The hydro-generator being study subject

this paper deduces the action characteristics of the static stability boundary impedance circle in detail.For specific excitation-loss protection criterion of generator transformer protection group

the research shows that putting into reactive reverse criterion does not reduce the static stability boundary circle range of the actual operation generator.Then through an example

by converting the low excitation limit curve from the power plane P-Q to impedance plane R-X

the cooperate relationship between excitation-loss protection and low excitation limit is illustrated in the impedance plane uniformly.