A new algorithm based on the morphological gradient for avoiding mal-operation of transformer differential protection

To avoid the mal-operation of transformer differential protection when external fault occurs with current transformer (CT) saturation, a new algorithm based on the mathematical morphology is put forward in this paper. Unlike the conventional Time Differential Method, the new algorithm only needs detecting the instant of fault occurring, while the instant of differential current appearance is no more needed. A new current waveform is obtained by a symmetrical transformation of initial part of differential current after fault occurrence, and then its characteristic is exacted by morphological gradient. In the case of CT serious saturation, a discrimination of external fault and internal fault can be clearly made by this algorithm. In addition, analysis shows that the new algorithm based on the mathematical morphology could also be applied for the identification of transferring fault (external fault transfers to internal fault). Simulation results of MATLAB validate its feasibility and reliability.