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, USA
e-mail: hvdiagnostics@mindspring.com
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ICC Educational Program Fall 2000.
Summary of Presentation
• Theory / Background
• Description of Methods used
• Field Examples
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ICC Educational Program Fall 2000.
Two Main Groups of On-Site
Cable Diagnostics Test
Equipment
Group 1:
• On Line (In Service)
• Off Line (out of service).
Group 2:
• Global / Integral Assessment Diagnostics eg.
TD.
• Discrete Location Diagnostics eg. PD
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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.
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ICC Educational Program Fall 2000.
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.

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ICC Educational Program Fall 2000.
PD- inception voltage levels for different
test voltage waveforms on XLPE
40
PD- Inception Voltage [kV]
35
wet
dry
30
25
20
15
10
5
0
50 Hz
0,1 Hz cosrectangular
0,1 Hz sin.
osc.wave
Reference:
Research and Development, Annual Report 1992, TU Berlin
High Voltage Institute
Prof.Dr.-Ing.Kalkner, Dipl.Ing.R.Bach
"Investigation on site test methods of M.V.cables with different H.V.waveforms"
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ICC Educational Program Fall 2000.
electrical tree growth rate
[mm/h]
Electrical Tree Growth Rate from PD Needle Point Fault.
Applied Voltage: 3V0 using Various
Waveforms
14
12
10
8
dry, new
wet, service aged
6
4
2
0
50 Hz
0,1 Hz cosrectangular
0,1 Hz sin
Reference:
Research and Development, Annual Report 1992, TU
Berlin
High Voltage Institute
Prof.Dr.-Ing.Kalkner, Dipl.Ing.R.Bach
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ICC Educational Program Fall 2000.
European Union Standard for Cable
Maintenance High Voltage Tests: CENELEC
HD 620 S1 1)
Cables from 6 to 36 kV
Frequency
PE/XLPE cables
Test voltage
Testing time
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 parameters
1) June 1996, 15 EU
coutries
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ICC Educational Program Fall 2000.
"WATER TREEING" in polymeric insulation
(schematic layout)
electrical field
 water
 disturbance field
 time

conditions for possible
development of
"water trees"
"bow-tie trees"
outer semiconducting
layer
insulation
inner semiconducting
layer
conductor
"vented trees"
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ICC Educational Program Fall 2000.
Vented- tree growing from the inner semicon
layer of a 30 kV- PE- cable
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ICC Educational Program Fall 2000.
Dielectric Failure of insulation originating
from a water tree
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ICC Educational Program Fall 2000.
Comparison of Electrical and Water treeing
Electrical 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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ICC Educational Program Fall 2000.
How Do We Detect:
Water Trees
and
Electrical Trees
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ICC Educational Program Fall 2000.
What is Dissipation Factor ( tan 
voltage
current
0
time/sec
dissipation factor tan =
10
U²/ R
true power
1
=
=
reactive power U².
C C.R
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ICC Educational Program Fall 2000.
Simplified Dielectric Equivalent Circuit of a
Water Tree
wt
R2
C1
C2
>>
R1
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ICC Educational Program Fall 2000.
Voltage Dependence of 0,1 Hz Dissipation Factor of New
and Service-aged XLPE-insulated Medium Voltage
Cables
tan. 10
-3
7
0,1 Hz dissipation factor
reference cable (new)
6
slightly service-aged (1)
moderately service-aged (2)
5
moderately service-aged (3)
strongly service-aged (4)
4
3
2
1
0
0
0,5
1
JICABLE, Versailles, June 1995, paper B.9.6.
1,5
2
2,5
3
U / U0
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ICC Educational Program Fall 2000.
Field Test Result for an 1974 XLPE Cable
Good Result
for an old
Cable.
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ICC Educational Program Fall 2000.
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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ICC Educational Program Fall 2000.
0.1 Hz Partial Discharge
Testing
• Measurement of partial discharge activity at site
including calibration
• Precise PD cable fault location
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ICC Educational Program Fall 2000.
Principle of P.D. Location using TDR
Techniques:
partial discharge fault (2)
joint with PD fault (1)
near cable end
far cable end
impulse directly reflected (1)
impulse reflected via far cable end (1)
impulse directly reflected
first reflection (1)
t
impulse directly reflected (2)
impulse reflected via far cable end (2)
impulse directly reflected
first reflection (2)
t
180,4 m
339,8 m
560 m
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ICC Educational Program Fall 2000.
Example of PD Location on PILC
cable.
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ICC Educational Program Fall 2000.
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
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ICC Educational Program Fall 2000.
Example of PD Location on XLPE Cable.
Nominal Operating
Voltage: Vo = 14kV
PDIV: only at 19kV
Note: TD values are
poor even at Vo.
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ICC Educational Program Fall 2000.
TD and PD Diagnostic System
A Good Combination:
TD:
PD:
Overall assessment of losses
in the cable.
Can Detect Water Trees
Discrete Location and
measurement of PD
activity in cables.
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