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Short‐circuit forces, which result in violent vibration of power transformer windings, are induced by the interaction between the short‐circuit currents and the leakage magnetic induction. During vibration, the rearrangements of disks affect the magnetic flux distribution, which leads to time‐varying equivalent reactance and further changes of currents and forces. When a short‐circuit accident occurs, these processes happen simultaneously and constantly. In this article, the influence of circuit parameters on short‐circuit currents is analyzed. The strongly coupled phenomenon between equivalent circuit, magnetic field and structural field is explicated, and the method to obtain vibration process with strongly coupled phenomenon considered is proposed. Then the strongly coupled effect on vibration process is investigated. The vibration process is characterized by the time‐varying currents, electromagnetic forces and displacements. The space coefficient is defined to represent the magnetic flux distribution. The changes of maximum value and its occurrence time of the characteristic parameters are obtained. The peak values of the short‐circuit forces and displacements increase and the waveforms change, which is caused by the distortions of leakage magnetic flux and the changes of spatial ampere‐turn balance during vibration. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

Accurately calculating axial electromagnetic force is essential to analyse transformer winding axial stability. Prior research has mainly focused on the effect of winding structure on static axial electromagnetic force and studying vibration by substituting the static force in a time‐varying function. However, the coupling effect between axial electromagnetic force and winding vibration has not been addressed, and no calculation method for the axial electromagnetic force that considers both winding meso‐structures and vibration coupling effects has been proposed. Previously the authors presented an electromagnetic force calculation model that considers winding structure characteristics, and an iterative algorithm for magnetic‐structure coupling calculation. Currently, the winding vibration model was first proposed and the dynamic calculation method was formulated. By applying the method to a typical 110kV transformer, the spatial‐temporal distribution of winding axial short‐circuit electromagnetic force was obtained. It was found that the peak value of the axial short‐circuit electromagnetic force of some windings appears at the second or third peak moment of the short‐circuit current, which is called as peak time shift phenomenon. Further stress analysis indicates that existing evaluation methods may overestimate the short‐circuit resistance of windings by only considering short‐circuit electromagnetic force under maximum peak of short‐circuit current. [ABSTRACT FROM AUTHOR]

When an external short circuit occurs, the axial electromagnetic force increases dozens of times. Under the effect of the axial short-circuit electromagnetic force, the transformer windings vibrate violently. The spatial distribution of the disks is constantly changing during vibration, which can change the temporal and spatial distributions of the leakage magnetic field and the axial electromagnetic force. This process is called the strong coupling phenomenon of the structural field and the leakage magnetic field. In this article, a strong coupled magnetic-structural model is proposed by using the analysis method. The strong coupling equations, where the independent variables are the axial position of the disks, are obtained. The vibration process with strong coupling phenomenon considered can be obtained. The accuracy of the strong coupling model is verified by the iterative method. By comparing the results calculated by the strong coupling model with those calculated by the weak coupling model, the influence of the strong coupling phenomenon on the vibration process is obtained. Due to the influence of the strong coupling phenomenon, the winding vibration intensity increases and the maximum displacement of the disks can be doubled, which implied that the strong coupling phenomenon cannot be ignored when investigating the short-circuit strength of power transformers. [ABSTRACT FROM AUTHOR]

• The Gap between spacers and adjacent disks can occur when the vibration of power transformer winding is intense. • The occurrence of the gap intensifies the vibration of power transformer winding and exacerbates the mechanical stress in winding components. • The residual deformation of the spacer increases unidirectionally under the effect of multiple short-circuit impacts, making the occurrence of the gap easier. • The positive feedback loop between gap occurrence and residual deformation of the spacer contributes to the mechanical failure of the power transformer winding. In this study, a positive feedback loop between gap occurrence during vibration and residual deformation of spacers is identified, based on which a transformer failure path under multiple external short-circuit impacts is demonstrated. A three-stage analysis is performed. First, two prestress application methods, namely, elastic and fixed methods, are differentiated. The nonlinear and nonstretchable characteristics of spacers are illustrated. The criterion of gap occurrence in the fixed-application method is investigated. A case study is conducted to analyze the possibility of gap occurrence. Subsequently, the effect of the gap on the vibration process is analyzed. Owing to the gap, the vibration intensity of the entire winding is amplified, and the stress on the conductors and spacers is exacerbated. The mechanism of this phenomenon is explicated. Finally, based on the experimental results in which the amplitudes of cyclic loads first increase and then decrease, the unidirectional change in the residual deformation is observed. Under cumulative impacts, the residual deformation is greater, and the gap occurs more easily, thus contributing to the winding collapse under multiple short-circuit impacts. [ABSTRACT FROM AUTHOR]

Transformer winding turns often consist of multiple parallel strands. The spatial position variation of each strand affects the leakage inductance of each branch, resulting in an uneven distribution of short‐circuit currents within the winding turns. And this unevenness persists even when transposition structures are implemented. Traditional methods in transformer analysis frequently overlooked the distribution characteristics of short‐circuit currents when calculating electromagnetic forces. A frequency‐domain calculation method for analysing the current distribution in winding turns was proposed, with a deviation of less than 3% compared to existing analysis methods. Two typical 110 kV transformer models were utilised to investigate the influence of uneven current distribution on the spatial distribution of electromagnetic forces. The spatial distribution of short‐circuit electromagnetic forces in low‐voltage (LV) windings exhibited significant changes, with maximum change rates of 10% and 61.2% for axial and radial electromagnetic force, respectively, in a LV winding with 4 parallel strands. The research also analysed how strand radial width and axial height affect current distribution unevenness and proposed specific design principles to mitigate these disparities in winding design. The findings offer valuable insights for selecting structural parameters and assessing short‐circuit stability during transformer design. [ABSTRACT FROM AUTHOR]

Winding transposition structures are commonly built into transformers to suppress winding circulation and minimize loss. They alter the distribution of the transverse leakage magnetic field and electromagnetic force. Researchers have not yet considered the winding transposition structures when investigating the transverse leakage magnetic field distribution. The relative position of conductors in a transposition structure is analyzed in this paper, and a scale model experiment is conducted to verify the effect of the structure. In the meantime, a finite element method model which includes the winding transposition structure of the scale models is established and verified by comparison with experimental observations. Then, the distribution of the transverse leakage magnetic field and the axial electromagnetic force of the 110 kV transformer under rated tapping are obtained. The transverse leakage magnetic field and axial electromagnetic force in the low‐voltage winding are affected by the transposition structure, while the high‐voltage winding is not affected. Compared with models without transposition structure, the amplitude of the axial electromagnetic force linear density of the 40th winding disk under rated tapping increases from a value close to zero to 25 N/m. The transposition structure needs to be considered when calculating the electromagnetic force and winding mechanical characteristics. [ABSTRACT FROM AUTHOR]

For continuous or spiral windings which feature multiple continuously transposed conductors (CTC) wound in parallel, the transposition structure is commonly used to suppress circulation between conductors. This introduces a local asymmetry in transformer windings. Fault analyses have shown that building an asymmetric structure in transformers increases deformation risk in these windings. Researchers have yet to fully investigate this asymmetric structure. This study was conducted to observe the influence of transposition structure in a 110 kV transformer. The conductor’s relative position forms three patterns during the transposition process; two parameters are established to describe the position-changing process. A finite-element method (FEM) model is built to investigate the magnetic field and electromagnetic force distributions. The results indicate that the transposition structure distorts the magnetic field distribution. The maximum distortion factor of the axial component of magnetic flux density ($B_{\mathrm {z}}$) along the axial direction caused by the transposition structure is 14.6%. The transposition structure only changes the radial change slope of $B_{\mathrm {z}}$. The gap caused by the transposition process aggravates imbalance in the ampere-turns distribution, increasing the amplitude of the radial component of magnetic flux density ($B_{\mathrm {r}}$) at the middle height of the transformer low voltage (LV) winding. The maximum amplitude of $B_{\mathrm {r}}$ at the transposition structure increases by 513%. The Lorentz force over the CTCs remains constant from before to after the transposition process, though its linear density over the CTCs is unevenly distributed throughout. The axial component $F_{\mathrm {z}}$ varies from −31 to 10 kN/m, while the radial component $F_{\mathrm {r}}$ varies from −146 to −51 kN/m. The results show that the initial stage of the CTC height rising segment is a weak point and should be strengthened in the manufacturing process. [ABSTRACT FROM AUTHOR]