Research On Residual Magnetism Characteristics Measurement And Elimination Simulation Of Single-phase Four-limb Transformer

It will produce residual magnetism under the conditions of direct resistance measurement,random switching,equipment maintenance,factory test,relay protection action after short circuit fault,and changing tap-changer.The reason of residual magnetism in transformer is the hysteresis characteristics of the ferromagnetic material.The existence of residual magnetism will lead to many adverse effects to the operation of the power transformer,such as the closing magnetizing inrush current,the high loss,the strong vibration and noise,wrong gas protection warning and differential protective malfunction.In order to eliminate its influence,the residual magnetism value of the transformer need to be accurately measured.Besides,the inrush current of the transformer also need to be recognized and suppressed.At present,the identification and suppression of transformer inrush current has achieved good results,but due to the complexity of transformer core magnetization process,the existing residual magnetism measurement methods are not mature enough to accurately measure it.In that case,this paper carries out the measurement and analysis of residual magnetism characteristics of single-phase four-limb core structure(referred to as single-phase four-limb transformer)commonly used in large transformers,as well as the simulation study of its elimination.The main research contents are as follows:Firstly,the magnetization process of ferromagnetic materials is analyzed.For eight kinds of single-phase four-limb transformers with different silicon steel sheets,the comparative experiments of magnetization characteristics measurement under AC and DC conditions are carried out,and the waveform distortion in the measurement process is analyzed and processed.The results show that AC and DC magnetization are similar in the linear region(H=0A/m-150A/m),and in the knee region(H=150A/m-450A/m),AC magnetization is slightly higher than DC magnetization,and in saturated region(H=450A/m-1000A/m),AC magnetization and DC magnetization are similar,but DC magnetization process is easier to make transformer core into a deep saturated region.Secondly,through theoretical analysis of residual magnetism,a measurement platform with baseline correction integral is built.The residual magnetism characteristics of single-phase four-limb transformer after direct resistance measurement and random switch off are studied,and their respective variation laws are analyzed.When DC excitation occurs,the residual magnetism increases firstly and then decreases with the increase of current.After random switch off,it presents a statistical characteristic of nonstandard cosine variation.A method for measuring initial residual magnetism of singlephase four-limb transformer based on Rayleigh region integral is proposed.The validity of the method is verified by experiments.The difference between the results of initial residual measurement using equal and unequal amplitude excitation currents is analyzed.It is found that under equal amplitude current,the relative errors of linear and knee segments are smaller than 2.0%,and the average relative error of saturation section is about 4.13%,which is larger than that of linear section and knee section.And under unequal amplitude current,the relative error of linear section,knee section and saturation section is close,and the average relative error is about 5.27%.It is found that the relative error of initial residual magnetism measurement is larger under unequal amplitude current.The effect of different direct resistance on the initial residual measurement of singlephase four-limb transformer is analyzed.The results show that the series resistance can shorten the measurement time,but too large resistance can also cause larger measurement error.Finally,based on the magnetic path-electric circuit duality principle and J-A hysteresis theory,the electromagnetic transient model of single-phase four-limb transformer is constructed.The equivalent inductance in the electromagnetic transient model is calculated by finite element analysis,and the J-A hysteresis model parameters of the non-linear inductance are calculated by particle swarm optimization.According to the theory of residual magnetism elimination of transformer,the method of commutative direct current is adopted to simulate and analyze the residual magnetism elimination.The results show that when the initial residual magnetism is known,the residual magnetism can be eliminated in five half cycles by applying the commutating DC power supply with amplitude of 10 V and frequency increment coefficient of 0.67,and the whole demagnetization time is about 8.4 seconds.Because its initial excitation direction is the same as the initial residual magnetism direction of the core,the demagnetization time is shortened.