Evaluation of the Effect of Quasi-Direct Currents on Power Transformers of Electric Power Systems

It is shown that at increased geomagnetic activity, the hot spot temperature of the metal structural components of electric power system transformers can reach the maximum permissible value that may cause the gas protection trip. The maximum permissible hot spot temperature of metal structural components of the power transformers in the conditions of short-term overload has been justified according to the requirements of international standards. For shell-core transformers, it has been established that at the increased geomagnetic activity, the tank hot spot temperature excess over the oil temperature depends on the magnitude of the quasi-direct currents in the grounded high voltage windings and the exposure time. As a criterion of power transformer vulnerability to geomagnetic disturbances, the maximum permissible value of the quasi-direct current in the grounded high voltage windings and the exposure time, at which the tank hot spot temperature can reach and exceed the maximum permissible value have been proposed. The occurrence of even harmonics (2nd, 4th, 6th, etc.) in the magnetizing current has been detected, which necessitates introducing an additional criterion characterizing the power transformer core saturation, i.e. a 50% excess of the total even harmonic distortion over the total odd harmonic distortion of current.