Estimating the Remaining Life of Water-Treed XLPE Cable
by VLF Voltage Withstand Tests
by Katsumi Uchida *, Yoichi Kato *, Masahiko Nakade *2, Daisuke Inoue *2,
Hiroyuki Sakakibara *3 and Hideo Tanaka *3

The authors have developed a method that uses very low frequency (VLF) voltages to estimate the remaining life of lines using 22- to 77-kV XLPE cables thathave undergone water-tree degradation. VLF testing has been considered the most effective
form of voltage withstand testing from the standpoints of the size of the test equipment, watertree detection capability, and test soundness. Breakdown tests were carried out on field-aged
cables and model cables, and the relationship between water-tree degradation and breakdown
level was investigated. A database of the results was subjected to statistical analysis, and a novel
method is proposed for determining a test voltage that makes it possible to estimate the remaining life of 22- to 77-kV XLPE cables; a suitable test voltage was determined. Based on theseresults, prototype field test equipment was built, on-site tests were carried out, and satisfactory
results were obtained.

ABSTRACT1.

INTRODUCTION

Water-treeing is typical of the processes involved in the
aging of XLPE cables, and it is known that it causes
degradation of insulation performance. The occurrence
and proliferation of these water trees interfere with theproper operation of the lines, and this has resulted in astrong need for techniques to diagnose water-tree degradation. Information on how many years of service remainin the cable is particularly essential to the stable management of power line operation.
In XLPE cables for 22- to 77-kV service, on the other
hand, there is a particular need to detect non-bridging
water trees--that is to say water trees that do not bridge
the insulation 1). In such cases the degradation signal fromthe water tree is extremely small, and such methods ofdiagnosing degradation have not been implemented onpractical lines. The authors' attention was therefore drawn
to voltage withstand testing as a method that enables both
the detection of non-bridging water trees and the estimating of the remaining life of a line.
First of all we considered the waveform of the voltage tobe used in the test from the standpoint of applicability tofield testing, and it was determined that the most effective
for water-treed cable was very low frequency (VLF) highvoltage. We then made a detailed investigation of the* Chubu Electric Power Company*2 Tokyo Electric Power Company*3 Energy Transmission Dept., Ecology and Energy Laboratory

breakdown characteristics of water-treed cable, andbased on a statistical analysis of the results proposed amethod of finding the most suitable test voltage. We also
confirmed that these voltages did not have an adverse
effect on aged XLPE cables.
Based on these research results prototype on-site testing equipment was built, and field tests were carried out.

2.

SELECTING VOLTAGE WITHSTANDTEST WAVEFORM

Normally voltage withstand testing is carried out using
either commercial-frequency alternating current (AC) or
direct current (DC). These waveforms are unsuitable toon-site testing, however, for the reasons set forth below.
First of all for AC, extremely bulky equipment is required
to apply voltages greater than the operating voltage, making on-site testing impracticable. For DC, on the other
hand, the equipment for providing the voltage is simple
enough but grounding trees are easily started, raisingproblems of soundness 1). We thus realized that in on-site
voltage withstand testing it is desirable that the test equipment should:
1) be lightweight and compact;2) have defect detection capability; and3) cause no damage to the lines.
The test waveform that most fully satisfies these criteria
may be said to be the most suited to withstand voltage
testing. The authors conducted experiments regarding
items 2 and 3 above using AC and DC, which have traditionally been used for voltage withstand testing, plus VLF

65

Breakdown voltage as multiple of AC

and oscillating wave (OSW), which raise the promise of
reduced equipment size 2), 3).
First of all, to determine detection capability using thevarious waveforms, breakdown tests were carried out
using cables 6), 7) which were provided with water-tree models. Figure 1 shows the breakdown values for the waveforms as multiples of AC. As can be seen the water-tree
breakdown voltage was lowest for AC, followed by VLF
and OSW, with DC being the highest.
Next we investigated damaging effects. Cables affectedby water-treeing have water trees of varying degrees ofdegradation, and we have assumed the presence both of
those that will not cause damage to operation in theimmediate future and those that are damaging to operation. In voltage withstand tests, when operationally damaging water trees break down, it is not permissible for thecharacteristics of water trees that are not immediately
damaging to be changed. Specifically if, as exemplified bygrounding trees, electrical trees are generated from nondamaging water trees, there will be a significant drop in
cable insulation performance. From the above considerations, the breakdown tests were carried out under various
voltages using cables provided with a plurality of watertree models. Depending on the breakdown test, breakdown might occur in one water-tree model, but not in
another using the same cable. The tip of the model that
did not suffer breakdown was then examined and a check
was made as to whether electrical trees had been generated or not.
It was found that AC and VLF presented a very low possibility of electrical trees being generated, but that with DC
and OSW they were generated readily.
Based on the test results described above we classified

3

2

1

0
AC

DC

VLF

OSW

Figure 1

Comparison of AC, DC, VLF and OSW breakdown
voltages during water-tree detection

Table 1

Suitability of selected waveforms in voltage withstand tests

Waveform

Scale of

Water-tree detec-

equipmenttion capability

AC
DC
VLF
OSW
: Suitable;

66

: OK;

: Unsuitable

Soundness

the four withstand voltage test waveforms according to
how they satisfied the three criteria. Table 1 shows theresults.
AC provides high water-tree detection capability but
bulky test equipment is required to apply voltages inexcess of the operating voltage, and can thus not be
regarded a suitable as the waveform for use in voltage
withstand tests.
DC test equipment is the most lightweight and compact,
but its water-tree detection capability is inferior, and it fails
in suitability with respect to causing damage.
In the case of VLF, the test equipment can be madelighter and more compact than for AC, and it is second
only to AC in terms of water-tree detection capability. It isalso superior in terms of cable soundness, and may beconsidered suitable as the voltage withstand test waveform.
Finally, OSW is the equal of VLF in terms of the size ofthe test equipment and water-tree detection capability, butwith respect to causing damage, cannot be considered
suitable.
It was therefore concluded that VLF was the most suitable waveform for use in voltage withstand testing ofwater-treed cables.

3.

AC AND VLF BREAKDOWN VOLTAGESOF WATER-TREED XLPE CABLES

The results obtained as described in Section 2 above
demonstrated that the VLF voltage provided the bestmeans for voltage withstand testing of cables with watertree degradation. To study the conditions for VLF voltage
withstand testing we conducted a detailed investigation of
AC and VLF breakdown voltage using field-aged cables
and model cables. The AC breakdown voltage wasobtained by carrying out AC breakdown tests on modelcables and by surveying the literature 1) relating to AC preinterruption tests on 22- to 77-kV field-aged cables. The
VLF breakdown voltage, on the other hand, was obtainedby carrying out VLF breakdown tests and pre-interruption
tests on model cables and field-aged cables.
Figure 2 shows the relationship between the AC breakdown value and the residual insulation thickness. Here,residual insulation thickness is an index of water-tree
degradation, being the length of the sound portion of insulation--the cable insulation thickness minus water-tree
length. Thus a smaller value of residual insulation thickness means that water-treeing is more advanced.
The data shown in Figure 2 is for different insulation
classes ranging from 22 to 77 kV and the dotted lines
show the cable insulation thickness for each of the classes. From the same figure it may be taken that AC breakdown voltage (nominal values shown) shows dependence
on residual insulation thickness (though with some deviation) for both model cables and field-aged cables, but may
be considered to be as shown by the curves in the figure.
That is to say in the region in which water-treeing is notparticularly advanced and the residual insulation thickness

Furukawa Review, No. 20

2001

33 kV

66 kV

77 kV

Insulation breakdown voltage

22 kV

AC breakdown voltage (kV)

200

100

VLF

Test voltage

Breakdowncharacteristics
AC

VN

OperatingvoltageαResidual insulation thickness (mm)
0

5

10

Remaining life =15αwater-tree growth rate

Residual insulation thickness (mm)Field-aged cable; 22 kV; 33 kV; 66 kV; 77 kV

Figure 2

Figure 4

Model cable; 6 mmt; 11 mmt

AC breakdown (pre-interruption) voltage as a function of residual insulation thickness

VLF breakdown voltage (kV)

200

100

5

10

15

Residual insulation thickness (mm)Field-aged cable; 6 kV; 11 mmt

Figure 3

down value (shown as peak value) and residual insulation
thickness. It can be seen that for VLF too, the breakdown
value is dependent on residual insulation thickness. It canalso be seen that since the data are for regions in which
the residual insulation thickness is less than half the insulation thickness, that breakdown voltage follows the same
curve as it does for AC.
The test voltage is found on the basis of this type ofdata on the rate of water-tree growth, plus the residual
insulation thickness dependence of AC and VLF breakdown values shown in Figures 2 and 3.

4.
0

Model cable; 6 mmt

VLF breakdown (pre-interruption) voltage as a function of residual insulation thickness

in the water-treed portion is substantially the same as theinsulation thickness, the breakdown voltage drops precipitously with a reduction in residual insulation thickness.
When water-treeing becomes further advanced and theresidual voltage thickness is about 2 mm less than theinsulation thickness, the breakdown voltage continues todrop gradually.
The breakdown values in this region where the residual
insulation thickness is small followed the same characteristics curve irrespective of differences in cable insulation
thickness. This means that the breakdown voltage isdetermined by the electrical field at the tip of the defect,
and is an acceptable result. When the residual insulation
thickness becomes larger, the breakdown voltage isaffected by insulation thickness due to the effect of the
background electrical field. Since life diagnosis is targetedat cables in which the breakdown voltage has dropped
within the operating voltage region, it is thought that evenfor cables of differing insulation class, breakdown value
characteristics can be evaluated using the same curve.
Figure 3 shows the relationship between the VLF break-

Basic method of estimating remaining life

CONCEPT OF ESTIMATING CABLE LIFE
BY VOLTAGE WITHSTAND TESTS

We then considered methods of estimating the remaining
life of water-treed cables by means of VLF voltage withstand tests.
Figures 2 and 3 demonstrated the characteristic that thesmaller the residual insulation thickness (i.e., the longer
the water trees), the lower the insulation breakdown voltage will be. Figure 4 shows the relationship between insulation breakdown voltage and residual insulation thickness, illustrating the behavior whereby, as water trees
grow and residual insulation thickness is reduced, theinsulation breakdown voltage drops until breakdown
occurs at the operating voltage.
Defining α in Figure 4 as the difference between thatvalue of residual insulation thickness at which breakdown
occurs at the test voltage and that value at which breakdown occurs at the operating voltage, we may estimatethat those cases passing at the test voltage have aremaining life greater than the product of α and the rate ofwater-tree growth. Conversely the required test voltage
can be set by deciding the years of remaining life that it isdesired to find.
As can be seen from Figures 2 and 3, however, there isa deviation or dispersion in breakdown voltage. Thus thevoltage withstand characteristics shown in Figure 3 have acertain breadth, depending on the deviation. Accordingly
when aged cable is subjected to voltage withstand tests
we can expect to produce four types of test results, as
shown in Table 2, depending on whether it passes the67

VLF voltage withstand test or not, and whether breakdown
occurs at AC operating voltage within the evaluation period or not (whether it is safe or not in the immediate
future).
Under an ideal voltage withstand test, cables that cancontinue in service will pass the voltage withstand test andonly those cables incapable of further service will suffer
breakdown in the voltage withstand test. In actual practice, however, cables capable of further use may suffer
breakdown in the voltage withstand tests and cables thatpass the voltage withstand tests may break down in operation. This is because of false results from the voltage
withstand tests.
With conventional voltage withstand tests, there was anemphasis on being sure to produce breakdown in damaged cables, and where there was a dispersion in breakdown voltages, it was more common to try to be on thesafe side, by, for example, taking the minimum value. This
made sure that damaged cables were indeed withdrawn
from service but there was a strong possibility that cables
capable of further service were also withdrawn. From thestandpoint of the efficient operation of the equipment,
however, what is necessary is to reduce false results andthereby raise evaluation accuracy, rather that working on
the safe side.
Accordingly we developed a technique 7) for determining
the appropriate withstand test voltage by carrying out astatistical analysis of breakdown voltages and calculating
the probabilities that the four possible types of result
described above would be obtained at various VLF voltages.
By using this statistical analysis to find test voltages forevaluating a remaining life of three and five years forwater-treed cables of 22- to 77-kV, the voltages shown in
Table 3 were deemed suitable, with the exception that for
66- and 77-kV cables there are still aspects to be studied,so the conditions are provisional.
When the voltage withstand tests at the above test voltage were passed by quantitative evaluation using statistical analysis, there was a 97% probability that breakdown

Table 2would not occur within the estimated remaining life, suggesting that estimating the remaining life can be achieved
with sufficient accuracy.

5.

FIELD TEST EQUIPMENT AND ON-SITE
VOLTAGE WITHSTAND TESTS

To apply the results of the above studies on site, prototype field testing equipment was built and voltage withstand tests were carried out on an practical line.
Figure 5 is a photograph of the field testing equipment,
and Table 4 gives its specifications. The equipment isowned by the Chubu Electric Power Co. The VLF generator is of a 2-stage cascade design. VLF voltage withstandtests for 22/23-kV class cables are performed by Stage 1,
and for 66/77-kV class cables by Stage 2.
On-site voltage withstand tests were performed in May
1999 on 33-kV XLPE cable for Chubu Electric Power andin February 2000 on 66-kV XLPE cable for Tokyo ElectricPower. The specifications of the lines on which the voltage withstand tests were performed were as shown in
Table 5.
In all cases, voltage was applied to the line 3 phases
simultaneously, as shown in Figure 6, by an energizing
from an air sealing end on the substation side.
Figure 7 is a photo of the voltage withstand test setup.
All tests ended without breakdown, and it was estimatedthat the line had a life expectancy of three years more.
Table 6 shows the time required for the testing of 66-kV
XLPE cable. The Table separates the work done with theStage 2 VLF
Control unit

Stage 1 VLF

Four examples of VLF test results
Test result
Fail

Pass

AC operating

UnsafeAccurate

False

voltage

Safe

False

Accurate

Table 3Manual control panel

Figure 5

VLF test voltages for XLPE cables rated 22~77 kV

VLF high-voltage generator

Years of life estimatedInsulation class
3

5

22 kV

50 kVp53 kVp

33 kV

60 kVpSpecifications of VLF high-voltage generator

Specification

Stage 1

Stage 2 cascade

65 kVpVoltage generated

VLF 85 kVp

VLF 170 kVp

2 µF

1 µF

66 kV

95 kVp100 kVp

Load capacitance

77 kV

110 kVp

115 kVp

Frequency

Note: Values for 66 and 77 kV are under provisional conditions

68

Table 4Weight of equipment

0.01~0.19 Hz
Approx. 2 t

Approx. 4 t

Furukawa Review, No. 20

2001Table 5

CH

Cable lines for on-site VLF voltage-withstand tests

Cable tested

VLF voltage generator

33 kV

66 kV

27

22

Years elapsed
Energizing leadLine undertest

Triplex type; 3x150 mm2 Triplex type; 3x80 mm2

Cable type
Length of cable

Approx. 150 m

Approx. 260 m

1

2

No. of straight200 V AC
Control panel

Figure 6

Setup for on-site VLF voltage withstand test

through joints
Laying configuration
Sealing ends

Part in conduit, part direct-laid
EB-A at both ends

EB-A at both ends

(one end on tower)

VLF 60 kVp for 10 min VLF 60 kVp for 10 min*

Test conditions* 66-kV cable tested under provisional conditions

Table 6Time required for VLF voltage withstand test
Power onProcedurePreparation

Power off
Time Procedure

Time

(min)

(min)

60
Connecting lead to tower90

Calibrating voltage

20(no load test)
Measuring insulation

20

resistance

Clear-up
Total

Figure 7

VLF voltage withstand test for 66-kV XLPE cable

power turned on from that done with power off. Thus we
see that VLF voltage withstand testing allows tests to be
carried out extremely easily.
VLF voltage withstand tests require no special measurements, and no special measures against on-site noise or
the like need be taken on site. The testing method is a
simple one, so that the remaining life of the line can be
estimated with a simple pass or fail.
It is also considered from the results obtained that performing VLF voltage withstand tests on-site will present no
problems, and that the way has been cleared for practical
VLF voltage withstand testing of XLPE cables. As our
track record in on-site VLF voltage withstand testing accumulates, we propose to establish techniques for estimating the remaining life of cable lines.

VLF voltage withstand test

20

Total

150

60
120
Grand total

270 mincable soundness.
A survey was made of the AC and VLF breakdown
characteristics of both field-aged cables and modelcables, and the relationship between water-tree lengthand breakdown voltage was investigated. These data
were subjected to statistical analysis, a new method of
estimating the remaining life of 22- to 77-kV water-treed
XLPE cables was proposed, and a suitable test voltage
was determined.
Based on the results obtained, prototype field testing
equipment was built, on-site tests were carried out, andgood results were obtained. We intend to continue accumulating field data.

REFERENCES

6.

CONCLUSION

Voltage withstand tests of XLPE cables with water-tree
degradation have been carried out using very-low frequency (VLF) voltage, and we have assessed testing
methods for estimating remaining cable life. It is felt that
VLF voltage withstand testing is the most effective methodof voltage withstand testing from the standpoints of size ofthe test equipment, water-tree detection capability, and

1) High Voltage XLPE Cable Insulation Diagnosis Technology
Research Committee: Degradation Mode of High Voltage XLPE
Cable Insulation and Trends in its Diagnosis Technology, IEEJ,Technology Report No.668, 1998 (in Japanese)2) K. Uchida, S. Kobayashi, T. Kawashima, H. Tanaka, S. Sakuma,
K. Hirotsu, and H. Inoue: "STUDY ON DETECTION FOR THE
DEFFECTS OF XLPE CABLE LINES", IEEE Trans. on Power
Delivery, vol.11, No.2, pp663-668 (1996)
3) P. Gronefeld and R. von Olshausen: "TREE INCEPTION AND
PROPAGATION IN POLYETHYLENE AT 0.1 Hz AND 50 Hz AC

69

VOLTAGE", 5th International Symposium on High Voltage
Engineering, 21.10 (1987)
4) R. Bach and W. Kalkner: "Comparison of different voltage types
for the evaluation of layed medium voltage cables", 8thInternational Symposium on High Voltage engineering, 61.04(1993)
5) J. Y. Koo, J. T. Kim, S. J. Kim, I. K. Song, J. Y. Kim, and Y. O. Cho:
"A study on the oscillating wave voltage test as an after-laying testfor distribution power cables", Conf. on Electrical Insulation andDielectric Phenomena, Minneapolis, October 19-22, (1997)
6) Uchida et al.: Remaining Life Estimation of Water Treed XLPE
Cables by VLF (Very Low Frequency) Voltage Withstand Test,
Report of Wire and Cable Research Group, IEEJ, EC-99-8, (1999)(in Japanese)
7) Uchida et al.: Life Estimation of Water Treed Cables by Very Low
Frequency Withstand Test, Trans. IEEJ, Vol.120-B, No.11 (inJapanese)

Manuscript received on November 27, 2000.

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2001