Improved differential protection method for flexible low-frequency transmission system based on additional current compensation

Low-frequency transmission systems (LFTS) emerge as a competitive solution to the problem of long-distance offshore wind transmission. Because the control strategy on both sides of the line will affect the phase and amplitude characteristics of the fault current, the working performance of the traditional differential protection (TDP) is weakened. Therefore, an improved differential protection (IDP) method based on negative sequence voltage cooperative control is proposed in this paper. Firstly, this paper focuses on the negative sequence control strategy of the modular multilevel converter (M3C). The fault characteristics of differential current and break current under the control strategy are discussed. Secondly, the boundary conditions and adaptability of traditional differential protection using a negative sequence cooperative control strategy are discussed. Then, this paper introduced an additional adaptive current with amplitude and phase angle compensation, which makes the change of differential current and braking current adaptive, so as to improve the sensitivity of the differential protection method. The boundary conditions and adaptability of the method are analyzed. Finally, the effectiveness of the improved method for differential protection of flexible low-frequency transmission systems was verified using PSCAD simulation. Simulation results verified higher sensitivity and 300 Ω fault resistance capability in the proposed method. In addition, the protection method satisfies the differential protection requirements for low-frequency transmission systems.