Electromagnets,
Induction cup,
Balance Beam
R, L, C, Transistors,
Analogue ICs
comparators
Microprocessors, Digital ICs, Digital
Signal Processors
Microprocessor s,Digital ICs, DigitalSignal
processors
Measuring
method
Electrical
Qtysconverted
intomechanical
force, torque
Level
detects,comparison
withreference value
in analogue
Comparator
A/D
conversion,Numericalalgorithmtechniques
A/D
conversion,Numericalalgorithmtechniques
Surrounding
Environment
Depend upon
gravitation and the
value changes to the
surrounding
magnetic fields also.
There value may
vary with respect to
temperature also.
Relay Size Bulky Small Small Compact
Speed of
Response
Slow Fast Fast Very Fast
Timing
function
Mechanical clock
works, dashpot
Static timers Counter Counter
Time of
Accuracy
Temp .Dependant Temp. Dependant Stable Stable
Reliability High Low High High
Vibration Proof No Yes Yes Yes
Characteristics Limited Wide Wide Wide
Requirement Required Required Not Required Not Required
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of Draw Out
CT Burden High Low Low Low
CT Burden 8 to 10 VA 1 VA 20 Pico Farads > 20 Pico Farads
Deterioration
due to
Operation
Yes No No No
Relay
Programming
No Partially Programmable Programmable
SCADA
Compatibility
No No Possible Yes
Operational
value
indication
Not Possible Possible Possible Possible
Visual
indication
Flags, targets LEDs LEDs, LCD LEDs, LCD
Self monitoring No Yes Yes Yes
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Parameter
setting
Plug setting, dial
setting
Thumb wheel,dual
in line switches
Keypad for numeric values,through
computer
Keypad for numeric values,through
computer
Fault
Disturbance
Recording
Not possible Not possible possible possible
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