Slide 1 Line Current Differential Application on Short Lines Presentation to SSCET October 26 th,...

Line Current Differential Application

on Short LinesPresentation to SSCET

October 26th, 2012

• Goals of Protection

• Definition of Short Lines

• Challenges Posed by Short Lines

• Line Current Differential Explained

• Benefits of Line Current Differential

• Application Example

Content

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: delays translate into increased damage: faster protection tends to compromise relay system security and selectivity.

Sensitivity

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

Selectivity

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

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

Challenges of Short Lines

Sensitivity of Overcurrent Elements


Coordination of Distance Elements


Operation Time of Distance Elements

Distance Relay Basics

For internal faults:

• IZ – V and V approximately in phase (mho)

• IZ – V and IZ approximately in phase (reactance)

RELAY (V,I)

IntendedREACH point

Z

F1

I*Z

V=I*ZF

I*Z - V


For external faults:

• IZ – V and V approximately out of phase (mho)

• IZ – V and IZ approximately out of phase (reactance)

RELAY (V,I)

IntendedREACH point

Z

I*Z

V=I*ZF

I*Z - V

F2


-0.5 0 0.5 1 1.5-100

-80

-60

-40

-20

0

20

40

60

80

100

Volta

ge [V

]

-0.5 0 0.5 1 1.5-3

-2

-1

0

1

2

3

4

5

Curr

ent [

A]

vA vB vC

iA

iB, iC

-0.5 0 0.5 1 1.5-100

-50

0

50

100

Reacta

nce c

om

para

tor

[V]

power cycles

SPOL

SOP


LineSystem

Relay

Voltage at the relay:SIRf

fVV

PULOC

PULOCNR

][

][

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


Line

SystemRelay

Voltage at the relay:SIRf

fVV

PULOC

PULOCNR

][

][

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

Current Differential Relay Basics

• Unit Protection• Communications Channel

Required


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 10

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 bitStore T1i-3=0

Send start bitStore 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=16.66Capture T1i=21.76

Store T2i-2=2.3

Store T1i-1=8.33Capture T2i=18.96

T2i-3=0T1i-2=5.1T1i-1=16.66T2i=18.96

a2=5.1-0=5.1b2=18.96-16.66=2.32=(5.1-2.3)/2== +1.4ms (behind)

T1i-3=0T2i-2=2.3T2i-1=16.66T1i=21.76

a1=2.3-0=2.3b1=21.76-16.66=5.11=(2.3-5.1)/2== -1.4ms (ahead)

Speed up

Relay 2

30°0°

Measure channel delay to shift local phasor by angle equal to the half of the round trip delay:


Clock Synchronization


Communications Channel Noise

window

time

A sum of squared differences between the actual waveform and 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


Traditional vs. Adaptive Restraint Differential

0

4 8 12

Irem 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 characteristic

s

Adaptive characteristics


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

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 and alleviates coordination issues.

Application Examples

Summary

51

51

51

51

51 51

SUB A

SUB B

SUB C

SUB 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.

Application Example

52

52

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

Application Example

Protection Scheme Needs

• High speed operation

• Weighted towards security

• Must protect short line without over-reaching

• Ability to handle weak source

Application Example

POTT Scheme

52 52

RO 85RTransmit

ReceiveReceive

Trip CB

RO85RReceive

Receive

Trip CB

Transmit

RO

RO

• Plus: good security, distance relay, simple comms

• Minus: Communications channel, weak infeed conditions

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 endidentical

Application Example

Line Differential52 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

References

• IEEE C37.113 Guide for Protective Relay Applications to Transmission Lines (1999) (draft 2011)Draft contains new information regarding short lines.

• Relaying Short Lines (Alexander, Andrichak, Tyska)GE Publication GER-3735.

Questions

Slide 1 Line Current Differential Application on Short Lines Presentation to SSCET October 26 th,...

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