What condition is described by steady state instability?

Steady state instability is characterized by a situation where the phase angle between the voltages or currents in an electrical system exceeds 90 degrees. In power systems, when the phase angle surpasses this critical threshold, it indicates that the system is no longer able to return to equilibrium after a disturbance. This can lead to increasing oscillations or unbounded growth in the system's behavior, ultimately risking loss of synchronization among components.

The state where the phase angle remains below 90 degrees signifies that the system can maintain stability and return to equilibrium after disturbances. When oscillations stabilize the system, this means that it has a mechanism in place to damp any deviations and maintain power system reliability. Likewise, while a constant frequency is an indicator of a stable system under certain conditions, it does not specifically define the type of instability that can arise when the phase angle exceeds 90 degrees. Thus, the identification of phase angle exceeding 90 degrees as a condition of steady state instability is accurate.