The polarized current travelling wave based directional relay（PCTDR）utilizing high frequency component of initial current wave and power frequency component of voltage performs effectively in transmission line protection.This relay，which has been successfully implemented in extra-high voltage（EHV）/ultra-high voltage（UHV）transmission lines，can overcome the high frequency transmission limitations of the capacitor voltage transformer（CVT）.However，the series capacitor has a significant impact on the travelling wave propagation characteristic in series compensated lines.In this regard，the application problems of PCTDR in series compensated lines are firstly analyzed.Moreover，an improved PCTDR algorithm is proposed based on an investigation of the frequency response of CVT.Further，extensive simulation results have shown that the improved algorithm has a good performance in series compensated lines and is well adaptive in various fault conditions.