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High-Impedance Fault Detection with the F60 Universal Relay
Bruce BuxkemperOptima Systems, Inc.College Station, Texas
Consultant to GE Power Management
Agenda
• Definition of a high-impedance (HiZ) fault
• Theory of operation
• Security and sensitivity analyses
• Settings
• Field tests to date
Optima Systems, Inc.
• Specialize in software and embedded systems
• Designed research prototypes for Texas A&M
• Designed first GE HiZ product
• Consultant to GE for integrating HiZ in UR
• Located in College Station, Texas (www.Optima-Systems.com)
Definition
A high-impedance (HiZ) fault is one that draws too little current to operate conventional overcurrent protection (fuses, relays, etc.).
Feeder Currents
HiZ Fault Load Bolted Fault
10,000
1,000
100
10
1
AMPS
Typical Fault Current Behavior
0
50
100
150
200
0 10 20 30 40 50 60 70 80 90 100 110 120TIME (SECONDS)
RMS AMPS
Ia Ib Ic InBolted Fault Current: 2,200 AFuse: 30K (no operation)Test Duration: 12-1/2 minutes
Causes
• Contact with tree limb or other object
• Broken hardware allowing primary to sag
• Contaminated or failing equipment (insulators, etc.)
• Broken line on ground
Misconception #1
Misconception: Properly set, overcurrent protection will clear all faults.
Reality: HiZ faults often draw less current than loads, making overcurrent protection impossible.
Misconception #2
Misconception: Sensitive ground protection will clear HiZ faults.
Reality: Unbalanced loads limit sensitivity of ground protection.
Misconception #3
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. O/C protection seldom operates after first minute or so.
Misconception #4
Misconception: Faults always clear on my system.
Reality: Engineering staffs believe HiZ fault rate is low, but line crews report many downed conductors are still hot when they arrive on scene.
Misconception #5
Misconception: TAMU/GE technology will solve all my HiZ problems.
Reality: This technology will detect many faults that overcurrent technology cannot, but no known technology can detect all HiZ faults reliably and securely.
Primary Protection Philosophies
• Overcurrent protection – Protect power system
• HiZ protection – Protect people and property
Research History
• EPRI targeted problem in late 1970s
• Constraint: Passive, substation monitoring
• Texas A&M University looked at non-fundamental frequency current
• Seven patents
• GE licensed technology in early 1990s
Detection Requirements
• Driven by utility workshops
• High speed operation not desired
• Allow conventional protection to operate
• Distinguish arcing on pole from downed conductor
• Don't false operate!
Characteristics of Arcing Faults
• Little effect on voltage
• Small fault current
• Current not steady state
• Significant harmonic and non-harmonic current
• No single parameter uniformly responsive
Detection Concepts
• Monitor multiple parameters simultaneously
• Use multiple detection techniques
• Use time to distinguish arcing from transients
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
Normal System Behavior
Normal System Behavior
Fault Behavior
Fault Behavior
Basic Arc Algorithms
• Energy algorithm– Monitor parameter continuously– Look for sudden sustained increase
• Randomness algorithm– Monitor parameter continuously– Look for sudden increase in variability
Expert Arc Detector Algorithm
• Monitor outputs of basic arc algorithms
• Increase confidence level…– Based upon multiple algorithms' indications– Based upon persistent indications
• Operate on per-phase basis
Load Pattern Analyzer Algorithm
• Monitor output of Expert Arc Detector• Monitor load flags
– Overcurrent– High rate of change– Three-phase events
• Perform coordination• Require continued arcing• Distinguish downed conductor from arcing
Simplified Block Diagram
ParameterProcessing
(DSP)
EnergyAlgorithm
RandomnessAlgorithm
ExpertArc
Detector
LoadPattern
Analyzer
1/0 %
%
%
%
Overcurrent
Loss of LoadThree-phase Event
Coordination Timeout
Arcing
DownedConductor
High Rate of Change
Even Harmonic LevelVoltage
12
12
12
12
Ia
Ib
Ic
In
High-Level Logic Behavior
High-Level Logic Behavior
Sensitivity Tests
• Texas A&M University's facility– 12.47/7.2 kV multi-grounded wye, overhead– 2000+ amps available fault current– 30K fuse to coordinate with upstream protection
• Substation monitoring point– 1-3 MVA nominal load– UR connected to existing CTs, PTs– UR installed long-term
Sensitivity Tests (cont'd)
• Three sets of tests to date– September 27, 2000– October 5, 2000– October 26, 2000
• Tests remaining (tentative dates)– November 1, 2000– November 8, 2000
Sensitivity Test Summary(Three-Day Totals)
Total Tests 34
Blew 30K Fuse Quickly (17)
No Fault Current (2)
Total Detectable 15
Total Detected 11 (73%)
Test Surfaces Used
• Grassy ground
• Bared ground
• Reinforced concrete
• Non-reinforced concrete
Response Procedure• Factors suggesting tripping
– Heavily populated areas (especially schools, etc.)– Highly flammable conditions
• Reasons to delay tripping
• Need written procedure
Response Procedure• Factors suggesting tripping
• Reasons to delay tripping– Loss of traffic signals, etc.– Personal injuries (darkened stairways, etc.)– Hospitals– Location difficulty when circuit not energized
• Need written procedure
Response Procedure• Factors suggesting tripping
• Reasons to delay tripping
• Need written procedure– IEEE Power System Relay Committee WG D15 (http://grouper.ieee.org/groups/td/dist/documents/highz.pdf)
Levels of Alarms
• Downed Conductor– Arcing following O/C or loss of load– Most serious
• Arcing Alarm
• Arcing Suspected Alarm
Levels of Alarms
• Downed Conductor
• Arcing Alarm– May indicate tree contact, failing equipment, etc.– May indicate downed conductor on lightly loaded
lateral
• Arcing Suspected Alarm
Levels of Alarms
• Downed Conductor
• Arcing Alarm
• Arcing Suspected Alarm– Possible intermittent tree contact, etc.– Least serious
HiZ User Settings
• Arcing Sensitivity
• OC Protection Coordination Timeout
• Phase/Ground OC Min Pickup
• Phase/Ground Rate of Change
• Loss of Load Threshold
• 3-Phase Event Threshold
HiZ User Settings (cont’d)
• Phase/Ground Event Count
• Event Count Time
• Voltage Supervision Threshold
• Even Harmonic Restraint
Arcing Sensitivity Setting
• Range 1-10 (10 = most sensitive)
• Determines arc confidence threshold
• Determine how many times to confirm
• Indirectly affects speed of operation
OC Coordination Timeout Setting
• Determines minimum operating time
• Arcing must continue after timeout
• Need to balance speed with reliability– Long enough to allow conventional protection
to sectionalize– Fault current often decreases over time, so
timeout must be short enough that significant arcing still exists
O/C Min Pickup Setting
• Used to recognize downed conductor
• Determines current at which to inhibit arc detection (temporarily)