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High Impedance Fault Detection on Rural Electrical Distribution Systems

High Impedance Fault Detection on Rural Electrical Distribution Systems Craig Wester Jakov Vico Mark Adamiak Ashish Kulsrestha GE Digital Energy Multilin Overview Definitions & causes of Hi-Z faults Importance of Hi-Z detection Misconception about Hi-Z faults

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High Impedance Fault Detection on Rural Electrical Distribution Systems

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  1. High Impedance Fault Detection on Rural Electrical Distribution Systems Craig WesterJakov VicoMark AdamiakAshish KulsresthaGE Digital Energy Multilin

  2. Overview • Definitions & causes of Hi-Z faults • Importance of Hi-Z detection • Misconception about Hi-Z faults • Characteristics of High Impedance (Hi-Z) Faults • Fault currents on various surfaces • Research and development at Texas A&M University • Hi-Z & Arc detection using Microprocessor-Based Technology • Implementation & response strategies • Field experiences • Summary

  3. Definitions & Causes of Hi-Z faults

  4. Definition of Hi-Z faults • An energized primary conductor in contact with a quasi-insulating object, such as a tree, structure or earth…..thus a high impedance (or Hi-Z) fault • Not detected by conventional phase or ground overcurrent protection (fuses or overcurrent relays) • Little threat of damage to power system equipment, but potential safety and fire hazard • Hi-Z faults produce primary current levels of 0 to 100 Amps • Seldom documented on trouble reports

  5. Causes of Hi-Z faults • Broken line on ground (Downed Conductor) • Broken pole allowing line to contact a ground or conducting surface • Broken pole or tree limb allowing primary to sag • Contact with tree limb or other objects (Intermittent Arcing) • Contaminated or failing equipment (insulators, transformers, conductors, etc.)

  6. Importance of Hi-Z detection

  7. Importance of Hi-Z detection Seasonal conditions impacts people & assets

  8. Importance of Hi-Z detection Extreme weather conditions impacts people & assets

  9. Importance of Hi-Z detection • If not detected and isolated, live Downed Conductors can be fatal to public and line crewmen • Hi-Z faults often arc and can be a significant fire hazard • Detect failing insulation before complete device failure which can lead to power outages and loss of production • Inability to detect Hi-Z faults can cost utilities liabilitiesand customer service issues • Performance has been verified under normal conditions (noisy feeders, arc furnaces, arc welders, capacitor switching, line switching and load tap changing) Hi-Z detection is about protecting people and assets

  10. Misconceptions about Hi-Z faults

  11. Misconceptions about Hi-Z faults Misconception: Properly set, overcurrent protection will trip and clear all faults on distribution feeder Reality:Hi-Z faults often draw less current (10 – 100 amps) than loads, making overcurrent protection unable to operate Misconception: Sensitive ground protection typically used to detect low ground current, will clear Hi-Z faults Reality:Unbalanced loads limit sensitivity of ground protection. Moreover, a down conductor can result in more balanced loads and reduced neutral current. This can easily mislead the low impedance protection element and no operation .

  12. Misconceptions about Hi-Z faults . . . Misconception: Over time, fault current will increase and operate protection Reality:In most cases, fault current decreases as conductor burns, moisture evaporates, sand fuses, etc. overcurrent protection seldom operates after first minute Misconception: Faults always clear on my system Reality:Engineering staffs believe Hi-Z fault rate is low, but line crews report many downed conductors are still hot when they arrive on scene

  13. Characteristics of Hi-Z Faults

  14. 10,000 1,000 AMPS 100 10 1 HiZ Fault Load Bolted Fault Introduction to Hi-Z Definition:A high-impedance (Hi-Z) fault is one that draws too little current to operate conventional overcurrent protection (fuses, relays, etc.). Hi-Z Fault Current Levels Conventional overcurrent protection is not able to detect Hi-Z

  15. Introduction to Hi-Z • Hi-Z arcing fault current is rich in harmonics and non-harmonics. • Hi-Z fault current is erratic but tends to decrease over time, often stopping completely after minutes. • Hi-Z faults persist seconds to minutes….. to days

  16. Fault currents on various surfaces Surface Fault current (A)Dry Asphalt 0Dry Sand 0Wet Asphalt 1Wet Sand 5Dry Sod 10Concrete (non-reinforced) 10Wet Sod 50Concrete (reinforced) 70 80-85% of all down conductors can be detected

  17. Basics of Hi-Z Faults • Down Conductor occurs when live conductor breaks and falls on the ground • A break in the conductor usually leads to: • Drop in the load or • Momentary overcurrent due to falling conductor contacting a grounded object. • A Hi-Z fault often is accompanied by arcing at the point of the fault • Hi-Z fault without a broken conductor is termed as Intact Hi-Z condition and is caused by: • Failure of the conductor mounting system • Insulation failure • Inadvertent contact with external element (tree limb)

  18. Characteristics of arcing faults • Little effect on voltage • Small fault current (10 – 100 Amps) • Current values will continue to fluctuate • Significant harmonic and non-harmonic current • No single parameter uniformly changing Normal System Behavior Hi-Z Fault Behavior

  19. Hi-Z & Arc Detection using Microprocessor-Based Technology

  20. Research and development lead by Texas A&M University (TAMU) • Hundreds of staged fault tests since early 1990’s • At dedicated local facility • At multiple utilities across US • Important note: fault current was not artificially limited • Characterization of Hi-Z behavior • Multiple generations of prototypes • Staged faults assessed sensitivity to faults • Long-term installations assessed immunity to false trips • Long-term prototype installations established criteria for success and formed high-level system concepts

  21. Communications • SOE • Oscillography • SCADA Trip Close Alarm Hi-Z & Arc detection • Apply on distribution breaker or recloser to detect Hi-Z faults • 4.16 to 34.5kV applications • Hi-Z detection will work on current alone - use relaying CTs • Voltage provides supplemental phase identification • Algorithm in service since 1992 on various hardware platforms

  22. The following nine sophisticated algorithms are used: • Energy • Randomness • Expert Arc Detector • Load Event Detector • Load Analysis • Load Extraction • Arc Burst Pattern Analysis • Spectral Analysis • Arcing Suspected Identifier Over 20 year matured algorithms Hi-Z & Arc detection • Uses signature based expert pattern recognition system developed at Texas A&M University • Harmonic energy levels of currents in the arc is used for Hi-Z fault detection • The expert system techniques are designed to assure security and dependability

  23. Hi-Z & Arc detection Detection Parameters • Odd harmonics (3rd, 5th, …) • Largest increase • Smallest relative increase • Even harmonics (2nd, 4th, …) • Small ambient level • Affected by inrush • Non harmonics (1/2, 1-1/2, 2-1/2, …) • Small ambient level • Voltage • Enhance security • Phase Identification • Learns ambient harmonic level and adjusts frequently

  24. Hi-Z & Arc detection High Impedance Fault Detection Block Diagram

  25. Implementation & Response Strategies

  26. Implementation Strategy Contrast in Detection Goals - Overcurrent vs. High Impedance • Sufficient current vs. low current • Equipment damage vs. safety/fire prevention Electrical or Mechanical Detection Options • Electrical options applied one per feeder or recloser • Mechanical options applied in certain areas (schools and churches) • Mechanical options can detect sagging conductors

  27. Implementation Strategy Customer Service • A priority due to increasing competition • Hi-Z faults can cause service interruptions and deliver substandard power to users • Applying electrical Hi-Z detection, allows utilities to respond quicker to Hi-Z occurrences • Accurate, dependable and secure operation is very important • Inform customers of potential problems • Response procedures can be created • 95-98% complete fault detection possible(low impedance + high impedance)

  28. Implementation Strategy Feeder Selection • Unreasonable to apply Hi-Z detection on every feeder at once due to expense Circuits with: • Past Hi-Z events • Population dense circuits • Fire prone areas • Older circuits with undersized conductors • 4 - 35kV circuits • Overhead construction

  29. Arcing fault response strategy Condition Primary Response Secondary Response Arcing Suspected Alarm -- Arcing Detected Alarm Trip Downed Conductor Trip Alarm • No device can protect from initial contact • Disable reclosing after detection of Hi-Z

  30. Hi-Z Based Feeder Sectionalizing • Coordination via settings & communication B R R R

  31. Field experiencewith High ImpedanceFault Detection

  32. PEPCO • Serves Washington DC and parts of Maryland • Covers an area of 640 sq. mi. • Provides power to over 2,000,000 customers • Distribution system of 620 13kV overhead feeders • Study based on 280 installed Hi-Z relays over a 2 year period • Hi-Z relays were set biased toward security

  33. Study Methodology • Candidate Faults for Study • Operator logs • Line Broken • Still Energized -- OR -- • Relay Hi-Z target - Checked Weekly Relays were set biased towards security

  34. Study Results • Study based on 560 relay-years of operation • Several hundred broken wires recorded during study • 48 “Downed/Energized” faults recorded • From the remaining 48 “Downed/Energized” faults • 46 of the 48 relays indicated “Arcing” – 96% • 28 of the 48 relays reported “Downed Conductors” – 58% • 80% of the 28 “Downed Conductors” were tripped by Low Set Instantaneous & successfully reclosed Only 2 false operations in 560 Relay-Years of Operation!

  35. Other Installations Experience to date of Hi-Z Algorithms • The ratio of “detected” downed conductors to the total population of downed conductors is 80-85%. • Investigation based on a periodic “arc detection” alarm lead to detect a motor failure at a customer site • Arcing due to loose transformer bushing was detected by Hi-Z algorithms • A house fire was successfully detected and feeder tripped and locked out • A downed conductor on an asphalt surface found paths through cracks in the asphalt, which lead to “down conductor” detection. • Intermittent arcing faults due to contact with tree limbs Secure determination of Down Conductor & Arcing Conditions

  36. Summary • HiZ faults happen . . . and result in: • Personnel hazard • Property damage • Poor customer service • Effective technology has been demonstrated to: • Reliably detect HiZ faults • Detect arcing conditions on system • Securely report Downed Conductors • Safely trip feeders with Downed Conductors

  37. Questions?

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