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EuroWire – November 2007
76
english
Light travelling down such a fibre will be
partially reflected at the index variations
but only for a small range of wavelengths,
where constructive interference occurs,
the light will be reflected (
Figure 2
).
The maximum wavelength of the reflected
light is the so-called Bragg wavelength:
λB =2•Λ•n
eff
(1)
where Λ is the grating’s period and n
eff
is
the effective refractive index.
From the equation
(1)
it can be deduced
that λB is affected by any variation of the
grating caused by external influences:
strain on the fibre causes changes in
both parameters via the elasto-optic
effect while temperature alters n
eff
via the
thermo-optic effect.
An example for a wavelength shift caused
by temperature changes is given in
Figure 3
. These dependences are used
to manufacture very small but highly
reliable and precise sensors for strain and
temperature
[4,5]
.
2.3 System Components
The following chapters describe in detail
the different components of the complete
system.
2.3.1 Jumper cable with sensor
The FBG sensor used for the temperature
measurements consists of the FBG itself
protected by a 1.5mm diameter stainless
steel tube sealed at both ends.
The outgoing fibre is protected by an
ordinary plastic tube. The length of the
steel tube housing depends on the jumper
cable length and ranges from 1.5m to 3m.
To use the sensor efficiently it has to
be placed into the core of the jumper
cable which is generally of the same type
as the phase conductor. In case of the
presented system, the phase conductor
was a steel/aluminium design with a
steel cross section of 39.5mm
2
and an
aluminium cross section of 243.1mm
2
.
Its designation according to EN 50182
[6]
is 243-AL1/39-ST1A.
Figure 4
shows
the cross-sectional view including the
FBG sensor.
Another possible way of creating a jumper
with an FBG sensor is the use of an OPPC
with steel tube design. The sensor can
then be placed into the steel tube. In that
case, the OPPC design has to be as close
as possible to the design of the phase
conductor to avoid a correlation mismatch
between the conductor and the jumper.
2.3.2 Strain sensor
The sensor for strain, as already mentioned,
is also using FBG sensor technology but is
specifically adopted to its main task: strain
measurement. It comes in a rectangular
shaped housing and is attached to a clevis
strap (
Figure 5
).
The existing configuration for the chosen
line was using two parallel insulators for
anchoring the phase conductor. Therefore,
two of the sensors were necessary.
2.3.3 Separator
For an ordinary power line, the jumper
cable is used to bridge the gap between
the ends of two phase conductors at a
tension tower. It stays on the same high
electrical potential as the conductors and
transports the same electrical current.
The idea of using a sensor in the jumper
raises two questions:
• How is the sensor’s fibre end coming
down to ground potential?
• Can an uninterrupted current flow be
ensured while exiting the sensor’s fibre
end?
The answer to both questions is simple:
Using a specially designed separator,
a so-called T-branch type. Separators are
normally used to terminate OPPC lines
with one cable entry at the ‘hot’ part.
Adding a second entry opposite to the first
one results in a T-branch type (
Figure 6
).
A T-branch separator splits the jumper
cable into two pieces with two ends
allowing for the sensor fibre to exit.
Optionally, a second sensor can be used in
the other jumper half.
Contrary to the separators for OPPC,
the splicing of the sensor fibres to the
connecting optical fibre cable can be done
on the grounded side of the separator,
easing the assembly procedure.
Figure 3
:
Bragg wavelength shift caused by temperature changes
▲
Figure 4
:
Cross section of 243-AL1/39-ST1A jumper cable including FBG sensor
▼