Power Cable Monitoring In A Tunnel

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Overhead Powerline – Strain & Temperature Monitoring
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In conjunction with Olex Australia – Sensornet monitored a 132kV cable within a tunnel installation using the Sentinel-DTS. The purpose of the DTS monitoring was to provide information on the temperature distribution of the cable along the tunnel, and on the behaviour of the cables when subjected to normal daily load cycles.
This case study describes the system set-up and provides examples of the temperatures along the full length of the tunnel and shows how the temperature of the cable varies throughout a full load cycle (i.e. a 24 hour period) thus providing information on the effectiveness of the tunnel ventilation system.

The key period of interest for monitoring the cable is during the peak loading hours –as this is when the cable is at it’s hottest and can potentially exceed the rating. This is also the most critical period for the utility company from an economic point of view, as it is the period at which the price of electricity is as the highest. It is therefore essential the operator is able to optimize the amount of electricity throughout the network and thus achieve the maximum return from the asset.


The DTS was setup inside the control room within the substation and the sensor cable ran through a switch room into the tunnel area (which was 750m in length)


The power cables were mounted on the side of the tunnel in a trefoil configuration and the fibre was installed inside the power cable (in the shielding layer) during cable manufacture.


The reason for installing the fibre inside the power cable is that it is closer to the conducting core and so provides a more accurate measure of the core temperature (than vs. attaching the fibre sensor to the outside of the cable).



The distributed temperature nature of the Sentinel DTS enables the user to obtain a measurement point every 1m along the length of the cable. Shown below is one of the temperature traces obtained by the Sentinel DTS from which you can clearly see the different temperature zones along the length of the installation.

Temp_profile (1)


Below is an example of the Sensornet thermal mapping visualisation view throughout a 24 hour period. The screenshot shows length along the tunnel along the vertical axis and the time during a 24 hour period along the horizontal axis. The colour of the data points is proportional to the temperature – with red representing the hottest zones (in this case 23.6°C and blue the coldest (19.4°C). This type of thermal mapping is a very effective way to qualitatively analyse large amounts of data.



As can be seen from the thermal map, there is a distinct temperature cycle throughout the 24 hours corresponding with the load cycle, with the hottest temperature along the length of the cable corresponding to about 16:00 in the afternoon. You can also see that there is a hotspot in the tunnel in the section between 150 and 250m along the length of the tunnel. The absolute temperatures are still low with relation to the cables thermal rating (90°C) – as the actual loading on the cables was quite light – but this hotter zone indicates that the ventilation at this point in the tunnel is not as effective as it could be and that at higher current loading levels this area is a potential trouble spot and should be monitored carefully.

In order to prevent over-heating occurring, Alarms can be set up within the Sentinel DTS and the Sentinel’s output can be linked to the ventilation system. Thus in the event that a warmer region starts to develop then the Sentinel can automatically increase the speed of the relevant ventilation fan and dissipate the heat effectively.