Using Protective Relay for Fighting Against Faults

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Using Protective Relay For FightingAgainst Faults

h ttp ://e lectri ca l- engineering -porta l.com/using -p ro tective -re lay- fo r- fi gh ting- against- faults December 20, 2012

Using Protective Relay For Fighting Against Faults

Content

1. Introduction to Protective Relay

2. Working Principle of Protective Scheme

3. What is Relay?

4. Functions of Prot ective Relay

5. Desirable qualities of protective relaying

6. Terminology of protective relay

7. History of Prot ective Relay

8. Types of Relays

9. Types of Relay based on Relay OperationMechanism

10. Prot ective relay testing: Test relays of allgenerat ions (VIDEO)

Introduction to Protective Relay

Protective relayworks in the way of sensing and

control devices to accomplish its function. Undernormal power system operation, a protective relayremains idle and serves no act ive funct ion.

But when fault or undesirable condition arrivesProtective Relay must be operated and functioncorrectly.

A Power System consists of various electrical components like Generator, transformers,transmission lines, isolators, circuit breakers, bus bars, cables, relays, instrument t ransformers,distribution feeders, and various types of loads.

Faults may occur in any part of power system as a short circuit and earth f ault . Fault may beSingle Line to Ground, Double Line to Ground, Line to Line, three phase short circuitetc.This results in flow ofheavy faultcurrent through the system.

Fault level also depends on the fault impedance which depends on t he location of fault referredfrom the source side. To calculate fault level at various points in the power system, faultanalysis is necessary.

The protection system operates and isolates the faulty section. The operation of theprotection system should be fast and select ive i.e. it should isolate only the faulty section in the

shortest possible t ime causing minimum disturbance to the system. Also, if main protection failsto operate, there should be a backup protection for which proper relay co-ordination isnecessary.

Failure of a protective relay can result in devastating equipment damage and prolongeddowntime.
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Basic connections o f circuit breaker contro l fo rthe opening operation

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Working Principle of Protective Scheme

Prot ective relaying senses the abnormal condit ion in a part of power system and gives an alarm

or isolates that part f rom healthy system. Protective relaying is a team work of CT, PT,prot ect ive relays, t ime delay relays, t rip circuits, circuit breakers etc.

Prot ective relaying plays an important role in minimizing the faults and also in minimizing thedamage in the event o f faults.

Figure above shows basic connections of circuitbreaker control for the opening operation. Theprotected circuit Xis shown by dashed line.

When a fault occurs in the prot ected circuit therelay connected to CT and PT actuates andcloses its contacts.

Current f lows f rom battery in the trip circuit . Asthe t rip coil of circuit breaker is energized, thecircuit breaker operating mechanism is actuatedand it operates for the opening operat ion.

Thus the fault is sensed and the trip circuit isactuated by the relay and the faulty part is

isolated.

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What is Relay?

A relay is automatic device which senses anabnormal condition of electrical circuit andcloses its contacts.

These contacts in turns close and complete the circuit breaker trip coil circuit hence make thecircuit breaker tripped fo r disconnect ing the faulty portion of the electrical circuit f rom rest ofthe healthy circuit .

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Functions of Protective Relay

These are the main functions of protective relay:

1. To sound an alarm or to close the trip circuitof a circuit breaker so as to disconnectFaulty Section.

2. To disconnectthe abnormall o eratin art so as to revent subse uent faults. For e. .
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Overload prot ection of a machine not only prot ects t he machine but also preventsInsulation failure.

3. To isolate or disconnect faulted circuits or equipment quickly from the remainder of thesystem so the system can cont inue to funct ion and to minimize the damage to the faultypart. For example If machine is disconnected immediately after a winding fault, only afew coils may need replacement. But if the fault is sustained, the entire winding may getdamaged and machine may be beyond repairs.

4. To localize the effect of faultby disconnecting the faulty part f rom healthy part , causingleast disturbance to the healthy system.

5. To disconnect the faulty part quickly so as to improve system stability, service continuityand system performance. Transient stability can be improved by means of improvedprotective relaying.

6. To minimize hazards to personnel.

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Desirable Qualities of Protective Relaying

1. Selectivity,

2. Discrimination

3. Stability

4. Sensitivity,

5. Power consumption

6. System Security

7. Reliability

8. Adequateness

9. Speed & Time

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Terminology of protective relayPickup level of actuating signal: The value of actuating quantity (voltage or current) which ison threshold above which the relay init iates to be operated. If the value of actuating quantity isincreased, the electromagnetic effect of the relay coil is increased and above a certain level ofactuating quantity t he moving mechanism of the relay just starts to move.

Reset level: The value of current or voltage below which a relay opens its contacts and comesin original position.

Operating Time of Relay: Just af ter exceeding pickup level of actuating quantity the moving

mechanism (for example rot ating disc) of relay starts moving and it ultimately close the relaycontacts at the end of its journey. The t ime which elapses between the instant when actuatingquantity exceeds the pickup value to the instant when the relay contacts close.

Reset time of Relay: The t ime which elapses between the instant when the actuating quantitybecomes less than the reset value to the instant when the relay contacts returns to its normal
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position.

Reach of Relay: A distance relay operates whenever the distance seen by the relay is less thanthe pre-specified impedance. The actuating impedance in the relay is the function of distance ina distance protection relay. This impedance or corresponding distance is called reach of therelay.

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History of Protective Relay

The evolution o f prot ective relays begins with the electromechanical relays. Over the pastdecade it upgraded f rom electromechanical to solid state technologies to predominate use ofmicroprocessors and microcontrollers.

The timeline of the development of protective relays is shown below:

1900 to 1963 1963 to 1972 1972 to 1980 1980 to 1990

Electromechanical Relay Static Relay Digital Relay Numerical Relay

1925=Single Disc TypeRelay (Single Input)

1963=Stat ic Relay(All Purpose)

1980=DigitalType Relay (AllPurpose)

1990=NumericalType Relay (AllPurpose)

1961=Single Cup TypeRelay (Impedance Relay)

1972=Stat ic Relaywith self checking

(All Purpose)

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Types of Relays

Types of protection relays are mainly:

A. Based on Characteristic:

1. Definite t ime Relays.

2. Inverse def inite minimum t ime Relays (IDMT)

3. Instantaneous Relays

4. IDMT with Instantaneous.

5. Stepped Characteristic

6. Programmed Switches

7. Voltage restraint over current relay
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B. Based on logic:

1. Differential

2. Unbalance

3. Neutral Displacement

4. Directional

5. Restricted Earth Fault

6. Over Fluxing

7. Distance Schemes

8. Bus bar Protect ion

9. Reverse Power Relays

10. Loss of excitation

11. Negative Phase Sequence Relays etc.

C. Based on Actuating parameter:

1. Current Relays

2. Voltage Relays

3. Frequency Relays

4. Power Relays etc.

D. Based on Operation Mechanism:

1. Elect ro Magnetic Relay2. Static Relay Analog Relay Digital Relay

Numerical /Microprocessor Relay3. Mechanical relay

Thermal OT Trip (Oil Temperature Trip) WT Trip (Winding Temperature Trip) Bearing Temp Trip etc.

Float Type Buchholz OSR PRV Water level Controls etc.

Pressure Switches

Mechanical Interlocks


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Pole discrepancy Relay

E. Based on Applications

1. Primary Relays

2. Backup Relays

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Types of Relay based on Relay Operation Mechanism

1. Electromagnetic Relay

Electromagnetic relays are further categorized under two following categories.

1.1 Electromagnetic Attraction RelayThis Relay works on Electromagnetic At traction Principle

1.2 Electromagnetic Induction RelayThis Relay works on Electromagnet ic Induct ion Principle

2. Solid State (Static) Relay

Solid-state (and static) relays are further categorized under following designations:

2.1 Analog RelayIn Analog relays are measured quantities are converted into lower voltage but similar signals,which are then combined or compared directly to reference values in level detectors to producethe desired output.

2.2 Digital RelayIn Digital relays measured ac quantities are manipulated in analogue form and subsequentlyconverted into square-wave (binary) voltages. Logic circuits or microprocessors compare t hephase relationships of the square waves to make a t rip decision.

2.3 Numerical RelayIn Numerical relays measured ac quant ities are sequent ially sampled and converted into numericdata form. A microprocessor performs mathematical and/or logical operations on the data tomake trip decisions.

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Protective relay testing: Test relays of all generations (VIDEO)

Cant see this video? Clickhere to watch it on Youtube.

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References

Handbook of Switchgear Bhel

Digital/Numerical Relays -T.S.M. Rao

Documents

Using Protective Relay for Fighting Against Faults