The ratio of E1 to I1 is the apparent impedance (R + jX) seen by a distance relay connected at the 
terminals of the generator. Keeping ‘x’ constant, the apparent impedance travels along the circle, 
crossing over from the first quadrant to the fourth quadrant. The characteristics of the impedance 
relays which define the steady stability boundary are also shown in Figure 10.4(b). It should be 
clear that as the field current of the generator drops, the generator goes from a lagging power 
factor to a leading power factor, and the apparent impedance seen by a distance relay approaches 
the steady-state stability boundary. Application of this principle to out-of-step relaying will be 
considered in Chapter 11.