Analysis of DC bias vibration of transformer core based on electromagnetic force field coupling

Monopole operation model of Ultra High Voltage Direct Current (UHVDC) system, DC bias has great influence on the safety and stability of the system, especially for transformers, including temperature rising and increased in vibration. However, there is less research have done on increased vibration of transformer due to influence of DC bias on HVDC system. This study explored transformer vibration caused by DC bias, including the principle of transformer DC magnetic bias and the related calculation method. The mathematical model of the electromagnetic force coupling of the transformer core was also studied and after that its core magnetic field and force field were analyzed. The displacement change of the transformer core which included core clamps and fixed structures were analyzed and discussed. Results revealed that the leakage flux of the clamps increases with DC bias and odd harmonics appeared in each displacement component. Specifically, the direction of the y displacement component changed, and the maximum displacement occurred with the 90-degree phase shift. Then, vibration experiment was done to analyze the DC bias characteristic of core, which also valid the effectiveness of the built simulation mode. Results indicate that amplitude and harmonics of the core limb vibration increase greatly under DC bias condition. The research of paper will also serve as references for addressing the vibration problems of transformers under DC bias conditions.