Research On Diagnosis Technology Of Internal Contacts Of On-load Tap-changer Based On Characteristic Ion Detection In Oil

At present,China’s power industry is developing rapidly.Transformer is an important voltage regulation equipment in power system,and the voltage regulation operation is completed by on-load tap-changer.The failure of the on-load tap-changer is one of the main reasons for transformer accidents,and the proportion has increased in recent years.The important reason for the failure of the on-load tap-changer is the deterioration of the metal contacts.Therefore,it is necessary to carry out research and health diagnosis of the contacts inside the on-load tap-changer.Corrosion is common in the insulating oil inside the on-load tap-changer.Under the high temperature environment,the corrosive sulfur activity in the oil increases,which accelerates the deterioration of the insulating oil and the erosion of the contacts,resulting in the diversification of ionic products in the insulating oil.It provides a new reference direction for internal state evaluation: the evaluation of the internal contact health of the on-load tap-changer using the detection experiment of characteristic ions in oil.This paper refers to the basic idea of condition-based maintenance,starts with the ion concentration in oil,explores the relationship between the characteristic ion content and the fault,and establishes the contact condition evaluation criterion,so as to achieve the purpose of evaluating the contact health status.In this paper,the generation mechanism of ions in oil was first studied.For typical oil arc-extinguishing on-load tap-changers,contact deterioration caused by arcing is one of the main reasons for the formation of metal ions in oil.The process of arc erosion of contacts to generate metal ions and the decomposition of insulating oil to generate non-metal ions during arc action were studied respectively,and the electrical and non-electrical factors affecting the arc action process were discussed.Based on the above analysis,metal copper ions are determined as the characteristic ions suitable for the diagnosis of the internal contacts of the on-load tap-changer.A simulation model of contact gap arc discharge in insulating oil is established.Streamer simulation can be used to describe the pre-breakdown process of the gap,citing streamer theory,considering the microstructure of the contact gap,and establishing a physical model of the pin plate.In order to simulate the contact gap discharge of the on-load tap-changer under different voltage levels,the simulation of current flow under different voltage amplitudes is carried out.The results show that the voltage amplitude has a very obvious effect on the development of the streamer.With the increase of the voltage amplitude,the space charge accumulation speed of the streamer head accelerates,the electric field distribution becomes more distorted,and the temperature increases more.The influencing factors of characteristic ion content in oil were studied,and the evaluation criterion of contact health was established.Experiments on non-electrical factors and electrical factors affecting the content of characteristic ions in insulating oil were carried out.The non-electrical factor experiment explores its influence on the copper ion content from the gas environment and temperature.The experimental results show that the gas environment mainly affects the copper corrosion path,and the temperature affects the copper corrosion rate.The experimental results of electrical factors show that the characteristic ion concentration in the oil is positively correlated with the arc breakdown times.In addition,the breakdown voltage of copper contacts also changes with the increase of the breakdown times.Finally,a diagnostic criterion for the internal contacts of the on-load tap-changer based on the detection of characteristic ions in oil is established.The corresponding early warning system is established by analyzing the characteristic ion concentration and the relative growth rate of the characteristic ion concentration,and the reference set value of the graded early warning is given.