Causes of Inrush Currents in Transformers

When a transformer is energized after a short interruption, the transformer may draw high inrush currents from the system due to core magnetization being out of synch with the voltage. The inrush currents will be as high as short-circuit currents in the transformer (almost 20 to 40 times the rated normal full load current of the transformer). Inrush currents may cause fuse, relays or re-closers to falsely operate. It may also falsely operate the faulted circuit indicators or cause sectionalizers to misoperate.

When the transformer is switched in, if the system voltage and transformer core magnetization are not in synchronize, a magnetic transient may occur. This transient may drive the core into saturation and drives a large amount current into the transformer causing transformer core to damage.

Factors Significantly Impact Inrush Currents in Transformer

  1. 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.
  2. Large transformers draw more inrush current. Large transformers will have smaller saturation impedance.
  3. Higher source impedance relative to the transformer size limits the currents that the transformer can pull from the system.
  4. 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°). 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.
  5. 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.
  6. Energizing a transformer can cause the nearby transformer to also draw inrush currents. The inrush currents into the switched transformer have a significant dc component that can cause the voltage drop. The dc component can push the other transformer into saturation and draws inrush current.
  7. A lightning flash near the transformer can drive the transformer core to saturation.

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