2. Considerations in Choosing Directional ... Considerations in Choosing Directional Polarizing Methods

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  • Considerations in Choosing Directional Polarizing Methods for Ground Overcurrent Elements in

    Line Protection Applications

    Technical Report to the Line Protection Subcommittee of the PES, Power Systems

    Relaying Committee

    Presented by Working Group D-3 Line Protection Subcommittee

  • Considerations in Choosing Directional Polarizing Methods for Ground Overcurrent Elements in

    Line Protection Applications

    Working Group D-3 Members:

    John Appleyard, Jeffrey Barsch, Gabriel Benmouyal, Art Buanno, Randy Crellin, Randy Cunico, Normann Fischer, Michael Fleck, Robert Frye, Charles Henville, Meyer Kao (Chair), Shoukat Khan, Gary Kobet, Alex Lee, Don Lukach, Walter McCannon, Joe Mooney, Jim O’brien, Cristian Paduraru, Suhag Patel, Russell

    Patterson, Frank Plumptre, Elmo Price (Vice-chair), Ryland Revelle, Sinan Saygin, Mark Schroeder, Steve Turner

  • What is Polarization

     The process of comparing a reference phasor, voltage or current, to line current phasor to determine the direction to a fault

     The reference phasor is generally referred to as the polarizing quantity

     Basis for design of directional elements

  • Why Use It for Ground Overcurrent Application

     In a network transmission system, ground overcurrent elements can be very difficult to coordinate

     Ground directional elements are used to supervise ground overcurrent elements so that they only operate for faults in a desired direction

  • Sequence Network for Ground Faults

    AV V VV

  • Sequence Network for Ground Faults, Cont.

    ZG0 ZTH0

    Zero

    ZL0 ZH0

    X A-G

    G H

    RELAY

    I0

    LINE

    IP

    V2, V0

    I1, I2, I0

    IP

    V0

    *

    *Tertiary not loaded

    ZTM0

    ZTL0

  • Polarizing Methods for Ground Directional Elements

     Zero sequence voltage  Negative sequence voltage  Zero sequence current  Dual Polarizing, combination of zero sequence

    voltage and zero sequence current  Negative sequence and zero sequence

    impedance  Virtual polarizing  Voltage compensation

  • System Vectors, Balanced and During a Single Line to Ground (SLG) Fault

  • Zero and Negative Sequence Voltage Polarizing Source

     Sequence voltages are obtained from the three phase to neutral voltages, or

     -3V0 obtained from broken delta connection

  • Zero and Negative Sequence Voltage Directional Operating Characteristics

  • Zero Sequence Current (Current Polarizing) Polarizing Source

    P+

    + P

    +P

    (a) Υ − ∆ (b) Υ – ∆ − Υ

    (c) Zig-zag

    +

    (d) Auto – with ∆ tertiary

    P

    RMRH

    RM

    RN

    + PVirtually always OK

    OK at times

  • Zero Sequence Current (Current Polarizing) Directional Operating Characteristics

    Transformer Zero Sequence Current Polarized Directional Ground Fault Function

    Forward Operating

    Region

    Reverse Operating

    Region

    Operating Sensitivity Minimum Levels

    3I0 (Operating) Ipol (Polarizing)

    Non-operating Region

  • Dual Polarizing, Combination of Zero Seq. Voltage and Zero Seq. Current

  • Dual Polarizing, Multiple Directional Element Designs

     Electromechanical Designs  Separate voltage polarized and current polarized

    units with their forward (closing torque) operating contacts arranged in parallel so that either unit may indicate forward ground fault direction

     A single directional unit that has both polarizing elements acting simultaneously on the same unit, so that a single contact that operates on the sum of the torque is developed by the two methods

  • Dual Polarizing, Multiple Directional Element Designs, Cont.

     Numerical Relay Designs  Paralleling (OR gate) of the appropriate outputs of the

    two methods, however, the voltage polarizing unit is blocked if polarizing current is available. The voltage unit will only operate if polarizing current is not available.

  • Dual Polarizing, Multiple Directional Element Designs, Cont.

     Numerical Relay Designs, Cont.  Dual polarization by summing the polarizing voltage

    phasor, –3V0, and the polarizing current phasor rotated by the angle α, the arg(V0/I0) for a strong forward fault.

  • Other Methods

     Negative and zero sequence impedance  Negative sequence Impedance:

    𝑍𝑍2 = 𝑅𝑅𝑅𝑅 𝑉𝑉2 � ∠𝜃𝜃2 � 𝐼𝐼2 ∗

    𝐼𝐼2 2

     Zero sequence Impedance:

    𝑍𝑍0 = 𝑅𝑅𝑅𝑅 𝑉𝑉0 � ∠𝜃𝜃0 � 𝐼𝐼0 ∗

    𝐼𝐼0 2

  • Other Methods, Cont.

     Virtual polarization  Based on phase selector has identified the faulted

    phase

    Phase Selector Pickup Virtual Residual, VN polarizing A Phase Fault VB + VC B Phase Fault VA + VC C Phase Fault VA + VB No selection VA + VB + VC

  • Other Methods, Cont.

     Voltage Compensation  Either negative or zero sequence voltage compensation  Long line and resistive fault applications; low operating

    current and strong source with low source impedance  Vpol = -V0A+ I0R* K*e jRCA ; care when choosing K so

    that the direction is not forward for a reverse fault

    Z0LZ0A

    REL A

    Z0B

    V0A = - I0AZ0A

    REL B

    I0BI0R = I0A

    I0A V0K = - I0A(Z0A+K)

    K

  • Application Consideration of Different Methods

     Zero sequence mutually coupled lines  Line and source impedance consideration  Mismatch of polarization methods on line

    terminals in communication assisted trip scheme  Other considerations

  • Zero Sequence Mutually Coupled Lines: Application Example (Zero Seq. Voltage and Zero Seq. Current Polarized)

    500kV Station 1 Station 2

    R R

    161kV

    Station 3

    57mi

    27mi Z0=0.25pu

    Z0=0.044pu

    Z0M=0.04pu

    40%

    85% Other linesOther

    lines

  • Fault on 500kV Bus at Station 1 (Neglect Mutual Coupling)

    500kV Station 1 Station 2

    R R

    161kV

    Station 3

    57mi

    27mi Z0=0.25pu

    Z0=0.044pu

    X 3I0=2510A

    3I0=480A

    Other linesOther

    lines

    100AWithout mutual coupling

    -3Vo

    Forward fault

    3Io-3Vo

    3Io

    Reverse fault

  • Fault on 500kV Bus at Station 1, Partial Zero Seq. Network (Neglect Mutual Coupling)

    Station 3

    69kV

    Station 1 500kV

    MidPt MidPt

    To positive sequence network

    To n

    eg at

    iv e

    se qu

    en ce

    ne

    tw or

    k

    Station 2 R

    R

    V0

    Z0LX

    2510A

    480A

    100A

  • Fault on 500kV Bus at Station 1 (With Mutual Coupling) At Station 3, forward direction asserted for both zero sequence voltage and current polarizing elements

    500kV Station 1 Station 2

    R R

    161kV

    Station 3

    57mi

    27mi Z0=0.25pu

    Z0=0.044pu

    Z0M=0.04pu

    40%

    85%

    X 3I0=2740A

    3I0=470A

    Other linesOther

    lines

    70AWith mutual coupling

    -3Vo

    3Io Forward fault-3Vo

    3Io

    Forward fault

  • Fault on 500kV Bus at Station 1, Partial Zero Seq. Network (With Mutual Coupling)

    Station 3

    69kV

    Station 1 500kV

    MidPt MidPt

    To positive sequence network

    To n

    eg at

    iv e

    se qu

    en ce

    ne

    tw or

    k

    Station 2 R

    R

    V0

    Z0LX Z0M

    1:1

    2740A

    470A

    70A

  • Evaluation of Polarizing Method Considering Line and Source Impedance, Z0 and Z2

  • Simplified Sequence Network at the Fault Location

    Z1S Z1L

    Z2S Z2L

    Z0S Z0L

    VA1

    VA2

    VA0

    Phase-to-ground

    VA1

    VA2

    VA0

    Phase-to-phase- to-ground

    Z1 = Z1S+Z1L

    Z2 = Z2S+Z2L ≅ Z1

    Z0 = Z0S+Z0L

  • Magnitude of Sequence Voltages Vary with Fault Type and the Z0/Z1 Ratio, As Viewed from the Fault Point

    𝑉𝑉𝐴𝐴0 = ⁄𝑍𝑍0 𝑍𝑍1

    ⁄𝑍𝑍0 𝑍𝑍1 + 2

    Single Line-to-Ground (SLG)

    𝑉𝑉𝐴𝐴2 = 1

    ⁄𝑍𝑍0 𝑍𝑍1 + 2

    𝑉𝑉𝐴𝐴0 = 𝑉𝑉𝐴𝐴2 = ⁄𝑍𝑍0 𝑍𝑍1 ⁄2𝑍𝑍0 𝑍𝑍1 + 1

    Line-to-Line-to-Ground (LLG)

  • Evaluation of Polarizing Method Considering Line and Source Impedance, Z0 and Z2

    Observations:  If the system is homogeneous, that is if the

    ratios of source to line impedances are similar in the zero and negative sequence networks then the ratios of voltages at the relay will be the same as at the fault. Because line zero sequence impedance is roughly three times that of positive/negative sequence impedance, zero sequence voltage polarizing is superior.

  • Evaluation of Polarizing M