Influence Of Stray Current On Harmonic Characteristics And Differential Protection Of Transformer

With the continuous development of urban rail transit,the problems caused by stray current of rail transit can not be ignored.Stray current of rail transit intrudes into transformer through neutral point,causing transformer magnetic bias phenomenon.On the one hand,a large number of harmonics appear in transformer excitation current and neutral point current,resulting in deterioration of power quality of power grid.On the other hand,the harmonic wave is transferred to the low voltage side and further amplified by the reactive power compensation capacitor bank,which threatens the normal operation of the capacitor bank.In addition,magnetic bias will have adverse effects on magnetizing inrush current characteristics and differential protection based on second harmonic braking criterion.Most studies on magnetic bias are considered from the perspective of constant DC injection,the amplitude direction of stray current in rail transit is constantly changing,so the characteristics of transformer magnetic bias induced by stray current in rail transit are not clear at present.Therefore,it is of great significance to study the influence of stray current of rail transit on transformer magnetic bias.This paper focuses on the influence of stray current of rail transit on transformer harmonic characteristics,excitation inrush current characteristics,differential protection and reactive compensation capacitor banks.The main research work is as follows:(1)By analyzing the coupling structure of rail transit and power grid,the path of stray current generation and intrusion into the power grid is given.The amplitude-frequency characteristics of stray current are studied,and the relationship between harmonic current,noise and low-frequency component of transformer neutral current is analyzed,which provides a basis for the accurate modeling of stray current source.(2)The transient change of the DC component of the flux under constant DC action is analyzed,and the correctness of the theoretical analysis is verified by simulation.On this basis,the transient changes of the DC component of the flux under the action of low-frequency alternating stray current are studied and the harmonic characteristics of the transformer field current and neutral current under the action of low-frequency alternating stray current are simulated and analyzed.Finally,the factors affecting the transformer magnetic bias level under the action of stray current are summarized.(3)The influence of stray current on transformer air-drop flux is revealed.Based on this,the influence of stray current on magnetizing inrush current characteristics is simulated and analyzed.The influence of stray current on transformer differential protection based on second harmonic braking criterion is analyzed by the simulation of single phase grounding fault,high resistance grounding fault and phase to phase fault.(4)A single phase equivalent circuit of transformer containing reactive compensation capacitor is established,and the way of harmonic transfer of excitation current to capacitor bank is analyzed.Combined with the analytical formula of excitation current under magnetic bias condition,the harmonic current flowing through two kinds of capacitor banks with series reactance of 5%and 12%under magnetic bias condition is calculated,and the amplification of harmonic of capacitor bank is analyzed.The law of the variation of the fourth harmonic voltage content in the low voltage side under the influence of stray current is simulated and analyzed,and the practical problems in the field operation are explained in a mechanical way,and the relevant strategies to restrain the influence of stray current on the capacitor bank are discussed.