The deterioration of winding defects is one of the important causes of power transformer fires and even explosion failures. The change of leakage magnetic field distribution is the most direct response to winding defects. Currently there are few sensors suitable for online measurement of the internal magnetic field of transformers. Based on the Faraday magneto-optical effect, a magnetic field sensor with wide range and high sensitivity is proposed in this paper, which is suitable for the interior use of transformers. The straight-through optical structure with interior polarizer is adopted, and the sensor has a measurement range of 1.5 T and a sensitivity of 1 mT. It also possesses a small size, with a length of about 30 mm after encapsulation. The influence mechanism of vibration and temperature is revealed through theoretical analysis and numerical simulation. It is proposed to filter out the interference of vibration by characteristic frequency analysis and to compensate for temperature by a two-probe structure. An anti-interference test verifies the effectiveness of this method, and it can reduce the error from 80.56% to 2.63% under the combined interference of vibration and temperature.

Pressure monitoring of a transformer oil tank can grasp the pressure change process caused by gas production when severe internal defects occur and take timely measures to ensure the safe operation of the transformer. Existing pressure sensors generally use metal encapsulation or have an air cavity structure, threatening the transformer’s insulation if it is directly used inside the transformer. To this end, this paper proposes a method for developing a high-sensitivity, large-range, and metallization-free optical pressure sensing device with temperature compensation. Fiber grating is encapsulated by fluorosilicone rubber and supplemented by an epoxy resin shielding shell on the outside. At the same time, a double-grating vertical arrangement is adopted to improve pressure measurement sensitivity, further avoiding the influence of temperature rise caused by a defect of the transformer on the measurement result of the sensor. In addition, by optimizing the geometric structure of the internal sensitizing element, pre-stretching length of the fiber grating, gap distance, and other parameters, probe size can be reduced while ensuring the sensor’s performance. Results show the proposed method can meet the requirements of sensor fabrication with different sensitivities and ranges, and to a certain extent, both high sensitivity and extensive ranges can be taken into account. The sensitivity of the fabricated prototype is 15 pm/kPa, and the range is about 0.2 MPa. At the same time, the metal-free feature of the sensor makes it suitable for use in various oil-immersed power equipment. It records oil pressure changes caused by oil discharge breakdown, making it sensitive to small pressure changes in early failures.