Mitigation of Transformer Inrush Current

Inrush Phenomenon

It is well known that energizing a transformer with rated voltage can produce a significant inrush current.  Inrush current is a random event which is not predictable and is in general not repeatable.  The magnitude of inrush current depends primarily on the following:

1. Level of residual flux in the core of the transformer;
2. Instant in the sine wave at which voltage is applied to the transformer;
3. Magnitude of applied voltage.

Although the actual inrush current is a random event, it is known that the maximum peak inrush current can occur under the following conditions:

1. Maximum positive value of residual flux in the core;
2. Energizing the transformer at the instant the voltage is at the zero crossing in the sine wave;
3. The applied voltage is increasing in the positive direction to add to the positive level of flux.

The following technical paper presents a method to mitigate inrush current when energizing large power transformers from a limited supply such as a generator or weak distribution system.

pdf Mitigation of Transformer Inrush Current (567 KB)