The protection schemes for large ultra-supercritical turbo-generators， the leading units in the near future in China， will suffer from the low sensitivities because it has the high reactance and low resistance. Several sorts of protection schemes with good performance such as transverse differential protection and incomplete longitudinal differential protection can not meet the requirements of ultra-supercritical turbo-generators. This paper investigates the laws governing the generator internal faults. And the second harmonic current in the field winding and the unit-transverse differential current of the faulty generator are also analyzed after the occurrence of asymmetrical faults. The results show that both of them have the similar changing trends and can be used to achieve the reliable action. According to that， a new directional relay based on the negative sequence power and the second harmonic current in the field winding is presented as one of the main protections. It can provide high performance at different conditions.