The Analysis Of Magnetic Induced Current Impact On Power Transformers

Solar storm phenomenon or monopole earth running of Extra-High-Voltage Direct Current transmission system both probably causes transformer DC bias magnetic effect, further producing potential safety hazard to electric power system. Transformer DC bias magnetic phenomenon is studied in depth in the thesis, and a new restraining measure is proposed, directing how to restraining DC bias magnetic effect more reliably and effectively in practice.1. In order to learn about transformer DC bias magnetic phenomenon more clearly, the effect is deduced meticulously out of mathematics angle, and the relation between the effect and the parameter inside transformer is reached, providing the theory basis for the following restraining and weakening effect. Set up the corresponding equivalent circuit model separately aiming at the transformer with the different iron core construction, by analysis draw the conclusion that three-phase block-type transformer is most easily affected by DC bias magnetic effect, and meanwhile provide the theory basis for the model selecting.2. Because excitation current of transformer will produce the serious distortion phenomenon after bias magnetic, the paper adopts the way of wavelet transform to analyze the distorted excitation current signal, according to the advantages on the wavelet transform processing the distortion signal. In the thesis, DC component value of distorted excitation current is obtained by digital simulation experiment and also to some extent, the value increases based on DC value inflowing into winding. The moment of signal distortion is accurately reflected in the high frequency detail signal. The conclusion that it may cause the malfunction of protective relaying device is reached, by analyzing high band energy spectrum of distortion signal.3.In order to reduce or eliminate the influence of DC bias magnetic effect on the transformer, we deduce the relation between the parameter inside transformer and the effect by analysis, strengthening immunity to the effect by optimizing design parameter inside transformer. Several methods of restraining bias magnetic effect, such as, series capacitor outside transformer, inserting dynamic current and series resistance, and so on, are compared and analyzed in the paper. Then the restraining project based on switch protection of bidirectional thyristor is put forward on the basis of the current existing problems of restraining device.4. The restraining device based on switch protection of bidirectional thyristor is tested and verified by digital simulation. The device can isolate DC, and meanwhile protect the capacitor effectively and real-timely with fast break-over of thyristor and closing-up of by-pass switch. Moreover, it can keep reliable neutral ground of transformer in the whole process of alternating current system fault. The simulation results supply the basis for practical production of device.