The possible reasons  ：

（1）The temperature sensor in transformer is damaged. 

（2）The oil temperature sampling line on high-voltage cabinet is connected reversedly. 

（3）The temperature adjusting potentiometer in controller is not correct or is damaged. 

（4）For reasons such as short circuit in rectifier transformer or inside and outside heat dissipation, the actual temperature is caused  too high and the alarm trips for protection.

Suggested solutions:

（1）Under the situation that safety can be guaranteed, have a try of the temperature on the surface of rectifier transformer by hand. Under normal situation, your hand can not press the surface for 5 seconds, its temperature is about 70-85℃. Judge whether the actual temperature is in accordance with the shown temperature, if not, judge that temperature sensor is damaged, and inspect and exchange the temperature sensor. 

（2）Inspect and connect wires correctly. 

（3）Adjust temperature and simulate resistance again .         

（4）Try the first solution, if the actual temperature is in accordance with the showed temperature, stop the operation of machine immediately, contact manufacturer, analyze the causes and then settle it.