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• Tellegen's theorem is introduced into the LCC-HVDC receiving-end AC power grid, and the instantaneous quasi-power characteristics are derived. • The quasi-power difference differential is defined to characterize differences between the internal and external faults and solve the problem caused by the distributed capacitance and reactive currents crossing through the transmission line. • The proposed pilot protection is immune to the control strategy of the inverter, commutation failure, receiving system strength and other factors. • The proposed protection can detect the fault within 5 ms. Affected by the control strategy and nonlinear characteristics of the inverter station, the traditional pilot protection relays employed in AC transmission lines do not perform well or even fail at the receiving end of the line commutated converter-based high-voltage direct-current system, thereby posing concealed threats to the security of the system. Leveraging the universality of Tellegen's theorem, this paper introduces the theorem to the realm of the AC-DC hybrid system. The characteristics of quasi-power difference under various conditions are analyzed, and a pilot protection criterion is proposed based on the quasi-power difference differential value. The AC grid model of the receiving end of the Tian-Zhong HVDC project in China is established in PSCAD/EMTDC to carry out simulation and performance analysis for the proposed protection under different fault cases. Furthermore, the proposed protection criterion undergoes validation using on-site fault recording data. The test results show that the proposed method is immune to the inverter control strategies, commutation failure, receiving system strength, and other factors. The new method is more adaptive to the AC system connected to the inverter station. [ABSTRACT FROM AUTHOR]