The variation of versus Pmo for different values of is shown in fig 24.2.
Now that we have reviewed, the rotor angle dynamics, we proceed to discuss the relay's perception of the dynamical system.
Determination of power swing locus
A distance relay may classify power swing as a phase fault if the impedance trajectory enters operating characteristic of the relay. We will now derive the apparent impedance seen by the relay R on the R-X plane. Again consider simple two machine system connected by a transmission line of impedance ZL as shown in fig 24.1(a). For the sake of convenience machine B is treated as a reference and it's angle is set to zero.
versus Pmo for different values of 
is shown in fig 24.2. 
