What problem is identified by excessive rotor angle swings?

Excessive rotor angle swings are primarily linked to transient instability. When a disturbance occurs in a power system—such as a fault or sudden load change—the rotor angles of synchronous machines can swing significantly as they attempt to regain equilibrium. If these swings are too large and the system cannot stabilize, it indicates that the system has lost transient stability, which means it can't return to a stable operating condition after being perturbed.

Transient instability refers specifically to the inability of a power system to maintain synchronism in the presence of disturbances, which is characterized by these excessive swings in rotor angles. This situation can lead to the disconnection of generators from the grid and even system collapse if not addressed properly. Understanding and managing this aspect is crucial for maintaining reliable operation of the electrical grid.