Optical Phase Detection Method for Measurements and Calibration of Pockels Cell-Based Sensors

Abstract

Nowadays, the most successful optical technique developed for measuring high voltages in 50-/60-Hz electric power lines utilizes the bulk lithium niobate Pockels cell, constituting the so-called optical voltage sensor (OVS). However, unlike the area of vibrometry, where there are ISO standards based on reliable and simple interferometric methods for calibrating the sensors, there is still no standardized procedure for measuring V-pi (half-wave voltage) of Pockels cells or for calibrating OVSs. As the Pockels cell-based OVSs can be considered as a polarimetric interferometer, and inspired by the ISO 16063-41 standard (for the calibration of vibration and shock transducers), specifically the technique called signal coincidence method (SCM), this work presents a new digital and real-time method for optical phase detection tailored for the measurement of V-pi in OVSs. Measurements were made in order to demonstrate the effectiveness of the new technique by applying a voltage signal to the OVS, composed by the superposition of a 60-Hz sinusoidal voltage, with amplitude equal to the reference value of V-pi (4.068 kV in this case) and a dc voltage high enough to provide a 90 degrees bias static phase shift, as specified by SCM, proving that the method can recover the value of V-pi in accordance with the estimated value. However, it is well known that the adjustment of the bias phase in a polarimetric interferometer can undergo undesired variations from time to time, due to drifts in ambient temperature and other external disturbances, taking the OVS out of its optimal operating point and thus not attending the ISO standard. As an advantage, experiments have shown that the new method is tolerant to variations in the 90 degrees bias static phase (ranging from 60 degrees to 120 degrees), as well as to variations in the amplitude of the voltage applied to the OVS, varying +/- 25% in relation to the 4.068-kV reference voltage. The V-pi values were accurately detected, with a maximum percentage error of 0.2% and, therefore, satisfying the specification of the ISO 16063 standard.