Study On Sensing Characteristics Of Optical Transformer Based On Radial Polarization Detection

Optical transformers based on optical sensing technology are in line with the development direction of smart grid and have good application prospects owing to the advantages of safety,environmental protection,outstanding electrical isolation,anti-electromagnetic interference,fast response,wide bandwidth,and digital signal.However,most of the conventional optical transformers are adopting light intensity detection mode,which can't realize the direct and linear measurement of current and voltage,and there are some problems such as small measurement range,optical power correlation,susceptibility to linear birefringence,and so on,which reduce the stability of optical transformers for long operation and seriously restrict the practical application of optical transformers.In view of the shortcomings of the existing detection mode,a new polarization detection scheme is presented in this paper.In this scheme,the polarization plane rotation of a polarized light is transformed into the synchronous rotation of a circular facula by the new designed radial polarization grating,and the direct and linear measurement of current and voltage can be realized by positioning the facula.Therefore the measurement range is greatly improved,the measurement result is not influenced by optical power fluctuation,and the problem of linear birefringence is basically solved.Based on the rigorous coupled wave theory and Jones matrix,the mathematical model of radial polarization grating is established and the principle of polarization detection is analyzed.The structural parameters of the grating are optimized and its optical characteristics are verified by experiments.Based on this radial polarization detection scheme,the new optical current transformer(OCT)and the new optical voltage transformer(OVT)are constructed.The magnetic concentration type sensor head of direct optical path is proposed for the new OCT,in which two magnetized plates with a hole in them are used to lead out the magnetic field of air gap,a magneto-optical film is placed between the magnetized plates,and the reflecting prism is not required,thus eliminating the reflection phase delay and the magnetic field distribution more uniform.An image converter is used to convert the circular facula into a linear one,and the current can be obtained by calculating the amount of stripe displacement.A 110kV prototype with the rated current 300A is completed,and its experimental results show that this new OCT can achieve a good linear measurement of Faraday rotation angle in the range of ± 45° with the measurement accuracy meeting 0.5 class,and the influence of linear birefringence is eliminated basically.In the new OVT,the voltage value is obtained by positioning the dark stripe center of the circular facula.The intrinsic birefringence and temperature characteristics of the electro-optic crystal are studied quantitatively by the interference fringes method based on a crystal wedge.The actual retardance of the quarter-wave plate is measured by simulation analysis method.The model of 110kV prototype is established.The experiments of BGO and LN are carried out at different wavelengths.The results show that this new OVT can achieve a good linear measurement of the electro-optic phase delay in the range of 360° with the measurement accuracy meeting 0.5 class,and has no limits of the half-wave voltage of crystal.Besides,the linear birefringence is greatly reduced.Finally,an image detection method based on four-quadrant detector is proposed.The experimental results show that the measured range of the rotation angle of the facula is ± 40°,and the measurement accuracy of the new OCT is improved to be 0.2S class,while that of the new OVT 0.2 class,making the optical transformer based on the radial polarization detection meet the engineering requirements.