High Impedance Fault Detection on Rural Electrical Distribution Systems

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High Impedance Fault Detection on Rural
Electrical Distribution Systems
Craig WesterJakov VicoMark AdamiakAshish
KulsresthaGE Digital Energy Multilin
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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 AM University
• Hi-Z Arc detection using Microprocessor-Based
  Technology
• Implementation response strategies
• Field experiences
• Summary

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Definitions Causes of Hi-Z faults
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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

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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.)

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Importance of Hi-Z detection
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Importance of Hi-Z detection
Seasonal conditions impacts people assets
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Importance of Hi-Z detection
Extreme weather conditions impacts people
assets
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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
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Misconceptions about Hi-Z faults
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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 .
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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
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Characteristics of Hi-Z Faults
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Introduction to Hi-Z
DefinitionA 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
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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

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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
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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)

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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
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Hi-Z Arc Detection using Microprocessor-Based
Technology
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Research and development lead by Texas AM
University (TAMU)

• Hundreds of staged fault tests since early 1990s
• 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

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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

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Hi-Z Arc detection

• Uses signature based expert pattern recognition
  system developed at Texas AM 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

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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

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Hi-Z Arc detection
High Impedance Fault Detection Block Diagram
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Implementation Response Strategies
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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

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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)

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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

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Arcing fault response strategy

• No device can protect from initial contact
• Disable reclosing after detection of Hi-Z

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Hi-Z Based Feeder Sectionalizing

• Coordination via settings communication

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Field experiencewith High ImpedanceFault
Detection
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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

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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
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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
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• 80 of the 28 Downed Conductors were tripped by
  Low Set Instantaneous successfully reclosed

Only 2 false operations in 560 Relay-Years of
Operation!
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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
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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

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Questions?