06 April 2013

Inrush Currents in Transformers - Causes

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 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 mis-operate
When the transformer is switched in, if the system voltage and transformer core magnetization are not in synch, a magnetic transient may occurs. 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:

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
  • 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
  • A lightning flash near the transformer can drive the transformer core to saturation

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