With an analysis of the change of excitation current for a current transformer with time and the change of operating characteristic for generator percentage differential protection with time when there is an external fault with current transformer saturation

this paper proposes a generator differential algorithm based on transient fitting restraint characteristic. The algorithm fits the slope K of restraint curve with the change of transient unbalanced current with time during current transformer saturation. The initial expression of slope K curve and its adjustment method are proposed. The proposed transient fitting restraint characteristic reflects both the transient unbalanced current characteristic with time during current transformer saturation period and the stable current transformer error characteristic enabling the operating threshold to envelop the unbalanced current to increase protection reliability. Meanwhile the restraint characteristic only needs to consider current transformer stable error when current transformer is working in linear condition and the slope K can be set lower than traditional percentage differential protection which makes this algorithm more sensitive than the traditional one. Theoretical analysis and simulation results based on MATLAB indicate that the algorithm proposed has better reliability during external faults with current transformer saturation and higher sensitivity during internal faults than traditional percentage differential protection. This work is supported by National Natural Science Foundation of China (No. 50977012

No. 51377022) and Graduate Research and Innovation Project of Jiangsu Province (No. KYLX_0122).