Research On Transformer Inrush Current Suppression And Fault Identification Based On Power Electronic Switch

As an important equipment in the power system,the safe and stable operation of the transformer is related to the reliability of the power supply of the entire power grid.The differential protection is the most important protection method for transformers,but the inrush current caused by air charging will flow into the differential circuit of the differential protection,which will cause the differential protection to malfunction and greatly reduce the correct rate of protection.Therefore,how to correctly distinguish the normal air-charging and air-charging faults of the transformer to prevent the influence of the inrush current on the protection performance is still a problem to be studied.To this end,the paper proposes a new idea that combines inrush current suppression and fault identification,and designs a set of control loops,pointing out the research work that the paper needs to complete.The thesis first proposes a control scheme for the three-phase voltage closing angle based on the mechanism of the transformer inrush current to achieve the suppression of the three-phase inrush current of the transformer.Based on this,the difference in electrical characteristics under normal and empty charging fault conditions is analyzed.Finally,the corresponding criteria are proposed.The main work and research results of the paper are as follows:(1)A transformer internal fault identification method based on power electronic switching devices is proposed,which combines inrush current suppression and fault identification.First replacing the traditional circuit breaker with a power electronic switch to improve the control accuracy of the closing angle.After comparing and analyzing various three-phase voltage closing angle control schemes,determining the final closing angle control strategy-delay Closing,that is,"the one-phase switch of the transformer is closed at its peak of the power supply voltage,and the remaining two phases are closed at the same time at the time of the zero-crossing of the first closing phase after waiting for about three to four power frequency cycles." The maximum amplitude of the magnetizing inrush current caused by the charging transformer is the first step in the transformation from the question "How to distinguish between normal empty charging and air charging failure of a transformer" to the question "How to distinguish between normal operating current and internal fault current of a transformer".(2)After analyzing the current waveform of the transformer’s power supply after suppression,according to the waveform characteristics of the transformer’s first closing phase(hereinafter referred to as the "first closing phase")before and after the closing phase of the two phases under different operating conditions,it is proposed that transformer fault identification criterion based on the first-phase phase current fundamental wave amplitude.This criterion can realize fault identification by only one electrical quantity of current,and it is hardly affected by the initial residual magnetism of the transformer.At the same time,the protection can operate reliably when the transformer is empty-filled with a small number of windings.The criterion is simple and the calculation is small,which is convenient for engineering application.Matlab simulation shows the effectiveness of the method.(3)Considering that a large current may occur when the transformer has an internal fault,this paper proposes to limit the fault current by using a series inductor or a series resistor to reduce the requirement for protection speed to a certain extent.On the basis of comparative analysis,combined with the research goal of this article-fault identification,the scheme of series resistance was finally determined,and the general expressions for the series resistance solution at different voltage levels were listed.The series resistance can limit the fault current to the rated current value to avoid damage to the transformer;on the other hand,it can accelerate the attenuation of transient components in the current.The second step of the transformation from "normal air-charging and air-charging failure" to the question "how to distinguish the normal operating current and internal fault current of the transformer" makes the fault characteristics more obvious and is helpful to distinguish between the air-charging failure of the transformer and the normal air-charging.When it is judged that the transformer has no internal faults,the conventional circuit breaker branch is three-phase closed,and the series branch of the power electronic switch and the resistor is withdrawn.At this time,the transformer operates normally and no inrush current occurs.When it is determined that there is an internal fault in the transformer,the series branch of the power electronic switch and the resistor is disconnected,the closing of the traditional circuit breaker is no longer performed,and the operating personnel performs maintenance.