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EEE 6903: Advanced Protective Relays

Dept. of EEEBUET

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Contents

• Review of characteristics of over current, directional, differential, distance and pilot relays.

• Principles of relay design.• Effects of transients on relay operation. • Harmonic relaying.• Static and digital relays.• Applications of static and digital relaying in

various protection schemes.

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Lecture #1: Introduction

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Power System

• It consists of – Generators– Transformers– Transmission &– Distribution Circuits

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• Failure– May occur somewhere in the system– Must be quickly detected and disconnected from the system– If not cleared quickly

• May cause unnecessary interruption of service to the customers• Rapid disconnection of faulted apparatus limits the amount of

damage to it and prevents the effects of fault from spreading into the system

• Can affect the stability of the power system– Fault in the bus-zone of a power station can cause tripping of all the

generator units in power station and can affect the stability of the interconnected system

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• Detection of a fault and disconnection of a faulty section or apparatus can be achieved by– Fuses– Relays in conjunction with circuit breakers (CBs)

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• Fuse– Performs both detection and interruption functions

automatically – Use is limited for the protection of low voltage

circuits only

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• Relay and CB– For high voltage circuits– Relay detect the fault and supply information to

the CB– CB performs the function of circuit interruption

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Protective Relays• A protective relay is a device that detects the fault and

initiates the operation of the CB to isolate the defective element from the rest of the system.– Distinguish between normal and abnormal condition– Detects the abnormal conditions in the electrical circuits by

constantly measuring the electrical quantities which are different under normal and fault conditions

– Electrical quantities which may change under fault conditions• Voltage• Current• Frequency• Phase angle

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Protective Relays• Actuating quantities for the relay can be one or more of the following parameters of

voltage/current derived from CT/PT– Magnitude– Rate of change– Direction– Phase angle– Frequency– Wave shape– Duration – Ratio

• Faults signal their presence, type and location to the protective relays through the changes in one or more of the above quantities– Relay operates to close the trip circuit of the CB

• Opens the CB and disconnect the faulty section

• Entire process is automatic

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• Relay circuit connections can be divided into three parts– 1st part: Primary winding of the current

transformer (CT)• Connected in series with the line to be protected

– 2nd part: Secondary winding of the CT and the relay operating coil

– 3rd part: Tripping circuit, consists of• A source of supply• Trip coil of the CB• Relay stationary contacts

– Protective relaying is a team work of these components

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• Operation– F is the fault (short circuit) point on the

transmission line– Line current increases to an enormous value– Heavy current flows through the relay coil,

causing the relay to operate by closing its contact– Closes the trip circuit of the CB, making the CB

open and isolating the faulty section from the rest of the system

– Ensures the safety of the circuit equipment from damage and normal working of the healthy portion of the system

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Fault Statistics in a System in a YearEquipment Causes of Fault % of total Faults

Overhead lines Lightning strokes Storms, earth quakes, icing Birds, trees, kites, aeroplanes, snakes

30-40

Underground cables Damage during digging Insulation failure due to temperature rise Failure of joints

8-10

Alternators Stator faults Rotor faults Faults in associated equipment Faults in protective system

6-8

Transformer Insulation failure Faults in tap changer Faults in bushing Faults in protection circuit Inadequate protection Overloading, over voltage

10-12

CT, PT Over voltages Insulation failures Breaking of conductors Wrong connection

15-20

Switchgear Insulation failure Mechanical defect Leakage of air/oil/gas Inadequate rating Lack of maintenance

10-12

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Desirable Qualities of Protective Relays

• Selectivity• Speed• Sensitivity• Reliability• Simplicity• Economy

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Selectivity• Ability of the protective system to select correctly that part of the system in

trouble and disconnect the faulty part without troubling the rest of the system• Should select and disconnect only faulty part without disconnecting the

remaining healthy part (able to detect the point at which the fault occurs and cause the opening of the CBs closes to the fault with minimum or no damage to the system)

• Should be inoperative and stable during faults and abnormal conditions beyond its protective zone

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Selectivity• If not selective and operates for faults beyond its protective zones

– a larger portion of the system gets disconnected unnecessarily, causing embarrassment to supplier and consumers

• In order to provide selectivity to the system: usual practice to divide the entire system into several protection zones

• Boundary of a protective zone is determined by the location of CT• CT are located such that CBs are covered in the protective zones• When fault occurs in a given zone, only CBs within that zone will be opened• Certain amount of overlap between adjacent protection zones• For a failure within the region where two adjacent zones overlap, more

breakers will be opened• If there is no overlap, no CBs will open when a failure in the region between

zones would not lie in either region

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Speed

• Should disconnect the faulty section as fast as possible. – Minimises damage. Electrical apparatus may be damaged if

they carry fault currents for a long time– Voltage is reduced, may cause shut down consumers’ motors

and the generators on the system may become unstable– Decreases the possibility of development of one type of fault

into the other more severe type– Short circuit on bus bar with 60 kA rms:

• Fault duration 0.07s: no damage• Fault duration 7s: completely destroyed

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Sensitivity

• Ability of the relay system to operate with low value of actuating quantity

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Reliability

• Ability of the relay system to operate under the pre-determined conditions

• Should not fail to operate in the event of faults in the protected zone

• Should not be any fault in the components (CB, relays, CT’s, PT’s secondary coils, trip circuit, battery system accessories etc) of the protective system

• Should not operate unnecessarily

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Economy

• Should not cost more than 5% of total cost

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Some Terms in Protective Relaying• Switchgear

– Apparatus used for switching, controlling and protecting the electrical circuits and equipment

• Relay– It is a device by means of which an electric circuit is controlled by

change in the other circuit• Protective Relay

– It is an electrical relay used for protective of electrical devices– Closes its contact when operating quantity reaches certain

predetermined magnitude and electric circuit (trip circuit) is controlled (closed), by change in the other circuit

– Closing of relay contacts initiates trip circuit

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• CT– Used for measurement and protective relaying purposes– Stepped down primary current to a secondary current of lower value– Secondary current ratings are of the order of 5A, 1A & 0.1A– Primary current rating vary from 10 to 3000A– Volt-ampere rating is low (5-150VA)

• PT– Stepped down primary voltage to a secondary voltage of lower value– Standard secondary rated voltage 110V, 240V, 440V– Volt-ampere capacity is small relative to power transformer (few kVA

to MVA for power transformer)

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• Fault clearing time– Time elapsed between the instant of occurrence

of fault and instant of final arc extinction in CB• Relay time– Time interval between occurrence of fault and

closure of relay contacts• Breaker time– Time interval between closure of trip circuit and

final arc extinction

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• Stability of Power System– Denotes a condition during which all the synchronous

machines in the system are in synchronism• Earth fault– Fault which involves earth: L-G, LL-G

• Phase fault– Fault which does not involve earth: L-L

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• Instantaneous Relay– A fast relay having relay time of less than 0.2s (0.1s) and

having no intentional time lag– Relay contacts are closed immediately after current in the

relay coil exceeds the minimum calibrated value• Inverse time relay– Operating time is approximately inversely proportional to

the magnitude of the actuating quantity

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• Definite time lag delay– Definite time elapses between the instant of pick up

and closing of relay contacts• Independent of the amount of current through the relay coil

– Same for all values of current in access of the pickup value

– Practically all inverse time relays are also provided with definite minimum time feature in order that relay may never become instantaneous in its action for very long overloads

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• IDMT Relay– Inverse definite minimum time relay– A relay having an inverse characteristic of current

vs time, upto certain increased value of current after which the time is definite

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• Pick up– Value of operating quantity which is on threshold (border) above

which the relay operates and closes its contacts– Normally relay setting corresponds to pick up value

• Current setting– Often desirable to adjust the pick up current to any required value– Usually achieved by the use of tappings on the relay operating coil– Pick up current=rated secondary current of CT x current setting

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• Plug settling multiplier (PSM)– Ratio of fault current in relay coil to the

pick up current– Suppose CT rating 400/5, current

setting 150%, primary fault current 2400A

– Fault current in relay coil=2400x5/400=30A

– Pickup current=5x1.5=7.5A– PSM=30/7.5=4

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• Time setting multiplier– Generally provided with control to adjust the time of

operation and known as time setting multiplier

– If time setting is 0.1 (vary from 0 to 1)and the time obtained from the time/PSM curve is 3s, then actual relay operating time=3x0.1=0.3s

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• Calculation of relay operating time, we need– Time/PSM curve– Current setting– Time setting– Fault current– CT rating

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• Over current Relay– Relay which responds to increase in current

• Earth fault Relay– Relay which sense earth fault

• Distance Protection– Protection scheme used for protection of transmission lines in which

the relay measurement is based on measuring V/I ratio at relaying point which gives a measure of distance between relay location and fault location

• Differential Protection– Protective system which responds to vector difference

(Phase/magnitude) between two or more similar electrical quantities

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• Protective scheme– A set of protective systems covering a particular

protective zone• Protective system– A combination of components which together

performs the protective relaying• Static Relay– Does not have movable parts in their system– Measurement is carried out by stationary circuits

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• Electro-mechanical relay– Measurement is performed by movable parts– Based on following effects of current

• Electromagnetic attraction• Electromagnetic induction• Thermal effect, heat generated by I2R

• Auto-reclosure– Process of automatic reclosing of CB after its opening

• Power Line Carrier– High frequency signal sent through the power line conductors– Used for purpose of communication, monitoring and protection

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• Types of protection– Primary protection– Back up protection

• Primary protection– Main protection and serves as the

first line of defence– Service record of primary protection

is very high (over 90% operations are correct)

– Sometimes may fail due to trouble within relay

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• Back up protection– Second line of defence in case of failure of the

primary one – Designed to operate with sufficient time delay so

that primary one will be given enough time to function

– A large part is disconnected than when primary relaying functions correctly

– So, greater emphasis should be placed on the better maintenance of primary protection