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:
- Level of residual flux in the core of the transformer;
- Instant in the sine wave at which voltage is applied to the transformer;
- 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:
- Maximum positive value of residual flux in the core;
- Energizing the transformer at the instant the voltage is at the zero crossing in the sine wave;
- 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.