Considerations in Choosing Directional Polarizing Methods for Ground Overcurrent Elements in Line Protection Applications

Transcription

1 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

2 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

3 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

4 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

5 Sequence Network for Ground Faults

6 Sequence Network for Ground Faults, Cont. G * LINE H I 1, I 2, I 0 X A-G *Tertiary not loaded I P RELAY V 2, V 0 Z G0 Z TM0 Z TH0 Z L0 Z H0 I 0 I P Z TL0 V 0 Zero

7 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

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

9 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

10 Zero and Negative Sequence Voltage Directional Operating Characteristics

11 Zero Sequence Current (Current Polarizing) Polarizing Source + P R H + P R M (a) Υ (b) Υ Υ R M Virtually always OK + P + P (c) Zig-zag OK at times R N + P (d) Auto with tertiary

12 Zero Sequence Current (Current Polarizing) Directional Operating Characteristics Operating Sensitivity Minimum Levels Non-operating Region Reverse Operating Regio n Forwa rd Operating Regio n I pol (Polarizing) 3I 0 (Operating) Transformer Zero Sequence Current Polarized Directional Ground Fault Function

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

14 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

15 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.

16 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.

17 Other Methods Negative and zero sequence impedance Negative sequence Impedance: ZZ 2 = RRRR VV 2 θθ 2 II 2 II 2 2 Zero sequence Impedance: ZZ 0 = RRRR VV 0 θθ 0 II 0 II 0 2

18 Other Methods, Cont. Virtual polarization Based on phase selector has identified the faulted phase Phase Selector Pickup A Phase Fault B Phase Fault C Phase Fault No selection Virtual Residual, VN polarizing V B + V C V A + V C V A + V B V A + V B + V C

19 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 = -V 0A + I 0R * K*e jrca ; care when choosing K so that the direction is not forward for a reverse fault Z 0A I 0R = I 0A Z 0L I 0B Z 0B I 0A REL A V 0A = - I 0A Z 0A K V 0K = - I 0A(Z 0A+K) REL B

20 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

21 Zero Sequence Mutually Coupled Lines: Application Example (Zero Seq. Voltage and Zero Seq. Current Polarized) Station 1 Station 2 500kV 57mi Z 0 =0.044pu 40% Other lines Z 0M =0.04pu 85% Other lines Station 3 161kV 27mi Z 0 =0.25pu R R

22 Fault on 500kV Bus at Station 1 (Neglect Mutual Coupling) Station 1 X 3I 0 =2510A Other lines 500kV 57mi 3I 0 =480A Z 0 =0.044pu Other lines Station 2 Station 3 161kV 27mi Z 0 =0.25pu R Without mutual coupling R 100A 3Io -3Vo Reverse fault -3Vo Forward fault 3Io

23 Fault on 500kV Bus at Station 1, Partial Zero Seq. Network (Neglect Mutual Coupling) Station 1 500kV 2510A Station 2 R X 480A Z 0L Station 3 To negative sequence network MidPt R 69kV MidPt 100A V 0 To positive sequence network

24 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 Station 1 X 3I 0 =2740A 40% Other lines 500kV 57mi Z 0M =0.04pu 85% 3I 0 =470A Z 0 =0.044pu Other lines Station 2 Station 3 161kV 27mi Z 0 =0.25pu With mutual coupling 3Io -3Vo Forward fault Forward fault -3Vo R 3Io R 70A

25 Fault on 500kV Bus at Station 1, Partial Zero Seq. Network (With Mutual Coupling) Station 1 500kV X 2740A 1:1 Station 2 Z 0L 470A R Station 3 To negative sequence network MidPt R Z 0M 69kV MidPt 70A V 0 To positive sequence network

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

27 Simplified Sequence Network at the Fault Location Z1S Z1L Z1 = Z1S+Z1L Phase-to-ground V A1 V A1 Phase-to-phaseto-ground Z2S Z2L Z2 = Z2S+Z2L Z1 V A2 V A2 Z0S Z0L Z0 = Z0S+Z0L V A0 V A0

28 Magnitude of Sequence Voltages Vary with Fault Type and the Z0/Z1 Ratio, As Viewed from the Fault Point Single Line-to-Ground (SLG) VV AAA = VV AAA = ZZ 0 ZZ 1 ZZ 0 ZZ ZZ 0 ZZ Line-to-Line-to-Ground (LLG) VV AAA = VV AAA = ZZ 0 ZZ 1 2ZZ 0 ZZ 1 + 1

29 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.

30 Evaluation of Polarizing Method Considering Line and Source Impedance, Z0 and Z2 Observations (Cont.): Where the system is not homogeneous, the relative zero and negative sequence voltages at the relay can be easily estimated for ground fault at any point on the line VV AA0RR VV AAAAA = ZZ 0 ZZ 2 1+ nn ZZ 1LL ZZ 1SS 1+ nn ZZ 0LL ZZ 0SS VV AAAAA = 1+ nn ZZ 1LL ZZ 1SS VV AAAAA 1+ nn ZZ 0LL ZZ 0SS SLG LLG

31 Evaluation of Polarizing Method Considering Line and Source Impedance, Z0 and Z2 Observations (Cont.): The magnitude of the negative- or zerosequence voltages are directly proportional to the current and the source impedance behind the relay location. Therefore, it can be seen that given a long line with a strong source behind the relay (Z2L >> Z2S or Z0L >> Z0S) the negative or zero-sequence voltage at the relay location can be relatively small.

32 Evaluation of Polarizing Method Considering Line and Source Impedance, Z0 and Z2 Observations (Cont.): To evaluate the choice of the sequence voltages at the relay, the magnitude of zero and negative sequence voltages at the relay can be estimated for ground fault at end of the line VV AAAAA = VV AAAAA = 1 1 ZZ 0 ZZ 1 +2 ZZ 2LL ZZ 2SS +1 ZZ 0 ZZ 1 1 ZZ 0 ZZ 1 +2 ZZ 0LL ZZ 0SS +1 per unit per unit

33 Inadequate Negative Sequence Voltage Example Z S+ =0.01pu Z S0 =0.02pu Phase-to-ground line- Station 1 3I 0 =760A end fault Station 2 240/1 161kV 96mi Z + =0.3pu Z 0 =0.74pu X 1400/1 V 2 =590V I 2 =250A R NOTE: All quantities at Station 1 are after the breaker at Station 2 has tripped Breaker trips by ground instantaneous

34 Sources of Error in calculating V2 and V0 Untransposed Lines V A V A V A V B av C 3V 0 3V 2 V C a 2 V b V C V B Compute 3V 0 Compute 3V 2 Normally Expected Phase and Sequence Fault Voltages, A Phase SLG

35 Sources of Error in calculating V2 and V0 Untransposed Lines, Cont. V A V A V A V B av C 3V 0 V C V C V B 3V 2 a 2 V b Compute 3V 0 Compute 3V 2 Phase and Sequence Fault Voltages Showing Reversal of 3V 0, A Phase SLG

36 Sources of Error in calculating V2 and V0 Untransposed Lines, Cont. V A V A V A av C V B 3V 0 3V 2 a 2 V b V C V B V C Compute 3V 0 Compute 3V 2 Phase and Sequence Fault Voltages Showing Reversal of 3V 2, A Phase SLG

37 Mismatch of polarization methods on line terminals in communication assisted trip scheme Bus F, relay system K: Zero sequence voltage and current polarized Bus H, relay system M: Negative sequence voltage polarized

38 Mismatch of polarization methods on line terminals in communication assisted trip scheme, Cont.

39 Mismatch of polarization methods on line terminals in communication assisted trip scheme, Cont. Sequence Currents (IFH2 and IFH0) in Per Unit of If on Line FH for Different Fault Locations on Line FG m Distance from Bus F

40 Mismatch of polarization methods on line terminals in communication assisted trip scheme, Cont. The negative and zero sequence currents will flow in opposite directions on line FH for faults between 0.45 and p.u. of line FG length as measured from Bus F m IFH IFH

41 Other Considerations Single pole open affecting an adjacent line Series compensated lines or lines near series compensated lines and static var compensator Inherently directional; strong source impedance behind and weak forward source impedance Current polarizing at stations with more than one transformer and split low-sides Modeling of ground directional element in software

42 Recommendation on Choosing Appropriate Method Configuration: Short and medium length lines (SIR > 0.5) Long lines** (SIR 0.5) DCB or POTT Relay at the remote terminal with zero sequence polarizing Significant Zero Sequence Mutual Coupling Negative Sequence Voltage Zero Sequence (Voltage, Current, or Dual) No OK* OK* Yes OK* NR No SR OK* Yes SR NR No NR OK* Yes NR SR * - OK, but study recommended NR - Not recommended SR - Study required ** - special compensation may be required or inherent directionality may make directional control unnecessary

43 Summary: Choice of polarizing method for directional ground elements is an important decision that should not be overlooked or reduced to a cookie-cutter approach. Each application should be carefully evaluated for adequacy. Manufacturer recommendations and/or traditional utility practices should never be used in lieu of thorough study of the particular relay in question on the protected line.