Transformer-energizing inrush current

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 Fig. N31).

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 (< 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.