A pilot protection principle for transmission lines based on fault component integrated impedance, defined as the fraction of the sum of the fault component voltage phasors across the two terminals of the transmission line to the sum of the current phasors through the same line is proposed in this paper. The magnitude of the fault component integrated impedance becomes large and reflects the capacitive impedance of the line when an external fault occurs on the line, whereas in case of internal fault on the transmission line, the magnitude of the fault component integrated impedance

becomes relatively small which reflects the impedance of the system and the line. Therefore, this characteristic distinguishes the external fault and the internal fault on the transmission line. The capacitive current of the transmission line has no effect on the criterion proposed in this paper. Extensive simulation studies are carried out to verify the high sensitivity and reliability of the proposed principle by using the power system simulation software PSCAD under internal and external line to ground (L-G) fault conditions.