Line Current Differential Protection

7/30/2019 Line Current Differential Protection

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Slide 1

Line Current DifferentialApplication on Short Lines

Presentation to SSCETOctober 26th, 2012


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Goals of Protection Definition of Short Lines

Challenges Posed by Short Lines

Line Current Differential Explained Benefits of Line Current Differential

Application Example

Content


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Goals of Protection

Security

Dependability: the degree of certainty that the

relay will operate correctly.Security: the relay will not operate incorrectly

Speed Very high power during fault conditions: delaystranslate into increased damage: faster protectiontends to compromise relay system security and

selectivity.

Sensitivit

y

The minimum operating quantities allows the relay

to detect an abnormal condition. High-impedance

ground faults, voltage unbalance and high source-

to- line impedance ratio affect the sensitivity

Selectivit

y

or coordination: ability of the relay system to

minimize outages as a result of a fault by operating

as fast as possible within their primary zone.

Simplicity simple to apply and to obtain maximum protection


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Slide 4

What is a short line?

Classification of line length depends on: Source-to-line Impedance Ratio (SIR),

and

Nominal voltage

Length considerations:

Short Lines: SIR > 4

Medium Lines: 0.5 < SIR < 4

Long Lines: SIR < 0.5


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Challenges of Short Lines

Sensitivity of Overcurrent Elements


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Coordination of Distance Elements


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Operation Time of Distance Elements


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Distance Relay Basics

For internal faults:

IZV and V

approximately in phase(mho)

IZV and IZapproximately in phase

(reactance)

RELAY (V,I)

Intended

REACH point

Z

F1

I*Z

V=I*ZF

I*Z - V


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For external faults:

IZV and V

approximately out of phase(mho)

IZV and IZapproximately out of

phase (reactance)

RELAY (V,I)

Intended

REACH point

Z

I*Z

V=I*ZF

I*Z - V

F2


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-0.5 0 0.5 1 1.5-100

-80

-60

-40

-20

0

20

40

60

80

100

Voltage[V]

-0.5 0 0.5 1 1.5-3

-2

-1

0

1

2

3

4

5

Current[

A]

vA

vB vC

iA

iB, i

C

-0.5 0 0.5 1 1.5-100

-50

0

50

100

Reacta

ncecomparator[V]

power cycles

SPOL

SOP


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LineSystem

Relay

Voltage at the relay:SIRf

fVV

PULOC

PULOC

NR

][

][

Consider SIR = 0.1

Fault location Voltage

(%)

Voltage change

(%)

75% 88.24 2.76

90% 90.00 0.91

100% 90.91 N/A

110% 91.67 0.76


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Lin

e

System

Relay

Voltage at the relay:SIRf

fVV

PULOC

PULOC

NR

][

][

Consider SIR = 30

Fault location Voltage

(%)

Voltage change

(%)

75% 2.4390 0.7868

90% 2.9126 0.3132

100% 3.2258 N/A

110% 3.5370 0.3112


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Current Differential Relay Basics

Unit Protection

Communications Channel Required


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Clock Synchronization

Communication path

Initial clocks mismatch=1.4ms or 30

8.33 ms

8.33 ms

8.33 ms

Store T1i-2=5.1

8.33 ms

t1 t2

Slow down

Relay 1

0

5.1

0

2.3

8.33

8.33 Send T2i-2=2.3

Send T1i-2=5.1

Capture T1i-2=5.1

8.33 ms

Send start bit

Store T1i-3=0Send start bit

Store T2i-3=0

13.4310.53

Send T1i-1=16.66

Capture T2i-2=2.3

16.66

21.76

16.66

18.96

Send T2i-1=16.66

Store T2i-1=16.66

Capture T1i=21.76

Store T2i-2=2.3

Store T1i-1=8.33Capture T2i=18.96

T2i-3=0

T1i-2=5.1

T1i-1=16.66

T2i=18.96

a2=5.1-0=5.1

b2=18.96-16.66=2.3

2=(5.1-2.3)/2=

= +1.4ms (behind)

T1i-3=0

T2i-2=2.3

T2i-1=16.66

T1i=21.76

a1=2.3-0=2.3

b1=21.76-16.66=5.1

1=(2.3-5.1)/2=

= -1.4ms (ahead)

Speed up

Relay 2

300

Measure

channel delay to

shift local

phasor by angleequal to the half

of the round trip

delay:


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Clock Synchronization


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Communications Channel Noise

window

time

A sum of squared differences between the actual waveformand an ideal sinusoid over last window is a measure of a

goodness of fit (a measurement error)

The goodness of fit is an

accuracy index for the digital

measurement

The goodness of fit reflects

inaccuracy due to:

transients

CT saturation

inrush currents and other

signal distortions electrical noise

The goodness of fit can be used

by the relay to alter the

traditional restraint signal

(dynamic restraint) and improve

security


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Traditional vs. Adaptive Restraint Differential

0

4 8 12

I rem pu

OPERATE

RESTRAINT

BP=8, P=2, S1=30%, S2=50%

BP=4, P=1, S1=30%, S2=50%

BP=4, P=1, S1=20%, S2=40 %

OPERATE

Iloc pu

16 20

0

4

8

10

16

20

Pickup

Restraint 1

Restraint 2

Traditional

characteristics

Adaptive characteristics


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Adaptive Restraint Differential

Total restraint= Traditional restraint + Adaptive restraint(Error factor)

Imaginary (ILOC/IREM)

Real (ILOC/IREM)

OPERATE

REST.

Error factor is high

Error factor is low


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Summary

SIR, not just line impedance, defines a short line. Overcurrent protection is less secure than alternatives.

The sensitivity and speed of distance relaying are adversely

impacted, and coordination becomes more complex.

Line current differential provides good sensitivity, speed andalleviates coordination issues.


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Application Examples

S


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Summary

51

51

51

51

51 51

SUB

A

SUB

B

SUB

CSUB

D

SUB

E

time

current

51 51

BLUE relay sees the most current.

Coordination time intervals are

acceptable.

If line between Sub B and Sub C

are out of service,coordination time interval between

D and C is unacceptable.

87L 87L

By eliminating one of the 51

elements, we have increased the

coordination time interval and

made system coordination easier.


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Application Example

5

2

5

2

500 kV

230

kV

ZS = 0.01 pu

500 kV ZS = 0.02 pu

ZS = 0.01 pu

ZL = 0.003 pu ZL = 0.013 pu

ZL = 0.01 pu50 miles

14 miles 62 miles

SIR = 3.33

SIR = 6.67 SIR = 1.54

SIR =

0.76

Short line, weak

source


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Application Example

Protection Scheme Needs High speed operation

Weighted towards security

Must protect short line without over-reaching

Ability to handle weak source


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Application Example

POTT Scheme

52 52

RO 85R Transmit

Receive

Receive

Trip CB

RO85R

Receive

Receive

Trip CB

Transmit

RO

RO

Plus: good security, distance relay, simple comms

Minus: Communications channel, weak infeed

conditions


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Application Example

Hybrid POTT

52 52

RO

Transmit

Receive

Receive

Trip CB

RO

RO

RU B

RUB

WI

RU

B

85R

0

T

Receive

Echo

Transmit

RO

WI

RU

B

This end

identical


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Application Example

Line Differential

52 52

R

Trip CB Trip CB

RCVR

XMTR

Local +

RemoteCurrent

R

RCVR

XMTR

Local +

RemoteCurrent

Plus: good security, good for short lines

Minus: Complex communications channel


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Slide 27

References

IEEE C37.113 Guide for Protective Relay Applications toTransmission Lines (1999) (draft 2011)

Draft contains new information regarding short lines.

Relaying Short Lines (Alexander, Andrichak, Tyska)

GE Publication GER-3735.


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Questions

Line Current Differential Protection

Documents

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