Line Current Differential Protection

Line Current Differential  Application on Short Lines Presentation to SSCET October 26th, 2012

Content •

Goals of Protection

•

Definition of Short Lines

•

Challenges Posed by Short Lines

•

Line Current Differential Explained

•

Benefits of Line Current Differen Differential tial

•

 Application  Applicati on Example

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.

Sensitivit y

The minimum operating quantities allows the relay to detect an abnormal condition. High-impedance ground faults, voltage unbalance and high sourceto- 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

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

Challenges of Short Lines Coordination of Distance Elements

Challenges of Short Lines Operation Time of Distance Elements

Distance Relay Basics

I*Z Intended REACH point

V=I*ZF I*Z - V

F1 Z

For internal faults: •

IZ  –  V and V

approximately in phase (mho) RELAY (V,I)

•

IZ  –  V and IZ

approximately in phase (reactance)

Distance Relay Basics

I*Z F2

V=I*ZF

Intended REACH point

I*Z - V

Z

For external faults: •

IZ  –  V and V

approximately out of phase (mho) RELAY (V,I)

•

IZ  –  V and IZ

approximately out of  phase (reactance)

Distance Relay Basics 100

v A

vB

100

vC

80 60 40    ]    V    [   e   g   a    t    l   o    V

20 0 -20 -40 -60 -80

-100

-0.5

0

0.5

1

1.5

   ]    V    [   r 50   o    t   a   r   a   p   m   o   c 0   e   c   n   a    t   c   a   e    R-50

SPOL

5

i A

SOP

4 3

-100

   ] 2    A    [    t   n   e 1   r   r   u    C

0

0.5

power cycles

0

iB, iC

-1 -2 -3

-0.5

-0.5

0

0.5

1

1.5

1

1.5

Distance Relay Basics Relay

Lin e

System Voltage at the relay:

V  R



V  N 

 f   LOC [ PU ]  f   LOC [ PU ]  SIR

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

Distance Relay Basics Relay System Lin e Voltage at the relay:

V  R



V  N 

 f   LOC [ PU ]  f   LOC [ PU ]  SIR

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

Current Differential Relay Basics Clock Synchronization Relay 1 Send start bit Store T1i-3=0

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

Relay 2

0

Initial clocks mismatch=1.4ms or 30°

Communication path 0

Send start bit Store T2i-3=0

8.33 ms

Capture T2i-2=2.3 5.1

Capture T1i-2=5.1

2.3 8.33 ms

Send T1i-2=5.1

8.33 8.33

Store T1i-2=5.1

8.33 ms

13.43 Store T2i-2=2.3

Send T2i-2=2.3

10.53 8.33 ms

Send T1i-1=16.66

16.66 16.66

Send T2i-1=16.66

8.33 ms

Store T2i-1=16.66 Capture T1i=21.76

T1i-3=0 a1=2.3-0=2.3 T2i-2=2.3  b1=21.76-16.66=5.1 T2i-1=16.66 1=(2.3-5.1)/2= T1i=21.76 = -1.4ms (ahead)

21.76

Store T1i-1=8.33 Capture T2i=18.96 18.96 T2i-3=0 a2=5.1-0=5.1 T1i-2=5.1  b2=18.96-16.66=2.3 T1i-1=16.66 2=(5.1-2.3)/2= T2i=18.96 = +1.4ms (behind)

Speed up Slow down 0°

30°

Current Differential Relay Basics Clock Synchronization

Current Differential Relay Basics Communications Channel Noise  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

window

The goodness of fit reflects inaccuracy due to: •

transients

•

CT saturation

•

•

time

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

Current Differential Relay Basics Traditional vs. Adaptive Restraint Differential Iloc pu 20

Restraint 2

OPERATE

16

RESTRAINT Restraint 1

BP=8, P=2, S1=30%, S2=50% BP=4, P=1, S1=30%, S2=50% 10

Pickup

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

Traditional characteristics

8

4

OPERATE I rem pu

0 4

8

12

0

 Adaptive characteristics

16

20

Current Differential Relay Basics  Adaptive Restraint Differential Total restraint = Traditional restraint + Adaptive restraint (Error factor ) Imaginary (I /I ) LOC

REM

OPERATE

Error factor is high Real (ILOC/IREM)

REST. 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 SUB

SUB C

A

51

SUB B

51 time

SUB E

SUB D

51

51

51

51

87L

87L

51

51

BLUE relay sees the most current. Coordination time intervals are By eliminating one of the 51 acceptable. elements, we have increased the If line between Sub B andand Sub C coordination time interval are outsystem of service, made coordination easier. coordination time interval between D and C is unacceptable.

 Application Example

50 miles ZL = 0.01 pu 500 kV ZS = 0.01 pu

14 miles ZL = 0.003 pu

62 miles ZL = 0.013 pu

SIR = 0.76

SIR = 3.33 SIR = 6.67 Short line, weak source

ZS = 0.01 pu

5

5

2

2

SIR = 1.54

500 kV ZS = 0.02 pu

230 kV

 Application Example

Protection Scheme Needs •

High speed operation

•

Weighted towards security •

•

Must protect short line without overreaching

 Ability to handle weak source

 Application Example

POTT Scheme RO

RO

52

52

Trip CB

RO

Receive Receive

85R

Transmit •

•

Trip CB

Receive Receive

85R

RO

Transmit

Plus: good security, distance relay, simple comms Minus: Communications channel, weak infeed conditions

 Application Example

Hybrid POTT RO

RU

B

RO B

RU

52

52

Trip CB

Receive This end identical

RO

RO

WI

WI

Receive

Receive 85R

Transmit RU

RU

B

B

Transmit 0 T

Echo

 Application Example

Line Differential 52

52

Trip CB

RCVR

Trip CB

RCVR

R

R XMTR

XMTR

Local + Remote Current •

Plus: good security, good for short lines

•

Minus: Complex communications channel

Local + Remote Current

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