Line Current Differential Protection

(1)

Line Current Differential

Application on

Short Lines

Presentation to SSCET

October 26

(2)

•

Goals of Protection

•

Definition of Short Lines

•

Challenges Posed by Short Lines

•

Line Current Differential Explained

•

Benefits of Line Current

Differen

tial

•

Applicati

on Example

Content

(3)

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 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.

(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

(5)

Challenges of Short Lines

(6)

Challenges of Short Lines

(7)

Challenges of Short Lines

(8)

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) Intended REACH point Z F₁ I*Z V=I*Z_F I*Z - V

(9)

Distance Relay Basics

For external faults:

•

IZ

_–

V

and

V

approximately out of phase

(mho)

•

IZ

_–

V

and

IZ

approximately out of

phase (reactance)

RELAY (V,I) Intended REACH point Z I*Z V=I*Z_F I*Z - V F₂

(10)

Distance Relay Basics

-0.5 0 0.5 1 1.5 -100 -80 -60 -40 -20 0 20 40 60 80 100 V o l t a g e [ V ] -0.5 0 0.5 1 1.5 -3 -2 -1 0 1 2 3 4 5 C u r r e n t [ A ] v_A v_B v_C i A i_B, i_C -0.5 0 0.5 1 1.5 -100 -50 0 50 100 R e a c t a n c e c o m p a r a t o r [ V ] power cycles

S

_POL

S

_OP

(11)

Distance Relay Basics

Lin

e

System

Relay

Voltage at the relay:

SIR

f

V

PU LOC PU LOC N R   ] [ ] [

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

(12)

Distance Relay Basics

Lin

e

System

Relay

Voltage at the relay:

SIR

f

V

PU LOC PU LOC N R   ] [ ] [

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

(13)

Current Differential Relay Basics

•

Unit Protection

(14)

Current Differential Relay Basics

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 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=0

Send start bit Store T2i-3=0 13.43 10.53 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=8.33 Capture 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 30° 0°

Measure

channel delay to

shift local

phasor by angle

equal to the half

of the round trip

delay:

(15)

Current Differential Relay Basics

(16)

Current Differential Relay Basics

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

(17)

Current Differential Relay Basics

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 characteristics Adaptive characteristics

(18)

Current Differential Relay Basics

Adaptive Restraint Differential

Total restraint = Traditional restraint + Adaptive restraint

(Error factor )

Imaginary (I_LOC/I_REM)

Real (I_LOC/I_REM)

OPERATE

REST.

Error factor is high

(19)

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.

(20)

(21)

Summary

51 51 51 51 51 51 SUB A SUB B SUB C SUB D SUB E time 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

(22)

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 pu 50 miles 14 miles 62 miles SIR = 3.33 SIR = 6.67 _{SIR = 1.54} SIR = 0.76 Short line, weak

(23)

Application Example

Protection Scheme Needs

•

High speed operation

•

Weighted towards security

•

Must protect short line without

over-reaching

(24)

Application Example

POTT Scheme

52 52 RO 85R Transmit Receive Receive Trip CB RO 85R Receive Receive Trip CB Transmit RO RO

•

Plus: good security, distance relay, simple comms

•

Minus: Communications channel, weak infeed

(25)

Application Example

Hybrid POTT

52 52 RO Transmit Receive Receive Trip CB RO RO RU B RU B WI RU B 85R 0 T Receive Echo Transmit RO WI RU B This end identical

(26)

Application Example

Line Differential

52 52 R Trip CB Trip CB RCVR XMTR Local + Remote Current R RCVR XMTR Local + Remote Current

•

Plus: good security, good for short lines

•

Minus: Complex communications channel

(27)

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.

(28)