DA Class About Relays 26122014

8/10/2019 DA Class About Relays 26122014

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BASIC

PROTECTION AND

RELAYINGSCHEMES


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Why protection is needed

Principles and elements of the protection

system

Basic protection schemes

Digital relay advantages and enhancements

THE BASIC QUESTIONS


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WHY STUDY THIS PROTECTION SCHEME??

Protection scheme plays a vital & important role for

the normal operation or the steady state operation of

different components of power system network, which

must be reliable, fast and efficient.

In order to achieve all these features, it is essential

that these should be proper care in designing and

choosing an appropriate and efficient protection

scheme.

The protective relays functions as the brain behind the

whole schemes


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DISTURBANCES: WHETHER LIGHT OR SEVERE

The power system must maintain acceptable operation 24

hours a day

Voltage and frequency must stay within certain limits

Small disturbances

The control system can handle these

Example: variation in transformer or generator load

Severe disturbances require a protection system

They can jeopardize the entire power system

They cannot be overcome by a control system


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TYPICAL SHORT-CIRCUIT TYPE DISTRIBUTION

Single-Phase-Ground: 7080

Phase-Phase-Ground: 1710

Phase-Phase: 108

Three-Phase: 3

2


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BALANCED VS.

UNBALANCED CONDITIONS

Balanced System Unbalanced System

cI

aI

bI

aI

cI

bI


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DECOMPOSITION OF AN UNBALANCED SYSTEM

Positive-Sequence

Balanced Balanced

Negative-Sequence

1bI

1cI

1aI

2bI

2aI

2cI

0aI

0bI

0cI

aI

cI

bI

Zero-Sequence

Single-Phase


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POWER SYSTEM PROTECTION

Operation during severe disturbances:

System element protection

System protection

Automatic reclosing

Automatic transfer to alternate power supplies

Automatic synchronization


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DAMAGE TO MAIN EQUIPMENT


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Loss of service in a large

area or population region

Hazard to human life

May result in enormous

economic losses

Overreaction of the

protection system

Bad design of the

protection system

BLACKOUTS

Characteristics Main Causes


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SHORT CIRCUITS PRODUCE HIGH CURRENTS

FaultSubstation

a

bc

I

IWire

Three-Phase Line

Thousands of Amps


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MECHANICAL DAMAGE DURING

SHORT CIRCUITS

Very destructive in busbars, isolators, supports, transformers, and machines

Damage is instantaneous

i1

i2

f1 f2

Rigid Conductors f1(t)= ki1(t) i2(t)

Mechanical

Forces


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ESSENTIAL QUALITIES OF PROTECTION:

Reliability

Selectivity-

Absolute or relative Fastness

Discrimination


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PROTECTION SYSTEM ELEMENTS

Protective relays

Circuit breakers

Current and voltage transducers

Communications channels

DC supply system

Control cables


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HOW RELAYS ARE DIFFERENTIATED?

Can be differentiated based on:

* Functional categories

* Input quantities

*Operating Principles

* Performance Characteristics.


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WHAT ARE VARIOUS TECHNIQUE USED?

* Electromechanical

*Solid state/Static

* Microprocessor/Numerical


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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.


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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.


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TYPES OF RELAYS

As per function:

Main

Auxiliary

Signal

As per actuating quantity

Over relays

Under relays


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METHODS OF DISCIMINATIONS:

To locate fault

by time

by current grading

by time and direction

by distance

by time, current and distance

by current balance

by power direction comparison

Type of fault

THREE PHASE DIAGRAM OF THE PROTECTION


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THREE-PHASE DIAGRAM OF THE PROTECTION

TEAMCTs

VTs

Relay

CB

Control

ProtectedEquipment


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DC TRIPPING CIRCUIT

SI

52

TC

DC Station

BatterySI

Relay

Contact

Relay

Circuit

Breaker

52a

+

Red

Lamp


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HOW DO RELAYS DETECT FAULTS?

When a fault takes place, the current, voltage, frequency, and

other electrical variables behave in a peculiar way. For example:

Current suddenly increases

Voltage suddenly decreases

Relays can measure the currents and the voltages and detect

that there is an overcurrent, or an under volt ge, or a

combination of both

Many other detection principles determine the design of

protective relays


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PRIMARY PROTECTION


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PRIMARY PROTECTION ZONE OVERLAPPING

Protection

Zone B

Protection

Zone A

To Zone BRelays

To Zone A

Relays

52 Protection

Zone B

Protection

Zone A

To Zone B

Relays

To Zone A

Relays

52


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BACKUP PROTECTION

AC D

E

Breaker 5

Fails

1 2 5 6 11 12

T

3 4 7 8 9 10

B F


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POWER LINE PROTECTION PRINCIPLES

Overcurrent (50, 51, 50N, 51N)

Directional Overcurrent (67, 67N)

Distance (21, 21N)

Differential (87)


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CHARACTERISTICS OF OVERCURRENT RELAYS:

Definite time

IDMT- inverse definite minimum

time

Very inverse

Extremely inverse


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APPLICATION OF INVERSE-TYPE RELAYS

tRelay

Operation

Time

I

Fault Load

Radial Line


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Distance

Distance

t

I

T

INVERSE-TIME RELAY COORDINATION

T T


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50/51 RELAY COORDINATION

Distance

Distance

t

I

T T T


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DIRECTIONAL OVERCURRENT PROTECTION

BASIC APPLICATIONS

K

L


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DISTANCE RELAY PRINCIPLE

Three-Phase

Solid Fault

d

L

RadialLine21

Suppose Relay Is Designed to Operate

When:||||)8.0(|| 1 aLa IZV

cba III ,,

cba VVV ,,


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THE IMPEDANCE RELAY CHARACTERISTIC

21

22rZXR

R

X Plain Impedance Relay

Operation Zone

Zr1

RadiusZr11rZZ


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NEED FOR DIRECTIONALITY

1 2 3 4 5 6F1F2

R

XRELAY 3Operation Zone

F1

F2Nonselective

Relay Operation


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THREE-ZONE DISTANCE PROTECTION

1 2 3 4 5 6

Zone 1

Zone 2

Zone 3

Time

Time

Zone 1 Is Instantaneous


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CIRCULAR DISTANCE RELAY CHARACTERISTICS

MHO

OFFSET

MHO (1)

PLAIN

IMPEDANCE

R

X

R

X

R

X

OFFSET

MHO (2)

R

X

LENS

(RESTRICTED MHO 1)

TOMATO(RESTRICTED MHO 2)

R

X

R

X


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DIFFERENTIAL PROTECTION PRINCIPLE

No Relay Operation if CTs Are Considered Ideal

ExternalFault

IDIF = 0

CT CT

50

Balanced CT Ratio

Protected

Equipment


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DIFFERENTIAL PROTECTION PRINCIPLE

InternalFault

IDIF > ISETTING

CTR CTR

50

Relay Operates

Protected

Equipment


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PROBLEM OF UNEQUAL CT PERFORMANCE

False differential current can occur if a CT saturates during a through-fault

Use some measure of through-current to desensitize the relay when high currents are present

External

Fault

ProtectedEquipment

IDIF 0

CT CT

50


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POSSIBLE SCHEME PERCENTAGE DIFFERENTIAL

PROTECTION PRINCIPLE

Protected

Equipment

RS

CTR CTR

Compares:

Relay

(87)

OP S RI I I

| | | |

2

S R

RT

I Ik I k

RPSP


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DIFFERENTIAL PROTECTION APPLICATIONS

Bus protection

Transformer protection

Generator protection

Line protection

Large motor protection

Reactor protection

Capacitor bank protection

Compound equipment protection


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DIFFERENTIAL PROTECTION

SUMMARY

The overcurrent differential scheme is simple andeconomical, but it does not respond well to unequal current

transformer performance

The percentage differential scheme responds better to CT

saturation

Percentage differential protection can be analyzed in the

relay and the alpha plane

Differential protection is the best alternative

selectivity/speed with present technology


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ADVANTAGES OF DIGITAL RELAYS

MultifunctionalCompatibility withdigital integrated

systems

Low maintenance

(self-supervision)

Highly sensitive,

secure, andselective

Adaptive

Highly reliable

(self-supervision)

Reduced burden

on

CTs and VTs

Programmable

VersatileLow Cost

THANK YOU


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THANK YOU

DA Class About Relays 26122014

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Transcript of DA Class About Relays 26122014