The protection schemes for the large ultra-supercritical turbo-generator will suffer the disadvantage of low sensitivity as it has the high reactance and low resistance. This paper investigates the changing laws of the longitudinal zero-sequence voltage which reflects the internal faults of generators. And the third harmonic voltage of the faulted generator is also analyzed. Analysis results show that when an inter-turn fault happens， the third harmonic voltage will decrease. However， it will increase if the fault locates out of the generator. According to that， the third harmonic voltage is introduced as the restricted component to improve the performance of the longitudinal zero-sequence voltage protection. Simulation results show that the new scheme is successful in detecting the generator inter-turn faults.