1

Copyright © SEL 2017

Distance Protection: Why Have We

Started With a Circle, Does It Matter,

and What Else Is Out There?

Edmund O. Schweitzer, III and Bogdan Kasztenny

Schweitzer Engineering Laboratories

What Is a Distance Protection Element?

• Uses local voltage and current only

• Responds to faults within a predetermined reach

• Operates independently of fault current level,

pre-fault load, fault type, or fault resistance

Reach PointZ

Reach Setting

I

V

2

Distance Element Applications

• Line protection without a pilot channel

▪ Underreaching element (Zone 1)

▪ Stepped distance (time coordinated)

• Directional comparison schemes

• Applications that require impedance elements

▪ Out-of-step, power swing, loss of excitation

Why Did We Start With a Circle?

𝑁1 ∙ 𝐼 >𝑉

𝑅∙ 𝑁2

𝑍𝑅 = 𝑅 ∙𝑁1𝑁2

Im(Z)

Re(Z)

ZR

𝑉

𝐼< 𝑍𝑅

Directional Supervision

Spring Pivot

RN2 VI N1

Trip

Contact

3

Innovation and ProgressDirectional Mho Characteristic

Im(Z)

Re(Z)

ZR

ZEXT

ZR – ZEXT

ZINT

ZR – ZINT

𝑍𝑅 − 𝑍𝐴𝑃𝑃 𝑣𝑠 𝑍𝑅 𝑍𝑅 −𝑉

𝐼𝑣𝑠 𝑍𝑅 𝐼 ∙ 𝑍𝑅 − 𝑉 𝑣𝑠 𝑉

·

Im(Z)

Re(Z)

ZR

Implementation With a Cylinder-Unit Relay

∠ 𝑆𝑂𝑃, 𝑆𝑃𝑂𝐿 < ±90°

𝑆𝑃𝑂𝐿 = 𝑉

𝑆𝑂𝑃 = 𝐼 ∙ 𝑍𝑅 − 𝑉

Replica Current

IPOL

IPOL

IOP

IOP

Trip Contact

4

Shaping Distance Characteristics Using

Phase Comparators

Reverse Offset Mho Forward Offset MhoDirectional Mho

Im(Z)

Re(Z)

ZR

SOP = I · ZR – VSPOL = V

Im(Z)

Re(Z)

ZR1

–ZR2

SOP = I · ZR1 – VSPOL = –(I · ZR2 + V)

Im(Z)

Re(Z)

ZR1

ZR2

SOP = I · ZR1 – VSPOL = I · ZR2 – V

Shaping Distance Characteristics Using

Phase Comparators

Reactance Resistive BlinderNondirectional Mho

Im(Z)

Re(Z)

ZR

SPOL = –(I · ZR + V)SOP = I · ZR – V

Im(Z)

Re(Z)

ZR

SOP = I · ZR – VSPOL = I · ZR

Im(Z)

Re(Z)RB

SOP = I · RB – VSPOL = I · RB

5

Shaping Complex Characteristics

Im(Z)

Re(Z)

ZR

Enhanced Resistive

Coverage

Immunity to

Load Encroachment

Need for Speed, 1969

A.R. van C. Warrington

Protective Relays Their Theory and Practice: Vol. 2:

“Faults on E.H.V. links must be cleared as fast as possible to

prevent instability on the H.V. system. Modern relays can trip in less

than 1 cycle but half-cycle tripping time is the desirable goal,

making an overall clearing time of 2½ cycles.

There is very little possibility of improvement in electromagnetic

relays in these respects and this may be a reason for accelerating

the acceptance of transistorized relays.”

Page 363

6

Static Implementations

• “Analog machines” with electronics

• Speed vs security is a design choice (filtering)

• Coincidence timing as a phase comparator

SOP

Both Positive

POS

NEG

cyc0.25

0

SPOLPOS

NEG

Both Negative

MHO

cyc0.25

0

Microprocessor-Based Implementations

• First mP-based relays sampled at low rates

• Phasors were the only practical solution

▪ Cosine filter, or

▪ Fourier with mimic prefiltering

• Full-cycle band-pass filtering set the speed vs security

balance

• Operating characteristics through calculations on

complex numbers, such as ∠ 𝐼 ∙ 𝑍𝑅 − 𝑉, 𝑉 < ±90°

7

TD Distance

Line Parameters (Z1,Z0)

Intended Z1 Reach

Dv, Di

Incremental Quantity Distance Element|vPRE| |DvF|

Time

Vo

lta

ge

|vPRE|

|DvF|

Internal Fault

DvDi

Remote

Bus

Calculated Voltage Change

at the Reach Point

Reach

PointLocal

Bus

Actual Voltage

Change at the FaultPre-Fault

Voltage

If calculated voltage change at the reach point is Greater than calculated

pre-fault voltage at the reach point, then OPERATE

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External Fault

Dv Di

Remote

Bus

Local

Bus

Calculated Voltage Change

at the Reach Point

Actual Voltage

Change at the Fault

Reach

Point

Pre-Fault

Voltage

If calculated voltage change at the reach point is Lower than calculated

pre-fault voltage at the reach point, then RESTRAIN

General Implementation

_

+Calculations

| DvREACH |

Directional

Supervision

Other

Security

Conditions

In-Zone Fault

Re

ach

| vPRE |v, i

Dv, Di

Lin

e D

ata

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Implementation Considerations

Pre-Fault Voltage Change in VoltageReach Point Voltage

Calculated voltage change can be the magnitude of a phasor

(hypothetical)

Time

vPRE

Time

vREACH

Time

DvREACH

Implementation Considerations

Pre-Fault Voltage Change in VoltageReach Point Voltage

Calculated voltage change can be an edge or a step obtained with

a high-pass filter (actual three-decade-old implementation)

Time

vPRE

Time

vREACH

Time

DvREACH

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Implementation Considerations

Pre-Fault Voltage Change in VoltageReach Point Voltage

Calculated voltage change can be a time-domain value obtained via

memory (actual modern implementation)

Time

vPRE

Time

vREACH

Time

DvREACH

CG Fault at 57% of Zone 1 on a 400 kV, 224 km Line

1.9 ms !

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CG Fault at 57% of Zone 1 on a 400 kV, 224 km Line

margin

Vo

lta

ge

, kV

CG Fault on a 400 kV, 224 km Line

Reach = 180% of fault location

Reach = 110% of fault location

Reach = 85% of fault location

Vo

lta

ge

, kV

12

Need for Speed, 2017

0 10 20 30 40 50 60 70 800

1

2

3

4

5

6

7M

ed

ian

Op

era

tin

g T

ime

(m

s)

210.50.1

SIR

Values

Fault Location in Percentage of Reach (%)

S F R

t1

M LL – MtFAULT = 0

B

t2

t3

t4

t5

Time Time Time

Traveling-Wave

Distance Zone 1

2 ∙ M = t4 − t1 ∙LL

TWLPT

M =LL

2∙t4 − t1TWLPT

1

2∙t4−t1

TWLPT< 0.99 pu → TRIP

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CG Fault at 78.8 km on a 400 kV, 224 km Line

CG Fault at 78.8 km on a 400 kV, 224 km Line

590 ms round trip

time or 78.8 km

one way distance

Raw Current

Current TW

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General Implementation

TW Detection and

Time-Stamping

Subsystem

tF – t1

Reach

Setting

Directional

Supervision

Other Security

Conditions

TW21–

+

Challenges

• Identifying the first return from the fault

• Faults close to either of the buses

• In-zone switching events

• TW attenuation and dispersion

• Same TW timing and polarity patterns at multiple

buses

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Measuring Voltage Traveling WavesIdeas for Retrofitting CCVTs

Tuning

Reactor

Primary Voltage

(a)

Data Acquisition and F/O

Communications

F/O to TW21 Relay

Re

sis

tive

Div

ide

r W

ith

Instr

um

en

tatio

n

Am

plif

iers

Data Acquisition and F/O

Communications

F/O to TW21 Relay

(b)

History of Distance Elements

• Electromechanical technology shaped the

characteristics we use today

• Static technology introduced wide design choices

• Initially limited by processing power, mP technology

reverted to speed of electromechanical relays

• Today’s mP relays with very fast sampling and vast

processing can implement any distance principle

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Three Types of Distance Principles

• Apparent impedance elements: 1 cycle

• Incremental quantity elements: 2 ms – 0.5 cycle

• Traveling-wave elements: 1 – 2 ms

Progress in Distance Element Performance

Security

Speed

Fast

Slow

Low High

EM

S

mP

Gen 1

mP

Gen 2

mP

TD

mP

TW

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Conclusions

• Distance elements are a cornerstone of line

protection

• We have not reached performance limits yet

• Today we have access to a phenomenal relay

technology (ms sampling and processing)

• We have an obligation to continue to innovate