Research On Power Transformer Vibration And Noise Under DC Bias Condition

With the development of extra-high voltage transmission project in China,the application of the long distance direct current transmission is generally more extensive.In the long distance high-voltage direct current transmission process,it is easy to cause the DC bias phenomenon within the power transformer,resulting in excitation current distortion,aggravation of the core saturation,increase of magnetic leakage,and enhancement of transformer vibration and noise.Aiming at this problem,taking a 240000 k VA,330 k V three-phase five-column power transformer as an example,this paper focuses on the power transformer vibration and noise problem under DC bias.The modal analysis of the core structure is carried out,and the natural frequencies and modal shapes of the core are obtained.The twelve natural frequencies of the core are susceptible to resonance,such as the first order and the fourteenth order.And the modal analysis of the cavity in the power transformer is carried out,and the natural frequencies and modal shapes are obtained.The first radial natural frequency(445.43Hz)is easy to cause resonance phenomenon.Then,based on the electromagnetic field theory,the electric field,magnetic field,structural force field and acoustic field of the prototype are coupled,and the influence of DC bias on the vibration and noise of the power transformer core is analyzed.According to the magnetic density and the density of the transformer core under different magnetic bias,the structural force field is solved,and the core displacement and the surface acceleration are obtained.Taking the surface acceleration as the excitation of acoustic field,the noise distribution of transformer under different magnetic bias is determined.The sound pressure level of the transformer is 63.72 d B without DC bias;the sound pressure level is increased to 66.81 d B with 1A DC current;and the sound pressure level is 70.16 d B with 2A DC current,which is 6.44 d B higher than the normal operation.The phenomenon of DC magnetic bias leads to the asymmetry of the core magnetic density in one cycle,which makes the vibration and noise of the core increase.Finally,the core vibration and noise of the prototype under the condition of no load and DC bias are tested and compared with the theoretical calculation,which proves the accuracy of the simulation method.