On-Line Partial Discharge Testing

Voltage Stress Related Failures in Extruded Insulation Voltage Stress Related Failures in Extruded Insulation z Overloads, defects or weak spots in insulation lead to chemical breakdown

What Is Partial Discharge?

Electrotechnical Commission (IEC) International Standard 60270, Section 3.1 published in 2000, the definition of Partial Discharge is:

“Localized electrical discharge that only partially bridges the insulation between conductors and which can or cannot

occur adjacent to a conductor.”

IEC 60270 Section 3.1 Notes

z “Partial discharges are in general a

consequence of local electrical stress concentrations in the insulation or on the surface of the insulation…”

z “Corona is a form of partial discharge that

occurs in gaseous media around conductors which are remote from solid or liquid

insulation…”

z “Partial discharges are often accompanied by emission of sound, light, heat, and chemical reactions…”

Paper-Insulated Lead Cable

Paper-Insulated Lead Cable

Extruded MV Cable Construction

EPR Cable Construction

EPR Insulation in Cable Accessories

Cones and other Cable Accessories are

manufactured from an EPR compound because

of it’s great resistance to treeing and the effects

of corona discharge around terminations and

splices.

Types of Cable Failures

Insulation

Laminar Cable Insulation Failure

Picture Provided By Pirelli Wire

Laminar Cable Insulation Failure

Picture Provided By Pirelli Wire

Tape removed showing electrical treeing

Voltage Stress Related Failures in

Extruded Insulation

Voltage Stress Related Failures in

Extruded Insulation

z Overloads, defects or weak spots in insulation lead to

chemical breakdown of the insulation, producing thermal runaway leading to decreased Insulation Resistance,

increased leakage current and failure

z Design, manufacturing or workmanship results in voids or contamination which produce Partial Discharge and

ultimate failure

z Water Trees Form and convert to Electrical Trees and

lead to ultimate failure

Voltage Stress Related Failures in

Extruded Insulation

Voltage Stress Related Failures in

trees Water Trees lead to decreased insulation resistance and very slight increase in leakage

current Water Trees do not directly produce

failure.

produce Partial Discharge and ultimate failure.

Water Trees

Bowtie Trees [9]

Bowtie Tree in XLPE Bowtie Tree in EPR

Water Tree Conversion [2]

and/or switching.

result in power dissipation in the water channels existing in insulation sufficient to raise the

temperature of the water and increase pressure

in the water tree channel.

support partial discharge.

Water Treeing

HMWPE Insulation

Picture Provided By General Cable XLPE Insulation

Picture Provided By General Cable

Water Tree Conversion

z PD Inception voltage is exceeded by lightning and other surges causing a PD in voids

z Voltage drops to below PD Extinction voltage and PD

stops

z Each occurrence at which PD occurs causes the size and geometry of the insulation void to change Lowering the

PD inception and extinction voltages

z When PD extinction voltage drops below system nominal voltage, PD’s are continuous leading to electrical tree

formation and ultimate insulation failure

Water Tree Conversion

Insulation Failure

Picture Provided By Pirelli Wire

Failures in Cable Accessories

occur in terminations, splices and other cable accessories.

Accessories are predominantly involve Partial Discharge Deterioration caused by voids,

contaminants and workmanship problems.

The Partial Discharge Process

ACOUSTICS HEAT/CHEMICAL

REACTIONS

IEEE Standard 400-2001 Notes:

IEEE Standard 400-2001 Notes:

z “Partial discharge measurement is an important method of assessing the quality of the insulation

of power cable systems…”

z “A partial discharge is an electrical discharge

(formation of a streamer or arc) that does not

bridge the entire space between two electrodes.”

z Partial discharges may occur in a “void…at a

contaminant…or at the tip of a well-developed water tree ”

The Partial Discharge Process

Microscopic spaces (“voids”) may be formed

in insulation systems due to water tree

growth, aging, installation or manufacturing

defects Continued stress and overvoltages

can initiate PD in voids.

Heat and other forms of energy released by PD cause erosion of the internal surface of the void.

Continued erosion forms channels that develop into so-called electrical trees in the insulation.

Insulation

System

Failure

Continued PD produces further erosion until the electrical tree bridges the insulation.

Why Test for Partial Discharges?

z Partial discharge testing can detect:

– Insulation defects that may have occurred during cable system installation or the manufacturing process

– Insulation deterioration due to normal service operating conditions

z Partial discharge testing is a PREDICTIVE qualitative analysis tool that can warn of a potential upcoming system failure.

z The Partial Discharge testing program can locate problem areas

(cable, splices, and/or terminations) in a cable system.

z Partial discharge test results can be trended over time.

z Unlike traditional High Potential Testing, partial discharge testing is not destructive to an insulation system (Hipot testing is typically a

“go-no-go” test the cable either fails or it doesn’t) In addition, “even

massive insulation defects in extruded dielectric insulation cannot be detected with DC ” according to the IEEE 400-2001 standard.

AEIC PARTIAL DISCHARGE

Historical Testing Methods

Off-Line Testing Techniques

Off-Line Testing Techniques

and in some cases, completely removed from the distribution system.

voltage, and if necessary raising the voltages

above the normal operation voltage.

voltage also minimizes noise, reducing the need for filtering and intense data interpretation.

High Potential Testing - DC

IEEE, Electric Power Research Institute (EPRI), Insulated Cable

Engineers Association (ICEA), and Association of Edison Illuminating Companies (AEIC) all agree on the following:

• “DC High Potential Maintenance Testing on aged (XLP) cables can damage the cable resulting in premature failure”

• “High Voltage DC Tests continue to be useful tests to check systems before they are placed in service When used as maintenance tests the possibility of damage to the cable should be considered.”

• “Cable Manufacturer’s must be consulted to determine acceptable voltage levels and recommendations typically include to test at your own risk”

High Potential Testing - DC

• IEEE 400-2001 standard:

• " even massive insulation defects in extruded

dielectric insulation cannot be detected with DC at

the recommended voltage levels."

In other words not only is DC High Potential testing likely destructive for field-aged extruded cables, but it may not tell you a darn thing

High Potential Testing - DC

WHAT’S THE BOTTOM LINE

SKIP??????????

QUIT DC HIPOT

TESTING

Power Factor/Dissipation Factor Testing

z Effective in locating weaknesses in insulation and

potential hazards before impending failure

z Not a “Go-No-Go” Test

z Testing does not overstress the insulation and can

determine if the insulation is slowly degrading through trending

z Testing limited to relatively short lengths of cables

z Not effective in detecting localized faults as the length of cable increases

Very Low Frequency (VLF) Testing

z VLF - High Potential Testing

– VLF damages the insulation less than DC Testing and has the

capability of locating potential failure sites.

– VLF has the advantage of portability with low energy

requirements, which results in much smaller test sets.

z VLF Partial Discharge Testing

– Voltage is raised to above the PD inception voltage to cause PD

to occur PD is then locatable using Time Domain PD Detection methods.

– Cable must be disconnected to test.

In-Service Testing Techniques

In-Service Testing Techniques

z Has the obvious advantage that the cable is not put at any additional risk from the test

z Technique has the advantage that the cable is not

removed from service, leaving it energized as it is for normal operation

z Removes the potential for damage due to

inappropriate switching and yields no system

contingency problems

Ultrasonic PD Testing

accessory is not directly buried or is at least

physically accessible

are extremely short wave in nature, fairly

directional and easy to isolate from background noise

On-Line Partial Discharge Testing System for Cable Systems

On-Line Partial Discharge Testing

System for Cable Systems

z There is zero down time associated with On-Line PD

testing because the the test is performed at normal

operating voltage Testing involves no external voltage or current sources

z Test equipment measures PD produced at voltages of

2400 volts and greater.

z The system is independent of load current.

z It is non-invasive testing that does not inject current into the system, nor does it subject the system to excessive voltage levels.

z Method is 100% destructive and 100%

non-invasive.

On-Line Partial Discharge Testing

System

On-Line Partial Discharge Testing

System

Two Part System Study

z Partial Discharge activity is

recorded from several Points

of Attachment (POA’s) along

the length of each cable run.

z If possible, the sensors are

clamped around all three

phases at once.

z In general, it is preferred to

attach sensors every 500 ft

z Partial Discharge data is processed utilizing pattern recognition software and analysis.

z Analysis is used to formulate a final report, which details findings

z Final Report detailing the analysis

of system is delivered

The ETI On-Line Partial Discharge

Testing System

The ETI On-Line Partial Discharge

Testing System

SPECTRUM ANALYZER

LAPTOP COMPUTER

3-PHASE AC CABLE COAXIAL CABLE

PD SENSOR

CUSTOMER FINAL TESTING REPORT

HISTORICAL TRENDING

DATABASE

DIAGNOSTIC AND REPORT GENERATION SOFTWARE

SCHEDULED MAINTENANCE

SOFTWARE

Partial Discharge Testing Equipment

z Custom made split core

sensors, are placed

around all three phases of

the cable (if possible).

z Sensors sense RF

Current and convert to a

millivolt (mV) output.

z Several images of partial

discharge activity are

recorded at several Point

of Attachments (POA’s)

along the length of the

cable.

Partial Discharge Testing Equipment

z Data from the sensors are fed through a coaxial cable, through an amplifier to a Spectrum

Analyzer

z The Spectrum Analyzer is used to evaluate the

magnitude patterns and frequencies of any partial discharge activity at the POA

z Partial discharge waveforms are captured for

analysis by our partial discharge analysis software and engineers

z Any obvious PD activity may be relayed to the

customer before leaving the site

Partial Discharge Predictions

z Characteristics of PD in electrical trees differ from those in spherical voids

z The pulse shapes, rise and fall times, width, and

amplitude depend on the applied field in the insulation

z Voids are identifiable by single discharges occurring each half cycle

z Electrical Trees are branchlike with the PD’s proceeding in steps along the branch Trees are identifiable by multiple discharge pulses occurring per half cycle

PD Characteristics

PD Characteristics

PD Characteristics

PD Characteristics

Electrical Treeing Electrical Treeing

PD Characteristics

Electrical Treeing Electrical Treeing

Final Partial Discharge Report

The data is then used to electronically generate a final report

that details the severity and location of partial discharge

activity PD activity is then classified into the following

category(s) and appropriate recommendations made.

Severity Level Classifications

Level A-Low levels of Partial Discharge activity occurring No

necessary actions at this time Retest in 3 years.

Level B-Moderate levels of Partial Discharge activity occurring Retest

within 12-18 months to monitor & trend PD activity.

Level C-High levels of Partial Discharge activity occurring Prioritized

repair or replacement is recommended.

Final Partial Discharge Report

z Software has been developed for pattern

recognition and conversion of data to usable format Software significantly reduces the

amount of time required to generate a final report

z Reports can show the severity level of any

observed partial discharge activities, where they are located, recommended testing

intervals, other recommended actions, etc

z In addition, the final report can provide the actual data used to formulate our

recommendations.

Partial Discharge Analysis

z All data recorded while on-site

is scrutinized to uncover any pending problems within the tested system, and analyze their severities and locations.

z A comparative analysis is generated from the data gathered while on-site This comparative analysis allows for prioritization/planning of cable replacements, scheduling

repairs, etc

Success Stories

- St Louis Correctional Facility

– Tuesday - Identified “Level C” Y-Splice located in PMH feeding 5KV Padmount Transformer -

Recommended Prioritized Replacement

– Friday - Y-Splice violently fails before facility could replace causing unplanned outage

• 5KV Distribution System was less than 2 years old and

failure determined to be workmanship related on the Splice

Y-Success Stories

-Bellamy Creek Correctional Facility

– Performed On-Line Partial Discharge Testing to establish baseline data for facility upon completion of Acceptance

Testing.

– 15KV Distribution System Passed Acceptance Testing that consisted of AC High Potential Testing

– On-Line Partial Discharge Testing found a workmanship

defect at a Transformer Termination which was not found by High Potential Testing.

– Identified “Level C” splice located in Tap Box on roof of

facility Repairs were not scheduled until July Outage 2003 – Cable Splice failed Dec 2002 causing 48 hr loss of plant production.

Success Stories

– Performed On-Line Partial Discharge Testing to determine condition of 34.5KV Distribution cables and accessories

– Testing was performed during normal plant

production with zero down time

– Several areas requiring prioritized attention were identified

– Replaced pothead connection with attached

results

Primcor Refinery - 8/2002 (Initial Test)

Primcor Refinery - 8/2002 (Initial Test)

Primcor Refinery - 2/2003 (After Repairs)

Primcor Refinery - 2/2003 (After Repairs)

z On-Line Partial Discharge Testing is a

non-invasive, non-destructive, predictive test

z Final report generated which provides

interpretations and recommended actions

Insulation Magazine, Vol 16, No 5

Conditions”, IEEE Trans PD-13, No 2, April 1998, pp 310 - 315

Magazine, Vol 11, No 2

No 5

AC Impulse Breakdown of Model EPR and TR-XLPE Cables as a Function of Wet Electrical Aging”, PES ICC, April 2002

of the IEEE Conference on Electrical Insulation and Dielectric Phenomena pp 547-551 (also submitted for review to Trans PD.)

Insulation Magazine, Vol 8, No 2

Voltage Solid Dielectric Cables”, IEEE Electrical Insulation Magazine, Vol 9, No 6