Research On Inrush Current Of Power Transformer

Power transformer realizes power transmission between different voltage levels through electromagnetic energy conversion,which is a very important equipment in power system.Statistical data of relay protection operation for many years show that the correct operation rate of transformer protection is the lowest.When the transformer closes in no-load,or when the voltage restores after the external fault is removed,the inrush current may be generated on one side of the transformer several times the rated current,and the corresponding inrush current may also be generated on the normal operation of the transformer around,which may bring potential malfunction to the differential protection of the transformer.When inrush current flows in the system,it may cause bipolar blockade,power drop and power oscillation in HVDC transmission system.Aiming at these problems,this paper makes a detailed study on transformer inrush current,analyses the influence mechanism of inrush current on AC and DC system,and puts forward the suppression strategy of inrush current.The second harmonic restraint method is the most widely used method to identify the inrush current in the field,but in operation,there are occasional cases when the inrush current is misoperated due to the low second harmonic ratio.Based on the generation mechanism of transformer inrush current,the influence factors of second harmonic ratio of transformer inrush current are studied theoretically with simplified transformer model,and various influence factors are analyzed with digital simulation and dynamic simulation test.Around the problem of transformer and sympathetic inrush current,the physical mechanism of sympathetic inrush current is systematically analyzed from the angle of current.Then,through dynamic model test,various influencing factors of responding inrush current are studied,including power supply impedance,operating voltage,capacity ratio of air-drop transformer to operating transformer,neutral grounding mode,operating transformer with generator,etc.The commutation failure mechanism of DC system is analyzed through theoretical analysis.In view of the influence of voltage distortion on commutation failure,the effects of voltage sag,voltage phase offset and harmonics on commutation failure are analyzed.Then the response process of DC system to inrush current is studied by digital simulation.In order to solve the influence of inrush current on AC and DC system from the source,a strategy of inrush current suppression based on phase selection control circuit breaker is obtained by means of dynamic model test,and the control methods of actual opening and closing angles are given.This strategy can avoid the most complex problem of inrush current suppression-remanence.The relationship between opening angle and closing angle can be obtained directly from dynamic model test data,which has a certain reference value for practical transformers.