IEEE Power & Energy Magazine - March/April 2020 - 46

Another way to improve grid operation is to reduce the asset
downtime that results from failures (with a duration of days to weeks)
and periodic maintenance actions that require de-energization.
4G (for use cases requiring a broader bandwidth). Powering
those systems generally required batteries whose lifespan was
only a few months. Adding energy-harvesting solutions such
as solar panels enhanced the systems' lifespan, but placing
those devices on a tower risked affecting the structure's integrity by adding weight, size, wind resistance, and additional
maintenance requirements.
Then appeared Internet of Things (IoT) solutions, promising to break this technological lock. In 2017, a two-year "sandbox" project was initiated to facilitate the experimentation of
monitoring use cases incorporating IoT solutions: in reference
to the place where little children safely learn how to play and
walk, the sandbox is a place and an organization that allows
for safe testing, failing, retrying, and thus learning.
The evaluations addressed the technical and value chain
from the connected object to the business use by visualizing
and correlating the data that were generated. The conclusions provided by the project helped design the company's
industrialized architecture and methods for IoT.
Powering Equipment Installed on OHL Towers

Electric transmission system towers are widely spread throughout
the country. Monitoring them and their environment, or using
them to support smart systems, might provide opportunities
for efficiencies, for instance, in maintenance planning. But the
energy to power smart devices is not easily available on towers.

Topsoil
Backfill
1.5 m

Warning Net

HDPE Duct
for EarthContinuity
Conductor

HDPE Duct
for OpticalFiber Cable

Power
Cables

HDPE Ducts
for Power
Cables
0.5 m

figure 2. The solution when the fiber-optic cable is
inserted in separate ducts close to the three-phase power
cables. HDPE: high-density polyethylene.
46

ieee power & energy magazine

However, today's microprocessors and microcontrollers need,
at most, a few watts to provide IoT connectivity and embedded
intelligence. RTE's R&D Department works with its partners
to design solutions that are easy to install, maintain, and use,
while having the least possible adverse impact on the tower
structures. These solutions will supply the few watts needed by
smart devices to sense and communicate on IoT networks and
be part of the so-called fog and edge computing of the TSO.

RTE's First Super-Digital
Underground Link
In a fashion similar to OHLs, underground links can ensure
greater flexibility for grid operation. The historical way by
which the company rates capacity for underground cable
systems assumes continuous loading at full capacity and
establishes the resulting impact on the cable temperature.
However, the main difference between overhead and underground circuits is thermal inertia. It could take up to several weeks for a cable to reach its steady-state temperature
after applying a load step, depending on soil characteristics,
whereas an OHL requires no more than half an hour.
From a TSO perspective, operating at maximum capacity
for long periods rarely occurs. In particular, operators need to
respect the N-1 criteria, which concern the ability to endure
a loading increase due to the outage of other equipment. The
result of this security rule is that many power circuits may
never reach such heavy-loading situations. The steady-state
loading hypothesis for cable ratings usually included conservative assumptions about the impactful parameters' values, such
as ambient temperature and soil thermal resistivity. These considerations may lead to oversized conductors and unnecessary
extra costs.

Real-Time Data Requirement
To optimize the modeling required to operate a cable closer to
its physical rating, real-time knowledge of the cable's surroundings and loading is required. A common way to obtain this
information is by using distributed temperature sensor (DTS)
measurements. By using an optical fiber, a DTS can measure
the temperature in tens of minutes, along tens of kilometers,
with an accuracy near 1 °C and a spatial resolution close to 1 m.
For the past 20 years, optical-fiber cables were systematically installed in dedicated ducts placed in parallel to all
new underground power lines (Figure 2). Intended for communication and protection purposes, the optical fibers can
also be used for thermal monitoring. The goal is simple.
By knowing the fiber's temperature in real time, one can
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IEEE Power & Energy Magazine - March/April 2020

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