Rotor angle in a 4-machine system.

[Mahmoud Salem]

When studying the rotor angle in the transient stability state in a 4-machine system, 2 of the machines display blank drawings. The other 2 are seemingly fine. I'm using ETAP as the simulation transfer. my question is, what could possibly be the reason in theory that this would happen , or reasons from the software's point of view for those who have experience dealing with it.
Thank you

 

[IrishPower]

Are you plotting relative rotor angles? If so, your swing generator will be the reference for the relative angles of the other generators and appear to be at zero degrees during the simulation. If you have a second identical generator at the same bus as the swing generator that could explain why it also has a relative rotor angle of zero.