Understanding Tensile and Compressive Forces in Winding Designs

In the realm of power transformer design, the behavior of conductors under various forces can significantly impact the longevity and reliability of the winding. This article delves into the mechanical stresses that arise in winding conductors, specifically tensile stress and compressive stress, and explores the potential failure modes that can result from these forces.

Tensile stress is observed in the outer windings of a transformer, where conductors experience outward forces. As long as this tensile stress remains below the material's proof stress, the winding retains its circular shape, ensuring structural integrity. However, if the tensile stress exceeds this limit, the conductors may stretch. This stretching can lead to insulation failure and a loss of axial stability, particularly if a local bulge forms beyond the spacer contour, which can compromise the entire winding.

Conversely, the inner windings are subject to compressive stress. When this stress surpasses specific thresholds, buckling can occur, deforming the winding shape. There are two main types of buckling: forced and free. Forced buckling occurs when conductors bend inward between supports due to excessive compressive stress, while free buckling can happen under lower radial forces without the influence of the number of spacers. Several factors, including tightness, initial eccentricity, and conductor geometry, determine the critical stress necessary to prevent free buckling.

Another failure mode associated with compressive forces is spiraling, which often affects helical windings. In configurations where conductors are axially stacked, particularly with a high pitch, the risk of spiraling increases. This deformation pattern is more pronounced when the winding is adjacent to main leakage flux channels, which exert additional radial forces. The design of the winding, especially when utilizing epoxy-bonded conductors, plays a crucial role in resisting spiraling effects.

Axial forces also present challenges, leading to the tilting of conductors. Winding configurations with thin conductors and fewer strands are more susceptible to this phenomenon. The coverage of radial spacers can enhance resistance to tilting, as greater coverage provides additional support. Notably, epoxy-bonded conductors exhibit remarkable stability against tilting, demonstrating the advantages of this bonding technology in modern transformer designs.

Understanding these mechanical stresses and potential failure modes is essential for engineers and designers. By carefully considering the properties of materials and the geometrical configurations of windings, it is possible to create robust transformer designs that effectively mitigate risks associated with tensile and compressive forces.

Understanding Axial Forces in Power Transformer Design

In power transformer design, axial forces play a significant role in the operational stability and efficiency of the unit. These forces arise from the interactions between the windings and can greatly influence the overall performance of transformers. A clear understanding of the factors contributing to these forces is essential for engineers and designers in the field.

The axial force, denoted as "ax," is measured in Newtons and is particularly impacted by the height of the windings. The average axial height of windings (Hwdg) directly correlates with the amount of axial force experienced. As the height of the windings decreases—often due to transportation constraints—the axial forces increase, leading to greater stress on the transformer structure. This is particularly relevant in large-rated power transformers, where the impedance affects both axial and radial forces generated during operation.

Further complicating the dynamics within a transformer are the varying ampere-turn distributions across windings. Unbalanced magneto-motive forces (mmf) between inner and outer windings can result in axial forces that tend to push the windings apart. For instance, if the inner low-voltage (LV) winding has no gap while the outer high-voltage (HV) winding has a tap gap, the resulting radial leakage flux generates additional axial forces. Designers must carefully balance the ampere-turns to mitigate these effects and enhance structural integrity.

Several strategies can be employed to manage axial forces effectively. One approach is to create a compensation gap in the LV winding to counterbalance the HV tap gap. Alternatively, the LV winding can be designed with fewer turns and thicker spacers, which can help in achieving a more balanced magnetic field and reducing excess axial force. Additionally, yoke laminations are used to direct leakage fluxes more axially, which can aid in diminishing axial forces at winding ends.

Despite efforts to achieve symmetry in winding designs, asymmetries due to manufacturing variances and tap positions are inevitable. These differences can lead to uneven ampere-turn distributions, generating challenges in maintaining equilibrium. Consequently, structural components like clamping rings may experience significant forces, which designers need to account for in the transformer’s overall structural design. Understanding these dynamics is crucial for ensuring reliable transformer operation and longevity.