Current differential protection is one of the major protections for extra-high-voltage/ultra-high-voltage transmission lines, but its performance is severely weakened by the prominent capacitive charging current along transmission lines. Travelling waves take the charging current into account inherently, hence travelling wave-based protection is not impacted by the charging current. Based on the principle of travelling waves, this study proposes the concept of integrated surge impedance (ISI), which is defined as the ratio of the difference of voltage quantities at two ends of a line to the sum of current quantities at two terminals of the same line. The ISI, closely related to travelling wave differential current, can discriminate between internal and external faults clearly. The ISI-based criterion is composed of power frequency quantities in order to ensure the robustness of the protection. The action zone is designed on an impedance plane. EMTP simulation verifies the correctness of the proposed pilot protection.