While active and reactive power change， the 3rd harmonic voltages at the terminals（U3t） and at the neutral（U3n） of large-sized turbo-generator also change. The basic reason is that the load change causes the 3rd harmonic voltage changing of stator windings（EΦ3）， so that the 100% stator ground fault protection based on the 3rd harmonic voltage will be influenced. By magnetic field analysis method and Fourier series， E Φ3 ， which is generated by field magnetomotive force（MMF） and armature MMF， is studied. By studying the MMF and electromotive force（EMF） phasors while active or reactive power changes， the influence relationship of the active or reactive power change to EΦ3 is obtained. The analysis results indicate that it is an important factor to affect EΦ3 ， due to the saturation of the rotor teeth at q axis. And it may be a reason of U3t/ U3nchanging that the parameters of the neutral grounding device are non-linear.