| For the low voltage distribution grid in China,it mostly adopts three-phase four wire structure,and there are a lot of unbalanced single-phase load in the grid.It is affected by the switching,temporary and seasonal power consumption of single-phase equipment,line failure and other problems,which leads to the long-term unbalanced operation of most distribution transformers,which directly leads to the increase of line loss and equipment utilization rate Lower.At the same time,due to the different phase loads of transformer,the winding current is different,which affects the internal electromagnetic environment and mechanical characteristics,and endangers the stability of the transformer structure,even causes serious accidents.Based on Jilin Science and technology development plan project(20190303007sf),Jilin Science and technology innovation development plan project(20166014),National Natural Science Fund Project(51507027)and several national grid science and technology projects,this paper explores the electromagnetic mechanical characteristics and change rules of winding under unbalanced operation mode of transformer,and studies the identification method of transformer operation state based on vibration signal.Firstly,the finite element method and its application in the calculation of transformer electromagnetic field are introduced.The effectiveness of FEM to the calculation of transformer magnetic field is analyzed by simulation.The electromagnetic direct coupling and electromagnetic mechanical indirect coupling method are described.Based on this,the electromagnetic mechanical coupling model of transformer is built.Then,the current,internal magnetic leakage and vibration characteristics of the transformer under different operating modes are studied in view of the unbalanced vibration of the winding caused by the unbalanced operation of three phases of the transformer.Based on the electromagnetic principle,the state equation of transformer mode state analysis is established.The vibration acceleration of winding is solved by electromagnetic mechanical coupling.On this basis,the three-dimensional finite element simulation model of transformer is built.The corresponding relationship between the internal electromagnetic parameters and the vibration characteristics of the winding under different operating modes is studied.The dynamic model experiment platform is built.The data of winding current and vibration acceleration in different operation modes and states of transformer are collected.The simulation results and experimental data show that under the condition of 20% unbalance,the stress and vibration acceleration of unbalanced phase winding can be increased by 1 times,and the unbalanced vibration acceleration of three-phase winding is about twice different,which seriously endangers the stability of transformer internal structure.In view of the problem of the change of winding vibration characteristics and low utilization rate of vibration data under different operating modes and states of transformer,based on the coupling principle of electromagnetic machinery and winding vibration mechanical model of different operation modes of transformer,the original vibration signal is reconstructed by wavelet packet transform,the energy distribution law of different frequency scales is studied,and the vibration feature identification method based on scale energy ratio is proposed.The vibration information of winding is obtained by dynamic model experiment,the signal is reconstructed by wavelet packet decomposition,the scale energy ratio characteristic value is extracted,and the vibration characteristics of unbalanced operation winding are studied,which provides a new diagnostic method for unbalanced abnormal condition of transformer.When the transformer is in normal operation,the frequency component signal accounts for 46.72% of the total signal energy.When the unbalanced operation a is-20% and 20%,the ratio of the second frequency energy decreases by 75.67% and increases 42.07%,respectively.In the unbalanced operation,the vibration characteristics of the three-phase winding are different,and the unbalanced vibration moment will harm the stability of the whole structure of the transformer winding. |
