## 3 Development of the DOFS Integrated Power Transformer and Platform Setup
### Pre-Experiments  
To ensure that the optical fiber works stably under the high temperature environment of transformer and has good compatibility with transformer oil, in our former series work [31], the safety test was performed through the accelerated thermal aging method (the optical fiber was immersed in the transformer oil at 130 \({}^{\circ}\)C for 576 h) which can be considered as equal to the fiber working continuously for 21 years under the normal operation of transformers according to IEC standards [13]. Tetrafluoroethylene (ETFE) and polyimide (PI) were finally selected as the optical fiber sheath and coating layer material due to their stable performance after the long-term aging process in the insulating oil. Meanwhile, the electric performance of the selected optical fiber was also qualified for the actual operating of a transformer due to its good insulation properties [11]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Power Transformer Prototype** & \multicolumn{2}{c}{**DOFS**} \\ \hline Rated capacity & 200 kVA & Temperature resolution & \textless{}0.5 \({}^{\circ}\)C \\ \hline Rated voltage & 35 kV/400 V & Temperature accuracy & \textless{}1.0 \({}^{\circ}\)C \\ \hline Rated current & 3.3 A/288.7 A & Positioning accuracy & 80 cm \\ \hline Core type & Shell & Operating temperature range & \textasci{}30–270 \({}^{\circ}\)C \\ \hline Cooling method & ONAN & Response time & 2–10 s \\ \hline \multirow{2}{*}{HV winding size} & Height: 465 mm & Fiber type & multimode fiber \\ \cline{2-4}  & Width: 310 mm & Fiber size & 50/125/250/900 \(\mu\)m \({}^{1}\) \\ \hline \multirow{2}{*}{LV winding size} & Height: 460 mm & Fiber material (shealth/coating) & ETFE/PI \\ \cline{2-4}  & Width: 186 mm & Effective monitoring range & 2 km \\ \hline \end{tabular} \({}^{1}\) The size is fiber core/cladding/coating layer/sheath in external diameter.  
\end{table} 
Table 1: Specifications of the transformer and the distributed optical fiber sensor (DOFS). To furtherly test the hot region measuring accuracy of the as designed DOFS composite transformer windings, a temperature-rise experiment was performed on an assembled winding. The schematic diagram of the test platform is shown in Figure 4.  
The distributed optical fiber sensor was fixed upon the outermost turn of winding with insulating paper, while the inner side of the winding wire was closely adhered to a heating tape to realize the temperature controlling. The heating tape adopted multi-point sampling through thermocouples to ensure that the temperature control error would reach 0.1 \({}^{\circ}\)C. The hot region detecting accuracy was explored by heating the discontinuous winding turns to different stable states (55 \({}^{\circ}\)C to 75 \({}^{\circ}\)C every 5 \({}^{\circ}\)C a state) and the measuring results are shown in Figure 5.  
The experimental test shows that the as designed DOFS composite winding has a spatial resolution of 0.8 m (each hot region is 3.2 m wide and consists of four sampling points) and a temperature accuracy less than 0.3 \({}^{\circ}\)C. For the continuously winded windings inside the transformer, this accuracy is sufficient to locate the exact local overheating turn. Meanwhile, there is no oil passages between each adjacent wire in a layered structure winding, thus, the temperature along windings will be uniform and continuous. In the case of densely winded DOFS, massive data can be hereby obtained and interpolation method can be utilized to estimate the temperature where located under the spatial resolution. The commercial product has already integrated a section of calibrating optical fiber inside and the reference temperature is stable according to the manufacturer. However, the equipment was also calibrated before the experiment. The temperature calibration was conducted through the thermostatic  
Figure 4: Schematic graph of the test platform. Figure 5: Hot region detecting accuracy test result.