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)