Effects of AC modulation frequency and amplitude on kerr electro-optic field mapping measurements in transformer oil

Kerr electro-optic field mapping technique with ac modulation is used in this work to measure the electric field distributions between parallel plate copper electrodes in transformer oil. The ac modulation method maximizes measurement sensitivity for small Kerr constant material like transformer oil. The measurements are conducted with ac peak-to-peak voltages set to various percentages of the dc voltages 1 kV-18 kV, with modulation frequencies in the range of 10 Hz-10 kHz, and at 3 different positions between the parallel plate electrodes with 2 mm spacing. When the dc voltage is low, there exist a wide range of modulation frequencies and amplitudes that ensure the accuracy of the measured results. When there is insignificant space charge effect a modulation frequency as low as 10 Hz and ac peak-to-peak voltage comparable to the dc voltage can still yield data of high accuracy. Some unreliable measurements occur at relatively low modulation frequencies and high ratios of ac to dc voltages when higher dc high voltage is applied, which establishes a space charge distribution in the gap which is disturbed by the ac modulation voltage. To increase the accuracy and reliability of the measurements the modulation frequency is increased and the applied ac voltage is made as small as possible. It is shown that an increasingly limited range of modulation frequencies and ac voltages is allowed for accurate measurements as the dc high voltage is increased. Although a photo diode and a lock-in amplifier are used in the measurements as light detector, this work identifies the requirements on key parameters, such as maximum frame rate and sensitivity, when upgrading to modern CCD cameras in the future work.