PREDICTION OF LOAD NOISE RADIATED FROM A POWER TRANSFORMER

Abstract

This dissertation is conducted to predict load noise generated in a large power transformer. To this end, two studies are performed independently. One is to calculate the equivalent stiffness of a winding that is a component of a transformer and causes load noise. The other is to predict the load noise of the transformer caused by the vibration of the winding. Main contents and issues for these two researches are follows: First, we predict the natural frequency of the winding assembly of a power transformer, and based on this, a method for calculating the equivalent stiffness of the winding is proposed. A modal experiment was conducted to obtain the mode shape and natural frequency of the winding, and it was confirmed that the boundary conditions can be affected by the weight of the winding. Meanwhile, the modal analysis using commercial finite element analysis code was performed, revealing that the effect of the boundary condition on the natural frequency is negligible. At the same time, the modal analysis was carried out by changing the main design parameters of the winding, which validated that the natural frequency is constant regardless of height of the winding. Based on these analysis results, it was concluded that the natural frequency of the winding assembly is the same as that of one ring-shaped layer constituting the winding. Through this conclusion, a regression equation for predicting the natural frequency was derived by utilizing the well-known theoretical equation that is able to predict the natural frequency, and the equivalent stiffness of the winding was calculated using those two equations. Finally, by comparing the measured natural frequency with one obtained by applying the equivalent stiffness to the finite element model, the validity of the equivalent stiffness was proven. Next, we calculate the winding vibration of the power transformer under load conditions and based on the calculated vibration, propose a new regression equation to predict noise. By measuring the load noise and tank vibration for multiple transformers with different rated powers and performing regression analysis, it was confirmed that there was a correlation between them, and based on the general fact that the winding vibration level is proportional to the tank vibration level, a regression equation for load noise and winding vibration was derived. To obtain the winding vibration required for regression equation, the electromagnetic force that is the excitation force of the winding was calculated using the equivalent magnetic circuit network method, and then the calculated force was applied to the finite element model for the winding to achieve the vibration response. It was verified that the winding vibration obtained by these methods closely correlated with the load noise, and it was also confirmed that the amount of winding vibration transferred to the tank could be changed according to the distance between the tank and winding. Based on this result, the load noise prediction equation was established considering the winding vibration and the correlation factors according to the distance of the transmission path, and it was verified that the proposed prediction equation is much more accurate than the previous prediction equation.