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© SEL 2019

Protection Basics 101

What Are We Protecting?

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Lightning Strikes Create Faults

3

4

Arcing Evidence on Pole Ground

Flashed C-Phase Insulator With

Melted Balloon Material

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Snake Causes Breaker to Fail

Flashed Insulators

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When Primary Met Secondary

Bϕ

H3J1 J2

Aϕ

H2

Cϕ

H1

Control

Cabinet

Digital Relay

Extra Slack

in CT Cable

Junction Box Scorched

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Arc Flash During Commissioning

Culprit Found!

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Typical Short-Circuit Type Distribution

Single-phase-to-ground 70–80%

Phase-to-phase-to-ground 10–17%

Phase-to-phase 8–10%

Three-phase 2–3%

Power System Protection Requirements

Reliability Selectivity Speed Simplicity Economics

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• Protection system will perform correctly

• Protection system will operate for in-zone

problems (dependability)

• Protection system will not operate for out-of-zone

problems (security)

Reliability

• Ability of protection system to disconnect minimal

amount of system

• Ability to isolate problem in shortest amount of time

Selectivity

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• Hazards to humans, equipment damage, and

stability problems are minimized when equipment is

isolated faster

• Speed and selectivity are frequently at odds –

allowing for additional delay before tripping gives

better selectivity

Speed

Simplicity

Reliability increases when protection

systems are kept simple

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Protection system should provide greatest amount of

protection, commensurate with components being

protected, for minimum total cost

Economics

• Correct performance

• Incorrect performance

▪ Failure to trip

▪ False tripping (misoperation)

Protection Performance Classification

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• Fuses

• Automatic reclosers

• Sectionalizers

• Circuit breakers

• Protective relays

Protective Devices

• Protective

• Regulating

• Reclosing and synchronism check

• Monitoring

• Auxiliary

Relay Classification

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IEEE C37.2 Device Numbers

51 Time-overcurrent relay

50 Instantaneous overcurrent relay

67 Directional overcurrent relay

21 Distance relay

87 Differential relay

52 Circuit breaker

Protective Relaying System

52

Relay

DC

Supply

Communications

Channel

DC

Supply

Circuit Breaker

Current

Transformers

(CTs)

Voltage

Transformers

(VTs)

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Protective Relays

• Monitor

• Detect

• Report

• Trigger

Circuit Breakers

• Interrupt

• Isolate from

abnormal condition

Protection System Elements

CTs

• Current scaling

• Isolation

VTs

• Voltage scaling

• Isolation

Instrument Transformers

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IPIS

H

VS ZB

Why Does a CT Saturate?C

urr

en

t

Time

Asymmetrical Saturation

b c d e f g h i j

IS IP

a

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DC Tripping Circuit

DC Station

Battery SI Relay

Contact

Relay

Circuit

Breaker

52a

(+)

–

SI

52

TC

Circuit Breaker

Transmission

Line

Single Bus

Transformer

Distribution

Lines

Motor M

Generator

Ring Bus

Example Power System

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Circuit Breaker

Transmission

Line

Single Bus

Transformer

Distribution

Lines

Motor M

Generator

Ring Bus

Example Power System

Overlapping Zones. No Gaps. Ever.

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52

Protection

Zone B

Protection

Zone B

To Zone B

Relays

To Zone B

Relays

Protection

Zone A

Protection

Zone A

To Zone A

Relays

To Zone A

Relays

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Circuit Breakers

Dead-Tank Breaker Live-Tank Breaker

Backup Protection

A C D E

B F

T

1 2 5 6 11 12

3 4 7 8 9 10

Breaker 5 Fails

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Remote Backup

A C D E

B F

T

1 2 5 6 11 12

3 4 7 8 9 10

Breaker 5 Fails

Remote Backup Trips

Local Backup

A C D E

B F

T

1 2 5 6 11 12

3 4 7 8 9 10

Breaker 5 Fails

Local Backup

Trips

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Instantaneous (50) and Time-Overcurrent (51)

CharacteristicsSTD

TD

Curve Shape

t

I

Istd Iinst

Ipu

Overcurrent Relay Coordination

Setting

S1

EI

Z

M

F Load

Load

Load

Relay

Relay

Load or Fault Current

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Distance Relay

Relay designed to operate when ( )A L1 AV 0.8 Z I

21 Three-Phase

Bolted Fault

L

d

IA, IB, IC

VA, VB, VC

Mho Characteristic

X

R

F2

1 2 3 4 5 6

F1

F2

F1

Directional Impedance

Relay Characteristic

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Quadrilateral Characteristic

X

R

Zone 3

Zone 2

Zone 1

RG1 RG2 RG3

XG1

XG2

XG3

TANGG

Distance Relay Timing and Coordination

Zone 1 is instantaneous

Operation Time

A B C

Zone 1

Zone 2

Zone 3

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Why Is Distance Not Enough?

Zone 1S

Zone 1R

High-Speed

Line Clearing

Time-Delayed Clearing at One End

S R

• Direction comparison

blocking (DCB)

• Permissive overreaching

transfer trip (POTT)

• Directional comparison

unblocking (DCUB)

• Direct underreaching

transfer trip (DUTT)

• Permissive

underreaching transfer

trip (PUTT)

Typical Protection Pilot Schemes

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• Advantages

▪ High bandwidth

▪ High reliability

▪ Noise immunity

▪ Safety

• Disadvantages

▪ Cost (offset by

shared use)

▪ Rerouting (SONET)

Communications ChannelsOptical Fiber

• Advantages

▪ Power line noise immunity

▪ Not affected by faults

• Disadvantages

▪ Placement

▪ Weather

Communications ChannelsMicrowave

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• Advantages

▪ Low cost

▪ Interference immunity

▪ No license required

▪ Not affected by faults

• Disadvantage –

low bandwidth

Communications ChannelsSpread-Spectrum Radio

• Advantage – reliable • Disadvantages

▪ Low bandwidth

▪ Affected by power

line noise

▪ Affected by faults

Communications ChannelsPower Line Carrier

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• Advantages

▪ Low installation cost

▪ Dedicated channel

• Disadvantages

▪ Ongoing cost

▪ Low reliability

▪ Short range

Communications ChannelsTelephone Lines

• Proprietary

• On / off

• Frequency shift keying (FSK)

Modulation Techniques

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DUTT Logic

S R

Zone 1S

Zone 1R

TX

RX

Trip SZone

1S

TX

RX

Trip RZone

1R

• Underreaching Zone 1 elements send tripping signal

▪ Limited fault resistance coverage

▪ Immunity to current reversals

• Scheme trips without supervision – susceptible to

misoperation with noisy channels

DUTT Scheme

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PUTT Logic

S R

Zone 1S

Zone 2R

TX

RX

Trip SZone 1S

TX

RX

Trip R

Zone 2SZone 1R

Zone 2S

Zone 1R

Zone 2R

• Underreaching Zone 1 elements send tripping signal

▪ Limited fault resistance coverage

▪ Immunity to current reversals

• Overreaching Zone 2 elements trip on receipt of signal,

making PUTT scheme more secure than DUTT scheme

PUTT Scheme

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POTT Logic

S R

Zone 2S

Zone 2R

TX

RX

Trip SZone

2S

TX

RX

Trip RZone

2R

DCB Logic

S R

Zone 2S

Zone 2R

TX

RX

Trip S

Zone

2S

CTD

0

TX

RX

Trip R

Zone

2R

CTD

0

Zone 3S Zone 3R

Zone 3S

Zone 3R

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Differential Overcurrent

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Differential OvercurrentExternal Fault

51

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Differential OvercurrentInternal Fault

51

51

Effect of CT Saturation

5/_0

5/_0

5/_0

3/_30

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Percentage Differential Protection Principle

Protected

Equipment

CTR CTR

Relay

I1 I2

= +1 2IOP I I

( )= +1 2IRT 0.5 • I I

Percentage Restraint Characteristic

IOP

Multip

les o

f T

ap

Multiples of TapIRT

IRS1

SLP1

SLP2Operate

Region

Restraint

Region

U87P

O87P

= +1 2IOP I I

( )= +1 2IRT k • I I

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Adaptive Slope

Slope 1

Internal Fault

IRST

IOP

Slope 2

External Fault

Restrain

Operate

87P

87Z

Trip Alarm

OUT1

OUT2

86

Trip and Short

87 Input

High-Impedance Differential Protection

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How Many Relays for One Line?

3 phase + 3 ground + auxiliaries

1 relay has it all, and much more!Digital Relay

• Multifunction devices

• Negative-sequence element

• Low-burden devices

• Programmable logic; very flexible

• Automatic self-tests

• Greater sensitivity due to high-accuracy metering

Microprocessor-Based Relay Advantages

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The Good Old Days?

I wonder

where the

problem is…

Root Cause Found!

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Digital Relay I/O Scheme

Computer-Based

Relay (digital)

Computer

Communications

Analog Inputs

Discrete Inputs

Auxiliary Inputs

(ac or dc)

Dry Contact Outputs

(trip and alarm)

Live Outputs

Digital Relay Architecture

Microprocessor

RAM ROM / PROM EEPROM

Discrete

Output

Subsystem

Operation

Signaling

Analog Input

Subsystem

Discrete Input

Subsystem

Analog-to-

Digital (A/D)

Conversion

Tripping

Outputs

Communications

Ports

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Signal Path for Microprocessor-Based Relays

Analog

Low-Pass

Filter

A/D

Conversion

Digital

Cosine

Filter and

Phasor

Magnitude

and

Impedance

CT

PT

A/D Conversion

A/D

Analog Signal Digital Signal

00000001

00000101

00001001

00100100

10010000

:.

Input Output

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Nonfiltered Signal

(samples)

Filtered Signal

(samples)

Digital Filtering

Digital Filtering

Phasor Samples:

Magnitude and Angle

Versus Reference

| I |

qReference

Phasor Calculation

Filtered Signal

(samples)

Phasor Calculation

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Logic

A

B

C

D

EA

B

DC

E

–

(+)

E = A AND B OR C OR NOT D

Equation Implemented

Programmable Logic

Digital Relay Algorithm

Read Present Sample k

Digital Filtering

Phasor Calculation

Protection Methods

Relay Logic

Modify if

Required

Trip Order

No Trip

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• Phasors depend on sinusoidal steady state

• Faults change the network

• We want to determine where … fast!

• We want to determine sinusoidal steady state and

filter out everything else

• Shorter windows make us faster but less accurate

It Takes a Cycle to Catch a CycleO

pe

ratin

g T

ime

in C

ycle

s

Fault Location in Percent of Set Reach

Line-to-Ground Fault, SIR = 1.0

Modern Distance Relays: 8–16 ms

0 10 20 30 40 50 60 70 80 900

0.5

1

1.5

100

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– 0 – 0 0 10 20 30 40–

–

–

0

2

4

Loca

l C

urr

en

t (A

)

–

–

0

2

4R

em

ote

Cu

rren

t (A

)

Time (ms)

Traveling Waves Bring Information Fast

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– 0 – 0 0 10 20 30 40–

–

–

0

2

4

Loca

l C

urr

en

t (A

)

–

–

0

2

4

Rem

ote

Cu

rren

t (A

)

Time (ms)

Traveling Waves Bring Information Fast

Internal Phase Fault (100 km, 400 kV)

BC fault at 50 km

TW32 0.1 ms

TD3 2.1 ms

TD21 3.7 ms

TW87 0.9 ms

Fault Location =

49.993 km

kV

A

0

200

400

–200

–400

0

4,000

8,000

–4,000

–8,000

TWDDTW32FTD32F

TW87PKPTD21P

TRIP

–5 ms 0 ms 5 ms 10 ms 15 ms 20 ms 25 ms 30 ms

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One-Second Power System ViewkV

A

0

–400

–800

400

800

0

400

800

–400

–800–200 ms 0 ms 200 ms 400 ms 600 ms

One-Cycle Power System View

kV

A

0

–200

–400

200

400

0

400

800

–400

–800–10 ms 0 ms 10 ms 20 ms 30 ms–20 ms

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One-Millisecond Power System ViewkV

A

0

–200

–400

200

400

0

400

800

–400

–800–1 ms –0.5 ms 0 ms 1 ms 1.5 ms–1.5 ms 0.5 ms

Submillisecond Power System View

kV

A

0

–200

–400

200

400

0

400

800

–400

–800–1 ms –0.5 ms 0 ms 1 ms 1.5 ms–1.5 ms 0.5 ms

600

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One-Microsecond Power System View!kV

A

0

–200

–400

200

400

0

400

800

–400

–800–20 μs 0 μs 40 μs 60 μs–40 μs 20 μs

600

This Is Only the Beginning

We’ll all learn more about

the power system!

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Thank You!