## 2 Materials and Methods  
### Monitor Principle and Design  
In order to obtain the real-time temperature data inside the dry-type transformer, in this study, a platinum resistance dry-type transformer temperature monitor was developed. A field experiment was carried out at a transformer manufacturer in Yunnan Province, China. The monitor could acquire the dry-type transformer temperature data in real-time and make timely and correct responses to abnormal temperatures, ensuring that the transformer worked within the rated temperature range. The dry-type transformer monitor had a total of six modules, and its physical diagram and architecture are shown in Figure 1.  
Figure 1: The dry-type transformer temperature monitor and its architecture.  
The dry-type transformer used in the experiment was cooled by forced air cooling. That is, the fan was arranged under the high-voltage winding. This method could keep the temperature of the transformer stable, especially under overload conditions, making it more reliable. However, it also led to the obvious step-by-step distribution of the internal temperature of the transformer, and the temperature distribution of the lower part showed an asymmetrical phenomenon. Therefore, the monitor needed to use multiple sensors for multi-point sampling and real-time monitoring. At the same time, according to the industry standard of China JB/T 7631-2005 [18], the accuracy of the thermostat must meet 0.1 \({}^{\circ}\)C, the range is 0-270 \({}^{\circ}\)C, it has a black-box function, and it could communicate with the host computer.  
Based on the above job requirements, the temperature controller designed in this study used three-way PT1000 platinum resistance as the temperature sensor. The resistance of the thermal resistance was 1000 \(\Omega\) at 0 \({}^{\circ}\)C, and its resistance changed linearly with temperature. The four-wire structure was used to eliminate the lead resistance. These were arranged at different positions at the lower end of the high-voltage winding, and the signals were amplified and transmitted to the single-chip microcomputer through three-way operational amplifiers. Its resistance value was converted by the operational amplifier and ADC, then finally converted into specific temperature data by FPGA operation. At the same time, the monitor used five digital tubes to scan and display the monitoring temperature of each channel and could communicate with the host computer through RS485.  
Because the national standard of China GB/T 1094.11-2007 [19] stipulates that the maximum temperature rise of the transformer shall not exceed 150 \({}^{\circ}\)C. To ensure that the transformer worked in the normal temperature range, the FPGA needed to select the maximum value of the three-temperature data as the reference value to determine whether the reference value exceeded the default monitoring threshold. The relay module realized the functions of turning off the fan below 80 \({}^{\circ}\)C, starting to cool down the fan when the temperature exceeded 100 \({}^{\circ}\)C, a high-temperature alarm by buzzer when the temperature exceeded 130 \({}^{\circ}\)C, a high-temperature trip when the temperature exceeded 150 \({}^{\circ}\)C, fault tripping, and fault alarm when the temperature exceeded the range from \(-\)30 \({}^{\circ}\)C to 240 \({}^{\circ}\)C.