Transformers used in power systems must be designed and constructed so that, in addition to achieving an acceptable cost, the following can also be achieved:

– A good voltage regulation: it means that they have reduced voltage drops. It is possible by the intensification of the magnetic coupling between windings to reduce the dispersion flows and corresponding reactive drops;

– High yields: Implies obtaining low energy losses in both copper and core iron. It is possible to limit the requests of the materials used (current densities in the copper and iron inductions) to levels compatible with the costs, while improving their properties;

– Low currents and losses in idle operation: Low idle currents can be achieved with high magnetization inductances using highly permeable cores. Lower vacuum losses mean, mainly, lower iron losses. Generally, star neutral and grounded and triangle feeds overhead lines or cable networks.

The actual transformer can be represented by an ideal transformer in which the primary and secondary windings are connected impedances representative of the phenomena that occur in the actual transformer:

– voltage drops due to resistance and magnetic leakage inductance;

– Joule energy losses in resistors, magnetization;

– losses in iron.