The large capacity power equipment should be equipped by absolutely independent duplicated main protections. For large capacity generators there are several main protections of different principle can be used. But for large capacity transformers only longitudinal current differential protection principle was used as main protection till now. It is seriously affected by the magnetizing inrush currents especially for large capacity UHV transformers. In this paper an impedance type main protection principle for large capacity UHV transformers is proposed. The construction of protection scheme of this principle, the ability to withstand magnetizing inrush currents, the reliability and the sensitivity of its operation are studied. Theoretical analysis and digital simulations proved the effectiveness and advantages of this main protection principle for large capacity UHV transformers.
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