Sensitivity Characteristics of Electro-Optical Sensors Under AC/DC Hybrid Electric Fields in Oil Medium Environment

Abstract

The measurement of ac/dc hybrid electric fields in converter transformers is important for the verification of electric field calculation model and the insulation optimization. Electro-optical (EO) sensors have obvious advantages for measuring strong electric fields, but their response characteristics and mechanisms under the action of ac/dc hybrid electric fields are not fully understood by researchers, which restricts a wide application of sensors. In this article, we first describe an improved dual-beam structure, which enables us to measure both ac and dc electric fields in oil simultaneously. Next, the complete output characteristics of the EO sensor under ac/dc hybrid electric field are obtained through experiments. The results show that measurement sensitivities of EO sensor under dc and ac electric fields are different; there is a coupling effect in measuring ac/dc hybrid electric fields: the increase of one electric field component will reduce the sensor's measurement sensitivity to the other electric field component, which is more obvious for a strong electric field. Finally, we propose the response mechanism with theoretical analysis. First, the sensitivity difference is due to the difference of electric fields inside EO crystal under ac and dc electric fields, and it is affected by the conductivity and dielectric constant of crystal and oil medium. Second, the optical nonlinearity of EO sensors under strong electric fields is the cause of coupling effect: the sensor's nonlinear modulation of a strong ac electric field will introduce coupling errors in measurement signal of the dc electric field and a strong dc electric field will move the zero point of the sensor's ac induction curve from the linear region to the nonlinear region, causing the ac sensitivity to decrease nonlinearly with the increase of the applied dc electric field. This article provides a measurement and analysis of EO sensors under ac/dc hybrid electric fields in an oil medium environment, which is beneficial to the wide application of EO sensors.