A Novel Differential Protection Of Transformers Based On Waveform Asymmetric Coefficient

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 usually produced. The differential protection relays will maloperate. So it is still a research focus for relaying researchers in the world. This paper analyses the origin of inrush current appearance, and introduces many methods based on electric parameters and modern signal processing technology. Then a new method of identifying inrush current based on waveform asymmetric coefficient is described in detail. According to this method, we build simplified mathematic models of one-sided inrush current and symmetrical inrush current separately and figure out the theoretical definite value to identify the inrush current. Because the internal fault current is symmetrical, so its waveform asymmetric value is less than 0.10, even if the waveform contains decay DC component and influenced by initial phase angle of source voltage. Meanwhile the dead angle of inrush current makes the inrush current asymmetric, so its asymmetric coefficient is big and greater than 0.15 in most conditions. "Or-gate" brake can be used to block the protection maloperation in which the asymmetry coefficient is small when the dead angle is obscure. In the end, many simulations prove the correctness of this method in an actual two-source power system by using MATLAB. The simulations indicate that this method is legible and easy to be used in microcomputer protection, otherwise it has the advantages of filtering decay DC component, suppressing disturbance of high harmonics, resisting CT saturation, and being unacted on the deviation of system frequency.