There are circumstances when the magnetising inrush current of a transformer can require the system designer to focus special attention on methods to mitigate the effect of the magnetising current on the system operation. For example, in island
operation when a single generator is used to energise a transformer with an MVA
rating significantly greater than the rating of the generator.
The magnetising inrush current occurs at the instant the transformer is switched on. There is an inrush current in the primary winding that is significantly greater than the full load current. If the secondary winding is not connected then there may be no secondary current. Furthermore, the characteristics of the phase currents, phase voltages and kVAR reactive load may vary from phase to phase and from one switching event to the next, even with the same source generator.
If there are multiple transformers, the phenomena of resonance between the transformers may be observed.
In such instances, the effect of the magnetising current may vary from a number of cycles to many seconds.
The situation may be more complex if one transformer is already connected and loaded at the time of switching on another transformer. In this case, the most important factors are the kW load with respect to the capacity of the generator and the power factor of the load.
One tool available to the designer, as an aid to mitigation, is the configuration of the generator control and protection systems. The modelling of the power system during the design may helpful in understanding the transient response of the system and the effect of remedial measures. This can reduce the risk of an unsatisfactory outcome during commissioning.
Each system should be evaluated on a case-by-case basis. More information on transformer resonance is given in the references below.
1. M. Martinez Duro, R. Denis, Electricite’ de France, France, “Parameter uncertainty assessment in
the calculation of the overvoltages due to transformer energisation in resonant networks” CIGRE C4-204 2012
2. [B17] Lin, C. E., Cheng, C. L., Huang, C. L., and Yeh, J. C., “Investigation of magnetizing inrush current in
transformers—Part II: Harmonic analysis,” IEEE PES Winter Power Meeting, New York, 92 WM 246-9 PWRD,
Jan. 26–30, 1992