Distinguishing Inrush Current Based On Characteristic Of Equivalent Instantaneous Inductance

Transformer, as one of the most important equipment in the power system, is the hub to transform electric energy among different voltages, and its running state affects the safety and stability of the power system. With the enlargement of electric system and the rising of voltage level, the environment of transformer's operation is more complex. So the function of transformer protection has declined in accuracy, and some mal-operations appear now and then. Therefore, it is very important to search for a more effective and more reliable protection scheme to improve the performance of transformer protection.Firstly, the paper introduces some new theories about transformer differential protection, and gives a further explanation and analysis about these theories. Then, using digital simulation, the paper analyses the correctness of this principle—the phase comparison principle based on the fault component of current, proves the effectiveness and feasibility of this principle in distinguishing internal fault and external fault. The simulation's results indicate the defect that it can not distinguish between inrush and internal fault. For this defect, the paper introduces a principle based on characteristic of equivalent instantaneous inductance. According to the characteristic that the equivalent instantaneous inductances of normal transformer is comparatively large before flux being Saturation while the fault transformer's equivalent instantaneous inductances is comparatively small when no-load closing, a novel transformer protection principle based on characteristic of equivalent instantaneous inductances is proposed. The improved principle can distinguish inrush and internal fault more quickly and effectively.The existed transformer protection principles based on equivalent instantaneous inductance cast off the influence of the magnetizing inrush, however, the principles do not consider the impact of the circulation in△side of the transformer which is Y/△connected as well as the problem that the vice-side data can not be obtained when no-load transformer is closed. Utilizing the primary-side data in 5ms after no-load transformer closed identifies the equivalent instantaneous inductances.In response to the demand of transformer protection research, based on the previous experiences, the digital simulation model and dynamic imitation model at the representative running states of transformer are established. And finally this simulated data is used to verify the proposed principle. Good performance is shown from the calculated results.