Axial stability analysis and simulation of power transformer windings

The stable operation of power transformers is important guarantee of power system stability. Nevertheless, accidents of winding instability occurs intermittently, which exerts huge harm to the secure operation of transformers and even power systems. Moreover, the axial instability is a major type of winding failure. When the natural frequency of windings approaches to the frequency of axial electromagnetic forces, winding structures will resonate intensively, causing the axial instability to occur. Therefore, the axial resonance of windings should be avoided. This paper presents the axial stability analysis and simulation of transformer windings to understand the resonance mechanism. At first, the theoretical model is put forward to demonstrate the axial vibration of transformer windings. Moreover, a 3-D modal analysis simulation model of winding structures are established and solved by means of finite element method (FEM). Furthermore, the natural frequency under different values of pre-stress has been calculated and the relationship between the axial natural frequency and pre-stress has been analyzed. Based on this, the first three axial vibration modes of windings are extracted and analyzed. The simulation results indicate that, natural frequencies will continue to decline as the pre-stress decreases over time, and thus maintaining the pre-stress in a relatively high level will be an effective axial vibration-proof measure contributing to avoiding the axial resonance. The simulation results and the method employed in this paper can provide reference for the transformer design, manufacture, operation and maintenance.