Maintenance diagnostics of transformers considering the influence of short-circuit currents during operation

The worst mechanical effects acting on the transformer are under the sudden short-circuit. Since the currents flows through the windings in the case of short-circuit are enormously, forces generated by these currents are also large and in many cases exceed the value of the maximum mechanical strength of windings or other parts of the transformer. Because of the transient and dynamics entity of this phenomenon the solution is very difficult. It is therefore necessary, in the absence of scientific and research potential in distribution and transmission organizations (e.g. power plants, heating plants), to achieve the objectives of the proposed activities, i.e. in-depth analysis of undesirable impacts of short-circuit currents on the state of transformers, design of modern methodology of measurements and its verification, and design of a new integrated system of diagnostics with the possibility of analysis and classification of possible failures by short-circuit impact on power transformers. Except the winding faults (inter-turn short-circuit, short time connection with the tank) it could lead to the tank damage because of the arc pressure. The direct cause of the acting forces on windings is the action of the magnetic field with current paths. In the case of the transformer it is the field of the leakage flux. Normal conditions, when the currents in the transformer do not reach the rated ones, the forces acting on the windings are generally small. On the contrary, the short-circuit currents reach the values which are multiple of the rated ones; these forces could be dangerous for the windings and fixing construction. Forces in transformer windings can be radial (transverse) and axial (lengthwise), since these two components of the force can be calculated and analyzed independently. Also, the two components have influence on different parts of the total transformer and it is necessary to obtain the two components for design purposes. The nomenclature axial and radial is applicable to concentric wound core type transformer. Action of the forces on the windings could be separated to the radial and axial forces (Fig. 1). Both of these forces exist not only in the short-circuit conditions but also in the rated condition [3].