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半波长线路无法采用传统差动保护电容电流补偿的方法，因此不能用补偿电容后本侧电流和对侧电流构成差动，必须采用新的方法构成差动保护。采用分布参数模型可以通过线路两端电压、电流计算得到线路上的任一点两侧的电流，构成差动保护，从而消除电容电流的影响。这个点定义为差动点。当差动点越接近故障点时，差动保护的灵敏度和安全性越高，因此将故障点选为最优差动点。针对半波长输电线路特殊的时空特性，提出时差法计算最优差动点的方法。最后在RT DS仿真系统上搭建半波长输电线路模型，对时差法和基于最优差动点的差动保护进行了仿真验证。“,”Capacitive current compensation method of traditional differential protection can’t be used on half- wavelength line, so the current after compensation capacitor can’t obtain, and a new method of differential protection must be proposed. With distributed parameter model the current of both sides of any point on line can be calculated by the voltage and current of both ends of line, and the current constitutes a new differential protection, in the meantime the influence of the capacitive current is eliminated. The point is defined the differential point. When the differential point is closer to the short-circuit point, the sensitivity and safety of the differential protection is higher, so the short-circuit point is defined the optimal differential point. Based on the space-time characteristics of half-wavelength line, this paper proposed the time difference method to calculate the optimal differential point. The simulation system of half-wavelength AC transmission line was established on RTDS, and the time difference method and the differential protection based on the optimal differential point was simulated and verified.