When a practical multimachine system is subjected to a large disturbance, loss of synchronism may take place if one or more groups of machines "separate out" from each other. It is not meaningful to see rotor angles relative to an infinite bus, since there isn't one! Instead they are plotted relative to the rotor angle of one machine, or to the center of inertia of the system which is defined as follows:
Note: Movement of all generator rotors together (or equivalently, movement of the center of inertia) does not indicate loss of synchronism. Instead it tells us that overall load and generation in the system are not balanced. This issue is discussed in the next module. Loss of synchronism means that the relative motion between generator rotors is unstable.
This is illustrated by the plot of rotor angles for a 10 machine system for a large disturbance (detailed machine model and constant impedance loads are considered). Machines 1 and 3 separate out from the other machines if the clearing time is greater than 0.34 s. |
