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