Abstract
If the wye-delta connected three-phase transformer energizes non-synchronously, the first energized phase will induce an electromotive force into the other two phases via delta winding. Then, the flux in transformer core maybe increasing greatly and causes severe saturation. The second harmonic ratio (SHR) of the three-phase differential currents in such a case is small, which can cause the conventional second harmonic restraint principle failure and the mal-operation of differential protection. In this paper, the mechanism of non-synchronous energization (NSE) is firstly introduced. Then, a novel method for detecting the NSE of unloaded transformer is proposed. If the change of neutral current does not coincide with the line currents of transformer, the NSE can be identified. To avoid the mal-operation of the transformer differential protection due to the inrush currents occurred during the NSE, a new method, taking advantage of the changing trend of the SHR in the differential currents, is also put forward to discriminate inrush current from fault current during NSE. The effectiveness of the proposed scheme has been verified by simulation results. Copyright © 2015 John Wiley & Sons, Ltd.
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