The voltage level and capacity of the temporary excitation power should be correctly choosed and the protection setting for the safe and smooth first-starting of units should be calculated. According to the starting test of No.1 unit in a power plant, in this paperthe selection of excitation power and the calculation of the protection setting for the self-shunt units are discussed, and the generation mechanism of magnetizing inrush current is analyzed in the case of non-full voltage impaction on excitation transformer. The results show that the magnetizing inrush current generated by non-full voltage impaction is far less than that by full voltage impaction, which is totally different from the magnetizing inrush current generated by the transformer no-load closing with nominal voltage, and is verified by the incident wave records. On that basis, combined with the incident instance, the conclusion that the large protection setting values for excitation temporary source could cause the protection failure under short circuit conditions of excitation transformer at the low voltage side is pointed out, and a correct protection setting value method for temporary relay is suggested, which could meet the requirements of starting test.