View source for Transformer-energizing inrush current

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At the moment of energizing a transformer, high values of transient current (which includes a significant DC component) occur, and must be taken into account when considering protection schemes (see {{FigRef|N31}}).

[[File:FigN31.jpg|none]]
'''''Fig N31:''' Transformer-energizing inrush current''

The magnitude of the current peak depends on: 

*The value of voltage at the instant of energization 
*The magnitude and polarity of the residual flux existing in the core of the transformer 
*Characteristics of the load connected to the transformer

The first current peak can reach a value equal to 10 to 15 times the full-load r.m.s. current, but for small transformers (&lt; 50 kVA) may reach values of 20 to 25 times the nominal full-load current. This transient current decreases rapidly, with a time constant θ of the order of several ms to severals tens of ms.

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