Power Transformer Inrush Current Analysis

As EHV power network is developing rapidly, large capacity power transformers are used widely in the transmission system and distribution system. Therefore, high performance and high reliability of power transformer protection are required urgently. At present, the main protection of internal faults of the power transformer is the differential protection. When the unloaded power transformer is switched on the power system or in the process that a fault, outside the protected zone, is cleared, the magnetizing inrush current will ususlly produced. The differential protection relays will maloperate. So it is still a research focus for relaying researchers in the world.In this paper the transient course after an external fault of transformer is cleared is analyzed from the variation of flux . It is proved from the method of simulation that the transformer differential protection and the current protection possibly maloperates due to the recovery surge .Based on the analysis of sympathetic inrush in operating transformers, the principle and characteristics of sympathetic inrush are explained . It is shown that the inrush current caused by the energized transformer running across the system resistance results in the deviation of voltage in the common node which lead to the saturation of the core of the transformer is the main reason for the sympathetic inrush on the adjacent transformer. Then the influence on transformer protection caused by sympathetic inrush are discussed. Finally, several countermeasure to sympathetic inrush are proposed.