01 -Intro - Protection

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> Basic Protection Philosophy - January 2004

Basic Protection Philosophy


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Protec tion - Why Is It Needed?

PROTECTION IS INSTALLED TO :

Detect fault occurrence and isolate the faultedequipment.

SO THAT :

Damage to the faulted equipment is limited;

Disruption of supplies to adjacent unfaultedequipment is minimised.

PROTECTION IS EFFECTIVELY AN INSURANCE POLICY- AN INVESTMENT AGAINST DAMAGE FROM FUTUREFAULTS.

All Power Systems may experience faults at some

time.


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FAULTS ON POWER SYSTEMS RISK :

Severe damage to the faulted equipment :

Excessive current may flow;

Causes burning of conductors or equipment

windings;Arcing - energy dissipation;

Risk of explosions for oil - filled switchgear, or whenin hazardous environments.

Damage to adjacent plant :

As the fault evolves, if not cleared quickly;

Due to the voltage depression / loss of supply.


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FAULTS ON POWER SYSTEMS RISK :

Danger to staff or the public :

Risk of shock from direct contact with the faultedequipment;

Danger of potential (voltage) rises in exposedmetalwork accessible to touch;

Fumes released by burning insulation;

Burns etc.

Disruption to adjacent plant :

Prolonged voltage dips cause motors to stall;

Loss of synchronism for synchronous generators /motors.


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SUMMARY :

Protection must :

Detect faults and abnormal operating conditions;

Isolate the faulted equipment.

So as to :

Limit damage caused by fault energy;

Limit effect on rest of system.


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Impo rtant Cons iderat ions When Applying

Protect ion

Types of fault and abnormal conditions to be protected against

Quantities available for measurement

Types of protection available

Speed

Fault position discrimination

Dependability / Reliability Security / Stability

Overlap of protections

Phase discrimination / Selectivity

CTs and VTs

Auxiliary supplies Back-up protection

Cost

Duplication of protection



Faults Are Mainly Caused By Insulat ion Fai lure

Underground Cables

DiggersOverloading

Oil Leakage

Ageing




Overhead Lines

Lightning

Kites

Trees

Moisture

Salt

Birds

Broken Conductors




Machines

Mechanical Damage

Unbalanced Load



Types o f Fault

a

b

c

e

/E

//E

/

e

a

b

c

3

3/E

e

a

b

c

a

b

c

a

b

c



Types o f Fault

CROSSCOUNTRYFAULT

a

bc

e

a'

b'c'

e



Types o f Fault

OPEN

CIRCUIT+ /E

FAULTBETWEEN

ADJACENTPARALLELLINES

a

b

c

e



Types o f Fault

CHANGINGFAULT INCABLE

a

b c



Types Of Protect ion



Types of Protect ion

Fuses

For : LV Systems, Distribution Feeders andTransformers, VTs, Auxiliary Supplies

Direct Acting AC Trip

For : LV Systems, Pole Mounted Reclosers

Overcurrent and Earthfault

Widely used in all Power Systems

Non-DirectionalVoltage Dependant

Directional




Differential

For : Feeders, Busbars, Transformers, Generators,

etc.

High ImpedanceRestricted E/F

Biased (or low-impedance)

Pilot Wire

Digital




DistanceFor : Distribution Feeders and Transmissionand Sub-Transmission Circuits

Also used as Back-up Protection for

Transformers and Generators

Phase Comparison

For : Transmission Lines

Directional Comparison

For : Transmission Lines




MiscellaneousUnder and Over Voltage

Under and Over Frequency

Special Relays for Generators,Transformers, Motors, etc.

Control Relays

Auto-Reclose, Tap Change Control, etc.

Tripping and Auxiliary Relays



Overcu rrent Protect ion Direct Ac t ing AC Trip

AC series trip

common for electromechanical O/C relays

51

IF

Trip Coil



Overcu rrent Protect ion DC Shunt Trip

IF'

IF

DC

BATTERYSHUNT

TRIP COIL

51

Requires secure DC auxiliary

No trip if DC fails



Overcu rrent Protect ion Co-ordinat ion Principle

Relay closest to faultmust operate first

Other relays must haveadequate additional

operating time toprevent them operating

Current setting chosento allow FLC

Consider worst case

conditions, operatingmodes and currentflows

T

IS1IS2 MaximumFaultLevel

I

R2R1

IF1

Diff t i l P t t i P i i l (1)


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Differential Protect io n Princ iple (1)

Protected

Circuit

R

Diff t i l P t t i P i i l (2)


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Differential Protect io n Princ iple (2)

Protected

Circuit

R


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Basic Principle of Distance Protect ion

LOADLR

RR ZZVZmeasuredImpedance

RelayPT.

Normal

Load

IR ZLZS

VRVS ZLOAD


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Basic Principle of Distance Protect ion

Fault

IRZ

S

VRVS ZLOAD

ZL

ZF

Impedance Measured ZR= VR/IR= ZF

Relay Operates if ZF < Z where Z = setting

Increasing VRhas a Restraining Effect VRcalled Restraining Voltage

Increasing IRhas an Operating Effect


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Plain Impedance Characterist ic

jX ZL

R

TRIP STABLE

Impedance Seen AtMeasuring LocationFor Line Faults


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Impedance Character ist ic Generat ion

Operate

IF

VF

Restrain

Spring

Trip

zF

Ampere Turns : VF IZ

Trip Conditions : VF < IFZ

jIX

IZV1V2

V3

IR

TRIP STABLE

Voltage to Relay = VCurrent to Relay = IReplica Impedance = Z

Trip Condition : S2 < S1

where : S1 =IZ Z

S2 = V ZF

Buchho lz Relay Ins tal lat ion


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Buchho lz Relay Ins tal lat ion

5 x internal pipe

diameter (minimum)

3 x internal pipe

diameter (minimum)

Transformer

3 minimum

Oil conservator

Conservator

Auto reclose Benef i ts (1)


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Improved continuity of supply

Supply restoration is automatic (does not requirehuman intervention)

Shorter duration interruptions

Less consumer hours lost

Use of instantaneous protection for faster fault clearance(NB: some healthy circuits may also be tripped)

Less damage

Less pre-heating of circuit breaker contacts (reduced

maintenance?) Less chance of transient fault becoming permanent



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Less frequent visits to substations

More unmanned substations

Reduced operating costs


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Defin i t ions & Considerat ions

C f


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Classes of Protect ion

Non-Unit, or Unrestricted Protection :

No specific point downstream up to which

protection will protect

Will operate for faults on the protected

equipment;

May also operate for faults on downstream

equipment, which has its own protection;

Need for discrimination with downstream

protection, usually by means of time grading.

Cl f P t t i


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Classes of Protect ion

Unit, or Restricted Protection :

Has an accurately defined zone of protection

An item of power system plant is protected as

a unit;

Will not operate for out of zone faults, thus no

back-up protection for downstream faults.

C di t i


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Co-ordinat ion

LOAD

LOAD

LOAD

LOAD F2 F3F1

SOURCE

Co-ordinate protection so that relay nearest tofault operates first minimises amount ofsystem disconnection.

ANSI R f N b


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ANSI Reference Numbers

2 Time Delay

21 Distance

25 Synchronising Check

27 Undervoltage

30 Annunciator

32 Directional Power

37 Undercurrent or Under Power

40 Field Failure

46 Negative Sequence

49 Thermal

50 Instantaneous Overcurrent

51 Time Delayed Overcurrent

51N Time Delayed Earthfault

52 Circuit Breaker

52a Auxiliary Switch - Normally Open

52b Auxiliary Switch - Normally Closed

59 Overvoltage

60 Voltage or Current Balance

64 Instantaneous Earth Fault (High Impedance)

67 Directional Overcurrent

67N Directional Earthfault

74 Alarm

79 Auto-Reclose

81 Frequency

85 Signal Receive86 Lock-Out

85 Signal Receive

86 Lock-Out

87 Differential

I t t C id t i Wh A l i


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Protect ion

Speed

Fast operation :

Minimises damage and danger

Very fast operation :Minimises system instability

Discrimination and security can be costly toachieve as it generally involves additionalsignaling / communications equipment.



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Protect ion

Fault Position Discrimination

Power system divided into PROTECTED ZONESMust isolate only the faulty equipment or section

Z f P t t i


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Zones of Protect ion

BUSBAR

ZONE

TRANSF-

ORMER

ZONE

BUSBAR

ZONE

FEEDER

ZONE

GENERATION ZONE

BUSBAR

ZONE FEEDER

ZONE



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Protect ion

Overlap of Protections

No blind spots

Where possible use overlapping CTs

Protect ion Overlap


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Protect ion Overlap

BBP

1

BBP

2

JH

Z

LG LPJ

LP

HK

LP

L

LP

K



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Protect ion

Dependability / Reliability

Protection must operate when required to

Failure to operate can be extremely damagingand disruptive

Faults are rare. Protection must operate evenafter years of inactivity

Improved by use of: Back-up protection andduplicate protection



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Protect ion

Security / Stability

Protection must not operate when not required to,e.g. due to : Load switchingFaults on other parts of the systemRecoverable power swings



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Protect ion

Phase Discrimination

Correct indication of phases involved in the fault

Important for single phase tripping and auto-reclosing applications

Cost


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Cost

The cost of protection is equivalent to an insurancepolicy against damage to plant, and loss of supplyand customer goodwill.

Acceptable cost is based on a balance of economics

and technical factors. Cost of protection should bebalanced against the cost of potential hazards.

There is an economic limit on what can be spent.

MINIMUM COST : Must ensure that all faultyequipment is isolated byprotection.

Cost


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Cost

TOTAL COST should take account of :

Relays, schemes and associated panels andpanel wiring

Setting studies Commissioning

CTs and VTs

Maintenance and repairs to relays

Damage repair if protection fails to operate Lost revenue if protection operates

unnecessarily

Cost


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Cost

DISTRIBUTION SYSTEMS

Large numbers of switching and distributionpoints, transformers and feeders

Economics often overrides technical issues

Protection may be the minimum consistent

with statutory safety regulations Speed less important than on transmission

systems

Back-up protection can be simple and is often

inherent in the main protection Although important, the consequences of

maloperation or failure to operate is lessserious than for transmission systems

Cost


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Cost

TRANSMISSION SYSTEMS

Emphasis is on technical considerations ratherthan economics

Economics cannot be ignored but is ofsecondary importance compared with the needfor highly reliable, fully discriminative high speed

protection Higher protection costs justifiable by high capital

cost of power system elements protected

Risk of security of supply should be reduced tolowest practical levels

High speed protection requires unit protection

Duplicate protections used to improve reliability

Single phase tripping and auto-reclose may berequired to maintain system stability

Impo rtant Considerat ions When Applying


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Impo rtant Considerat ions When Applying

Protect ion

Current and Voltage Transformers

These are an essential part of the protection schemeto reduce primary current and volts to a low levelsuitable to input to relay.

They must be suitably specified to meet therequirements of the protective relays.

Correct connection of CTs and VTs to the protection isimportant. In particular for directional, distance, phase

comparison and differential protections. VTs may be electromagnetic or capacitor types.

Busbar VTs : Special consideration needed whenused for line protection.

Current Transfo rmer Circui ts


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Current Transfo rmer Circui ts

Never open circuit a CT secondary circuit, so :

Never fuse CT circuits;

VTs must be fused or protected by MCB.

Do wire test blocks in circuit (both VT and CT) toallow commissioning and periodic injection testing of

relays.

Earth CT and VT circuits at one point only;

Wire gauge > 2.5mm2recommended for mechanicalstrength.

Auxi l iary Supp l ies


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Auxi l iary Supp l ies

Required for : TRIPPING CIRCUIT BREAKERS

CLOSING CIRCUIT BREAKERS

PROTECTION and TRIP RELAYS

AC AUXILIARY SUPPLIES are only used on LV and MV systems.

DC AUXILIARY SUPPLIES are more secure than AC supplies.

SEPARATELY FUSED SUPPLIES used for each protection.DUPLICATE BATTERIES are occasionally provided for extrasecurity.

MODERN PROTECTION RELAYS need a continuous auxiliarysupply.

During unoperated (healthy) conditions, they draw a smallQUIESCENTload to keep relay circuits energised.

During operation, they draw a larger current which increases due tooperation of output elements.

Relay Outputs


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Relay Outputs

TRIP OUTPUT CONTACTS :

Check that these are rated sufficiently to make and

carry the circuit breaker trip coil current. If not, aheavier duty tripping relay will be needed.

Use a circuit breaker normally open (52a) contact to

interrupt trip coil current. This extends the life of the

protection relay trip contacts.TYPE OF CONTACTS :

Can be break before make (BBM)

or make before break (MBB).

Changeover (C/O)

Close when de-energised.Break (B) /

Normally Closed (NC)

Close when energised, typically

used for tripping.

Make (M) /

Normally Open (NO)


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Documents

01 -Intro - Protection