Research On Magnetizing Inrush Of Transformer In Traction Substation

Traction transformer is one of the most important equipments in traction power supply system, and plays an important role in the operation of electric railway traction. As the main protection of traction transformers, some problems exist in differential protection applied in the field. It is important to find out the reasons affecting the performance of differential protection and propose the relative solutions.Firstly, the construction and principle of V/V connected traction transformer is introduced. The protection configuration of differential protection used as the main protection and the limitation of differential protection are described. Also, the principle of magnetizing inrush produced when a transformer energizing is introduced. To recognize the magnetizing inrush from fault current, the equivalent instantaneous inductance based algorithm is modified according to the special structure of V/V connected traction transformer to recognize magnetizing inrush from fault current. The experimental results show that the algorithm can distinguish magnetizing inrush and sympathetic inrush reliably and is immune to the saturation of current transformer.Secondly, magnetizing inrush would be caused by the bad insulation and AT power supply model. The series sympathetic inrush might be produced by the magnetizing inrush running through the operating transformer according to the special structure of the traction substation. The influence of series sympathetic inrush on differential protection is also presented. Time difference exists between the voltage unbalance jumping and the differential current appearing. Based on the above analysis, a differential-time method is proposed to distinguish the series sympathetic inrush. The algorithm detects the magnitude of the ratio and state of differential current, and regards the moment that the ratio exceeds the threshold as a switch operation. The algorithm recognizes series sympathetic inrush by detecting the differential time between the switching operation and the appearance of differential current. Some special cases are analyzed, such as adding or shedding load suddenly, transferring fault between inside and outside of transformer and appearing internal fault during series sympathetic inrush. Dynamic experiments verify the feasibility and reliability of the proposed algorithm.Finally, the mathematic model and saturation process of current transformer are discussed. The phase-shift saturation of current transformer would take place when the current transformer transfers a small current with the frequency of 50Hz. According to the characteristic of flux linkage accumulation during the phase-shift saturation, an algorithm based on the variance of relative flux linkage accumulation in phased-shift saturation is proposed. Dynamic experiments and field data show that the algorithm has good performance in detecting the saturation.