protection primer

Copyright © SEL 2008

Power System Protection Fundamentals

What should we teach students about power system protection?


Agenda

Why protection is needed

Principles and elements of the protection system

Basic protection schemes

Digital relay advantages and enhancements


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 protection system They can jeopardize the entire power system

They cannot be overcome by a control system


Power System Protection

Operation during severe disturbances: System element protection

System protection

Automatic reclosing

Automatic transfer to alternate power supplies

Automatic synchronization


Electric Power System Exposure to External Agents


Damage to Main Equipment


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”


Blackouts

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

Characteristics Main Causes


Short Circuits Produce High Currents

FaultSubstation

abc

I

IWire

Three-Phase Line

Thousands of Amps


Electrical Equipment Thermal Damage

I

t

In Imd

Damage Curve

Short-Circuit Current

Damage Time

Rated Value


Mechanical Damage DuringShort Circuits

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

Damage is instantaneous

i1

i2

f1 f2

Rigid Conductors f1(t) = k i1(t) i2(t)

Mechanical Forces


The Fuse

Fuse

Transformer


Protection System Elements

Protective relays

Circuit breakers

Current and voltage transducers

Communications channels

DC supply system

Control cables


Three-Phase Diagram of the Protection TeamCTs

VTs

Relay

CB

Control

Protected Equipment


DC Tripping Circuit

SI

52TC

DC StationBattery

SIRelay

Contact

Relay

CircuitBreaker

52a

+

–

RedLamp


Circuit Breakers


Current Transformers

Very High Voltage CTMedium-Voltage CT


Voltage Transformers

Medium Voltage

High Voltage

Note: Voltage transformers are also known as potential transformers


Protective Relays


Examples of Relay Panels

Old Electromechanical

Microprocessor-Based Relay


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 undervoltage, or a combination of both

Many other detection principles determine the design of protective relays


Main Protection Requirements Reliability

Dependability Security

Selectivity

Speed System stability Equipment damage Power quality

Sensitivity High-impedance faults Dispersed generation


Primary Protection


Primary Protection Zone Overlapping

ProtectionZone B

ProtectionZone A

To Zone BRelays

To Zone ARelays

52 ProtectionZone B

ProtectionZone A

To Zone BRelays

To Zone ARelays

52


Backup Protection

AC D

E

Breaker 5Fails

1 2 5 6 11 12

T

3 4 7 8 9 10

B F


Typical Short-Circuit Type Distribution

Single-Phase-Ground: 70–80%

Phase-Phase-Ground: 17–10%

Phase-Phase: 10–8%

Three-Phase: 3–2%


Balanced vs. Unbalanced Conditions

Balanced System Unbalanced System

cI

aI

bI

aI

cI

bI


Decomposition of an Unbalanced System

Positive-Sequence

Balanced BalancedNegative-Sequence

1bI

1cI1aI

2bI

2aI

2cI

0aI

0bI

0cI

aI

cI

bI

Zero-Sequence

Single-Phase


Power Line Protection Principles

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

Directional Overcurrent (67, 67N)

Distance (21, 21N)

Differential (87)


Application of Inverse-Type Relays

tRelay Operation Time

I

Fault Load

Radial Line


Distance

Distance

t

I

T

Inverse-Time Relay Coordination

T T


Addition of Instantaneous OC Element

tRelay Operation

Time

I

Fault Load

Radial Line


50/51 Relay Coordination

Distance

Distance

t

I

T T T


Directional Overcurrent ProtectionBasic Applications

K

L


Directional Overcurrent ProtectionBasic Principle

F2

Relay

F1

Forward Fault (F1)Reverse Fault (F2)

V

IV

I

IV


Overcurrent Relay Problem

11 )8.0( LSSETTING ZZ

EI

11)( )8.0( LS

LIMITFAULT ZZE

I

Relay operates when the following condition holds:

SETTINGaFAULT III

As changes, the relay’s “reach” will change, since setting is fixed

1sZ


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 ,,


The Impedance Relay Characteristic

21

22rZXR

R

X Plain Impedance RelayOperation Zone

Zr1

Radius Zr11rZZ


Need for Directionality

1 2 3 4 5 6

F1F2

R

XRELAY 3Operation Zone

F1

F2Nonselective Relay Operation


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


Mho Element Characteristic (Directional Impedance Relay)

MTMZZ cos

ZM

Z

R

X

MT

MTMZIV cosOperates when:


Three-Zone Distance Protection

1 2 3 4 5 6

Zone 1

Zone 2Zone 3

Time

TimeZone 1 Is Instantaneous


Line Protection With Mho Elements

E

X

RA

B

C

D


Circular Distance Relay Characteristics

MHO

OFFSETMHO (1)

PLAIN IMPEDANCE

R

X

R

X

R

X

OFFSETMHO (2)

R

X

LENS(RESTRICTED MHO 1)

TOMATO(RESTRICTED MHO 2)

R

X

R

X


Semi-Plane Type Characteristics

REACTANCE

OHM

DIRECTIONAL

R

X

R

X

R

X

RESTRICTEDDIRECTIONAL

R

X

RESTRICTEDREACTANCE

QUADRILATERAL

R

X

R

X


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


Directional ComparisonPilot Protection Systems

CommunicationsChannel

Exchange of logic information on relay status

RL

Relays RelaysT

R

R

T

LI RI


Permissive OverreachingTransfer Trip

1 2 3 4 5 6

FWD

FWD

Bus A Bus B


Basic POTT Logic

Zone 2 Elements

RCVR

Key XMTR

TripAND


Directional ComparisonBlocking Scheme

1 2 3 4 5 6

FWD

FWD

RVS

RVS

Bus A Bus B


Basic DCB Logic

Zone 2

RCVRTrip

CC

0

Carrier Coordination

Time Delay

Key XMTRZone 3


Differential Protection Principle

No Relay Operation if CTs Are Considered Ideal

ExternalFault

IDIF = 0

CT CT

50

Balanced CT Ratio

ProtectedEquipment


Differential Protection Principle

InternalFault

IDIF > ISETTING

CTR CTR

50

Relay Operates

ProtectedEquipment


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

ExternalFault

ProtectedEquipment

IDIF ¹0

CT CT

50


Possible Scheme – Percentage Differential Protection Principle

ProtectedEquipment

ĪRĪS

CTR CTR

Compares:

Relay(87)

OP S RI I I

| | | |

2S R

RT

I Ik I k

ĪRPĪSP


Differential Protection Applications

Bus protection

Transformer protection

Generator protection

Line protection

Large motor protection

Reactor protection

Capacitor bank protection

Compound equipment protection


Differential ProtectionSummary

The overcurrent differential scheme is simple and economical, 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


Multiple Input Differential SchemesExamples

Differential Protection Zone

Bus Differential: Several Inputs

ĪRPĪSP

OP

ĪT

I1 I2 I3 I4

Three-Winding TransformerDifferential: Three Inputs


Advantages of Digital Relays

MultifunctionalCompatibility withdigital integrated

systems

Low maintenance(self-supervision)

Highly sensitive,secure, and

selectiveAdaptive

Highly reliable(self-supervision)

Reduced burden on

CTs and VTs

ProgrammableVersatile

Low Cost


Synchrophasors Provide a “Snapshot” of the Power System


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-machine interface

Much more

protection primer

Technology