The angle of a synchronous generator's internal voltage can be directly calibrated against the rotor position acquired by the phasor measurement unit(PMU). However

presently

there is no direct method to calculate the amplitude of internal voltage. Based on accurate measurement of the internal voltage angle

this paper proposes a direct method to calculate internal voltage amplitude. The proposed method has nothing to do with the generator's parameters

its accuracy depends only on the measuring accuracy of the electrical measurements(including terminal voltage

terminal current and power angle). Furthermore

a method of calculating xd(d-axis synchronous reactance)and xdf(mutual reactance between field winding and stator winding)is proposed

which is capable of taking into account the influence of magnetic saturation. Based on the statistical theory of normal distribution

a robust parameter identification algorithm is proposed to give the confidence interval and point estimation of xd and xdf. Examples based on the wide area measurement system(WAMS)at North China power grid show that the proposed method is feasible and reliable.