Cumulated Effect of Spacers in Transformers Based on Equivalent Short-circuit Test

Spacers are one of key materials in the axial supporting structure of coils in oil-immersed transformers. The pattern of cumulative effects on transformer spacers under multiple short-circuit conditions is not yet clear. The proposed method is an equivalent test for spacers under multiple short circuits, capable of simulating dynamic short-circuit forces with a transient component of 50 Hz. This method overcomes the limitations of static load tests in the field of transformer insulation materials. The test results provide insights into the variation in plastic deformation of spacers under different conditions such as short-circuit force magnitude, aging degree, type of short-circuit method, and times of short-circuits impact. The results indicate that plastic deformation is positively correlated with the magnitude of the short-circuit force and exhibits saturation characteristics as the times of short circuits impact increases. The interrelation between plastic deformation and interval types of short circuits is not significant. Based on the plastic deformation pattern of spacers aged for 15 years, the cumulative effects are categorized into three stages: rise, stability, and rapid accumulation. Under the combined influence of aging and load, the fiber state of the spacers corresponds to different stages of cumulative effects. The elastic modulus of the spacers shows a nonlinear relationship with the short-circuit force. Furthermore, a method for calculating the loss of axial pre-tension force in transformers is proposed. Case calculations demonstrate that the axial pre-tension force of each coil decreases or redistributes after multiple short circuits. The conclusions of this study provide technical support for a deeper understanding of the impact of multiple short-circuit cumulative effects on transformers. ©2024 Chin.Soc.for Elec.Eng.