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2016 China International Conference on Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
Technics of Locating Underground Cable Faults
inside Conduits
Gilbert Cheung, Yuan Tian, Tobias Neier
Representative Office Hong Kong
Baur GmbH
Austria
Abstract—A cable fault can be defined as any defect,
inconsistency, weakness or non-homogeneity that
affects the performance of a cable. All faults in
underground cables are different and the success of a
cable fault location depends to a great extent on
practical aspects and the experience of the operator.
To accomplish this, it is necessary to have personnel
trained to test the cables successfully and to reduce
their malfunctions.
In many countries, many underground cables are laid
inside PVC pipes or conduits. Pin-pointing cable
faults would be much easier when the cables are
buried underground directly. However, when the
cables are laid inside a conduit, it becomes much more
difficult to pin-point the exact fault location. There
are certain ways to identify the cable fault location by
applying different technics. This paper describes the
use of different equipment and technics to locate
underground cable faults inside conduits.
Index Terms—Cable Fault Location (CFL), PVC
Conduit, PVC Pipe, Pre-location, Pin-pointing.
I. I NTRODUCTION
Power supply networks are growing continuously and
their reliability is getting more important than ever.
The complexity of the whole network comprises
numerous components that can fail and interrupt the
power supply for the end user. For most of the
worldwide operated low voltage and medium voltage
distribution lines, underground cables have been used
for many decades. Underground high voltage cables
are used more and more because they are not
influenced by weather conditions, heavy rain, storm,
snow and ice as well as pollution. Even though the
cable manufacturing technology is improving steadily,
there are still many influences which may cause cables
to fail during operation or test.
Locating
underground cable faults under direct buried cables
has been quite successful during the past. However,
for underground cables which are laid inside conduits,
it can become much more difficult to locate cable
faults. It is important to fully utilize the technics of
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locating underground cable faults inside conduits.
II. PRE-LOCATION METHODS
A. Multipule Impulse Method (SIM/MIM)
The Multiple Impulse Method is the most advanced
cable fault pre-location method available. Every
cable fault that is either a high resistive or intermittent
fault cannot be indicated by means of the Time
Domain Reflectometer (TDR) method. The low
voltage impulse sent out by the TDR is not reflected at
the faulty position, as the fault impedance compared to
the insulation impedance of the healthy part of the
cable is not significantly lower. Based on this fact,
the Multiple Impulse Method is supported by a single
high voltage impulse that is generated by the coupled
surge generator. Like this it is possible to change the
high resistive fault temporarily into a short circuit
(flash over, temporary low resistive fault condition)
and therefore can be detected by a second TDR
impulse (SIM) or multiple secondary Impulses (MIM).
The low voltage TDR impulse is coupled to the high
voltage output of the surge generator via the coupling
unit. For many years, the Secondary Impulse Method
was considered to be the most advanced method.
Problems were figured out, as faults with difficult
characteristic had to be located. Those influences
like water in a joint, oil-reflow in oil filled cables, etc.
either shorten the duration of the flash over or delayed
the ignition time of the flash. All these effects are
influences that make the timing for the triggering and
release of the secondary impulse, to reach the fault
exactly at the short time frame of arcing, very difficult.
Manual trigger delay settings had to be varied and
therefore requested the user’s skills significantly.
The method of “trial and error” results in giving extra
stress to the cable due to the continuous high voltage
impulses sent out from the surge generator.
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2016 China International Conference on Electricity Distribution (CICED 2016)
Fig.2 Measurement graph
Xi’an, 10-13 Aug, 2016
Fig.3 Example of
CFL Unit (up to
4kV) - STG600
Fig.1 SIM/MIM graph sequence, display of
automatically measured Multiple Sequence
Multiple Impulse Method (MIM) is basically a much
more advanced development of the Secondary Impulse
Method (SIM). The big advantage of MIM is that it
can monitor a wider timeframe of the fault condition
which described earlier. Therefore, no manual
trigger delay time adjustment is needed and no “trial
and error” is required any more.
B. Impulse Current Method (ICM)
The previously mentioned cable fault pre-location
methods based on a TDR impulse are generally
affected by either damping of the signal in very long
cables or reflections from the joints along the cable.
The damping influences can be caused by corrosion of
the cable sheath or any other influences in the joint.
In other words, anything which has an influence to the
length resistance of the cable can affect the damping.
In very long cables the natural damping of the cable
may cause the TDR impulse to be damped off before
returning back to the Time Domain Reflectometer.
Thus, the TDR (or SIM/MIM) method is not always
successful. Impulse Current Method (ICM) can be
applied in such conditions. When a surge generator
releases a HV impulse, it causes a flashover at the
fault point. This flashover/discharge causes a transient
current wave travelling along the cable sheath between
the surge generator and the flashover point. This
current wave will repetitively travels back and forward
from the fault point to the beginning of the cable.
The interval of this wave/pulse can then be determined
as the fault distance. For the coupling unit, an
inductive coupler (SK1D) is connected to the sheath of
the Surge Generator (SSG) cable. The Time Domain
Reflectometer (IRG 2000 or IRG 3000) can then be set
to automatic adjustments/settings and display the
graphs on the screen. Since the pulse width of the
transient current wave is very wide, ICM method is
more suitable in long cables.
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Fig.4 Example of Cable Fault Location Unit (up to
32kV) - Syscompact 2000 with IRG2000 portable and
vehicle mountable version
C. Bridge Method
Cable faults happening between two defined cores can
usually be pre-located using the reflectometry method.
Certain cable structures allow cable faults to happen
between a core to outer sheath and then to soil.
Unshielded cables such as high voltage DC cables
used in railway supply, low voltage cables, signal
cables or so called pilot cables, faults mainly happen
between the core and the surrounding soil. As the
cable ground or metal screen cannot be accessed
anymore, the theory of reflectometry is no more useful.
The reflectomerty method can only travel when there
are two parallel conductive paths. Cable sheath fault
usually means a defect in the outer protective PVC
insulation.
Cable sheath faults do not directly
influence the electrical performance of a shielded
cable, but it will have a negative effect in long term
operation of the cable. The damages of the outer
sheath enable water from the surrounding soil to
penetrate into cable. Corrosion of the cable sheath as
well as development of water trees will lead to sooner
cable breakdowns. Therefore, according to IEC
60229, protective oversheaths have to be tested and
fault repair has to be done to ensure the long term
performance of the cable. These kinds of cable faults
can only be pre-located by using a measuring bridge.
Bridge methods are basically used for pre-location of
low resistive faults. By using a high voltage source
that is integrated in the latest generation of measuring
bridge instruments, even high resistive faults can be
pre-located.
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2016 China International Conference on Electricity Distribution (CICED 2016)
Fig.5 Measuring circuit according to Murray (left)
and Glaser (right)
Xi’an, 10-13 Aug, 2016
Electromagnetic Trigger and Acoustic Trigger.
The two signals have different propagation velocity.
Further the distance to the fault influences the
difference in triggering the acoustic trigger and
electromagnetic trigger. The digital receiver
automatically converts the measured time (propagation
time) from the fault location and indicates the distance
with a digital meter. According to the meter indication,
where the distance indication is the nearest, the fault
position can be found.
By detecting both electromagnetic and acoustic signal,
the exact final location of the fault position can be
determined. This special feature increases the
performance compared to other ‘acoustic only’ pick-up
sets. Furthermore, the electromagnetic indication can
also assist in doing simple cable tracing.
Fig.6 Example of 10kV Cable and Cable Sheath
Testing and Fault Location System - Shirla
III. PIN-POINTING METHODS
A. Acoustic Method
For cables laid inside conduits, which have similar
characteristics with direct buried cables, pin-pointing
high resistive and intermittent faults are often difficult.
Acoustic method can be used to pin-point the exact
fault location. As signal source, a surge generator is
used in repetitive pulsing mode. High energy pulses
from a surge generator (SSG) create a voltage pulse to
travel along the cable. Flashover happens at the point
of the fault. These repetitive noises/sounds are
detected from the surface of the ground by using a
ground microphone, receiver and headphone. The
closer the distance from the fault to the microphone,
the higher amplitude it will detect from the flashover
noise. If it is placed straight on top of the fault position,
the highest level of flashover noise can be detected.
Using a higher energy surge generator would have a
better effect on creating higher flashover noise. The
flashover noise may vary depending on how deep the
cable conduits are buried, but the flashover noise is
usually audible even when the cable fault is inside the
conduit.
The acoustic fault location set comprising the receiver
(UL30) and the ground microphone (BM30) contain
the special feature of digital propagation time –
distance measurement.
Firstly, the ground
microphone measures the electromagnetic signal that
can be recorded all along the cable where the HV
impulses are travelling. As this signal is available all
along the cable trace towards the fault, it can further
be used to make sure that the “cable trace” is followed.
The maximum signal can confirm that the position of
the ground microphone is now directly above the cable.
Secondly, the ground microphone detects the flashover
noise coming from the fault via the ground when it
gets closer to the fault location. Therefore, every
flashover
activates
two
different
triggers,
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Fig.7 Schematic connection and shape of acoustic
signal – acoustic fault
Fig.8 Electromagnetic signal along the whole cable,
acoustic signal at point of flashover
Fig.9 Example of signal pick up set BM30 and UL30,
UL30 digital receiver indicates electromagnetic and
acoustic signals and the distance to the fault location
(e.g. 6.9 meters).
B. Step Voltage Method
Using a surge generator would not be able to create a
flashover at the fault point when the cable faults have
direct contact to the soil or ground. When there is no
acoustic signal or no flashover noise is audible, cable
fault pin-pointing using the acoustic method is not
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2016 China International Conference on Electricity Distribution (CICED 2016)
possible. This condition is mainly resulting from a
completely burnt cable fault that is furthermore
resistive to the surrounding soil. These kinds of cable
faults can then be pinpointed by using the step voltage
method.
Faults in low voltage cables as well as pilot cables
(signal lines) are often difficult to pin-point, this is
because the high voltages that applied to the cables do
not have sufficient surge energy to create a strong
audible flashover.
As these cables are mainly
unshielded, cable faults in most cases appear to have a
direct contact to the surrounding soil. The step voltage
method becomes more suitable in such case.
Another fault type showing similar conditions is the
cable sheath fault. As there is no definite potential
point, a fault at the outer protective PVC insulation of
a XLPE cable cannot be located via the acoustic
method. Accordingly, it is again difficult to create a
strong flashover noise.
The step voltage method enables the localization of the
cable sheath faults. This method also enables to locate
several sheath faults along a cable. In here, two
potential earth probes are used together with the
receiver. As soon as the earth probes are placed on
the ground along the cable trace, the potentiometer in
the receiver will point to the direction with higher
potential which is the fault point.
Follow the
direction until the potentiometer points to zero. The
fault position is then determined.
Fig.10 Step voltage method, two earth probes
connected to KMF1 Receiver
i. Pin-pointing of cable sheath faults - for cables
laid in Short PVC conduit
Cable conduits usually contain water or humidity
inside. Therefore sheath faults can be pre-located and
pin-pointed. The leakage current exits from the
nearby conduit joints and creates voltage funnels that
can be detected and located. For pin-pointing, it is
important to use low current or voltage setting of
maximum 20-30mA or 1kV for pulse output. This
enables to keep the signal only in the close area from
the fault. Too high current setting can cause voltage
funnels to appear at every conduit joint over a long
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Xi’an, 10-13 Aug, 2016
distance.
Fig.11 Example of cable fault in short PVC conduit
ii. Pin-pointing of cable sheath faults - for cables
laid in Long PVC conduit
Some cables and conduits are laid by using ‘Direct
Horizontal Drilling’ method. In this case, the length
of the conduit can range up to 100m or above. An
additional tool will be needed to pin-point the cable
fault. One of the popular ways is to use a conduit rod
combined together with the step voltage method.
Knowing the fact that cable conduits usually contain
water or humidity inside, step voltage method can be
used in a same way to find the point of leakage from
the conduit joints. This particular section has to be
excavated once the leakage point is found. Once
excavated, it is necessary to cut out an opening from
the PVC conduit. A voltage pulse generator can then
be used to inject a sequence of voltage pulses
(100-200V) into the cable sheath from one of the cable
ends. For safety reasons, it is no more required to
inject high voltage pulses and it is still necessary to
wear a pair of high voltage protection gloves during
this stage.
A conduit rod should then be prepared according to the
length of the conduit.
The receiver cables and
conduit rod should be tied together approximately
every 1-2m apart. It has to be noted that the conduit
rod and the first receiver cable are aligned in the same
position and the tip of the receiver cables are exposed.
The conduit rod can then be slid inside the PVC
conduit. While the conduit rod is sliding in, it is
required to read the potentiometer in the receiver.
The conduit rod should be slid inside until the
potentiometer points to zero. The fault position can
then be determined. The conduit rod should be
marked to make sure how much distance has been
pushed into the conduit. The conduit rod can then be
pulled out and the distance can be measured, this
should be the distance until the cable fault location.
This method should be very convenient to pin-point
cable faults inside long PVC conduit.
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2016 China International Conference on Electricity Distribution (CICED 2016)
Xi’an, 10-13 Aug, 2016
REFERENCES
[1] “Cable Fault Location in LV, MV and HV Underground Cable
Networks Practical Experience, Author: Ing. Tobias Neier Version 2,
01-2013” BAUR GmbH, Austria.
[2] “Shirla Application Guide - All in one - Fully Automatic Cable
Sheath Testing and Fault Location System, Author: Ing. Tobias
Neier Version 1, 04-2010” BAUR GmbH, Austria
[3] IEC Standards IEC 60229, Edition 3.0 2007-10 “Electric cables
– Tests on extruded oversheaths with a special protective function”
Fig.12 Application of using Receiver & Conduit Rod
in long PVC conduit
Fig.13 Example of KMF1 Receiver (left), Conduit Rod
(middle), Photo of cable fault in conduit (right)
IV. CONCLUSION
There are many ways of using different technics to
locate cable faults inside conduits. By knowing and
applying different cable fault location technics, it
would become easier and faster for the electrical
technicians to locate cable faults even when the cables
are laid inside conduits. The important points are to
understand the environmental conditions, for example
soil conditions, weather conditions, road conditions,
etc., as well as understanding the cable fault conditions.
Furthermore, using the correct equipment can shorten
the time to find the cable fault location.
ACKNOWLEDGMENT
The authors would like to give special thanks to Mr.
Chew Chin Seang from CJ Hi-Tech Sdn Bhd Malaysia
who supported us on providing useful experiences and
technics on pin-pointing cable faults in different
circumstances.
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