Research On Vibration And Noise In Transformer Core Under DC Bias Conditions

The status of current energy field presents higher overall output, less consumption per capita, area distribution unbalanced, as a result, HVDC transmission line with large capacity and long transfer distance becomes inevitable. But with the large capacity long distance HVDC applied, under the earth returning operation mode, HVDC system earth current will flow over the transformer winding through the neutral point, this causes the transformer DC bias phenomenon. More and more research scholars noticed this phenomenon. The impact to the transformer caused by DC bias is mainly reflected as increase of leakage flux caused by the severe iron core saturation, a high degree of distortion of the excitation current, a large number of harmonics, and thus initiated loss of structural parts, partial overheat, insulation damage, core vibration increase, noise increase. Meanwhile it will also cause power grid voltage dropping, voltage wave distortion, false trip of relay protection. This article will focus on core vibration and noise under DC bias condition.Based on finite element numerical analysis, COMSOL Multiphysics software is used for simulating real physical phenomena through the solution of partial differential equation (single field) or partial differential equation group (multiple fields), which is named by current world scientists as A real direct coupling analysis software to any physical field combinations.There are two major reasons caused the vibration of transformer core, magnetostrictive of silicone steel slice and the electromagnetic attraction generated by magnetism leakage existed between joint of silicone steel slice and folded sheet. This article stimulate the core vibration and noise under DC bias condition by using COMSOL Multiphysics. The COMSOL Mutiphysics software conduct multi physical field coupling analysis to electric field, magnetic field, structural stress field and acoustic field, set up equation of single field by using three D transient equation solver, conduct coupling analysis to above four fields, thus get distribution status of vibration and noise of the transformer core. By using field-circuit coupling method, you will get transformer core vibration and noise distribution under DC bias condition, and compare the influence to the core vibration and noise between DC current and no DC current, and among different levels of current.Based on modal analysis theory, modal analysis is conducted to the transformer core under two situations– core as single or as whole body. The COMSOL Multiphysics software will help get the first four vibration modals and resonance frequency of whole core body.