Sustained operation with a power angle above ________ can lead to system instability.

Sustained operation with a power angle above 90 degrees can lead to system instability due to the relationship between the power angle and the stability of the power system. The power angle, often referred to as the rotor angle in synchronous machines, reflects the difference in angle between the rotor's magnetic field and the stator's magnetic field.

When the power angle exceeds 90 degrees, the system enters into a region where the generator's ability to produce and maintain power becomes compromised. In this condition, the generator may not be able to react sufficiently to changes in load or disturbances, and a loss of synchronism can occur. This could lead to sudden voltage drops, frequency fluctuations, and potentially a cascading failure within the power system.

Therefore, maintaining the power angle within safe limits is critical for ensuring system stability and reliability. Understanding this relationship helps operators recognize the importance of monitoring power angles and addressing any conditions that may lead to excessive power angles to prevent instability in the power network.