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.

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油介质环境交直流混合电场下电光传感器的灵敏度特性

换流变压器交直流混合电场的测量对于电场计算模型的验证和绝缘优化具有重要意义。电光（EO）传感器在测量强电场方面具有明显优势，但其在交直流混合电场作用下的响应特性和机理尚未被研究人员充分了解，限制了传感器的广泛应用。在本文中，我们首先描述了一种改进的双光束结构，它使我们能够同时测量油中的交流和直流电场。接下来，通过实验得到交直流混合电场下电光传感器的完整输出特性。结果表明，电光传感器在直流和交流电场下的测量灵敏度不同；在测量交直流混合电场时存在耦合效应：一个电场分量的增加会降低传感器对另一个电场分量的测量灵敏度，对于强电场更明显。最后，我们通过理论分析提出了响应机制。首先，灵敏度差异是由于EO晶体内部在交直流电场作用下的电场不同，受晶体和油介质的电导率和介电常数的影响。其次，强电场下EO传感器的光学非线性是耦合效应产生的原因：传感器对强交流电场的非线性调制会在直流电场的测量信号中引入耦合误差，强直流电场会使传感器交流感应曲线的零点从线性区移至非线性区，导致交流灵敏度随着施加的直流电场的增加而非线性地降低。本文提供了油介质环境中交直流混合电场下电光传感器的测量和分析，有利于电光传感器的广泛应用。