A novel approach to transformer differential protection using sequence component based algorithm

This paper presents a new unit type protection for power transformer by combining differential protection and phase angle comparison of sequence components of currents. The simple differential protection scheme alone may ma-operation during current transformer (CT) saturation and high resistance internal fault conditions. Hence, the sequence component based phase angle comparison technique is combined with percentage biased differential protection scheme. Discrete Fourier Transform (DFT) is used to calculate the phase angles of sequence components and harmonics of primary and secondary current signals of transformer. Low pass butter worth filter is used to correct the current signals before passing it to DFT. During internal fault, the phase angle difference of primary and secondary current is very low whereas large angle difference observed during external fault/other conditions. The performance of the proposed algorithm has been evaluated by considering various transformer and system parameters. Numerous simulations are carried out on three phase power transformer using PSCAD/EMTDCTM software to validate the technique. The proposed algorithm perfectly distinguishes internal fault and external fault condition. Moreover, various situations such as magnetizing inrush current, high resistance internal fault and CT saturation conditions are also tested for validation.