The Research On Discriminating Magnetizing Inrush In Power Transformer Protecting

As rapid development of electric undertaking, ultra high voltage,large capacity and long range transmitting electricityhave become very popular now and the large-sized power transformer has a more significant status in power supply system than before, so, it brings forward a big project of how to protect the power transformer more precise. In protecting of transformer differential, it is always the hotspot of researching for distinguishing excitation surge current and internal fault current, which is congenital and unavoidable. The existing method of transformer differential protection has some drawbacks, the main reason of which is that people can not know the principle of transformer internal fault mechanism exactly. Based on that, there is great important meaning in the improvement of transformer relaying protection level and reduce accident based on developing and improving of existed transformer differential protection.In this paper, a detailed introduction on causes of excitation surge current has been mentioned, after which, the paper analyzes and appraises various of distinguishing methods based on electric value and modern signal process by sorts.Based on these analyses for magnetic circuit and electrocircuit of transformer, the paper finds out the differences between excitation surge current and internal fault current which are produced by transformer and sets up a model for transformer. Then, a action equation grounded on single phase and three phase transformer model has been gotten to work out a protecting design for transformer model. The method distinguishes excitation surge current and internal fault current by calculating practical calculation difference and threshold value. As protection function both sides practical calculation difference are less than setting threshold value, excitation surge current is determinated. As practical calculation difference are greater than setting threshold value, internal fault current is determinated.The paper states that two kinds of transformer (single-phase and three-phase three-coil ) running under several states (empty load switch on, turn-to-turn fault, turn-to-turn ground, resuming power supply after switching external malfunction) simulated by MATLAB and PSB tools kit. The simulation results demonstrate the scheme effectively recognizes excitation surge current and internal fault current. Differential protection method based in transformer module reasons protection function and submits protection scheme. The protection scheme does not affected by excitation surge current, also not acquire accurately internal parameter of transfomer. Its simple execution provides a new line for differectial protection existed.