## 1 Introduction
Thus, the infrared detection could only provide an external overall thermal situation. In comparison, the contact measurement is a method to detect the temperature of a certain position inside the transformer by thermocouples, fluorescent fiber, fiber gratings, etc. It is simple and effective, but the location of the sensor has a strong empirical dependence and it still cannot realize the whole area temperature sensing, leaving a large monitoring blind area [11]. The remarkable achievement was a measurement performed in 2012 by [PERSON] on a 154 kV transformer during the temperature-rise test. The fluorescent fiber sensors were placed in advance on the windings for a local temperature measurement [22]. This was followed by [PERSON], who had arranged large quantities of fluorescent fiber sensors in the oil passages between each adjacent winding wire for the detection of overheated regions in a 1.5 MVA transformer [23]. However, affected by the complicated circumstances inside transformer, the different positions chosen for the sensors may lead to different results. There are also leaving numerous sensor probes and complex internal leads. Some new horizons are thereby required to minimize the monitoring blind zones. The distributed optical fiber sensing technology, developing rapidly in recent years, has gradually applied in some super projects like geo-environmental monitoring, bridge and tunnel monitoring, petroleum and gas lines monitoring, power lines monitoring, and so forth, due to its spatiotemporally continuous monitoring and excellent real-time performance. Among these, the ROTDR (Raman Optical Time Domain Reflection) technology has been successfully applied by NASA (National Aeronautics and Space Administration) and ANDRA (French National Radioactive Waste Management Agency) in the harshest environments such as a space shuttle [24] and an underground nuclear waste repository [25]. [PERSON], from Japan, has further obtained a 0.01 \({}^{\circ}\)C temperature resolution through coherent OTDR(Optical Time Domain Reflection) technology [26]. [PERSON] has applied the DOFS to a laboratory energized transformer core based on Rayleigh scattering, but its limited detecting range is hard to monitor the field power transformer [27]. In commercial products, the Sensornet ltd (Hertfordshire, British) has reached a resolution of 0.02 \({}^{\circ}\)C per meter along the sensing fiber for 45 km in each direction, and can withstand up to 700 \({}^{\circ}\)C in a corrosive situation [28]. Therefore, it exhibits great potential for DOFS (Distributed Optical Fiber Sensor) application in the electrical apparatus field. In this contribution, according to the actual structures of an oil-immersed 35 kV power transformer, different laying schemes were designed for the DOFS. Verified by the corresponding tests, the transformer prototype with built-in distributed sensing fibers was successfully developed and was up to standard for power grid operation through the ex-factory type tests. Moreover, the internal real-time online temperature of an operating power transformer was also revealed in a distributed manner. The hotspots were accurately located and continuously monitored. Assisted by numerical simulations, the actual detected data were also compared with the IEC traditional calculation results. These first-hand data may provide a solid reference for the delicate management of power transformers.