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Ground-Distance Concepts for R elay Technicians

This presentation aims to enhance attendee knowledge by reviewing ground-distance relay fundamentals, exploring testing of basic characteristics, and filling in conceptual gaps between phase-distance and ground-distance relaying.

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Ground-Distance Concepts for R elay Technicians

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  1. Ground-Distance Concepts for Relay Technicians Steve Laslo System Protection and Control Specialist Bonneville Power Administration

  2. Special Thanks Quintin ‘Jun’ Verzosa Jr. Doble Engineering Company Schweitzer Engineering Laboratories, Inc.

  3. Presentation Information • Our Objectives for this presentation are to enhance attendee knowledge by: • Reviewing Ground-Distance relay fundamentals that affect relay settings and relay decision-making. • Review calculations for test quantities for testing basic Ground-Mho and Ground-Quad characteristics. • Explore testing of basic Ground-Mho and Ground-Quad characteristics.

  4. Some Background • Historically, ground-distance relays were not commonly used in the era of electro-mechanical relaying. • In that ‘era’ protection was commonly: • Phase-distance • Directional Ground-overcurrent • With the advent of solid-state and now microprocessor relays, the ground-distance functions are more easily accomplished and have become more commonly applied.

  5. Some Background • Training Programs for Relay Technicians have historically educated new technicians well in how to test and work on phase-distance relays because they have been around for so long. • Not all training programs have done as well when it comes to ground-distance concepts and practical application. • This presentation will attempt to fill in some of the conceptual gaps that might be present between phase-distance and ground-distance relaying.

  6. Protection Key Concept • Relays make impedance calculations based on the values of voltage and current at the relay location.

  7. Phase-Distance Decisions • Fault current is limited by the line impedance. • Relays ‘see’ the correct impedance to the fault location.

  8. Phase-Distance Decisions • Test values are relatively easy to calculate: • Assume test voltage = 40.4V • I = E / Z = 40.4V / 9.36Ω = 4.32A • Test Quantities: • VA = 40.40°V, VB= 40.4-120°V, VC= 40.4120°V • IA = 4.32-83.97°A, IB= 4.32-203.97°A, IC= 4.3236.03°A

  9. Relay Settings for our example - Positive and Zero-Sequence impedance of the protected line (magnitude and angle) in Secondary Ohms. - Reach of Zone 1, Zone 2, and Zone 3 for Phase Mho Characteristic in Secondary Ohms. - Overcurrent Supervision settings; ignored for our discussion. - Reach of Zone 1, Zone 2, and Zone 3 for Ground Mho Characteristic in Secondary Ohms. - Reactive* Reach of Zone 1, Zone 2, and Zone 3 for Ground-Quadrilateral Characteristic in Secondary Ohms. - Resistive Reach of Zone 1, Zone 2, and Zone 3 for Ground-Quadrilateral Characteristic in Secondary Ohms. - Zero-Sequence Compensation Factor Settings; magnitude and angle. ‘T’ is a correction factor we will ignore in our discussion. Note that all settings above (for this manufacturer) are ‘Per-Phase’ Values…

  10. Associated with ‘ground’, as in ground-faults. Does not exist when we are under ideal normal conditions. Associated with ‘Normal’ conditions where we only have positive sequence. This diminishes when things go wrong. Associated with ‘unbalance’, in any form. Does not exist when we are under ideal normal conditions.

  11. Zone 2 Settings • If the impedance to the fault is 9.3683.97° Ohms secondary, and the reach of our relay based on our setting is the same value, then our relay is reaching right up to the fault and should operate.

  12. ‘Per-Phase’ Values • Are just what they sound like: In this case, they are the values of impedance ‘per-phase’, as shown in the picture below.

  13. Ground-Distance Decisions • Fault current is limited by conductor impedance + ground impedance. • Relays ‘see’ an ‘incorrect’* ‘loop’ impedance to the fault location.

  14. Ground-Distance Decisions • If the relay ‘sees’ an impedance of 16.1582.42°Ω, then the fault appears to be much farther away than it actually is on the transmission line. • How does the relay properly locate the fault?

  15. Zero Sequence Compensation • We factor out the ‘ground impedance’ using a ‘compensation factor’: • KN • What does KN do for us? • First remember that for a ground fault a relay sees a combination of line impedance + ground impedance. • Compensation factors allow a relay to factor out the portion of impedance seen at the relay location that is the ground impedance. • This allows the relay to estimate the transmission line impedance to the fault location.

  16. Ground-Distance Decisions • How does the relay properly locate the fault? • Relay ‘sees’ an impedance of 16.1582.42°Ω • Relay uses KN to factor out 6.8080.28°Ωof ground impedance. • Relay knows impedance to fault is 9.3683.97°Ω • Relay takes proper logical actions

  17. How does thisrelay properly locate the fault? • This relay uses the settings highlighted on the right to make compensation calculations. • The first two are used for Zone 1 while the second two are used for other Zones. • From the SEL-321 Manual:

  18. Ground-Mho • Where did this test current come from? • Since relay manufacturers vary in their exact form of compensation we need the form specifically used by this relay: • ZAG = (VA / IA) / (1+k0) • ZPer-Phase= ZLoop / (1+k0)

  19. Ground-Mho • How do we use this? • Using our knowledge that the loop impedance is the per-phase impedance times the compensation factor we can take the Zone 2 settings, multiply the compensation factor and our result is the relay reach for Zone 2. Let’s do it: • Zone 2 per-phase setting/reach is Z2MGZ1ANG°Ω • Z=9.3683.97°Ω • The compensation factor=1+k0 where k0=k0mk0A° • KN = 1 + 0.726-3.69° = 1.725-1.552° • ZLoop= 9.3683.97°Ω * 1.725-1.552° = 16.1582.42°Ω • Look familiar?

  20. Ground-Distance Decisions • How does the relay properly locate the fault? • Relay ‘sees’ an impedance of 16.1582.42°Ω • Zloop / KN = Zper-phase • 16.1582.42°Ω / (1+ 0.726-3.69° ) = 9.3683.97°Ω • Relay knows impedance to fault is 9.3683.97°Ω • Relay takes proper logical actions -Relay uses KN to factor out 6.8080.28°Ω of zero sequence impedance

  21. Ground-Distance Graphical Analysis • Here we see: • ZPh = the per-phase impedance: • Z=9.3683.97°Ω • ZKN = the compensated impedance: • Z=6.8080.28°Ω • KN = 1 + 0.726-3.69° = 1.725-1.552° • ZLp = the loop impedance: • Z=16.1582.42°Ω • Note that the relay response is defined by the Loop impedance…

  22. Ground-Distance Graphical Analysis • Any point on the per-phase characteristic can be translated to an equivalent point on the loop characteristic. • Any per-phase impedance multiplied by the compensation factor gives the equivalent loop impedance.

  23. Ground Quadrilateral • Key Ground-Quadrilateral Settings: • XG2 = ‘Reactive’* reach of the quadrilateral element. • Z=9.3683.97°Ω • RG2 = Resistive reach of the quadrilateral element. • Z=5.000°Ω

  24. Ground Quadrilateral XG2 RG2

  25. Ground Quadrilateral XG2 RG2

  26. Ground Quadrilateral Z2MG XG2 RG2

  27. Testing Ground-Distance Relays • Review: • Relays must make decisions based on the voltage and current at the relay location. • Raw relay response to ground-impedance functions happens in the loop-impedance plane. • The relay has to factor out the zero-sequence impedance to calculate a fault position on the protected line. • Testing: • Essentially happens in the loop-impedance plane. • Once a test voltage is determined, one divides the loop impedance to calculate the test current for the impedance point being tested.

  28. Calculating Test Quantities • Ground-Mho Characteristic: • Test voltage = 300°V • What is our test current at: • The Line Angle? • Z@ Line Angle = 16.1582.42°Ω • I = E / Z = 300°V / 16.1582.42°Ω • I = 1.86-82.42°A • At 45°? • Z@45° = 16.15Ω * COS(82.42°-45°) = 12.83Ω • I = E/Z = 300°V / 12.8345°Ω • I = 2.34-45°A • At 90°? • Z@90°= 16.15Ω * COS(82.42°-90°) = 16.01Ω • I = E/Z = 300°V / 16.0190°Ω • I = 1.87-90°A • Hint: ZR = ZMAX * COS(MTA-θ)

  29. Interpreting Test Results • Examine how your results are reported:

  30. Interpreting Test Results

  31. Calculating Test Quantities • Ground-Quad Characteristic: • Test voltage = 300°V • What is our test current at: • The Line Angle? • Z@ Line Angle = 16.1582.42°Ω • I = E / Z = 300°V / 16.1582.42°Ω • I = 1.86-82.42°A • At 90°? • Z@90°= ‘y’ component of: 16.1582.42°Ω • 16.1582.42°Ω = (2.13 + j16.01)Ω; y = 16.01Ω • I = E/Z = 300°V / 16.0190°Ω • I = 1.87-90°A

  32. Calculating Test Quantities • Ground-Quad Characteristic: • Test voltage = 300°V • What is our test current at: • At 0°? • Z@0° = 5Ω • I = E/Z = 300°V / 50°Ω • I = 60°A • At Top Right Corner of Quad? • Z@TRC= 16.1582.42°Ω + 50°Ω= 17.5265.99°Ω • I = E/Z = 300°V / 17.5265.99°Ω • I = 1.71-65.99°A

  33. Calculating Test Quantities • Ground-Quad Characteristic: • Test voltage = 300°V • What is our test current at: • A point in the per-phase plane: • ZP1 = 4.6260°Ω? • ZP1= 4.6260°Ω = 2.31 + j4.00Ω • Z1´X = Tan(90-83.97)°*4.00 = 0.422Ω • Z1´ = 0.422 + j4.00Ω = 4.02283.97°Ω • Z1´´ = Z1´*KN = 4.02283.97°Ω * 1.725-1.552° • KN= 1 + 0.726-3.69° = 1.725-1.552° • Z1´´ = 6.9482.42°Ω = 0.915 + j6.88Ω • ZL1 = Z1´´+ (1.89 + j0)Ω = 2.80 + j6.88Ω = 7.4367.82°Ω • I = E/Z = 300°V / 7.4367.82°Ω • I = 4.04-67.82°A

  34. Some Test Considerations

  35. Some Test Considerations

  36. Synopsis - Fundamentals • Relays make decisions based on voltage and current at the relay location. • Relays are generally set in terms of ‘per-phase’ impedance. • For ground faults relays respond to ‘loop’ impedance. • Relays are tested in the loop impedance plane. • A Compensation Factor (KN) is used to factor out zero sequence impedance to allow a relay to make a fault location decision based on the per-phase settings. • Relay manufacturers use a variety of forms of compensation but their fundamental application is the same. • Relay test software may need interpretation to resolve discrepancies between raw values of voltage and current and per-phase settings.

  37. Thanks for your time! Please provide feedback to the school on this presentation and its content. 

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