Background: The large-scale distributed new energy integrations brings about various non-ideal grid situations, which could deteriorate the control performance of grid-connected converters. Objective: This study aims to improve the control performance of this grid-connected converters when the grid is non-ideal. Methods: Firstly, a neutral point clamped (NPC) three-level grid-connected converter was studied, and the characteristics of non-ideal grid were analyzed in detail. Then, a real-time signal extraction method called generalized sliding discrete Fourier transform (GSDFT) is proposed to extract the non-ideal signals, e.g., harmonic components and negative sequence components, based on which a double closed loop control strategy was designed to improve the control performance of this grid-connected converters when the grid is non-ideal. Conclusion: Firstly, this paper analyzes the characteristics of non-ideal grid and establishes the mathematical model of NPC three-level grid-connected converter in non-ideal grid environment. Secondly, aiming at the problem of characteristic harmonics and negative sequence components in non-ideal power grid, a real-time and fast extraction method of non-ideal signals based on generalized sliding discrete Fourier transform is proposed. Finally, taking the three-phase grid current of grid-connected converter as the control target, the closed-loop control strategy of non-ideal signal extraction based on GSDFT is designed, and good control effect is achieved.