low-frequency transmission, line protection, differential protection, sampling value differential, Electrical engineering. Electronics. Nuclear engineering, TK1-9971

As a fault occurs in a low-frequency transmission line zone， the modular multilevel matrix converters （M3Cs） at both ends of the line limit the fault current amplitude and phase. Under such a circumstance， the current on both sides displays the characteristics of through current， which in turn leads to poor sensitivity or even failure to operate of the traditional phase-based line differential protection. To solve the above problems， the electrical characteristics of the line after a fault and the adaptability of the phase differential protection are analyzed first based on the architecture of the low-frequency transmission system. Then three logical criteria based on the sampling values are given in combination with the differential principle of sampling values， which together with the steady-state differential threshold current and the low ratio braking equation constitute a differential protection method applicable to low-frequency transmission lines. Finally， a model of the low-frequency transmission system is built in RTDS （real-time digital simulation） system， and the effectiveness of the method used in low-frequency transmission lines is verified through the simulation analysis of different types of faults within and outside the lines， which lays the foundation for subsequent engineering applications.