Power System Stability

- Ability to remain in operating equilibrium

- Equilibrium between opposing forces

Angle Stability

- Ability to maintain synchronism

- Torque balance of synchronous machines

Voltage Stability

- Ability to maintain steady acceptable voltage

- Reactive power balance

Transient Stability

- Large disturbance

- First-swing aperiodic drift

- Study period up to 10 s

Small-Signal Stability

Non-oscillatory Instability

- Insufficient synchronizing torque

Oscillatory Instability

- Insufficient damping torque

- Unstable control action

Local Plant Modes

Interarea Modes

Control Modes

Torsional Modes

Mid-term Stability

- Severe upsets; large voltage and frequency excursions

- Fast and slow dynamics

- Study period to several min.

Long-term Stability

- Uniform system frequency

- Slow dynamics

- Study period to tens of min.

Large-Disturbance Voltage Stability

- Large disturbance

- Switching events

- Dynamics of ULTC, loads

- Coordination of protections and controls

Small-Disturbance Voltage Stability

- Steady-state P/Q - V relations

- Stability margins, Q reserve