Basic Relaying Iiee

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BASICPROTECTIVE RELAYING


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BASICPROTECTIVERELAYING

I. INTRODUCTION

A. Classification of Relays

B. Philosophy of Protection

C. Operating Principles

1. Electromechanical Relays

2. Static and Digital Relays

II. DIFFERENTIAL RELAY PROTECTION

A. Transformer Protection

B. Bus Protection

III. LINE PROTECTION

A. Distance Protection

B. Ground Protection

IV. Breaker Failure Protection

V. Auto Reclosing


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BASIC PROTECTIVE RELAYING

Relays are compact analog, digital, and numerical devices

that are connected throughout the power system to detect

intolerable or unwanted conditions within an assigned area.

They are, in effect, a form of active insurance designed to

maintain a high degree of service continuity and limit

equipment damage. They are Silent Sentinels.

I . I NTRODUCTI ON


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A. CLASSI FI CATI ON OF RELAYS

1. Protective Relays

2. Monitoring Relays

3. Reclosing Relays4. Regulating Relays

5. Auxillary Relays

6. Synchronizing Relays

BASIC PROTECTIVE RELAYING(I. INTRODUCTION)


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1. PROTECTI VE RELAYS

Detect defective lines, defective apparatus, or other

dangerous or intolerable conditions. These relays generally

trip one or more circuit breakers, but may also be used tosound an alarm

2. VERI FI CATI ON OR MONI TORI NG RELAYS

One whose functions is to verify power system conditions

with respect to prescribed limits and to initiate or permit

automatic functions other than opening a circuit breaker

during fault conditions. These relays includes fault detectors,

alarm units, channel-monitoring relays, synchronism

verification, and network phasing.




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3. RECLOSI NG RELAYS

Establish reclosing sequence for a circuit breaker following

tripping by protective relays.

4. REGULATI NG RELAYS

Are activated when an operating parameter deviates from

predetermined limits. Regulating relays function throughsupplementary equipment to restore the quantity to the

prescribed limits.

Ex: OFR, UFR




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5. AUXI LLARY RELAYS

Operates in response to the opening or closing of the

operating circuit to supplement or assist another relay or

device. These include timers, contacts-multiplier relays,

sealing units, isolating relays, lockout relays, closing relays

and trip relays.

6. SYNCHRONI ZI NG RELAYS

Assure that proper conditions exist for interconnecting two

sections of a power system.

Ex: Synchrocheck relay


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Branch of electric power engineering concerned with the

principles of design and operation of equipment (called

relays or protective relays) which detect abnormal

power system conditions, and initiate corrective action as

quickly as possible in order to return the power system to

its normal state.

B. PHI LOSOPHY OF PROTECTI ON


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Relays are the Brains of the Fault Clearance System

Breakers are the Muscles

A CRUDE ANALOGY



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Continuously monitor the power system

Respond to abnormal conditions

Prevents equipment damage

Minimizes service interruption

WHAT PROTECTI VE RELAYS DO


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MINIMIZES DAMAGE

MINIMIZES EFFECT ON SYSTEM OPERATION

MAXIMUM POWER TRANSFER

HI -SPEED CLEARI NG OF FAULTS


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To prevent of limit damage to apparatus and all other

components of a transmission and distribution system.

PURPOSES OF PROTECTI VE RELAY

To isolate the affected part from the remainder of the

power system.


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SENSOR - Feeds system information to the relay

Ex: CT, PT

RELAY makes a decision as to the need for action

Ex: OCR, Distance/Differential/Voltage relays

SWITCHING OR CONTROLLING DEVICE physically

isolates or control the problem Ex: Circuit Breaker

THREE MEMBERS OF PROTECTI ON


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Fault Clearance System

PROTECTIONEQUIPMENT

Currenttransformer

Voltagetransformer

TELECOM DC-system

Circuit

BreakerMechanism

TripCoil

CIRCUIT BREAKER



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PowerSystem

Voltageand CurrentTransformer

RelayCircuit

Breaker

Decides whether systemquantities are normal or

abnormal

These devices changeelectrical quantities to level

relay can use, i.e. 5A,115V

*If quantities are normal,no signal sent.

*If quantities are abnormalsignal is sent to breaker toopen.

Functional Diagram of Relaying


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Electrical Diagram of Relaying

Protected

Device

Circuit Breaker

Trip Coil

Current Transformer

Relay

Operating

Coil

Relay Contacts

Station Battery

Power

System

Conductor



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Dependability

Relays must detect designated abnormalsystem conditions on the power system

Speed

Relay systems should operate as quickly aspossible in order to minimize the duration of

system disturbances

Economy

The cost of the relay system should be as lowas possible

Security

Relays must never misinterpret normalcurrents and voltages as conditions which

require tripping

Selectivity

Relay systems should only trip as much ofthe system as necessary to de-energize

distressed components

Reliability

It is not possible to build a system which will never fail.

DESI GN CRI TERI A


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1. Loss of Dependability

relay R1 does not operate on this fault

Reliability of Protection System

2. Loss of Security

relay R5 operates through breaker B5 for the same fault

before breaker B2 clears the fault

Example :

R1

R2

R3

R4

R5

B1

F

B3

B4

x

B5

B2



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Correct and Desired

Correct but undesired

Incorrect Tripping

Failure to trip

GENERAL CLASSI FI CATI ON OF RELAY OPERATI ON


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Poor application

Incorrect setting

Personnel error

Equipment malfunction

CAUSES OF I NCORRECT RELAY OPERATI ON


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usually associated with major system components usually delimited by circuit breakers

must overlap to prevent any system component from being

unprotected

ZONES OF PROTECTION



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Generator ProtectionG

M

Low Voltage Switchgear Protection

Power Transformer Protection

High Voltage Switchgear Protection

Transmission Line Protection

High Voltage Switchgear Protection

Motor Protection

ZONES OF PROTECTI ONS



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System Configuration

Existing system protection and difficulties

Degree of protection required

Existing preferences, operating procedures and practices

Possible future expansion

Fault study

Maximum load and current transformer ratios

Potential transformer locations, connections and ratios

Impedance of the lines and transformers

I NFO NECESSARY FOR RELAY APPLI CATI ON


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1. Thou shall trip all faults within thy zone of protection

irrespective of changes in generation.

2. Thou shall trip these faults at high speed, yea, thou

shall make thy tripping decisions in terms of a second

split into a hundred parts.

3. Thou shall not trip faults outside thy zone of protection

except in back-up assistance to a failing brother.

4. Thou shall not trip under heavy load conditions even

though thy coils do carry much current.

5. Thou shall not trip during power swings, denying always

the temping surges of current and voltage.

MAJOR COMMANDMENTS FOR AN I DEAL RELAY


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Principle of Overlapping Protectionaround a Circuit Breaker

CT for Zone B

CT for Zone A

ZONE A ZONE B


Dead Tank Breaker and Breakers With SeparateCurrent Transformers on Both Sides of Breakers


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Principle of Overlapping Protectionaround a Circuit Breaker


Live Tank Breaker and Breakers With SeparateCurrent Transformers on One Side Only

CT for Zone BCT for Zone A

ZONE A ZONE B


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PRIMARY PROTECTION

A set of protective functions assigned to a zone

Optimum speed and sensitivity

Main protective function for the zone



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BACKUP PROTECTION

A second set of protective functions assigned to a zone

Anticipates failure of primary function

May sacrifice speed or sensitivity for economy



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Primary relays for zone which also provide backup protection for

an adjacent zone.

Remote Backup:


Local Backup:

Use of additional relays on a zone to provide backup

protection separate considerations for relay and

breaker failure.


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C. Relay Operating Principles

1. Plunger - Electromagnetic Attraction

2. Hinged Armature - Electromagnetic Attraction

3. Induction Disk - Induction

4. Induction Cup - Induction

5. Static Relay


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DEFI NI TI ONS

Operation - Relay output changes state.

Pickup - Relay operating quantities of sufficientmagnitude to cause operation.

Reset - Relay restored to quiescent (at rest) state.

Dropout - Relay operating quantities reduced to belowmagnitude required to cause operation.



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Relay Contact Sense:

Normally open "a"

Normally close "b"

Defined as the state of the contact with the relay de-energized.

DEFI NI TI ONS



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Fig. 1 PLUNGER CONSTRUCTI ON

SOLENOID

COIL

ARMATUREMOVING

CONTACT

FIXED CONTACT

Type of Relays

C. Relay Operat ing Principles


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Fig. 2 HINGED ARMATURE CONSTRUCTI ON

ARMATURE

ELECTROMAGNET

MOVING

CONTACT

FIXED

CONTACTHINGE

POINT

Type of Relays



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FIXED

CONTACT

MOVING

CONTACT

DISK

SHADING

RING

COIL

MAGNETSHAFT

Fig. 3 SHADED POLE INDUCTION DISK

Type of Relays



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MAGNET

UPPER COIL

DISK

LOWER COILS

Fig. 4 WATTMETER INDUCTION DISK

Type of Relays



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II. The Differential Concept

Reliability Selectivity

Economy

Security Speed

Most relays system achieve objectives thru:

Desensitize for security

Slowdown for security & selectivity

Increase sensitivity for reliability

and other compromises

Differential Protection

Profoundly simple

Inherently secure

Highly reliable

Fast

Reasonably economical

S C O C G


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BASIC PROTECTIVE RELAYING(II. THE DIFFERENTIAL CONCEPT)

87

Identify a PROTECTIVE ZONE

Line Transformer Generator

Motor Bus

Provide CTs at all terminals of the zone Interconnect the CTs appropriately Install a relay to measure the error current



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87

For an EXTERNAL fault or through load:

CTs see identical primary current

Produce similar secondary currents Secondary currents circulate in CT secondary No current flows through relay




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87

For an INTERNAL fault:

CTs see different primary current

Produce different secondary currents Secondary currents OPPOSE in CT secondary Current in relay is the sum of secondary currents




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87

Requirements:

CT at each power connection to the connected zone All CTs have same rating Careful attention to CT connection Note that differential protection provides only PRIMARY protection

cannot provide backup protection for adjacent zones. Operating quantity is i1-i2 Provides protection on a per phase basis requires three sets ofequipment for three phase protection

Prefer identical (similar) current transformer

i1-i2

I1 I2

i1 i2




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Gas Detection and Analysis

Rate of Change of Pressure

Current Differential

Overcurrent

Overexcitation





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Type 900 Fault Pressure Relay





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A. Transformer Prot ect ion

Differential ConsiderationsInrush Current

Op

R R

Typical FaultCurrent

Typical InrushCurrent

20

0

20

0

When the transformer is firstenergized, there is a magnetizingcurrent inrush which will appearonly on the source side.

Inrush current is rich in harmonics, specially low order events. Harmonic restraint provides relay security.

Environmental Harmonics. CT performance.



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A very important part of the power system at various point

along the bus, transmission line and or other circuits are

connected through circuit breakers.

B. Bus Protection




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300A

300A

300A

4

5

6

300A

1

300A

2

300A

3

ALL C.T. RATIOS 100/1

5

5

5

5

55

55

55

55

3A

3A

3A3A

3A

3AZEROAMPS

DIFF RELAY

OP.COIL

SIMPLE CURRENT DIFFERENTIAL CIRCUITCONFIGURATION

B. Bus Protection




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3000A

3000A

3000A

4

5

6

18000A

3000A

1

3000A

FAULT OUTSIDE THE PROTECTED BUS

2

3000A

3


5

5

5

5

55

55

55

55

30A

30A

30A150A

30A

30A

DIFF RELAY

15000A

ZEROAMPSOP.

COIL

B. Bus Protection




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3000A

3000A

3000A

4

5

6

3000A

1

3000A

2

3000A

3


5

5

5

5

55

55

55

55

30A

30A

30A30A

30A

30A

DIFF RELAY

18000A

180AOP.

COIL

FAULT INSIDE THE PROTECTED BUS

B. Bus Protection




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Distance Relays

V

Z1 Z2 Z3

Zload

Fault

I

Er

Normal Conditions:

I =V

Z1 + Z2 + Z3 + Zload

Er = VZ2 + Z3 + Zload

Z1 + Z2 + Z3 + Zload

and Zr = I

Er

-1

= Z2 + Z3 + Zload

and

With Fault:

I =V

Z1 + Z2

Er = VZ2

Z1 + Z2

Zr =

I

Er

= Z2-1

I I I . LI NE PROTECTI ON



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X

R

+ load with lagging pf

- load with leading pf

The R-X Diagram the language of relay engineering

Distance Relays

BASIC PROTECTIVE RELAYING(III. LINE PROTECTION)



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Transmission Line Applications

(First Zone)21

21

Provide fault detection for middle section of the line withno intentional delay for speed

Must under reach end of the line for selectivity (typicallyset 80-90% of line impedance)

Distance Relays




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21

21

Provide fault detection in the end zones

Must over reach the end of the line for selectivity(typically set 120% of line impedance)

Ideally will not over reach first zone in adjacent line segments

But must be selective with first zone of adjacent lines

Distance Relays

Transmission Line Applications(Second Zone)




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Source of Failure:

Relay Failure

Communication Channel FailureInstrument Transformer FailureBattery FailureBreaker Mechanism FailureBreaker Electrical Failure

Wiring Failure, Relay Breaker

Backup

X

X

RemoteBackup

X

XXX

BreakerFailure

XXX

Local Backup


I V. BREAKER FAI LURE PROTECTI ON



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Primary/Secondary

Main A

Main B

Redundant Relaying Systems

Fully functional both high speed Complementary technology

Different manufacturers

Separate current transformers Separate breaker trip coils

Separate station batteries

Local Backup

BASIC PROTECTIVE RELAYING(IV. BREAKER FAILURE PROTECTION)



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Breaker Failure

FD

Main B

Main A

Fault Detector:

Must detect any fault which the primary/secondaryrelays can detect

Usually an instantaneous overcurrent relay

Local Backup

Rs

Rp

BASIC PROTECTIVE RELAYING(IV. BREAKER FAILURE PROTECTION)



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Causes of Most Transmission Line Faults:

Lightning induced flashover

Insulator contamination Trees or brush growing into the line

Animals and birds

Most (80-90%) of Transmission Line Faults are non-persistent if the fault is de-energized promptly

After the Ionized Air in the Vicinity of the Fault Dissipatesthe line can be re-energized

V. Automatic Reclosing




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Single Pole Trip and Reclose:

Trip and reclose only the faulted phase

Applied most often on EHV lines

Greater assurance of successful reclose

Maintain power transfer and synchronism throughtwo healthy phases

Require more complex relaying (phase

discrimination)

Require more expensive breakers

Deionization time is considerably longer

BASIC PROTECTIVE RELAYING(V. AUTOMATIC RECLOSING)


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Basic Relaying Iiee

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