Monitoring of Electric Power Network Using Opto-electronic Sensor for Measuring Electric and Magnetic Field

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A new Pockels sensor for measuring a rotating electric-field vector observed in a three-phase alternating current electric power network was developed by utilizing opto-electronics technique. The sensor consists of a light emitting diode, optical fibers, polarizers, a quarter wave plate, a beam splitter, micro lenses and a BGO Pockels crystal. The sensor was successfully applied to the measurement of a rotating electric field produced in a model of three-phase ac conductor systems. In the model, three conductors connecting to a three-phase transformer was placed on the vertices of a regular triangle. A mercury relay switch was inserted between one of the conductors and the ground of the system in order to simulate a line-to-ground fault.In the Pockels sensor, a linear characteristic of its output signal against electric field was confirmed under the condition of the field intensity up to 5 kV/cm. A wide frequency band width from 10Hz to 20MHz was also confirmed. The dependence of these characteristics on the kind of light sources (LED,LD,SLD) and the kind of optical fibers (single mode, graded index, phase maintaining) was examined to select a better combination of the components. Furthermore a method for correcting the error based on the change in polarizer orientation from a specified value was proposed and its validity was experimentally tested.A sensing system in which the track of the tip of field vector is analyzed has been developed and been worked on a personal computer. By using this system it was confirmed that the track for the three-phase conductor model mentioned above showed a circular shape. When a line-to-ground fault occurs in the model, both the detection of the fault conductor and the estimation of the fault time is successfully realized by observing the track of field vector.