The study based on the two-phase coordinate system shows that when an asymmetric fault occurs within a generator the unbalanced power component of the instantaneous power output increases while the balanced power component decreases. A new definition of instantaneous power asymmetry degree is presented. It is defined as the ratio of the unbalanced power to the balanced power and it sensitively reflects the asymmetry of faults internally happening in a generator. A new generator protection principle is proposed based on the instantaneous power asymmetry degree and the phase mutation of the unbalanced power. Dynamic simulation results show that the principle can remove small-number-turn faults with high sensitivity. In case of large-number-turn faults， phase to phase faults or faults happening during a system swing the protection can operate quickly and reliably. The wire connection is simple and the protection is easy to be embedded into other protection functions as a complementary function.