Research On Electromagnetic-mechanical State Of Transformer Windings Under Unbalanced Operation

In the power supply and distribution system,three-phase four-wire power distribution and the randomness of single-phase load power consumption will cause the distribution transformer to operate in a three-phase load unbalanced state for a long time.This can lead to neutral point shift,increased losses,reduced equipment utilization,which in turn affects power quality,and even burns out equipment due to increased single-phase voltage.At the same time,the asymmetry of the winding current caused by the unbalanced three-phase load will cause changes in the internal electromagnetic and mechanical characteristics of the transformer.The cumulative effect will affect the structural stability of the transformer,and in severe cases,it will lead to internal faults of the transformer and even power grid accidents.Relying on the National Natural Science Foundation of China（51507027）,the Science and Technology Development Program of Jilin Province（20190303007SF）,and some State Grid Science and Technology Projects,this paper explores the electromagnetic field and mechanical field characteristics of the transformer windings during unbalanced operation and summarizes the rules.Based on vibration signal processing to obtain multi-dimensional characteristic parameters,the identification method of transformer winding vibration characteristics is studied.Firstly,the electromagnetic coupling method is introduced.The validity of electromagnetic simulation calculation is verified by an example.The electromagnetic-mechanical sequential coupling method for unbalanced operation of transformers is studied based on electromagnetic coupling.Aiming at the problem of aggravated winding vibration during transformer unbalanced operation,the mathematical model of transformer unbalanced operation winding state is established.The mechanical field parameters are solved by electromagnetic-mechanical sequential coupling,and the winding current,winding flux leakage,electromagnetic force and vibration changes in the unbalanced mode are simulated and analyzed.Then,the electromagnetic-mechanical coupling characteristics of the windings are deeply studied,and the changing laws are summarized.A dynamic model experimental platform for unbalanced operation of transformers was built to collect winding current and vibration information.By comparing the simulation and experimental results when the load is unbalanced,the validity of the three-dimensional finite element model of transformer in simulating the changes of electromagnetic and mechanical characteristics of windings is verified.The results of winding force and vibration show that when the unbalance degree of the transformer reaches 20%,the electromagnetic force and vibration acceleration of the unbalanced phase windings are increased by about 1.5 times compared with the normal phase.The windings bear a large deflection moment at this time.Its cumulative effect will destroy the structural stability of the transformer during long-term operation.Based on the study of the electromagnetic-mechanical characteristics of the unbalanced running windings of the transformer,the feature extraction and state division of transformer vibration information are studied aiming at the identification of winding vibration characteristics.A method for identifying the vibration state of transformer unbalanced windings based on Hilbert-Huang transform and multi-class support vector machine is proposed.Through the identification of winding vibration state based on multi-dimensional feature parameters,the accuracy of unbalanced vibration feature identification could reach 98.5%.It shows that the method proposed in this paper can accurately and effectively identify the vibration state of transformer unbalanced windings,and provides a new idea for transformer state evaluation and identification.