Improved Differential Protection Scheme Based on Attenuation Index of Intrinsic Main Mode for Main Transformer of DFIG Wind Farm

When the main transformer of a wind farm fails and the crowbar circuit is put into operation, the fault current provided by the wind farm will have a rotational frequency component, and the performance of the traditional transformer differential protection based on the Fourier algorithm will be affected to a certain extent. In addition, there is a risk of delayed protection action due to the presence of a large second harmonic component in the fault current, which can cause the transformer differential protection to block incorrectly. In order to avoid the above situation, this paper proposes a new protection scheme of the main transformer. When a fault occurs, the permutation entropy difference between the currents on both sides of the transformer is used to quickly determine the transformer excitation inrush current or the moment of fault occurrence, and then the intrinsic frequency component in the differential current is obtained by performing the variational modal decomposition on the current signal on the system side of the main transformer. Finally, the identification of transformer excitation inrush and fault is achieved based on the attenuation index of the intrinsic main mode. In this paper, a doubly-fed wind farm system is constructed in PSCAD/EMTDC, and the applicability of the proposed protection scheme is verified under different working conditions, which shows that the scheme has the advantages of fast response, sensitive action and reliable protection.