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VLF-MWT – How to apply the new way
of cable condition assessment
Jenny, M., BAUR Prüf- und Messtechnik GmbH, Sulz, Austria, m.jenny@baur.at
Gerstner, A., BAUR Prüf- und Messtechnik GmbH, Sulz, Austria, a.gerstner@baur.at
Daniels, T.A., HV Technologies, Manassas, U.S., daniels@hvtechnologies.com
Abstract
Condition-based maintenance is an important and necessary strategy for coping with today's asset
management requirements for an electric utility system operator. However, this requires an exact
knowledge of cable conditions. Testing and diagnostic methods which provide meaningful results and are
simple and economical to apply in the field are a prerequisite. The Monitored Withstand Test (MWT)
meets these requirements. The MWT consists of a combination of established methods for cable testing
and diagnostics and provides the system operators additional information on the cables condition for
optimal planning of repairs or replacements to minimize downtimes.
I.
Introduction
Operators of medium voltage networks and distribution networks worldwide are facing similar
challenges: Existing cable systems must be maintained most economically and investments in new lines
must be secured while maintaining or improving the quality of the network. Many operators today use
diagnostic procedures to resolve the conflicts in these objectives in the best manner from technical and
economic perspectives. Simple cable testing is a common method described in various IEC, IEEE,
CENELEC and other national standards. Various test levels and times are used which depend on the
voltage type (Direct Current DC, Very Low Frequency VLF, 50/60Hz). Faulty locations are forced to
breakdown by application of a test voltage higher than nominal voltage (x*Uo).
The wide acceptance of this method and the years of testing experience have also shown its limitations.
The simple “passed” or “failed” statement allows no estimation about the remaining lifetime of the cable.
This circumstance has led to a broader acceptance of cable diagnostics, which provides information on the
cable's condition. As [1] indicates, VLF testing, tan-delta measurement and partial discharge
measurement have become established methods for this.
Evaluation of single measurement results and the combination of tan-delta and partial discharge (PD)
measurement provides the operator with important information about the condition of a particular cable.
Although cable diagnostics provides more relevant information for decision-making than a simple cable
test, it cannot reveal how the cable would respond to the application of an increased test voltage over a
longer period (15 minutes to an hour). Section 3 shows a practical example of how this can lead to
misinterpretations in specific cases.
Up to now, cable testing has lacked the ability to adapt the test duration to the condition of the cable and
thus reduce overstress by the increased test voltage and save time and money.
Page 1 / Jenny, Gerstner, Daniels
To avoid the disadvantages of these individual methods, the National Electric Energy Test, Research and
Applications Centre (NEETRAC) developed the VLF Monitored Withstand Test (MWT). A combination
of VLF cable testing and diagnostics enables the measurement limitations described to be compensated in
the best manner and significant additional value to be generated through additional information with a
flexible test period.
II.
VLF cable testing – a field proven method
VLF (Very Low Frequency) was introduced to test the insulation of Medium Voltage (MV) underground
cables after new installations, after repairs or as a routine measure at regular intervals.
It became important when it was recognized, that testing of PE/XLPE-insulated cables with DC voltages
is ineffective in detecting hidden defects in XLPE insulations. It was found, that DC testing could induce
trapped space charges in the polymeric material. After successfully passing the DC voltage test, these
cables would breakdown shortly after being re-energized. This behavioural pattern was observed for
medium voltage cables failures. [4]
The reasons for voltage testing are according to [5]:
 Detection of weak points which put reliable operation at risk using low test voltage levels
 Conversion or evolution of conductive inhomogeneous defects (water treeing) at low test levels
into first partial discharge channels (electrical treeing)
 Bringing partial-discharge defects rapidly to breakdown by means of high channel growth speeds
Figure 1 Development of electrical tree out of a water tree
By comparing different voltage sources (VLF Sinus, VLF Cos-Rect, 50/60Hz AC, Oscillating Voltage) it
was found, that especially the VLF Sinus voltage is suitable for testing medium voltage- and especially
PE/XLPE cables. The combination of a low PD incipient voltage, high channel growth speed and the
capability to perform diagnostics must be considered [5]. These are the preconditions, to convert
inhomogeneous defects and to bring partial-discharge defects rapidly to breakdown.
A typical VLF withstand test is performed with voltages between 2 and 3*Uo for the maximum time of
one hour. Due to the representation in different standards (IEC60060-3 (horizontal standard), CENELEC
HD620/HD621, IEEE 400.2) and the easy application on site, VLF cable testing became a widely adopted
method worldwide.
But voltage withstand testing has its limitations. The simple result (Pass/Fail) only offers the statement
that the cable was ready for operation or damaged at the time of testing. But it provides no estimate of
how long the cable can remain in operation nor when the next check should be performed.
That’s the reason, why diagnostic methods like tan-delta- or partial discharge measurement became more
popular in the last few years.
Page 2 / Jenny, Gerstner, Daniels
III.
VLF tan-delta diagnostics – more valuable information
The tan-delta measurement is an important extension to the simple withstand test, because more
information about the cable condition is available. This can be used, to optimize the maintenance strategy
of a utility.
The tan-delta method is an integral measurement which can be adopted for all cable types and gives a
statement about the condition of the whole cable line. Although there is no location information available,
the interpretation of various tan-delta parameters allows differentiating between different types of defects
of the cable line. These measurements allow the system operator to define follow-up measurements like
partial discharge- or cable sheath testing. With the combination of these methods it is possible, to interpret
and locate different types of defects.
For modelling, the cable insulation system is simply represented by a capacitor (representing the cable
with a perfect insulation material) and a resistor (representing the defective insulation).
Figure 2 Equivalent cable circuit
When a voltage is applied to the cable, the total current is the sum of the capacitive- (Ic) and resistive
current (IR) through the cable. (Figure 2) The measured angle δ increases with decreasing value of R,
which represents the imperfections of an insulation material.
The tan-δ is ratio between the resistor current and the capacitor current. If the resistor current is 0 due to a
perfect insulation material, the tan δ also becomes 0.
It is also possible to measure different tan-delta values at different voltage stages (e.g. 0.5, 1.0, 1.5 and
2.0*Uo)
 MTD: Mean tan-delta: Average or mean value of tan-delta values at constant test voltage
 ΔTD: Delta tan-delta: Change in tan-delta with changing test voltage
 SDTD: Stability or standard deviation of tan-delta values at constant test voltage
The measurement of these values allows an interpretation of different types.
A high MTD value is an indicator of the presence of water trees.
If the ΔTD is high (increasing TD over test voltage), this could be an indicator for partial discharges or
also for water trees. A negative ΔTD (decreasing TD over test voltage) could be an indicator for a
vaporisation effect, e.g. in terminations.
And the SDTD (stability at a voltage level) is another helpful indicator.
A low SDTD indicates that the cable is in a good condition. An increasing SDTD indicates the presence
of partial discharges. High SDTD values are an indicator for water ingress in joints.
Page 3 / Jenny, Gerstner, Daniels
IV.
The Monitored Withstand Test (MWT) – an ingenious combination
Before describing the MWT, let us examine the disadvantages of simple cable testing once again. As [2]
explains, there are essentially three disadvantages:
 No estimate of the cable line's quality can be made before the test voltage is applied.
 The duration cannot be adapted to the condition of the cable.
 No estimate can be made of how well the cable test was passed nor whether the cable will fail in
an hour or in ten years.
Combining VLF cable testing and VLF tan-delta diagnostics can avoid these limitations. It makes sense,
to perform the MWT in two stages:
 a “ramp-up”- and
 a "MWT"-or "hold" stage
A.
Ramp-up stage
Non-destructive tan-delta measurement as described before is performed prior to the actual MWT stage.
Continuous monitoring of the measurement values (mean tan-delta, tan-delta stability, delta tan-delta)
enables an initial estimation of the cable's condition to be made. As Figure 3 shows, tan-delta
measurements are performed typically at 0.5xUo, 1.0xUo and 1.5xUo.
Figure 3 Sequence of the ramp-up stage
Various tan-delta indicators are determined and evaluated at each stage:
Ramp-up stage
Indicator
Calculation
tan δ stability (SDTD)
Standard
deviation
of
6-10
measurements at Uo
delta tan δ (ΔTD)
Difference of the average values at
1.5 Uo and 0.5 Uo
mean tan δ (MTD)
Average
value
of
6-10
measurements at Uo
Table 1 Indicators during the ramp-up stage
The advantages of the ramp-up stage are apparent:
 An initial assessment of the cable line's condition is enabled.
 Excessive stress from high test voltages on aged cable lines can be avoided by an initial condition
evaluation.
 Tan-delta measurement is an established, commonly used method. Application experience and
limit values are available.
Page 4 / Jenny, Gerstner, Daniels
B.
MWT or hold stage
Cable testing and diagnostics are combined in the MWT stage.
According to [2], the MWT is only passed if
 no breakdown occurred during the MWT
 the tan-delta values determined prove to be stable (i.e. have a low standard deviation)
 the average tan-delta value is low
Figure 4 Sequence of the MWT stage
Figure 4 shows the sequence of the MWT stage. Various tan-delta measurement values are also
determined and evaluated during application of the voltage. (See Table 2.)
MWT stage
Indicator
Calculation
tan δ stability (SDTD)
Standard
deviation
of
6-10
measurements at Uo
mean tan δ (MTD)
Average value of 6-10 measurements
at Uo
Change in tan δ vs. time
The difference in the tan δ value from
(tΔTD)
0 to 10 minutes.
Table 2 Indicators during the MWT stage
Continuous evaluation of the measurement data from the ramp-up and MWT stages enables the optimum
test duration for the cable line to be determined during testing. The user can adapt the time to the cable's
condition based on the measurement results or the test system can suggest optimal test duration. In
addition to the time saved, shorter tests have the advantage of exposing the cable to the higher test voltage
only for the time actually necessary. But the user can also extend the test to cause existing weak points in
the insulation to break down.
The benefits of the MWT stage can be summarized as follows:
 The condition of the cable line can be evaluated.
 The test duration can be adjusted to the cable's condition.
 The influence of the higher test voltage on the cable can be assessed.
 MWT is a useful combination of established, accepted methods.
Page 5 / Jenny, Gerstner, Daniels
V.
Case study
Here is a practical example of why monitored withstand testing represents an important advance of
previous testing and diagnostic measurements.
The cable tested (11 kV) has a total length of 234 metres and is composed of various cable types (in other
words, a mixed cable line).
Figure 5 Structure of the cable line tested prior to the first repair in June 2010
In June 2010, there was a cable fault in an XLPE-insulated cable line produced in 1989 (first generation).
Cables produced during this period are known to develop water trees. An 11 metre section of this line was
replaced by an XLPE cable of a newer type.
Figure 6 Structure of the cable line tested after the first repair in June 2010
Diagnostic measurements (VLF tan-delta and partial discharge measurement) were performed after the
repair.
The tan-delta results showed that the cable line was heavily aged by service. (See Figure 7)
Although the measurement values were below the TD limits for mixed cable lines, for the section of line
at risk for water trees, the delta tan-delta (DTD) limit for XLPE cable was applied (DTD > 1.0E-3 as high
operating risk). Here L2 and L3 showed a strong rise with increasing voltage, indicating water tree
damage to the cable.
Figure 7 Tan-delta measurement after repair
Page 6 / Jenny, Gerstner, Daniels
The TD standard deviations (SDTD) for L1, L2 and L3 were also used to assess the situation (Figure 8).
Figure 8 SDTD – tan-delta standard deviation for conductors L1-L3
In Figure 8 it can be seen that the SDTDs for L2 and L3 increase. This indicates the presence of water
trees. Partial discharge measurement was carried out afterwards (Figure 9).
Figure 9 PD measurement result
The PD measurement data show partial discharges at the transition joints (on the PILC cable line) at 199
and 224 metres. Evaluation of the partial discharge and tan-delta measurement revealed that the high tandelta values were caused by water trees. This is indicated by higher TD standard deviations for L2 and L3
at voltages below 1.0xUo and the increasing trend of tan-delta without partial discharge. Moreover, the
partial discharge level is of an order of magnitude which does not affect the delta tan-delta.
Afterwards, a 15 minute VLF cable test was performed at 2xUo. The result was that all three conductors
passed the test despite the high tan-delta values. So the cable was put back into operation.
Four days later there was a cable fault at 125 metres, i.e. in the section endangered by water trees. Severe
water tree damage was found in this part of the line (Figure 10).
Figure 10 Cable line severely damaged by water trees
This example shows quite clearly how a VLF Monitored Withstand Test would have been helpful at this
location to avoid the cable fault shortly after restoration of service.
1) The 15 minute VLF test made the water trees more severe, but at the end of the test the progress
could not be determined. Here a VLF sinusoidal MWT would have indicated by the progression of
the mean tan-delta (rising TD values) and tan-delta standard deviation that the faults had been
exacerbated.
2) The test duration could have been extended during the measurement (to 30 minutes, for example).
The weak points (water trees in this case) would have grown worse and finally led to breakdown.
Page 7 / Jenny, Gerstner, Daniels
3) Thus the MWT could have shown the influence of the test voltage on the cable.
4) It would have been possible to estimate the "margin" of passing from the condition of the cable at
the end of the MWT.
5) Tan-delta measurement and cable testing as described in the example would have been possible in
a single automated run.
VI.
Application
It is important for the application of the VLF MWT, that the measurement is simple and automated. This
requires a VLF sine voltage, because this voltage shape allows a precise and combined tan-delta
measurement. Additionally it is possible to perform the tan-delta measurement at a constant frequency,
where limits are available and where a comparison of different measurement results is possible. This fact
allows the electric utility system operator to gain the experience with cable diagnostics.
As seen from Section V, it is useful to split the MWT into two stages: Ramp-up and MWT (or hold).
For an easy application in the field it is necessary to automate the whole measurement sequence.
An example of how these requirements can be implemented is the portable VLF truesinus® generator
with an integrated tan-delta measurement like frida TD from BAUR (Figure 9).
Figure 9 MWT application with the BAUR frida TD
Here an integrated tan-delta measurement function enables the same connection to be used for cable
testing and tan-delta diagnostics. This facilitates fully automated measurement runs without additional
external devices.
It is also important for the various measurement results to be displayed clearly and continuously so the
user can make decisions (for example, regarding the length of the MWT) during measurement.
An example can be seen in the screenshot in Figure 10. The evaluation of results is also displayed
continuously (with smileys) along with the details of individual measurement results.
Figure 10 Screen display during MWT measurement (BAUR frida TD)
Page 8 / Jenny, Gerstner, Daniels
frida TD also allows to consider surface currents in open terminations (subject to pollution, humidity and
mechanical damage) during the tan-delta measurement. These unwanted surface currents can heavily
influence the tan-delta result, especially for XLPE cables.
Figure 11 Application example: Collection and consideration of unwanted surface currents
VII.
Conclusion and outlook
The Monitored Withstand Test (MWT) is being promoted in North America and has already found a
place in various standards.
The latest revision of IEEE400-2012 [3] (the IEEE Guide for Field Test and Evaluation of the Insulation
of Shielded Power Cable System Rated 5kV and above) defines and describes the Monitored Withstand
Test.
The IEEE400.2-2004 standard (IEEE Guide for Field Testing of Shielded Power Cable Systems Using
Very Low Frequency (VLF)) is also currently undergoing revision, and MWT will play a role in it as
well.
A key factor for evaluating the condition of various cable types is comparison with defined limit values
(see the examples in Section VI as well).
Limits for various types of cable are shown in [2]. These were developed recently for the North American
region and will probably be included in the latest version of IEEE400.2.
The prerequisites for using the tan-delta MWT have been met. The first versions of the standards and the
necessary measurement technology are available. Now it is a matter of using tan-delta MWT in the field
and applying the experience from this in future discussions of limit values, also for various regions.
VIII.
Bibliography
[1] Diagnostic Testing of Underground Cable Systems (Cable Diagnostic Focused Initiative, CDFI),
December 2010
[2] Fletcher, Hampton, Hernandez, Hesse, Pearman, Perkel, Wall, Zenger: First practical utility
implementations of monitored withstand diagnostics in the USA, Jicable 11, A.10.2
[3] IEEE400-2012 IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power
Cable Systems Rated 5kV and Above
[4] S.C. Moh: Very Low Frequency Testing – it´s effectiveness in detecting hidden defects in cables.
CIRED 17th international Conference on Electricity Distribution, Barcelona 2003
[5] Bach: Testing and Diagnostic Techniques for assessing medium-voltage service aged cables and new
cable techniques for avoiding cable faults in the future
Page 9 / Jenny, Gerstner, Daniels
IX.
Vita
Martin Jenny was born in Austria in 1972 and is Product Manager for Cable
Testing & Diagnostics.
Martin is leading the product management for BAUR’s cable testing and diagnostics
product portfolio since more than four years. BAUR’s portable VLF testers were
one of the innovations that Martin drove forward in the last years. He has more than
ten years of experience in testing and measurement in different industries.
Alexander Gerstner was born in Germany in 1969 and is the Head of Global
Marketing and Product Management at Baur in Austria.
Alexander is an Electrical Engineer with more than 16 years of experience in
Product Management and Product Development for technology products in
global markets. For more than four years he is responsible for BAUR’s
innovation initiatives, Product Management and global customer
communication. His special focus is on customer value focused solution design,
User Experience, Communication and Information Technology.
Timothy “Tad” Daniels is currently the HV Sales and Marketing Manager for
HV TECHNOLOGIES Inc. in Manassas, VA. Tad has worked in the Electric
Utility Industry since 1984 with McGraw Edison, Cooper Power Systems, SPX
Transformer Solutions formerly Waukesha Electric, and Weidmann Electrical
Technology. Tad holds a BSEE from Tulane University. He is a member of the
IEEE and is active in ICC IEEE PES and IEEE Transformers Committee
Standards Groups.
Page 10 / Jenny, Gerstner, Daniels
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