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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    Challenges of Short Lines

    Coordination of Distance Elements

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    Challenges of Short Lines

    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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    Distance Relay Basics

    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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    Distance Relay Basics

    -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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    Distance Relay Basics

    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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    Distance Relay Basics

    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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    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

    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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    Current Differential Relay Basics

    Clock Synchronization

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    Current Differential Relay Basics

    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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    Current Differential Relay Basics

    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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    Current Differential Relay Basics

    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