considerations for mv plant cables -...

PD Testing

Considerations for MV Plant Cables

Cable Aging Management- SLIDE 2

Cable Testing Philosophy

Manu-facturing

Dam

age

Mis

take

Rep

air

Agin

g

Acce

ptan

ce

Test

ing

Mai

nten

ance

Te

stin

g

Trans-portation

In- stallation

Fact

ory

Test

ing Power

frequency 50/60 Hz

Operation Power frequency 50/60 Hz


Various Manufacturing Defects

1. Cavity at shield 2. Cavities due to shrinkage or

gas-formation in insulation 3. Defects in the insulation

shield 4. Loosely bound solid particles

5. Protrusions 6. Splinters 7. Fibers 8. Contaminants in the

insulation shield


Electrical Trees (XLPE)

• Often begins near – Protrusions at the

shield – Protrusions at the

conductor – Near impurities – At water trees

• Two Types – Branch type trees – Bush type trees


Partial Discharge Theory

rRx

xEE

rRx

xE

void

ln

121

3

)(21

3

ln

1)(

εε

εε

+=

+=

=

5 © Kinectrics Inc., 2011


Electrical Field Conditions

-6

-4

-2

0

2

4

6

0 45 90 135 180 225 270 315 360

Phase Angle

Vo

ltag

e/E

lect

ric

Fie

ld

Appl. Voltage [kV]

Background Field [kV/mm]

Void Breakdown Strength

Void Field [kV/mm]

Partial Discharge Theory


Partial Discharge Theory Electrical Field Conditions

-20

-15

-10

-5

0

5

10

15

20

0 45 90 135 180 225 270 315 360

Phase Angle

Vo

ltag

e/E

lect

ric

Fie

ld

Appl. Voltage [kV]



Void Field [kV/mm]

Initial Discharge

Return Discharge Second

Discharge


Partial Discharge Theory Electrical Field Conditions

-40

-30

-20

-10

0

10

20

30

40

0 45 90 135 180 225 270 315 360

Phase Angle

Vo

ltag

e/E

lect

ric

Fie

ld

Appl. Voltage [kV]



Void Field [kV/mm]


PD Testing

• Diagnostic Test • Can be performed on-line or off-line • With respect to commissioning testing often performed either

during a HiPot test or after HiPot test • Should preferably be done at..

– At near power frequency – Elevated voltages to identify problem prior to in-service

failure • Most PD monitors require experts for operation and data

interpretation ⇒ expert test • No standard for field PD testing – (IEEE 400.3 is a guide) • Should be performed by independent third party

© Kinectrics

Cable Aging Management- SLIDE 10 © Kinectrics Inc., 2008

Electrical Trees (XLPE)


PD Theory: Apparent Charge Apparent Charge at discharge location

Measured Apparent Charge

R

C

VPD

iPD

iPD

VPD

R

Measured Apparent

Charge

Attenuation Attenuation

VPD

R

© Kinectrics


TDR on Tape Field Cable


Cable Testing Philosophy

Manu-facturing

Dam

age

Mis

take

Rep

air

Agin

g

Acce

ptan

ce

Test

ing

Mai

nten

ance

Te

stin

g

Trans-portation

In- stallation

Fact

ory

Test

ing Power

frequency 50/60 Hz

Operation Power frequency 50/60 Hz


Acceptance & Maintenance Tests

Types of Tests • Soaking • DC HiPot Testing • AC HiPot Testing • VLF Testing • Damped AC • Partial Discharge

Testing

Standards • IEC 60502, IEC 60840,

IEC 62067

Guides • IEEE 400

– 400.1 DC Testing – 400.2 VLF Testing – 400.3 PD Testing – 400.4 Damped AC – 400.5 AC Withstand

• Cigre 182 • Cigre 279


VLF Testing

Pros • Inexpensive compared to AC

HiPot Non-Resonant testing • Effective in finding watertree’d

cable for field aged, distribution class cables

• Easy to perform • Non-expert test • IEEE Std 400.2

Cons • Still discussion on test levels versus

test time • Issues with charge injection and

trapped charge when testing at frequencies less than 0.1 Hz

• Not as effective in finding weak spots in the insulation as a conventional AC HiPot.

• Voltage wave may not be sinusoidal (depends on test kit).

• Not straight forward to interpret PD data acquired during VLF conditions

• Research still to be done on the effectiveness in detecting, for instance, interfacial phenomena

After-laying field tests



VLF vs. AC Testing After-laying field tests

-1

-0.5

0

0.5

1

0 1 2 3 4 5 6 7 8 9 10

-1

-0.5

0

0.5

1

0 1 2 3 4 5 6 7 8 9 10

0.1HZ

20HZ


-1

-0.5

0

0.5

1

0 1 2 3 4 5 6 7 8 9 10

60HZ


VLF vs. AC Testing Withstand Voltage of XLPE Cables



VLF vs. AC Testing

Withstand Voltage of XLPE Cables

VLF AC Withstand

Waveform Cable System Rating ( φ-φ)

Installation (φ-∅)

Acceptance ((φ-∅))

Maintenance (φ-∅)

Acceptance (φ-∅)

Maintenance (φ-∅)

[kV] [kV rms] [kV rms] [kVrms] [kVrms] [kVrms]

Sinusoidal

5 9 10 7 13 9 8 11 13 10

15 19 21 16 26 23 25 291 32 241 28 32 361 27 42 36 35 39 44 33 46 51 57 43 67 58 69 75 84 63 72


AC HiPot Testing

• Stipulated in IEC 60502 (MV), IEC 60840 (HV) and IEC 62067 (EHV) as well as most cable testing standards

• Testing at power frequency from 20 Hz to 300 Hz • Recommended high potential test level for acceptance testing

ranges from 1.25U0 to 2U0 but typically 1.7U0. • High potential voltage applied for 60 minutes • AC HiPot testing is gaining ground in Europe and North-

America • Proven test methodology for identifying installation related

defects.



AC HiPot Testing Pros • Proven test methodology;

test levels and duration well established

• Based on IEC standards • Reliable equipment

available. • Decades of experience with

tests • Can perform PD testing

during AC HiPot tests • Gets ‘soft’ failure so

meaningful forensics can be performed

Cons • Not a diagnostic test • Requires heavy equipment • Not necessarily cheap



The Phenomenon of Resonance

iR

iC

iL

L R

CCable


MV Test Spec (Plant Cables)

Equipment

Rated Voltage

(kV)

AC Testing DC Testing Hipot Test Partial Discharge Test

Test Level (kV)

Time (mins)

Extinction Level (kV)

Allowable Pulse Mag. (pC)

Time (mins)

Test Level (kV)

Time (minutes)

Power Cable

Unshielded:

Shielded: (including cable bus)

Bus Duct

Isolated Phase Bus

Metal-Enclosed Rigid

Switchgear

4.16

4.16 8

13.8

25

13.8

4.16

13.8

N/A

9 (13) 13 (18) 19 (27)

45 (60)

27 (36)

14 (19)

27 (36)

N/A 5 5 5 1 1 1 1

N/A

4.5 (4.5) 7.5 (7.5) 13 (13)

19

10.5 (10.5)

3.5 (3.5)

10.5 (10.5)

N/A

3 (3) 3 (3) 3 (3)

100

100 (100)

100 (100)

100 (100)

N/A

2 2 2

2

2

2

2

25 (35)

25 (35) 35 (45) 55 (70)

64

38 (50)

N/A

N/A

5

15 15 15

1

1

N/A

N/A


PD Interpretation Criteria Comm. Testing (Off-Line) • Measure PDIV, PDEV • Measure PD Level & Intensity • Identify signature partial

discharge pattern • Identify location of partial

discharge source ⇒ Guiding criterion is that the

cable system should be PD free at the specified test voltage at commissioning fro HV &EHV systems

⇒ Less then 3pC or 3pC for MV systems using conventional systems

Maintenance Testing (On-Line) • Assess individual PD sources

separately • Identify location of partial

discharge source • Assess impact of individual PD

sources (insulation materials, experience)

⇒ Develop individual assessment/ranking of joints and cable sections



PD Testing

Pros • Can help identify insulation

defects which does not cause failure during an AC HiPot test

• Can use external, non-invasive sensors

• Can be performed continuously at each accessory using daisy chained PD systems

Cons • No standard or acceptance

criteria • For long lengths of cable a

distributed PD measurement should be performed (at each accessory)

• Requires an expert to perform the test and evaluate the data

• Cannot compare factory PD tests to field tests

• May be practically difficult to perform (manhole access, traffic control)


Off-Line vs. On-Line

Off-Line • Requires cable disconnected and

cable conductors exposed. • Requires external power source • Done at elevated voltage • Can perform meaningful

sensitivity assessment • Can measure PDIV, PDEV • For cross-bonded cable systems,

SVLs must be shorted and, preferably, the cross links removed thus creating a continuous sheath

On-Line • Non-invasive test • Done at rated voltage • PDIV and PDEV cannot be

measured • Often, multiple PD sources from

various types of HV equipment are detected

• Can only detect sources present at the time of testing!

• Can be performed continuous over long lengths of time thus characterizing the dynamic PD behaviou

© Kinectrics


PD Monitors

Measurement Parameters • Magnitude • Phase Angle ⇒ Derive pulse count

rate • Signal Shape

Types of Monitors • Traditional PD

Monitors • Conventional PD

Monitors • Modern Partial

Discharge Monitors

© Kinectrics


AC HiPot & Conv. PD Testing


Broad Band PD Measurements


HFCT For Voltage Reference LFCT For Voltage Reference

Sensor Connections


AC HiPot & Unconv. PD Testing

• The frequency content of a partial discharge pulse depends on… – Duration

• Strength of the electrical field

• Length of the partial discharge it self

– Pulse propagation path

⇒ different PD sources have different frequency content


• Off-line Hi-Pots (ac, VLF)

Dielectric loss measurements

Partial Discharge testing


Summary of Partial Discharge Data

Cable ID (Phase) AC Hipot DIV (kV) DEV (kV) PD @ 13 kV (pC) 2ENA02NC1-AA/AG (Red) 2ENA02NC1-AA/AG (Black) 2ENA02NC1-AA/AG (Blue)

Passed Passed Passed

10.8 16.8 13.9

10.6 16.1 13.7

185 0 0

2ENA02NC1-AB/AH (Red) 2ENA02NC1-AB/AH (Black) 2ENA02NC1-AB/AH (Blue)


11.9 17.5 16.5

10.7 16.0 15.0

190 0 0

2ENA02NC1-AC/AI (Red) 2ENA02NC1-AC/AI (Black) 2ENA02NC1-AC/AI (Blue)


18.9 >19 14.2

17.3 >19 13.4

0 0 0

2ENA02NC1-AD/AJ (Red) 2ENA02NC1-AD/AJ (Black) 2ENA02NC1-AD/AJ (Blue)


15.7 11.6 16.8

14.8 11.2 15.4

0 330 0

2ENA02NC1-AE/AK (Red) 2ENA02NC1-AE/AK (Black) 2ENA02NC1-AE/AK (Blue)


>19 >19 >19

>19 >19 >19

0 0 0

2ENA02NC1-AF/AL (Red) 2ENA02NC1-AF/AL (Black) 2ENA02NC1-AF/AL (Blue)


>19 18.6 17.7

>19 18.0 16.1

0 0 0


Example 1: On-Line PD • 142 circuits of single conductor 5 kV

EPR

• 19 years to 21 years old

• All cables feeding 4.2 kV induction motors


Example 1: On-Line PD (5kV)

• Partial discharge activity detected through out the system • However, of 142 cables, only 7 subjected to partial discharge

activity from the cable system it self – In one case, partial discharge activity was detected from a cable splice – The remaining six cases related to tracking along cable termination

surfaces

• The remaining discharge activity detected originated from the 4.2 kV induction motors, the bus system or other connected HV equipment


Example 2: On-Line PD (27.5)

• Industrial client • 42 cable sections, 27.5 kV XLPE • 12 years old • Cable circuit constitute single supply line to the plant

⇒ crucial to avoid unforeseen failure • On-line partial discharge measurements performed

routinely as a maintenance test


Example 2: On-Line PD (27.5)


Ex 3: Periodic/Remote Monitoring

• Custom made sensor – Cable length – Cable specifications

• Non-invasive install, can be performed live

• Connected to termination box



• Permanent installation at substation

• 1 day installation • Suspected faulty

equipment • PD measured from

vacuum bottle • Verified during

subsequent off-line testing

PROCESSING


Remote Monitoring Setup – Example 1


Cable Testing also involves… Being nice to stray doggies at work sites - even while you’re busy!

“The greatness of a nation and its moral progress can be judged by the way its animals are treated.. I hold that, the more helpless a creature, the more entitled it is to protection by man from the cruelty of man" - Mahatma Gandhi (1869 - 1948)


Contact information

Terry Herrmann Structural Integrity Associates [email protected] (303) 542-1415 Steve Biagiotti Structural Integrity Associates [email protected] (303)542-1402 www.structint.com

Rick Easterling Kinectrics [email protected]

(804) 550-3109

Mark Fenger Kinectrics [email protected]

(416) 300-8464

Howard Sedding Kinectrics [email protected]

(416) 207-6000 x6172

www.kinectrics.com

mailto:[email protected]


http://www.structint.com/




http://www.kinectrics.com/

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