Dr.Latif Shaikh

Protective Relays

1.Introduction2.Basic Current Relays Electromechanical Mechanism3.Solid State Relays4.Micrprocessor Controlled Relays5.Over Current Relays6.Under Voltage Relays7.Differential relays8.Distance Relays9.Microcomputer Controlled Relays

Protective Relay

Protective relays are the devices that detect abnormal conditions in electrical 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.

Protective RelayA typical relay is shown

.When short circuit occurs at point F on transmission line CT current increases .This increases current in CT coil which closes trip coil of circuit breaker and CB contacts get opened. Thus faulty section is isolated.

Requirements and Qualities of Relays

Selectivity and DiscriminationSpeedSensitivityReliabilitySimplicityEconomy

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 level – It 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 target – It is a device which indicates the operation of relay.

6.Burden- It is power absorbed by the relay expressed in VA.

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 to pick up value until the relay operates its contacts.

8.Over –Reach –The 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 current to 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

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 to close the contacts .

Attracted Armature Type RelaysThese 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 to close the contacts .

Attracted Armature Type Relays

Applications

1.For over current protection2.Differential Protection3.Auxiliary Relays4.Definite time lag over

current and earth fault protection

Static RelaysA 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.

Static Relays

Figure shows the essential components of a static relay1.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.

Static Relays

Advantages:1.More accurate and fast in operation2. No moving parts.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.

Static RelaysDisadvantages 1.Reliability cannot be predicted2.Construction is not very robust.3.Easily affected by surrounding interference.4.Auxiliarty DC supply is required 5,.Affected by voltage transients.

Applications1.Ultra high speed protection of EHV AC transmission lines

utilizing distance protection.2.In over current and earth fault protection schemes3.As main element in differential relay

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 flexibility – Various 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.

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.

Microprocessor Based Static Relays

Microprocessor Based Static Relays

Under Voltage Relays

Under voltage protection is provided for AC circuits ,bus –bars ,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 inverse characteristics 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 voltage relays. The relay coil is energized by the voltage to be measured either directly or through a PT.

Over Current Protection

Over current protection is that protection in which the relay picks up when the magnitude of the current exceeds the pick up value.

The over current relays are connected to the system usually through a CT..

Over Current Protection

Protective Devices used for over current protection areFusesMiniature circuit breakersCircuit breakers fitted with overloaded coils or tripped

by over current relaysOver current relays along with current transformers

Applications of Over Current Protection

Motor protectionTransformer protectionLine protectionProtection of utility equipment

Characteristics of Relay Units for Over Current Protection

Definite CharacteristicsInverse CharacteristicsExtremely Inverse Very inverseInverse

Connection scheme with three over current relays

Connection scheme with three over current relaysThe 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 earth fault. Therefore such schemes are used with solidly earthed systems where phase to phase and phase to earth faults are likely to occur.

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

and after a definite time.

Over current relay with time delay relay

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 protection2.Voltage balance protection

Differential Relays (Current Differential Relay(

Current differential relay

Figure shows the arrangement of an over current relay connected to work on the current differential protection. In this arrangement a pair of current transformers are fitted either ends of the element to be protected and secondary windings of CTs are connected in series so that they carry induced currents in the same direction.

Under normal conditions, where there is no fault or there is external fault ,the currents in the two CTs secondary are equal and relay operating does not carry any current.

Whenever there is an internal fault ,currents in the two secondaries of CTs are different ,the relay operating coils gets energized by the current equal to their difference and the trip circuit is completed to operate the circuit breaker

Current Differential RelayThis 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.

Percentage Differential relay

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 the restraining 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 greater differential current through operating coil is required for operation than under light load.

Percentage Differential Relay

Since the relay has a rising characteristic that is the pick up value increases as the magnitude of through current increases, the relay is restrained or biased against operating inaccurately.

Due to this the relay is called a biased relay.The advantage of this relay is that it is less likely to operate

inaccurately than a differentially connected over current relay when a short circuit occurs external to the protected zone.

Voltage Balance Differential Relay

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 not 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.

Distance RelaysThe relays in which the operation is governed by the ratio of

the applied voltage to current in the protected circuit are called Distance 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 a predetermined 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.

Basic Principle of Operation of Impedance Relay

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.

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.