Study On Transient Characteristics Of Electrical And Thermal Insulation Defects Of New Type Current Transformers Used In Distribution Networks

As the key equipment of signal acquisition in power system,current transform can provide accurate primary current signal for secondary measurement,control,protection and other equipment in power system,so as to ensure fair electric energy trade settlement and safe and reliable operation of power system.Among them,the cast current transformer has been used more and more widely in China’s distribution network system due to its advantages of small size,long maintenance period,safety and environmental protection.However,with the improvement of the reliability of signals such as measurement and protection,once the current transformer fails,the interaction effect on primary and secondary equipment is very prominent,which seriously threatens the stable operation of the distribution network.Among them,current transformer faults caused by insulation defects and insulation damage caused by long time operation under load account for a considerable proportion,which has become a key problem to be solved urgently.This thesis takes the new type of cast current transformer for distribution network as the research object,and calculates the transient electric field and temperature field distribution of the current transformer.In addition to this,partial discharge(PD)of different types of defects were carried out with PD detection platform.The research results are helpful to reveal the deterioration law of electrical and thermal insulation of the cast current transformer and provide some reference for the internal defect detection and insulation optimization design.The main research contents of this thesis are as follows:(1)The three-dimensional model of the new cast current transformer for distribution network is established.The electric field distribution of the new cast current transformer for distribution network under the chopped lightning impulse is calculated when the transformer has defects of surface burr,inner bubble and attached metal particles and without defects and the insulation damage characteristics of the current transformer caused by the defects are obtained.The results show that the electric field distortion is the most serious when there is a burr defect on the surface of the primary guide rod,the maximum field strength reaches 501kV/cm,which is far greater than the defects of internal bubbles and attached metal particles.The existence of burr defects under the chopped lightning impulse will seriously harm the longitudinal insulation of the current transformer.(2)Adopting the frequency-steady-state electric-thermal coupling calculation method,the temperature field distribution characteristics of the new cast current transformer for distribution network under different loads and different environmental conditions were obtained through simulation,and the primary guide rod of the transformer was optimized.The results show that the temperature distribution of primary guide rod,which is the main heat source of transformer,is "parabolic".Under the maximum current,the hot spot temperature of transformer reaches 334K,and the temperature gradient is 2111K/m.Moreover,the maximum temperature difference and temperature gradient difference have no obvious correlation with the external ambient temperature.When the core diameter of the guide rod is between 18 and 20mm,the hot spot temperature and temperature gradient change tend to be stable,which meets the specification requirements of class A insulation and is the optimal core diameter range.(3)In this thesis,a test platform for partial discharge detection of current transformer is established,and the partial discharge under different defect types and different defect conditions is measured by using pulse current and UHF methods.The results show that the electric field in the area where the burr defect located is more prone to distortion and then partial discharge occurred,which causes damage to the insulation performance of the current transformer.Compared with the single needle,the two-needle defect can aggravate the degree of partial discharge,and the intensity of partial discharge gradually decreases with the increase of the two-needle distance.Metal particle defects almost have no partial discharge signal.