EoW November 2007

english Power Line Monitoring System for Force and Temperature By Reinhard Girbig and Norbert Fink, Draka Comteq Germany GmbH & Co KG, Mönchengladbach, Germany

1. Introduction The deregulation of energy markets with its increasing numbers of wind parks and small power plants is forcing power utilities to look for new strategies in planning and operation of overhead lines. One of the strategies is the optimisation of power transmission on the existing infrastructure. For such considerations the main parameters are the temperature of the conductor and the mechanical stress of the wire. They determine the existing reserves in transmission capacity limited by the maximum allowed tem- perature of the metals and the critical sag and ground clearance. Until now, operation of overhead lines required safety margins for the temperature which is usually evaluated through almost obsolete calculation pro- cedures and assumptions.

An economical use of the reserves of an existing line is hardly possible. The presented fibre based overhead line monitoring system allows for on-line and remote measurement of the inner temperature and the mechanical stress of a conductor. The use of such a system generates a return of investment in very short time on highly loaded lines inside a power grid. High mechanical stress due to ice can also be detected and preventive measures can be taken before the towers collapse. In addition, it can verify the planning data and assumptions for the construction of grid extensions. 2. System Description 2.1 General Overview Existing temperature and force monitoring techniques for phase conductors are based either on mechanical or on optical fibre systems. The former have limited lifetime and reliability and are less accurate than optical fibre systems. Fibre systems, so far, use Raman scattering where the ratio of intensity of the Stokes and anti-Stokes line of the scattered spectrum is proportional to the temperature. For such a system [1] , usually a phase conductor has to be replaced by a complete OPPC (Optical Phase Conductor) cable length making the system expensive. To avoid the installation of a new cable, the presented system uses the correlation between the conductor temperature and temperature of the jumper cable bridging two sections of a line at a tension tower. Instead of replacing a whole cable length, only a short jumper cable housing a sensor fibre is used.

Figure 2 : Fibre Bragg Grating - Principle

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Contrary to the Raman-based fibre system, the sensor is realised as a Fibre Bragg Grating (FBG) using the thermo-optic effect to measure temperature. One end of the jumper cable is entering a separator where the sensor fibre is spliced to an ordinary fibre leading down the tower for further data transmission; the other end is connected to the phase conductor as usual. Figure 1 shows the principle of the temperature monitoring system. By adding strain sensors, also using FBG technology, and a small weather station mounted on the tower, a complete power line monitoring system has been realised. The signals from the FBG sensors can be either processed in a small unit mounted to the tower or transported to another location by an optical underground cable or an existing OPGW link. In both cases, one processing unit can handle signals from several locations. 2.2 Fibre Bragg Grating – Principle Fibre Bragg Gratings are made by creating a periodic variation in the refractive index of an optical fibre. This can be realised by irradiation of the fibre with intense UV laser light [2,3] .

Figure 1 : Temperature monitoring – Principle set-up ▲

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EuroWire – November 2007