There are a considerable number of aspects affecting transformer inrush currents, and there is no “one size fits all” criteria that allow predicting the phenomenon in a given facility without customized, case-by-case analysis. In some instances, different aspects affect each other, or one aspect affects itself, compounding things in the beneficial or detrimental direction.

• The impedance of the source that energizes the transformer: the weaker the source, the higher its impedance, and the more limited the inrush current is, but also the more dramatic voltage sags are. Curiously, voltage sags diminish the saturation of the core, which effectively decreases inrush currents.
• The material of the transformer core, and its construction: not only affect the saturation of the core on both ends of the hysteresis loop, but the remanence flux density, which is a significant factor as explained in Figure 3. The quality of steel and core construction may intuitively be associated with better inrush performance, but such characteristics are mostly driven by motivations of efficiency, for which reducing losses (eddy currents and hysteresis loop size) is necessary. Two transformers with distinctively different quality materials and/or construction may not imply significantly different inrush performance.
• The maximum flux density in the hysteresis loop: it has been shown in the specialized literature that, the higher the maximum operational flux density (vertical axis in the hysteresis loop), the lower is the magnification of inrush current due to residual flux density.
• The magnitude and frequency of the voltage applied on the transformer: the higher the voltage, the closer the hysteresis cycle approaches saturation. An increase in frequency will elongate the hysteresis loop in the horizontal direction, therefore decreasing the value of the residual flux density. This, however, is not a common occurrence in power transformers.
• Whether the transformer is single phase, three-phase, and the type of connection (delta, wye): contrary to a single-phase transformer, in which the connection of the voltage source could theoretically be synchronized to fully eliminate inrush currents, three phase transformers are impossible to be connected without producing some inrush, as the right connection time in one phase is detrimental to the other two. The fact that the currents in a three-phase transformer must add up to zero complicates the inrush phenomena, but literature reports the intensity of inrush is less in transformers connected in three-phase as compared to the same transformers when considered single.
  On the matter of connection types, it can be highlighted that:
  • Single phase units connected in isolated Y-Delta bank, and three-legged core three phase transformers, would generally show 2/3 of the maximum inrush currents they would if they were used as single-phase units.
  • Transformers connected in delta in their primaries would see, on a winding-by-winding basis, the same inrush behavior as they would if they were single-phase. However, from the perspective of line currents, they would see inrush currents which are ~58% as severe as they would in equivalent single-phase units.

To illustrate how the connection of three phase transformers can alter the inrush current performance, a series of inrush simulations were performed in a 600V/480V, three-phase transformer bank of 1500 kVA, exposed to a large power source. For all simulations, all variables were fixed (including voltage angles to de-energize and energize), except the connection of the transformers themselves. There is distinctive differences in line currents between all six connections tested, in both magnitude and the predominant inrush phase.