VLF Testing of Cables Using IEEE 400.2. Presentation by John Densley that describes the in situ testing of distribution cables using very low frequency (VLF) voltages according to IEEE Standard 400.2. The Standard includes criteria to assess the condition of XLPE, EPR and paper insulated cable circuits. How the criteria were established and their limitations will be discussed, along with Partial Discharge (PD) testing using VLF. John, the President of ArborLec Solutions Inc. has worked on electrical ageing mechanism and diagnostics techniques in electrical insulation for more than 35 years. He has a sound practical and theoretical knowledge of ageing and failure mechanisms of insulation systems in high voltage equipment such as medium and high voltage cables, transformers and switchgears.

1. VLF Testing of Cables Using IEEE 400.2™
John Densley
ArborLec Solutions Inc.

2. Outline
• Aging/diagnostics basics
• Diagnostic testing of MV cables
• IEEE 400.2 – 2013
• Future trends
• Conclusions

3. Aging
• Changes in properties that occur with time
due to the applied stresses, electrical,
thermal, mechanical and environmental
(chemical). Can act singly or synergistically
• Changes may cause permanent damage that
may affect ability to perform specified
functions. This is sometimes referred to as
degradation
• Changes can lead to failure

4. Polymer cable insulation aging & failure
mechanisms
• Insulation aging usually at defects (voids, contaminants
or protrusions) → partial discharge (PD), water treeing
(WT), thermal aging
• Failure mechanism → Electrical treeing (ET), thermal
runaway
Aging, e.g. PD, can take a long time initiate failure →
possible to detect with diagnostic test
Failure, e.g. ET, usually takes place quickly → not easy to
pick up with diagnostic test
• Other types of failure:
– voltage transients → intrinsic BD
– abnormal high temperature → mechanical or electrical failure
– Mechanical damage (dig-in, thermomechanical)

5. Partial discharges
• Local BD of gas in voids,
delaminations at interfaces
or protrusions (electrical
trees)
• A partial discharge is a
current pulse of some
nanoseconds duration that
causes minute high
frequency currents to flow
in the circuit (100’s of MHz)

6. Simplified PD sequence
Initial
Discharge
Return
Discharge Second
Discharge
Electrical Field Conditions
-20
-15
-10
-5
0
5
10
15
20
0 45 90 135 180 225 270 315 360
Phase Angle
Voltage/ElectricField
Appl. Voltage [kV]
Applied Field in Void [kV/mm]
Void Breakdown Strength
Void Field [kV/mm]

7. 7
-1.5
-1
-0.5
0
0.5
1
1.5
-15
-10
-5
0
5
10
15
0 45 90 135 180 225 270 315 360
ReferenceVoltage[kV]
PDMagnitude[mV]
Phase Angle
Partial discharge pulse sequence
PD Signal Reference Voltage

8. Electrical treeing
• Localized breakdown of
insulation from voids,
sharp protrusions or
water trees that increase
the local electrical stress
• Local stress at tip at tree
initiation can be >100
kV/mm
• Once tree has initiated its
growth is caused by PD in
the channels (<20 μm
dia.)
Electrical
tree
Water
tree

9. Electrical treeing
Growth
• The PD leave charges on the
walls of the channels → stress
that can be > applied stress
• ET shape changes with
applied field/frequency in
insulation → non-linear
growth with time, e.g., rate at
60 Hz less than at 0.1 Hz and
rate at 0.1 Hz is less than 0.01
Hz
Breakdown through electrical
tree
9

10. Water treeing
 Occurs at stress enhancements such as
protrusions, contaminants or voids
 Moisture always present
 Soluble contaminants such as salts are
particularly harmful
 Degradation is slow taking several years
 Final failure due to electrical tree (PD) from
the water tree or thermal runaway for
bridging trees
 Vented tree from semicon surface or bow-tie
tree from contaminant or void
 Responsible for failures of many older
vintage cables (extruded insulation)
 No PD during initiation or growth

11. Other stresses
• PD and WT are the main aging mechanisms of
extruded MV cables
Therefore:
• Will not consider aging due to mechanical (M)
and thermal (T) stresses
• M and T accelerate electrical aging
We can now look at diagnostic testing

12. Diagnostic tests
• A diagnostic test is a field test made during the
operating life of a cable system that is intended to
determine and locate aging or degradation that may
cause failure of cable system - Defined in CIGRE WG21-
05 Report, 1994
• May be on-line or off-line
• Tests may be one-time only or periodic to establish
trends
• Can look at metallic and/or insulating parts
The main purposes of a diagnostic test are to reduce the
number of failures in service and provide information
on the serviceability of the cable system

13. How can we do diagnostics on cables?
Must know details of the cable system to be tested
• Need to know probable aging and failure mechanisms
• Need to measure properties or characteristics that give
an indication of how much aging has occurred
• Would like to know rate of aging
• Need to interpret data to assess circuit condition and
give an indication of serviceability
One of the most important diagnostic tests is a periodic
visual inspection to establish trends
Let’s look at water treeing

14. Properties of water treed insulation
1. Lower AC Breakdown Voltage
2. Small increase in capacitance (C) with voltage
3. Increase in Tan Delta (dissipation factor (DF))
4. Tan Delta (TD) increases with voltage
5. Tan Delta changes with time at constant V
6. Decrease in insulation resistance (IR)
7. Loss Current Waveform distorted
8. Increased DC leakage current
9. Space charge/polar species in trees
10. No PD until just prior to failure
Numbers 2 to 8 are bulk measurements

15. IEEE Std 400™-2012
(Revision of IEEE Std 400-2001)
IEEE Guide for Field Testing and
Evaluation of the Insulation of
Shielded Power Cable Systems
Rated 5 kV and Above

16. IEEE Std 400™-2012
The guide lists the various field test methods that are
currently available, or under development, to evaluate
the insulation of shielded power cable systems.
The guide:
• Rated 5 kV and above
• Describes the tests, gives advantages and
disadvantages, suggested applications, and typical
results
• Does not give test voltage levels or test durations
Guides covering particular test methods are available in
the form of IEEE 400 “point” documents. These Guides
give voltage levels and durations.

17. Field test categories (1)
Installation tests
• Conducted after cable installation but before jointing
(splicing) or terminating or energizing
• Intended to detect shipping, storage or installation damage
Acceptance tests
• Made after cable system installation, including terminations
and joints, but before the cable system is placed in normal
service
• Intended to detect installation damage and to show any gross
defects or errors in installation of other system components

18. Field test categories (2)
Maintenance tests
• Performed on cable systems that have been aged in service
for an extended period of time
• Intended to detect deterioration and to check the
serviceability of the system
• Need to consider failure history, testing and installation
history, cable system component makeup, importance of
cable in system

19. Voltage Sources
• 60 Hz AC (used for transmission class)
• VLF (used for MV and lower voltage
transmission class) – IEEE 400.2
• Damped ac (DAC) – IEEE 400.4 (in prep.)
• DC – IEEE 400.1
These sources may be used in one of two
modes: diagnostic (Tan δ, PD, etc.) or
withstand (Simple and Monitored)

20. 400 “Point” documents (1)
IEEE Std 400.1 ™ – 2007: IEEE Guide for Field
Testing of Laminated Dielectric, Shielded Power
Cable Systems Rated 5 kV and Above with High
Direct Current Voltage
• Oil/paper cables only, not to be used for extruded dielectric
cables
• Includes a table of withstand HVDC test voltages (acceptance
and maintenance) for cables rated 5 kV to 500 kV
• Usual to measure current after 2 and 15 minutes

21. 400 “Point” documents (2)
• IEEE Std 400.3™-2006: IEEE Guide for Partial Discharge Testing of
Shielded Power Cable Systems in a Field Environment
• No tables of test voltages or maximum allowable discharge
magnitudes for cables with different ratings, unlike factory/routine
tests
• Describes principles of methods capable of detecting, measuring
and locating partial discharges from defects and damage in installed
shielded power cable systems
• Usually cannot assume lumped capacitance as for factory tests as
long cable lengths have to be treated as transmission lines
Guide is presently under revision – more comprehensive than original
version and will probably contain test voltages for both distribution
and transmission class cables

22. 400 “Point” documents (3)
• P400.4™/D8: Draft Guide for Guide for Field-
Testing of Shielded Power Cable Systems
Rated 5 kV and Above with Damped
Alternating Current Voltage (DAC)
• Presently out for ballot
• Recommended test values are included in an Annex for
engineering information as not enough practical data are
available
• A number of damped oscillations between 20 Hz and 500 Hz
applied at each test level

23. 400 “Point” documents (4)
IEEE Std 400.2™-2013: IEEE Guide for Field
Testing of Shielded Power Cable Systems Using
Very Low Frequency (VLF) (less than 1 Hz)
• Cables rated 5 kV to 69 kV
• Guide valid for extruded & oil/paper cables
• Includes tables of withstand (installation, acceptance and
maintenance) tests
• Includes tables of tangent delta (dissipation factor) criteria –
“no action required”, “further study required” & “action
required”

24. VLF testing
• Mostly used at distribution voltage levels
• Can be a simple or monitored (tan δ and/or PD) withstand
test
• VLF defined as < 1 Hz but most common frequency is 0.1 Hz.
Very long lengths tested at 0.05 Hz or 0.02 Hz.
• Must be careful when interpreting partial discharge data
acquired at VLF conditions since the physics of PD has a
complex relationship with frequency
• Maintenance testing of medium voltage cables with VLF is
popular in North America (IEEE 400.2™), Asia and Europe
(Cenelec Standard)
• Present voltage sources limited to cables rated 138 kV and
lower (200 kV peak)
• Two waveforms – sinusoidal and cosine-rectangular
24

25. VLF waveforms
Sinusoidal waveform
HVA30 @5kV RMS with 280feet XLPE Load
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
Voltage(kV)
Cosine rectangular waveform
SEBAKMT VLF40 @5kv RMS with 280feet XLPE Load
-8000
-6000
-4000
-2000
0
2000
4000
6000
Voltage(kV)
25

26. Examples of commercially available VLF Sources
- courtesy of NEETRAC

27. VLF withstand testing (1)
• Test integrity of circuit
• Test frequency is between 0.01 Hz and 1 Hz (0.1 most
common). <0.1 Hz used for longer length circuits
• As electrical tree grows faster at 0.1 Hz than at 50/60
Hz, more likely to cause failure during test
• Test values based on most used practices
• Recommended test duration is 30 minutes with 60
minutes allowed for critical circuits. European standard
is 60 minutes
• A test time within the range 15 – 30 minutes may be
considered if the monitored characteristic remains
stable for at least 15 minutes and no failure occurs

28. VLF withstand testing (2)
• IEEE 400.2 -2013 test voltages are based on peak to
peak voltages being equal for sinusoidal and cosine-
rectangular voltage waveforms (based on breakdown
tests on aged XLPE cables)
• European standard test voltages based on rms values
being equal for the two waveforms (based on electrical
tree growth data)
• If a circuit fails during test → repaired or replaced →
re-tested using a complete 30 minute test, preferably
monitored withstand test. It is recommended to retest
each section with VLF-TD, VLF-DTD, VLF-TDTS or VLF-
PD

29. VLF withstand test voltages for sinusoidal
waveform1 IEEE 400.2 - 2013
• Note 1: If the operating voltage is a voltage less than rated voltage, it is recommended that the
maintenance test voltages should be those corresponding to the operating voltage class.
• Note 2: The maintenance voltage is about 75% of the acceptance test voltage magnitude.
• Note 3: Some existing test sets have a maximum voltage that is up to 5% below the values listed
in the Table. These test sets are acceptable to be used. However, there is a risk that the cable
may be "undertested" due to a combination of lower test voltage and allowed uncertainty of the
measuring circuit.
Cable System
Rating
(Phase to Phase)
[kV]
Installation
(Phase to Ground)
Acceptance
(Phase to Ground)
Maintenance2
(Phase to Ground)
[kV rms] [kV peak] [kV rms] [kV peak] [kV rms] [kV peak]
5 9 13 10 14 7 10
8 11 16 13 18 10 14
15 19 27 21 30 16 22
20 243 343 26 37 20 28
25 293 413 32 45 243 343
28 32 45 363 513 27 38
30 34 48 38 54 293 413
35 39 55 44 62 33 47
46 51 72 57 81 43 61
69 75 106 84 119 63 89

30. VLF withstand test voltages for cosine-
rectangular waveform1 IEEE 400.2 - 2013
• Note 1: If the operating voltage is a voltage class lower than the rated voltage of the
cable, it s recommended that the maintenance test voltages should be those
corresponding to the operating voltage class.
Cable System
Rating
(Phase to Phase)
[kV]
Installation
(Phase to Ground)
Acceptance
(Phase to Ground)
Maintenance2
(Phase to Ground)
[kV
rms]
[kV peak] [kV rms] [kV peak] [kV rms] [kV peak]
5 13 13 14 14 10 10
8 16 16 18 18 14 14
15 27 27 30 30 22 22
20 34 34 37 37 28 28
25 41 41 45 45 34 34
28 45 45 51 51 38 38
30 48 48 54 54 41 41
35 55 55 62 62 47 47
46 72 72 81 81 61 61
69 106 106 119 119 89 89

31. Note on dissipation factor
Tan , Dissipation Factor, or Loss
Factor  Power Factor
Tan  = Ir/Ic = / = 1/[2CR]
• Tan  is a measure of polar
and dipolar movement
• Polar/dipolar species  bulk
test  sensitive to water,
high density of WT), ageing
• Tan  /dielectric
spectroscopy are bulk
measurements. The average
of the whole length of the
cable and its accessories is
measured.
31
• Careful interpretation of tan
 may be needed:
– Short sections of high Tan 
in a long cable may not
significantly increase the
overall Tan 
– In field measurements,
certain types of accessories
can give high tan  values

32. Voltage dependence of 0.1 Hz dissipation factor
of new and service-aged XLPE-insulated
medium voltage cables - Baur (1995 JICABLE),
tan  . 10-3
0
1
2
3
4
5
6
7
0 0.5 1 1.5 2 2.5 3
U / U 0
0,1Hzdissipationfactor
reference cable (new)
slightly service-aged (1)
moderately service-aged (2)
moderately service-aged (3)
strongly service-aged (4)
.
32

33. VLF tan δ – aged cables
IEEE 400.2 - 2013 gives:
• Criteria for aged (WT) XLPE and EPR cables
– No action required
– Further study recommended
– Action required
• Criteria based on tan  time stability (TDTS),
voltage tip-up (DTD) and mean value of tan 
(TDM)
33

34. Criteria
• no action required
– Cable system can be returned to service, but may be
retested at some later date to observe the trend in tan
delta
• action required
– Unusually high set of tan delta characteristics which may
indicate poor insulation condition and should be considered for
further testing or replacement or repair immediately
• further study advised
– Additional information is needed to make an assessment
→ failure history, additional testing

35. Parameters used to establish criteria
• Time stability of tan delta at Uo (TDTS)
– At least six single TD measurements at intervals of 10
seconds between each measurement at 0.1 Hz
– Measure mean and standard deviation
• Voltage tip-up of tan delta between 0.5 Uo
and 1.5 Uo (DTD)
• Mean value of tan delta at Uo (TDM)
TDTS most sensitive to degradation, then DTD
which is more sensitive than TDM

36. How numerical values of criteria derived
• Derived from cumulative distribution function
of the maintenance measurement data of tan δ
(time stability (TDTS) voltage tip-up (DTD) and
mean value (TDM)) obtained for aged cable
systems, mainly in North America
• Tables use the probability criteria of 80% based
on the Pareto principle (best ranked 80%
account for only 20% of the problems) and 95%
probability
36

37. Cumulative distribution of all cable system TDTS
values at U0 from IEEE 400.2-2013

38. Cumulative distribution of all cable system DTD
Criteria from 400.2 - 2013

39. Cumulative distribution of all cable systems
TDM values from IEEE 400.2 - 2013
• Criteria for PE, filled
polymer and PILC are
values at 80% and 95%
probabilities
• Utilities are encouraged
to develop their own
data plots
39

40. Assessment criteria for PE based insulations - IEEE
400.2 - 2013
Historical figures of merit for condition assessment of service-
aged PE-based insulations (i.e. PE, XLPE, and TRXLPE) using 0.1 Hz
40
Condition
Assessment
VLF-TD Time
Stability (VLF-TDTS
measured by
standard deviation
at Uo
[10-3]
Differential VLF-TD
(VLF-DTD)
(difference in mean
VLF-TD) between
0.5 Uo and 1.5 Uo
[10-3]
Mean VLF-TD at
Uo
[10-3]
No Action
Required
<0.1 and <5 and <4
Further Study
Advised 0.1 to 0.5 or 5 to 80 or 4 to 50
Action
Required >0.5 or >80 or >50

41. Assessment of service-aged OIP insulations
Condition
Assessment
VLF-TD Time Stability
(VLF-TDTS measured
by standard deviation
at Uo
[10-3]
Differential VLF-TD
(VLF-DTD) (difference
in mean VLF-TD)
between 0.5 Uo and
1.5 Uo
[10-3]
Mean VLF-
TD at Uo
[10-3]
No Action
Required
<0.1 and -35 to 10 and <85
Further Study
Advised
0.1 to 0.4 or -35 to -50
or
10 to 100
or 85 to 200
Action
Required
>0.4 or <-50
or
>100
or >200

42. No action required criteria for service-aged
filled insulations - IEEE 400.2 - 2013
Condition
Assessment
Filled Insulation
System
VLF-TD Time
Stability (VLF-
TDTS measure
d by standard
deviation
at Uo
[10-3]
Differential
VLF-TD (VLF-
DTD)
(difference in
mean VLF-TD)
between 0.5
Uo and 1.5 Uo
[10-3]
Mean VLF-
TD at Uo
[10-3]
No Action Required
* If it is not possible
to definitively identify
a Filled Insulation
<0.1
a
n
d
<5
a
n
d
<35
Carbon-filled (Black)
EPR
<0.1 <2 <20
Mineral-filled (Pink)
EPR
<0.1 <4 <20
** Discharge resistant
EPR
<0.1 <6 <100
** Mineral-filled XLPE - - <100

43. Further study and action required criteria for
service-aged filled insulations - IEEE 400.2 - 2013
Condition
Assessment
Filled Insulation System VLF-TD Time
Stability (VLF-
TDTS measured
by standard
deviation
at Uo
[10-3]
Differential VLF-
TD (VLF-DTD)
(difference in
mean VLF-TD)
between 0.5 Uo
and 1.5 Uo
[10-3]
Mean VLF-TD at
Uo
[10-3]
Further Study
Advised
* If it is not possible to
definitively identify a Filled
Insulation
0.1 to 1.3
o
r
5 to 100
or
35 to 120
Carbon-filled (Black) EPR 0.1 to 2.7 2 to 120 20 to 100
Mineral-filled (Pink) EPR 0.1 to 1 4 to 120 20 to 100
** Discharge resistant EPR 0.1 to 1 6 to 10 100 to 350
** Mineral-filled XLPE - - 100 to 350
Action
Required
* If it is not possible to
definitively identify a Filled
Insulation
>1.3
o
r
>100
or
>120
Carbon-filled (Black) EPR >2.7 >120 >100
Mineral-filled (Pink) EPR >1 >120 >100
** Discharge resistant EPR >1 >10 >350
** Mineral-filled XLPE - - >350

44. If high values of TDTS, DTD and or TDM
• 1. Can compare results between different phases of the same
segment or sequential sections to better place the result in context
• 2. Can divide circuits into sub sections and retest, perform a visual
inspection of circuit components where accessible and replace
suspect parts, or replace the accessories, especially if they appear
to be old, and re-measure
• 3. Can perform additional testing in the form of a monitored
withstand, non-monitored withstand, or partial discharge test
should they wish to identify a localized problem
• 4. Can separate the response of terminations and other
components if connected from cables plus splices, by, if practical,
adding guard circuits at the terminations

45. VLF testing - advantages
• Inexpensive compared to AC HiPot testing
• Power supply light in weight
• Simple withstand test easy to perform
• Non-expert test
• IEEE Std 400.2™ gives test voltages
• Effective to measure water tree degradation
• Used for PE and EPR based, and OIP cables
• Monitored withstand testing can measure
tangent delta, insulation resistance and partial
discharge characteristics → better assessment of
cable insulation by picking out defects that do not
fail during the test
•

46. VLF testing - disadvantages
• Cable systems must be taken out of service for
testing
• Possible issues with charge injection and trapped
charge when testing at frequencies less than 0.1 Hz
• Two voltage waveforms available, sinusoidal and
cosine - rectangular
• Not straight forward to interpret PD data acquired
during VLF conditions
• When testing cables with extensive insulation
degradation, simple VLF withstand testing can result
in repeated failures, although this rarely occurs in
practice
• Simple withstand testing does not monitor the effect
of the test on the cable during the voltage
application and can fail to detect a potentially
destructive defect

47. Recent work - effect of test frequency
• Long lengths tested at <0.1 Hz due to power
limitation
• Should lower frequency be tested longer?
• NEETRAC grew WT in PE & EPR and subjected
samples to BD tests at 0.1 Hz and 0.05 Hz
• BD strength was lower at 0.05 Hz (6.7 kV/mm)
than at 0.1 Hz (8.5 kV/mm) – tree growth rate?
• Conclusion – no need to test for longer time at
0.05 Hz

48. Future work
• Encourage utilities to do diagnostic tests
• Encourage utilities to develop and use their own
cumulative distributions for VLF
• Retest cables to verify performance (trends)
• Re-establish Discussion Group FD03 to discuss
data
• Develop other criteria, e.g., Delta tip-up
(TD(1.5Uo) - TD(Uo)) - (TD(Uo) - TD(0.5Uo))
• Prepare standard for continuous AC test for MV

49. Conclusions
• VLF simple withstand and monitored withstand are
most commonly used diagnostic tests on MV cables
• Larger utilities have a cable diagnostic program (why?)
• The majority of utilities do not do diagnostic tests on
their MV cable systems
• Testing at less than 0.1 Hz does not require a longer
test time
• Refinements to the tan delta criteria are under study
and should be incorporated in the next revision of
400.2