Asymmetric Overload of Synchronous Generators with Negative Sequence Currents at Geomagnetic Disturbances

Abstract

It is demonstrated that at the occurrence of geomagnetic disturbances the value of quasi-direct current acting on a generator step-up transformer depends on the type and number of blocks "synchronous generator – generator step-up transformer" as parts of a power plant, phase wire parameters, geographic direction of overhead power lines extending away from high voltage collecting buses. It is established that, other things being equal, the generator step-up transformers of simple and enlarged power plant blocks are subject to a more intensive action of quasi-direct currents. The saturation of the magnet system of a generator step-up transformer under the action of quasi-direct currents causes simultaneous occurrence of symmetric overload at a synchronous generator with a stator reactive current induced by the increase in the magnetization capacity and non-symmetric rotor overload induced by the higher harmonics of positive and negative sequence magnetization current. The authors define a harmonic composition of the current of the synchronous generator stator at a unilateral saturation of the generator step-up transformer magnetic system. It is shown that the second current harmonic of the stator is the most significant one, as its current can reach 50% of the rated stator current. The authors obtain the ranges of a permitted duration of the asymmetric mode for all types of considered synchronous generators beyond which the rotor is damaged because of inadmissible thermal action.