Partial Discharge and Tan Delta On-Site Diagnostic … ICC Educational Program Fall 2000. Partial...

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ICC Educational Program Fall 2000.

Partial Discharge and Tan Delta On-Site Diagnostic Testing of

extruded cable with the integrated PD and TD BAUR

VLF test instrument. By:

Craig Goodwin, HV Diagnostics Inc. GA, USAe-mail: [email protected]

22


Summary of Presentation

• Theory / Background

• Description of Methods used

• Field Examples

33

ICC Educational Program Fall 2000.Two Main Groups of On-Site Cable Diagnostics Test EquipmentGroup 1:

• On Line (In Service)

• Off Line (out of service).

Group 2:

• Global / Integral Assessment Diagnostics eg. TD.

• Discrete Location Diagnostics eg. PD

44

ICC Educational Program Fall 2000.Why use Alternating Field instead of DC for HV Cable

Testing?Why not DC ?• Potentially damaging to aged extruded cable. • Only able to detect gross / major insulation defects. • No further internal Partial Discharges after DC threshold.

Example: 10-kV-XLPE-cable, with penetrated needle remaining insulation thickness approx. 0,8 mm breakdown higher than 12Vo.

55


Benefits of Voltage Testing with VLF Sinusoidal Waveforms in the

field. Pure sinusoidal, load independent alternating

voltage without any DC offsets. Low Power requirement Generally Higher PD inception voltage High electrical tree growth rate Compact portable design of test equipment Easier to integrate Cable diagnostic interfaces

such as TD and PD.

66


PD- inception voltage levels for different test voltage waveforms on XLPE

Reference:Research and Development, Annual Report 1992, TU BerlinHigh Voltage InstituteProf.Dr.-Ing.Kalkner, Dipl.Ing.R.Bach"Investigation on site test methods of M.V.cables with different H.V.waveforms"

50 Hz 0,1 Hz cos-rectangular

0,1 Hz sin. osc.wave0

5

10

15

20

25

30

35

40


0,1 Hz sin. osc.wave

wetdry

PD-I

ncep

tion

Volta

ge [k

V]

77

ICC Educational Program Fall 2000.Electrical Tree Growth Rate from PD -

Needle Point Fault.Applied Voltage: 3V0 using Various

Waveforms


0,1 Hz sin0

2

4

6

8

10

12

14

elec

tric

al tr

ee g

row

th ra

te [m

m/h

]


0,1 Hz sin

dry, newwet, service aged

Reference:Research and Development, Annual Report 1992, TU BerlinHigh Voltage InstituteProf.Dr.-Ing.Kalkner, Dipl.Ing.R.Bach

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European Union Standard for Cable Maintenance High Voltage Tests: CENELEC

HD 620 S1 1)

Cables from 6 to 36 kVFrequency Test voltage Testing time

PE/XLPE cables 0,1 Hz 3 x Uo 1 hour

50 Hz 2 x Uo 1 hour

For Paper insulated mass impregnated cables CENELEC HD 621:

DC test or above mentioned parameters1) June 1996, 15 EUcoutries

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"WATER TREEING" in polymeric insulation(schematic layout)

electrical field conditions for possible water development of disturbance field "water trees" time

outer semiconductinglayer

insulation

inner semiconductinglayer

conductor

"bow-tie trees"

"vented trees"

1010


Vented- tree growing from the inner semicon layer of a 30 kV- PE- cable

1111


Dielectric Failure of insulation originating from a water tree

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Comparison of Electrical and Water treeingElectrical Treeing (ET): high localized field fast tree growth in PE or XLPE insulation accompanied by partial discharges long channel structures (visible trees) clear indication of an electrical breakdown

Water Treeing (WT): can be present even at small electric field

strengths (e.g. < 1 kV / mm) extremely slow tree growth (e.g. over 6-10 years) no partial discharge associated.

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How Do We Detect:Water Trees

andElectrical Trees

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What is Dissipation Factor ( tan

time/sec0 10

voltage

current

= U RU C

²/².

1CR.

dissipation factor tan = true power reactive power

=

1515


Simplified Dielectric Equivalent Circuit of a Water Tree

w t

R1C1

R2 C2

>>

1616


Voltage Dependence of 0,1 Hz Dissipation Factor of New and Service-aged XLPE-insulated Medium Voltage

Cablestan . 10-3

0

1

2

3

4

5

6

7

0 0,5 1 1,5 2 2,5 3

U / U 0

0,1

Hz

diss

ipat

ion

fact

orreference cable (new)slightly service-aged (1)moderately service-aged (2)moderately service-aged (3)strongly service-aged (4)

JICABLE, Versailles, June 1995, paper B.9.6.

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Field Test Result for an 1974 XLPE Cable

Good Resultfor an old Cable.

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Conclusions of TD Diagnostics A TD cable assessment system can clearly distinguish

between new, medium and strongly aged extruded cable insulation.

Water trees which are a major original source of insulation failures in aged extruded cables can be detected in a non-destructive manner.

The TD cable assessment system provide reliable results in field use for both extruded and PILC cables.

To detect singular defects, a combined assessment system including the application of partial discharge measurement and location in addition to TD is necessary.

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0.1 Hz Partial Discharge 0.1 Hz Partial Discharge TestingTesting

• Measurement of partial discharge activity at siteincluding calibration

• Precise PD cable fault location

2020


Principle of P.D. Location using TDRTechniques:

560 m

339,8 m

180,4 m

joint with PD fault (1)

t

t

near cable end

partial discharge fault (2)

far cable end

impulse reflected via far cable end (2)

impulse directly reflected first reflection (2)

impulse directly reflected (2)

impulse directly reflected first reflection (1)

impulse reflected via far cable end (1)

impulse directly reflected (1)

2121


Example of PD Location on PILC cable.

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TD and PD: A good combination of Diagnostic Methods:

• Dielectric Dissipation Factor (TD) Measurement:Overall assessment of the cable insulation.Non-destructive test method

•Partial Discharge Measurement:Discrete Location of PD in cables, accessories and terminations.

•Dielectric Withstand Test (“Hi Pot”):High Voltage Acceptance or Maintenance Dielectric testing

2323


Example of PD Location on XLPE Cable.

Nominal OperatingVoltage: Vo = 14kV

PDIV: only at 19kV

Note: TD values are poor even at Vo.

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TD and PD Diagnostic SystemA Good Combination:

TD: Overall assessment of lossesin the cable.Can Detect Water Trees

PD: Discrete Location and measurement of PD activity in cables.

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