Method and apparatus for measuring the hot-spot temperature in an electric apparatus containing an oil

Method for establishing hot spot temperature TPC* of oil contained in an electric apparatus, according to different operating conditions of the apparatus, the method comprising:

• a) operating the electric apparatus according to at least one predetermined operating condition in order to increase temperature TH of the oil, the oil comprising at least one chemical compound soluble in the oil, each compound forming in a irreversible way a residue soluble in the oil at a residue transition temperature TR;
• b) taking at least one sample of oil during or after step a) while measuring the oil temperature TH;
• c) performing an analysis of the at least one sample taken in step b) in order to determine whether or not the residue is present;
• d) from the analysis of the step c), evaluating a hot spot temperature TPC under the at least one predetermined operating condition having allowed the presence of said at least residue in the sample, as being equivalent to the residue transition temperature TR of said at least one residue;
• e) calculating a difference ΔT between the hot spot temperature TPC and the oil temperature TH under the at least one predetermined operating condition;
• and
• f) establishing that the hot spot temperature TPC* according to any other operating condition of the apparatus is obtained according to the following equation:
• TPC*=TH*ΔT;
• TH* being the temperature of the oil under said other operating condition.