At present, due to more uncertainty in the energy market and increasing loads in the power grids, a real-time thermal rating (RTTR) system and other measurement techniques aid the asset manager in making optimal decisions in operational and planning investments. Based on measurement, a grid operator can on the one hand decide if a hotspot in network should be removed to increase the capacity or if the hotspot should be managed with a RTTR system. This enables the Operator to understand the load and overload possibilities in real time and into the near future (few hours or days). The currently predominant technology for thermal measurement on power cables is DTS (Distributed Temperature Sensing) often used in combination with

1. a RTTR system. Both applications are discussed in
this brochure.
With the use of DTS and RTTR grid owners can
overload critical cable systems in a safe and controlled
way. Three major categories can be formed based on
the load profile of the cable system in the grid and the
complexity of the solution a grid owner wants to apply
(figure 1).
Context
In 2004, CIGRE issued a Technical Brochure on the
subject (TB 247). In the meantime, the technology has
developed and matured and significant experience has
been gained by users and manufacturers of DTS and
RTTR systems and a replacement of the Technical
Brochure 247 was agreed upon by the CIGRE B1
Study Committee. The part of TB 247 dealing with
the point sensor is not discussed in this brochure. For
completeness, however, this information is included in
Appendix.
Technologies
The brochure starts with an introduction that presents
its scope, the definitions, the acronyms and a
literature review. The objective of the literature review
is to give an overview of experience and applications
of DTS and RTTR systems. There is considerable
amount of published literature about DTS technology
in scientific journals such as Journal of Physics and
IEEE publications. Because of the huge amount
of publications, the authors of this report had to
make a choice of how much of the information is •••
Introduction
At present, due to more uncertainty in the energy
market and increasing loads in the power grids, a
real-time thermal rating (RTTR) system and other
measurement techniques aid the asset manager in
making optimal decisions in operational and planning
investments. Based on measurement, a grid operator
can on the one hand decide if a hotspot in network
should be removed to increase the capacity or if the
hotspot should be managed with a RTTR system.
This enables the Operator to understand the load and
overload possibilities in real time and into the near
future (few hours or days).
The currently predominant technology for thermal
measurement on power cables is DTS (Distributed
Temperature Sensing) often used in combination with
Thermal monitoring of cable circuits
and grid operators' use of dynamic
rating systems
Members
B. HENNUY, Convenor (BE) J. SMIT, Secretary (NL), P. LEEMANS (BE),
S. CHERUKUPALLI (CA), M. CABAU (FR), M. STUERMER (GE),
R. REINOSO (ES), E. CHIMI (CH), R. SVOMA (UK), L. MOLIMBI (US)
Corresponding Members
S. DAMBONE SESSA (IT), M. NAKANISHI (JP)
756WG B1.45
technical brochure
Figure 1 - Categories of strategy
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2. transmission cable corridors are discussed. Some
power cable manufacturers can provide optical fibres
in metal tubes, for integration into power cables during
manufacturing. The different options and related
challenges (advantages and disadvantages) are
presented.
In figure 2 an example is shown of an integrated
optical fibre is shown.
The optical connectors used in the context of DTS
applications, how they may be incorporated in
power cable transmission corridors and the potential
challenges that one may encounter during installation
particularly when fibres are embedded in the power
cable system are part of the chapter. The testing, ageing
mechanisms and maintenance are briefly discussed.
The following part describes the functionalities of a
DTS system, which is comprised of a laser light source,
detection electronics, computer system controlling
the electronics to inject light into an optical fibre and
detect light backscattered from a hot spot along the
cable and correlate this information with temperature
and distance from the light source. The computer is
also used to set-up, acquire, process and display
the temperature versus distance. This computer
also may be used to provide pertinent data about
hot spot location, status alarms of the electronics,
optical fibre etc. which in turn may be connected
to a communication protocol system (SCADA)
and integrated into the utility control centre. The
different technologies of a DTS system e.g. Raman,
Rayleigh and Brillouin are explained. The challenge
related to the strain influencing the measurement is
discussed. A result of a comprehensive survey among
DTS suppliers is presented. Topics like testing,
maintenance, calibration and ageing are discussed. •••
included in the review. Therefore, only articles
published in Jicable and CIGRE were considered.
There is a description of the reasons why DTS and
RTTR systems may be harnessed to allow optimum
power cable assets utilization for (normal) operation,
emergency and strategic planning purposes. The
following reasons to apply DTS technology were
identified: the need to increase (or optimize) the power
transfer through a cable corridor without incurring
significant expense to replace the cable system(s),
the need to identify hot spots along cable corridors,
establish the limit of power transfer through a cable
corridor that intersects other heat sources (such as
steam pipes, other transmission, and or distribution
circuits), unknown soil conditions or changes in soil
conditions due to seasonal changes, new underground
construction that may introduce soil changes adjacent
to existing cable assets, lack of proper information
about historical circuit classical “book” ratings and
the need to establish operating power transfer
limits, the need to verify thermal calculation models,
confirmation of the calculated circuit ratings with “as-
installed” condition and to monitor the submarine
cables and sudden changes in temperature caused by
any potential exposure post installation of buried cable
due to scouring of the seabottom.
A specific chapter introduces the types of fibres and
fibre connections (splices or connectors) available in
the industry for DTS applications. Typically for DTS
applications either SM (single mode) or MM (multi
mode) fibres are the most commonly used. SM fibres
are used where temperatures have to be measured
over long distances (to a maximum of about 70 km).
MM fibres can be used when the measuring distances
are less than 30 to 40 km. Different methods of
incorporating DTS sensing optical fibres in power
Figure 2 - Optical fibres integrated in the power cable between the metal wire screen
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3. thermal parameters of cable and surroundings are
based on the assumption that these parameters are
independent of temperature. However in case of
drying out the soil around the cable and in extreme
case of thermal runaway in the insulation the thermal
parameters change. This chapter ends with the
results of the survey send out by the WG to different
RTTR manufacturers, the differences in analyses,
data stream management, input and output needs
and how this may be integrated with a DTS system.
In figure 3 a schematic representation of the DTS and
RTTR system is shown as working principle of the
overall system.
Applications
Since the RTTR application is installed on a server
the installation and integration process of such a
server and the secure communication between
all components (DTS, RTTR, software) is also
important. In this chapter the relevant computer
parameters are discussed: Installation parameters,
Communication network, IT-security, remote
access and Software updates and upgrades.
A chapter of the Technical Brochure is devoted to
the operation and maintenance issues related to
both DTS and RTTR systems including some of
the challenges with obsolescence of technology
(particularly computers) and how control software
needs frequent upgrading, depending on firmware
operating system upgrades, which is common to
many software packages. The DTS and RTTR
operation, maintenance and training are described.
This allows the reader to better understand which
actions he has to perform in order to maintain
the whole system reliable during its operational
lifetime. •••
Real-time rating systems (RTTR) are described
and detailed in a dedicated chapter. The DTS
measurement system provides the raw data (=
temperature profiles) about temperature over the
entire route of the HV cable system. The measured
temperature of the DTS can be transferred to the
RTTR system: software package which calculates,
taking into account the thermal model for the cable
system, the corresponding conductor temperature
over the cable route. The RTTR gives valuable
information about the overload capability of the cable
system rather than the temperature as the operator is
unaware where the temperature is being measured
along the cable corridor and how it relates power
transfer capacity. They also do not know the impact
surrounding soil may have on the cable’s temperature
and thus are unsure about the relationship between
the maximum load current and the DTS temperature.
In general, the RTTR system will use thermal
equivalent circuits for the monitored cable system
based on the IEC 60287 and IEC 60853 standards.
The complete cable link is divided in different thermal
zones, as many as required, and for each thermal
zone the equivalent thermal model will be assigned
for the calculations. The RTTR system will use the
maximum calculated temperature of a thermal zone
to define the maximum ampacity for that particular
zone. The most critical hot-spot of the entire cable
defines the minimum ampacity of all zones and is
the limiting parameter for the load of the entire cable
link. The most common cable parameters to build a
thermal model are the detailed design of the cable,
the thermal characteristics of the controlled backfill
around the cables (thermal resistivity), detailed
drawings of the cable route with exact information of
special crossings via horizontal directional drilling,
road crossings, tunnels, dimensions of duct banks,
position and installation of the optical fibre used for
temperature measurements. The calculation of the
Figure 3 - Example of possible schematics showing working principle of RTTR
system and all related components
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4. measured parameters e.g. moisture or resistivity of
the soil is presented. It also discusses monitoring
devices more and more space saving and cost
saving. The development of prognostic algorithms
which improve the reliability of the measurements
and, consequently, of the cable power transmission
systems is presented. Some new techniques, such
as the intermodal interferometric one, are studied and
documented. Some studies describe the possibility
to measure not only the temperature but also partial
discharges by means of optical fibres. Another point of
attention is the supply system. 
Case studies and user experiences are then presented.
This section of the Technical Brochure includes the
results from a survey of end users. Information from19
different grid operators with a combined total of 68
unique projects with DTS and/or RTTR was received.
From this survey, it can be concluded that DTS is the
preferred way of monitoring temperature (96%), the
majority of users have fibre strapped to the power
cable. DTS/RTTR is mainly used on new power cable
systems and problems are for 79% due to hardware
issues. Six case studies are presented. The first one
is a case with a congestion issue on an onshore cable.
The use of a DTS allows to avoid an unnecessary
reduction of the load during congestion periods, which
occur at least 2 times per year. The second case study
reports how a submarine cable deburial was detected
by a DTS system. The third case study describes the
measurement of a hot spot, due to a leaking steam
pipe. This was discovered before the commissioning
of the cable, which allows to perform remedial work
before the commissioning (figure 4).
The fourth case study explains how a DTS
measurement on a cable partially protected by a
metallic cover allowed to refine the thermal model
of the cable system. The fifth case reports the
detection of a failure in a cross bonding box, which
led to the increase of the temperature. The last case
study describes the implementation of a real time
transmission rating on a 525 kV cable system.
Future trends
The final chapter describes the main research trends
in cable monitoring in order to give the reader an
idea of what will be the contribution of the future
monitoring technologies in improving cable system
reliability, precision and smartness. The research
activities include Different topics. The improvement
of the RTTR accuracy by increasing the number of
Figure 4 - DTS temperature trace for identify unknown hot spots
756WG B1.45
technical brochure
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