# Power Transformer Inrush Current Analysis: Simulation, Measurement and Effects
## 6 Discussion--Effects of Inrush Current on Transformers in Service  
To illustrate the effect of inrush current on high power transformers, data is presented from a 440 MVA three-phase oil-filled power transformer in service [13; 34]. The inrush current varies in amplitude depending on the magnetic state of the core and the angle of connection of the voltage supplied to the transformer. This inrush current increases rapidly, reaching values up to 100 times higher than the no-load current, causing saturation of the magnetic core. Figure 8: The instantaneous values of the component oscillating at the fundamental frequency (SWT vs. FFT) for U_UV. Figure 9: **Left—**first 2 periods from U_UV after the first zero-crossing on ascending slope, overlapped with the instantaneous values of the component oscillating at fundamental frequency computed with SWT. **Right—**the difference: U_UV- instantaneous values of the component oscillating at fundamental frequency computed with SWT. After carrying out specific maintenance work (analysis of oil samples), it was found that electrical discharges had been detected inside the transformer. The fault identified in the test consisted of high energy electrical discharges, indicated by excessive carbonization, metal melting and possible triggering of equipment protection. There were also visible traces of electrical discharges outside the transformer, so a series of on-site tests were carried out. During the transient phenomenon caused by the connection of the transformer to the mains, a significant difference in electrical potential occurs between the two metal terminals of the conservator tank halves, leading to the occurrence of electrical discharges (Figures 10 and 11). This indicates that the earthing and equipotential bonding work has not been carried out correctly, that part of the magnetic flux is blocked at the junction between the two halves of the transformer tank (upper and lower tank) and that different electric fields are generated [13; 34]. The displacement current therefore represents the rate of change of the electric flux. The electric flux through a closed surface is proportional to the electrical load within that surface. The electric flux depends on the values of the electric induction (and the electric field) at the points of a closed or open surface. The area between the two halves of the tank is considered equivalent to three capacitors connected in parallel, with different insulation: the first capacitor is oil-insulated, the second is rubber-insulated (the middle one) and the outer one is air-insulated (Figure 12). Figure 11: Traces caused by external arcing. Figure 12: Area between the two halves of the tank. Figure 10: External electrical discharges. The variation of the electric field in the area between the two half-tubes (the capacitor described above) changes the values of the electric flow and the electric charges stored in the capacitor armatures. If we connect a high voltage level, we generate very high electrical loads. The difference in potential that appears as a consequence of the fact that the size of the electric field of the two armatures is different, causing electric arcs especially in areas where there is air. We can conclude that the observed discharges are dependent on the level of the supply voltage, the duration of the voltage and the internal configuration of the insulation and the screens. Another cause of visible discharges is the phenomenon of series resonance between the inductive impedance of the transformer and the capacitive impedance of its supply system (encapsulated bars, capacitive transducers, etc. ), can be seen in Figure 13.  
These discharges were the result of the superposition of the transient current, which occurs when the transformer is connected to the mains, with the achievement of the series resonance condition [34].