Field test of interferometric optical fiber high-voltage and current sensors

Abstract

. IntroductionOptical fiber sensors are of considerable interest to the electric power industry. Particularly attractive features as compared to conventional instrument transformers include the inherent galvanic isolation of the sensor head from ground potential, less sensitivity to electromagnetic interference, smaller size, and higher safety. Here we report on an ongoing field test of interferometric current and voltage sensors which have been integrated into the gas-insulated highvoltage switchgear (GIS) of a 220 kV high-voltage switching station. The voltage sensor exploits the converse piezoelectric effect in quartzl. The piezoelectric deformation of several crystals induced by the applied voltage is transduced to a dual-mode sensor fiber which is remotely interrogated by using 'white light" interferometry2. The current sensor makes use of the Faraday effect in an appropriately modified commercial fiber gyroscope. The test is to determine the long term performance of the sensors under field conditions with adverse environmental effects. . Voltage sensorThe gas-insulated switchgear has for each electric phase a current-canying high-voltage bus bar inside a concentric enclosure at ground potential (see Fig. 1 for a cross-section). The bus bar and the enclosure are electrically isolated from each other by SF6 gas. A fraction of the 220 kVrms voltage is capacitively coupled out and applied to the optical sensor by means of a ringshaped electrode which is concentric with the bus bar. The voltage sensor ( Fig. 1) has four disk-type quartz transducers each with a diameter of 30 mm and a thickness of 5 mm. The transducers are electrically in parallel with the voltage applied along their longitudinal axis which coincides with a 2-fold crystal axis. An ac electric field E along this direction produces a periodic modulation of the circumferential length £ of the quartz disks (converse piezoelectric effect) given by Ai!/t =-(1/2)d1 1E where the piezoelectric coefficient d11 is equal to 2.3 1 •1012 piJV. The piezoelectric deformations are transduced to an elliptical-core dual-mode sensor fiber which is wound onto the circumferential quartz surfaces with 12.2 fiber turns per transducer crystal. The sensor fiber is operated in series with another dualmode fiber which is part of the detection system and which acts as a receiving interferometer. Insensitive polarization maintaining single-mode fibers serve as optical interconnections. The fibers are joined by fusion splices that are made with a small lateral offset of the fiber cores such that the LP01-mode of the single-mode fiber couples about equally to the LP01 and even LP11 modes of the dual-mode fibers (or vice versa). The light source is a low coherent 5 mW, 780 nm multimode laser diode. The length of the sensor fiber and hence the optical path imbalance which the two modes accumulate is chosen such that the modal interference at the end of the fiber is incoherent. Fiber length and path imbalance are 6.82 m and 27. 1 mm, respectively. The imbalance corresponds to a lo...

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