Static transformers — Principles and operation

Recent developments in power electronics in the field of static electrical energy conversion have led to a reconsideration of the transformer approach, traditionally based on harmonic operating regimes. It is now necessary to consider temporal behavior models capable of taking instantaneous account of the electrical and magnetic quantities of these systems.

To achieve this objective, the equation mechanisms of the single-phase transformer are analyzed, then extended to the three-phase case, taking into account the diversity of magnetic circuits encountered. This study then leads to the definition of ideal models based on the concept of unit efficiency energy modulator, which is the basic element of any energy conversion chain, whatever the nature of the converters used - electromagnetic, electronic or electromechanical.

Under these conditions, the choice of a model can be discussed in the light of a given electrical environment, both for the study of unbalanced sinusoidal and non-sinusoidal modes, particularly when the transformer is associated with a static converter. In this case, the transformer can be placed upstream or downstream of the converter, or fully integrated into its operation.

The aim of this article is to familiarize the reader with the methods used to study electrical circuits that transfer electrical energy via three-phase links.

The aim is not to describe the techniques and equipment used in electrical distribution, but to explain the theoretical basis for the simplified models of lines and transformers used in the industrial sector, and the main assemblies used to measure the power "transported".

In this respect, the symmetrical component method is particularly useful, since it allows us to reproduce the reasoning and diagrams used in the single-phase case in the unbalanced three-phase case. Thanks to these results, numerous unbalanced configurations (wire breaks or short circuits) can be analyzed and calculated, whether or not transformers are present.