Large-scale renewable energy bases located in deserts, the Gobi, and other arid regions often lack synchronous generation support and therefore adopt grid-forming modular multilevel converter-based high voltage direct current (MMC-HVDC) transmission schemes. In such systems, an isolated AC grid with 100% renewable energy and full power electronics is formed at the sending end. When faults occur on the outgoing AC lines, severe transient fluctuations significantly degrade the reliability of AC line protection. To address this issue, a fault characteristic reshaping strategy is proposed from the perspective of control-protection coordination to enhance protection reliability. First, the mechanism of fault transient fluctuations under conventional control schemes and its correlation with the degradation of protection performance are analyzed. Then, through theoretical derivation, a sliding mode control-based inner-loop controller for negative-sequence current is designed to replace the conventional proportional-integral controller, along with a fast transient response strategy. Finally, an electromagnetic transient simulation model of the sending-end grid of a renewable energy base with grid-forming MMC-HVDC is established in PSCAD/EMTDC. Simulation results verify that the proposed fault characteristic reshaping strategy can effectively suppress transient current fluctuations, thereby enhancing protection reliability.