Factors Significantly Impact Inrush Currents in Transformer:

  • A transformer that is designed to operate lower on the saturation curve draws less inrush currents as there is more margin between the saturation point and the normal operating. The extra flux during switching is less likely to push the core into saturation
  • Large transformers draw more inrush current. Large transformers will have smaller saturation impedance
  • Higher source impedance relative to the transformer size limits the currents that the transformer can pull from the system
  • The point where the circuit breaker close (position of flux wave in sine wave). The worst case will be when the flux is at maximum (peak) and voltage is minimum (in transformer the applied voltage lag behinds the flux by 90 deg).
  • Other factors have little significance. The load on the transformer does not significantly change the inrush currents. While switching transients cause high inrush, other voltage transients especially voltage transients with dc components can saturate the core of the transformer and cause inrush currents
  • A lightning flash near the transformer can drive the transformer core to saturation
  • When the nearby fault was cleared and transformer voltage is recovering from the voltage sag, the sudden rise in voltage can drive the transformer to saturation
  • Energizing a transformer can cause the nearby transformer to also draw inrush currents. The inrush currents into the switched transformer has a significant dc component that can cause the voltage drop. The dc component can push the other transformer into saturation and draws inrush.