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UBC Theses and Dissertations

An integrated-optic current sensor for relaying and metering in high-voltage power systems Bull, Jeffrey David

Abstract

Optical instrumentation has attracted considerable interest over the years for high-voltage applications. Optical fibers, by virtue of their all dielectric construction, are highly resistant to electrical breakdown and electro-magnetic interference, making them ideally suited for signal transmission in substation environments. A novel hybrid current sensor for high-voltage instrumentation is developed, constructed, and tested here using a Rogowski coil and an integrated-optic Pockels cell (IOPC). The Rogowski coil generates a low-level voltage signal in proportion to the derivative of the primary current. A fully passive integrator is constructed from ultrastable components, permitting integration in the high-voltage environment without the compromised reliability associated with active components and their power supplies. A key aspect of this work is placing the integrator before the optical path to avoid amplification of low-frequency noise and drift. The penalty for using the passive integrator is high attenuation, necessitating a high sensitivity IOPC. High sensitivity IOPCs with integrated electrodes are fabricated on both Xcut and Y-cut lithium niobate substrates using titanium indiffusion. Only the X-cut configurations are able to provide the needed phase and magnitude stability because they can be fabricated without an optical buffer layer. The buffer layer is shown to be problematic due to mobile charge. IOPCs are pigtailed and packaged in an ultralow stress configuration to provide thermal stability. High current testing shows the hybrid sensor to exceed IEEE and IEC linearity standards for 0.3 and 0.2 % metering accuracy at a nominal current of 3 kA. The same sensor is also shown to achieve better than 0.5 % instantaneous accuracy when measuring transient over-currents up to 30 kA. The thermal stability of the sensor is shown to be capable of achieving 0.3 % accuracy over a temperature range from —30 to +70° C. Some of the IOPCs tested in this work exhibit a significant degree of mode conversion. The mode conversion is not intentional and results in distortion of the IOPC transfer function from its ideal sinusoidal shape. The mechanisms responsible for the mode conversion are explained by a coupled-mode formulation of the IOPC that takes into account the anisotropy caused by the electro-optic effect as well as the intrinsic anisotropy of lithium niobate. It is shown that deviations on the order of 0.5° of the propagation direction with respect to the crystallographic axes can induce 100 % mode conversion with waveguide lengths of a few centimeters. The importance of crystallographic alignment is identified and a solution to suppress mode conversion by maintaining a minimum amount of modal birefringence is proposed. Lastly, a new method for measuring birefringence is described based on mode conversion analysis. The method takes advantage of parasitic mode conversion and is non-destructive and unambiguous.

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