power system protection fundamental

7/29/2019 Power System Protection Fundamental

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Power System Protection

Fundamentals

What should we teach students

about power system protection?


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Agenda

Why protection is needed

Principles and elements of the protection

system

Basic protection schemes

Digital relay advantages and enhancements


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Disturbances: 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 protectionsystem

They can jeopardize the entire power system

They cannot be overcome by a control system


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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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Electric Power System Exposure to

External Agents


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Damage to Main Equipment


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Protection System

A series of devices whose main purpose

is to protect persons and primary electric

power equipment from the effects of faults

The Sentinels


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Blackouts

Loss of service in a

large area or

population region Hazard to human life

May result in

enormous economiclosses

Overreaction of the

protection system

Bad design of theprotection system

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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Electrical Equipment Thermal Damage

I

t

In Imd

Damage Curve

Short-Circuit

Current

Damage

Time

Rated Value


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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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The Fuse

Fuse

Transformer


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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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Three-Phase Diagram of the Protection Team

CTs

VTs

Relay

CB

Control

Protected

Equipment


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DC Tripping Circuit

SI

52

TC

DC Station

Battery SI

Relay

Contact

Relay

CircuitBreaker

52a

+

Red

Lamp


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Circuit Breakers


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Current Transformers

Very High Voltage CTMedium-Voltage CT


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Voltage Transformers

Medium Voltage

High Voltage

Note: Voltage transformers

are also known as potential

transformers


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Protective Relays


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Examples of Relay Panels

Old Electromechanical

Microprocessor-

Based Relay


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How Do Relays Detect Faults?

When a fault takes place, the current, voltage,

frequency, and other electrical variablesbehave 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 undervoltage, or acombination of both

Many other detection principles determine the

design of protective relays


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Main Protection Requirements

Reliability

Dependability

Security

Selectivity

Speed

System stability

Equipment damage

Power quality

Sensitivity

High-impedance faults

Dispersed generation


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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

A

C D

E

Breaker 5Fails

1 2 5 6 11 12

T

3 4 7 8 9 10

B F


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Typical Short-Circuit Type

Distribution

Single-Phase-Ground: 7080%

Phase-Phase-Ground: 1710%

Phase-Phase: 108%

Three-Phase: 32%


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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 Line Protection Principles

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

Directional Overcurrent (67, 67N)

Distance (21, 21N)

Differential (87)


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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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Addition of Instantaneous OC

Element

tRelay

Operation

Time

I

Fault Load

Radial Line


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50/51 Relay Coordination

Distance

Distance

t

I

T T T


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Directional Overcurrent ProtectionBasic Applications

K

L


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Directional Overcurrent ProtectionBasic Principle

F2

Relay

F1

Forward Fault (F1)Reverse Fault (F2)

V

IV

I

IV


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Overcurrent Relay Problem

11 )8.0( LSSETTING

ZZ

EI

11

)()8.0(

LS

LIMITFAULTZZ

EI

Relay operates when the following condition

holds:

SETTINGaFAULTIII

As changes, the relays reach will change,since setting is fixed1s

Z


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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

cbaIII ,,

cbaVVV ,,


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The Impedance Relay Characteristic

21

22rZXR

R

X Plain Impedance Relay

Operation Zone

Zr1

Radius Zr11rZZ


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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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Directionality Improvement

1 2 3 4 5 6

F1F2

R

XRELAY 3Operation Zone

F1

F2The Relay Will

Not Operate for

This Fault

Directional Impedance

Relay Characteristic


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Mho Element Characteristic

(Directional Impedance Relay)

MTM

ZZ cos

ZM

Z

R

X

MT

MTM

ZIV cosOperates when:


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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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Line Protection With Mho Elements

E

X

RA

B

C

D


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Circular Distance Relay Characteristics

MHO

OFFSETMHO (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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Semi-Plane Type Characteristics

REACTANCE

OHM

DIRECTIONAL

R

X

R

X

R

X

RESTRICTED

DIRECTIONAL

R

X

RESTRICTED

REACTANCE

QUADRILATERAL

R

X

R

X


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Distance ProtectionSummary

Current and voltage information

Phase elements: more sensitive than 67

elements

Ground elements: less sensitive than 67N

elements

Application: looped and parallel lines


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Directional Comparison

Pilot Protection Systems

Communications

Channel

Exchange of logic information

on relay status

RL

Relays Relays

T

R

R

T

LI RI

O


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Permissive Overreaching

Transfer Trip

1 2 3 4 5 6

FWD

FWD

Bus A Bus B

B i POTT L i


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Basic POTT Logic

Zone 2 Elements

RCVR

Key XMTR

TripAND

Di ti l C i


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Directional Comparison

Blocking Scheme

1 2 3 4 5 6

FWD

FWD

RVS

RVS

Bus A Bus B

B i DCB L i


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Basic DCB Logic

Zone 2

RCVRTrip

CC

0

Carrier CoordinationTime Delay

Key XMTRZone 3

Diff ti l P t ti P i i l


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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

Diff ti l P t ti P i i l


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InternalFault

IDIF > ISETTING

CTR CTR

50

Relay Operates

Protected

Equipment

P bl f U l CT P f


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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

P ibl S h P t


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Possible Scheme Percentage


Protected

Equipment

RS

CTR CTR

Compares:

Relay

(87)

OP S RI I I

| | | |

2

S R

RT

I Ik I k

RPSP

Diff ti l P t ti A li ti


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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

Diff ti l P t ti


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Differential ProtectionSummary

The overcurrent differential scheme is simpleand economical, but it does not respond well to

unequal current transformer performance

The percentage differential scheme respondsbetter 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

M ltiple Inp t Differential Schemes


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Multiple Input Differential SchemesExamples

Differential Protection Zone

Bus Differential: Several Inputs

RPSP

OP

T

I1 I2 I3 I4

Three-Winding Transformer

Differential: Three Inputs

Advantages of Digital Relays


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Advantages of Digital Relays

MultifunctionalCompatibility withdigital integrated

systems

Low maintenance

(self-supervision)

Highly sensitive,secure, and

selective

AdaptiveHighly reliable

(self-supervision)

Reduced burdenon

CTs and VTs

Programmable

VersatileLow Cost

Synchrophasors Provide a


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Synchrophasors Provide a

Snapshot of the Power System

The Future


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The Future

Improvements in computer-based

protection

Highly reliable and viable communication

systems (satellite, optical fiber, etc.)

Integration of control, command,

protection, and communication

Improvements to human-machineinterface

Much more

power system protection fundamental

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