The power-electronics-based renewable power plants are currently experiencing rapid growth in various power systems. Their variable control strategies have altered the power transmission line fault characteristics, which leads to problems for the protections that rely on a linear estimation of the power frequency fault information, even to the point of the refused operation risk of protection. Therefore, this paper proposes a new protection algorithm based on a current energy ratio for the transmission line connecting to renewable power plants. The current fault characteristics are investigated in detail, and their energy relationship is adopted to fully distinguish internal and external faults. When an internal fault occurs, there are large differences in energy ratio of transient current on both sides. Energy ratio can be used to measure the degree of variation to distinguish the faults inside and outside the line. Simulation results show that faults of different fault resistances can be distinguished within 10 ms. Moreover, the proposed method requires a conventional sampling frequency and a computation speed that can be met by most protection devices. Thus, the proposed algorithm provides a better solution to the transmission line protection problem. These features make the proposed method a good choice for industrial application.