Research On Nondestructive Testing Method Of Transformer Material Based On High-Frequency Skin Effect

Transformers are one of the core equipment in the power grid,and their quality has a significant impact on the safety and stability of power system operation.In recent years,some manufacturers have taken the liberty of using aluminium windings instead of copper windings for the production of transformers when the procurement contract clearly stipulates the production of copper windings,which has brought huge safety hazards to the power system.Transformers have a closed structure,making it difficult to directly observe the winding material from the outside.Most existing non-destructive testing methods have problems such as complex equipment,high environmental requirements,and low detection accuracy,making it difficult to promote them in large-scale engineering applications,resulting in the phenomenon of"replacing copper with aluminum" being repeatedly prohibited.In order to achieve rapid in-situ non-destructive testing of transformer winding materials,this paper studies the difference in harmonic AC resistance of copper aluminum winding transformers under high-frequency skin effect,and proposes an identification method based on swept frequency resistance.The specific research content is as follows:(1)A theoretical analysis was conducted on the non-destructive testing scheme for transformer winding materials based on high-frequency skin effect.Firstly,based on the principle of skin effect,the difference in harmonic AC resistance of copper aluminum conductors under high-frequency excitation was analyzed,and a method of identifying transformer winding materials using high-frequency excitation response was proposed.Then,the harmonic model of a long straight conductor under high-frequency AC excitation was derived through theoretical analysis,and the model was analogized to the transformer winding.Finally,a material identification scheme was proposed by comparing the theoretical calculation data of the harmonic model of the transformer winding with the actual test data,using the difference in harmonic resistance of the copper aluminum winding transformer under the influence of high-frequency skin effect.(2)A swept frequency harmonic resistance measurement system based on the automatic balance bridge method has been developed.Firstly,in order to accurately test the high-frequency harmonic resistance of transformers,this paper proposes corresponding short-circuit testing circuits and experimental models based on the commonly used structures of distribution network transformers,and proposes a harmonic AC resistance sweep frequency testing method based on automatic balance bridge.Then,a harmonic resistance sweep frequency testing system based on an automatically balanced bridge was developed,and the repeatability and accuracy of the testing system were tested through experiments,and potential errors in the experiment were analyzed.Finally,the system is used to test several dry-type transformers,oil immersed transformers and amorphous alloy transformers,and the experimental results show that the harmonic resistance identification method based on high-frequency skin effect can effectively distinguish copper aluminum winding transformers.(3)To promote the promotion and application of the classification method for copper aluminum winding transformers proposed in this article in engineering scenarios,a transformer winding material detection system based on the frequency sweep resistance method has been developed.Firstly,based on the collected data of copper transformers,a standard database for harmonic resistance curves of copper transformers was established,and specific detection and processing steps were proposed.Then,a copper aluminum transformer identification system based on high-frequency skin effect was developed,and the system was used to detect high aluminum and low copper transformers.The experimental results verified the feasibility of the proposed scheme in this paper.Finally,a impedance angle identification scheme using existing data for further analysis was proposed to assist in verifying the identification results of transformer winding materials.