Chapter3 Relays Sept 2012

8/13/2019 Chapter3 Relays Sept 2012

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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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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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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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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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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.

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Chapter3 Relays Sept 2012

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