05-06-2013, 12:56 PM
Cable Fault Location in LV, MV and HV underground cable networks
Cable Fault Location.pdf (Size: 5.96 MB / Downloads: 422)
Introduction
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. During the last years, also high voltage lines have been
developed to cables. To reduce the sensitivity of distribution networks to environmental influences
underground high voltage cables are used more and more. They are not influenced by weather
conditions, heavy rain, storm, snow and ice as well as pollution. Even the technology used in cable
factories is improving steadily certain influences may cause cables to fail during operation or test.
Cables have been in use for over 80years. The number of different designs as well as the variety of
cable types and accessories used in a cable network is large. The ability to determine all kind of different
faults with widely different fault characteristics is turning on the suitable measuring equipment as well as
on the operator’s skills. The right combination enables to reduce the expensive time that is running
during a cable outage to a minimum.
Cable types and their characteristics
Cable types are basically defined as low-, medium- and high voltage cables. The most common designs
of medium- and high voltage cables are shown below. According to the cable type, different
requirements to cable testing, cable fault location as well as maintenance strategy are defined.
Three-conductor cables have been in use in the lower voltage ranges. The tendency of the last years
show the shifting to single-core systems as they are lower in price, lower in weight and cheaper in
regards to repair costs. Furthermore oil impregnated or oil filled cables are used less and less, as the
environmental sustainability can not be guaranteed. Especially in industrialized countries, these cable
types have been replaced and are no more installed. On the other hand a high demand for maintenance
of those cables is given as the installed oil-insulated networks do show up a lifetime of 50 years and
more.
Cable Faults
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. The development of refined techniques in the field of high voltage testing and
diagnosis, in addition to the variety of methods for locating power cable faults, makes it imperative that
qualified and experienced engineers and service operators be employed.
In addition, it is important for the trained personnel to be thoroughly familiar with the fundamentals of
power cable design, operation and the maintenance.
The purpose of this document is therefore to be an additional support to the user manuals of the different
equipments concerning all aspects of the fault location in order to make up a volume of reference which
will hopefully be useful for operators and field engineers. The technology used and the experience that
can be shared is based on the BAUR expertise collected over more than 60 years.
The Advanced Secondary Impulse Method (SIM-MIM)
Impulses which are sent out from the Time Domain Reflectometer into a cable show no reflection at high
impedance cable faults.
Therefore the positive reflection of the far cable end is detected. In a second step the fault is ignited by a
single high voltage pulse or DC voltage of a surge generator and the discharge shows up as an arc at the
faulty spot. Exactly at the time of arcing (short circuit condition) a second measuring pulse sequence is
sent from the Time Domain Reflectometer into the cable which is reflected from the arc with negative
polarity because the arc is low resistive.
The modern Time Domain Reflectometers (IRG 2000 and IRG 3000) are using a 200MHz transient
recorder and send out 5 low voltage impulses considered as the Multiple Impulse MIM (compared to
one single secondary impulse SIM) which are reflected at the faulty spot and are recorded individually.
The effect of this Multiple Impulse Method is that on one single high voltage impulse, 5 faulty graphs
recorded in a sequence are shown.
Differential Impulse Current Method / Differential Decay Method
Basically for these methods one healthy auxiliary core is required. The coupling unit for both, the
Differential Impulse Current Method and the Differential Decay Method the 3phased surge coil SK3D is
used as a coupler. As the HV need to be applied to two cores simultaneously, these methods are used
only in combination with 3phased cable test vans. The coupling coil is mounted, so that all three cores
L1, L2 and L3 are contacting the triangular shaped 3phased coupler on one edge each. Independent
which two phases are selected, the remaining signal recorded is the differential signal coupled.
In a first step the HV impulse is released into the healthy core and the faulty core simultaneously. The
recorded signal will show up the first differential picture.
In a second step, at the cable end, the two cores are linked together. Therefore the effective length of
the healthy core is extended with the length from the far end to the faulty position in the faulty core. As
this reflection characteristic is now different compared to the open end in the first step, the impulse is
reflected differently, whereas the reflection taking place in the faulty core stays the same.
Due to physical extension of the healthy core in the second step, also the resulting differential picture
appears different. Laying both graphs on top of each other the deviation point that is influenced by the
extension of the healthy core to the fault, shows the faulty distance from the far end.
Burn Down Technique
High resistance cable faults lead to very small or even no impedance
changes at the faulty spot. Therefore the pulse reflection method TDR
is not suitable for location of this fault type.
For many years powerful burn down units had been used successfully
for treatment of high resistance cable faults in paper mass
impregnated cable (PILC).
The high voltage burn down unit treats the fault by forcing a high
current and carbonizes the insulation material. This carbon link is
changing the fault to become low resistive and therefore can be
prelocated with a Time Domain Reflectometer IRG according to the
pulse reflection method TDR.