EoW November 2007

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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 ▲

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: 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. λB =2•Λ•n eff (1)

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.

Figure 4 : Cross section of 243-AL1/39-ST1A jumper cable including FBG sensor ▼

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EuroWire – November 2007