Effect of inductance on the measurement of winding temperature in electrical plant

The normal method used to measure the temperature of the windings of transformers, reactors and other heavy electrical plant is by change of resistance. Resistances are measured before and after the heat run by simple d.c. measurements. Using the known temperature coefficients of resistance for copper, the temperature at shut-down is calculated. It is shown that inductance causes errors, leading to apparently higher temperatures than actually exist. The errors become increasingly serious the larger the unit under consideration. The inductive effects are examined by means of analysis and digital computer, when it is shown that controlling factors are the electrical and thermal time-constants of the winding. Ameliorative measures are suggested. It is important to include external resistance in series with the test winding so as to lower the electrical time-constant. The test current must be sufficient to saturate the core of the magnetic circuit. Though mainly applicable to transformers, the conclusions are relevant to any heavy winding.