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An Introduction to On-line Partial Discharge Surveys

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PARTIAL DISCHARGE

When high voltage insulating material breaks down Partial Discharges are created, measured in units of charge

known as pico-Coulombs or millivolts.

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WHAT IS PARTIAL DISCHARGE?

• Electrical discharges occurring inside medium and high voltage insulation (flaws, cracks, voids, irregularities).

• These imperfections create voltage stresses and cause eventual failure of the insulation.

• Insulation failures begin with and are characterized by small but detectable releases of energy or Partial Discharge.

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• For many years partial discharge testing has been an IEEE/ANSI standard to identify insulation problems in electrical apparatus before it leaves the factory floor.

• Many U.S. and International standards have been developed for partial discharge testing.

• Until recently Partial Discharge testing was limited to testing at manufacturer’s laboratories.

• Now computers and sophisticated equipment can isolate and detect Partial Discharge occurrences in the field.

PARTIAL DISCHARGE TESTING USED FOR QUALITY

ASSURANCE

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NO OUTAGE EVALUATION

These proven technologies exist today to evaluate medium or high voltage systems while the system is energized.

1. Oil testing/DGA – limited to oil transformers

2. Infrared testing – visible loose connections only

3. Visual inspections – problems may not be visible

4. Power Quality – cannot detect insulation failures

5. Vibration – limited to motors and transformers

6. Partial Discharge Testing – single best all-around test

IQ Services PARTIAL DISCHARGE VS. INFRARED

How are Partial Discharge Inspections different from Infrared Inspections?

• Infrared identifies electrical current problems (loose connections) that result in heat. The specimen must be visible to the naked eye for the infrared camera to detect a problem.

• Partial Discharge identifies voltage problems that result in insulation breakdown and generation of Partial Discharges. The specimen does not need to be visible for Partial Discharges to be detected.

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INSULATION BREAKDOWN

Why should we be concerned with insulation breakdown and Partial Discharges?

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• NFPA 70B states that insulation breakdown is the number one cause of electrical failures.

• In medium and high voltage equipment, partial discharges are the first indication of insulation breakdown.

INSULATION BREAKDOWN

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Component Percentage of insulation failure Transformers 84% Circuit Breakers 21% Disconnect Switches 15% Insulated Switchgear Bus 95% Bus duct 90% Cable 89% Cable Joints (splices) 91% Cable Terminations 87% Based on IEEE Gold Book Table 36

Total Failures due to Insulation Breakdown

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WHAT TYPE OF FACILITES ARE AFFECTED?

• INDUSTRIAL - Large Manufacturing, Cogeneration

• COMMERCIAL - Research Parks, High Rises• INSTITUTIONAL - Campuses, Hospitals• GOVERNMENTAL - Military Bases, Large Office Complexes• UTILITY - Investor Owned, Municipalities

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WHERE DO PARTIAL DISCHARGES OCCUR?

• Electrical Systems > 600V (2.4KV, 4.16 KV, 12KV, etc.)

• Types of Equipment Subject to Partial Discharge

-Cables -Transformers-Circuit Breakers - Instrument Transformers

-Switchgear (CT’s, PT’s)

-Insulators - Bushings-Generators - Motors-Surge Arrestors - Capacitors

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ADVANTAGES OF PARTIAL DISCHARGE SURVEYS

• PREDICTIVE - failures can be prevented.• NO OUTAGE NECESSARY - does not interrupt operations.• NON-DESTRUCTIVE - no damage to electrical system.• TRENDING - comparison to previous tests possible• PRIORITIZE MAINTENANCE ACTIVITIES - determine which equipment to service first.• PLANNING - allows time to schedule repairs.• FINANCIAL CONTROL – repairs can be budgeted.

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TYPICAL MEDIUM VOLTAGE POWER CABLE

CONSTRUCTION

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Source Switchgear

Ground Level

Underground Cable in Conduit (not visible)

Visible Cable Terminations

Load Unit Substation

Visible Cable Terminations

TYPICAL HIGH VOLTAGE UNDERGROUND CABLE

SYSTEM

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3 STAGES OF CABLE INSULATION FAILURE

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STAGE 1

Imperfections and voids

voids

INSULATION

/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

INSULATION

No problems, minor voids therefore minor partial discharges

Jacket, Shield, Semi-conducting shield

Jacket, Shield, Semi-conducting shield

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INSULATION

/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

INSULATION

Tracking begins

Jacket, Shield, Semi-conducting shield

Jacket, Shield, Semi-conducting shield

STAGE 2

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INSULATION

INSULATION

CABLE FAILURE!

Jacket, Shield, Semi-conducting shield

Jacket, Shield, Semi-conducting shield

Blow-upPhase-to-ground

/ / / /CONDUCTOR / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /

STAGE 3

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HOW CAN PARTIAL DISCHARGES BE DETECTED?

Discharges are detected by special signal processing equipment designed to eliminate outside interference.

Capacitive Methods - For Unshielded Components - Dry Type Transformers - Instrument Transformers - Switchgear

Inductive Methods - metal enclosed components - Shielded Cables and Components - Oil Filled Transformers - Rotating Apparatus Acoustic Emission Methods - Dry & Oil Transformers - Switchgear - Unshielded Cables

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Test Equipment & Sensors

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Partial Discharge detection of a flaw in a dry type transformer using

capacitive coupling

flaw

Transformer Coil

Capacitive sensor

Detector

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High Voltage Bus

Metallic Switchgear cover (signal detected outside)

Partial discharge site (inside)

Propagation of Transient Earth Voltages (TEV) in Switchgear

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Measurements

using Capacitive PD

sensors

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Tracking on epoxy resin busbar

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Start of tracking on insulation

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Failed Switchgear Insulation

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Detecting PD Using Airborne Acoustic (Ultrasonic) Sensors

Insulator

Busbar

Sensor

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Airborne Acoustic Sensor

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Insulator Failure Prevented

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Partial Discharge detection of a void in a shielded cable using inductive coupling.

Insulation

+

VoidShield (grounded)

Conductor

Detector

Inductive Sensor

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High Frequency Current Transformer Used to Test Cable in High Voltage Substation

IQ ServicesFailing Terminations

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Molded Cable Accessory Failure

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Splice Void

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Shield Delamination Causing Partial Discharge

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Poor Workmanship Causing Partial Discharge

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Testing 15kV cables

On-line testing of cables PD damage to termination

Case Study

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Identifying & Prioritising Circuits Most at Risk Using On-Line PD Survey

LIVE LINE PARTIAL DISCHARGE SURVEY RESULTS OF 629 HV FEEDERS

0

100

200

300

400

500

600

700

800

900

1000

1 21

42

63

84

105

126

147

168

190

211

232

253

274

295

316

337

358

379

400

421

442

463

484

505

526

547

568

589

610

FEEDERS

mV

Dis

ch

arg

e

ACTION

CONCERN

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PD Survey - Worst 30 ResultsLIVE LINE SURVEY WORST 30 RESULTS

0

100

200

300

400

500

600

700

800

900

1000

mV

DIS

CH

AR

GE

Dischargingjoint

Tripped

Tripped

Wet cable

Tripped

Tripped

and consequences within 3 months

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PERFORMING A PARTIAL DISCHARGE SURVEY

• Analyze the single line drawing to develop plan• Onsite Measurement

– Removal of equipment covers.– Record field data.– Obtain signatures of atypical components.

• Offsite Data Analysis– Correlate measurements to single line drawing.– Compare atypical results to like insulation from database.– Analyze signatures.– Trend Results.

• Report Generation– Recognize immediate failure possibilities.– Recommend possible repairs.– Recommend possible resurvey frequency.

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SUMMARY

• Electric Insulation will fail.

• Predicting failures is now possible through Partial Discharge Analysis.

• No outage is required to survey the equipment.

• Results can be prioritized and trended.

• Partial Discharge Surveys should be performed annually.

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We apologize to every electrical professional for taking 252 yearsto prevent your failures.

    

Benjamin Franklin discovered electricityin 1752. This year we launched an easy

to use, cost effective “no-outage” electricalinspection technology that detects

medium and high voltage equipmentproblems before they fail!