Stability definitions and improvement methods

Steady state stability is the ability of a power system to remain in synchronism (return to its operating point) after small and gradual changes in load or generation. Its limit, for a machine connected to an infinite bus, is $P_{max} = EV/X$ at $\delta = 90^\circ$.

Transient stability is the ability of a power system to remain in synchronism after a large and sudden disturbance such as a short circuit, switching out of a line, or sudden loss of a large generator or load. It is judged over the first rotor swing.

Methods of improving transient stability

1. High-speed fault clearing using fast relays and circuit breakers, reducing the accelerating area.
2. Reducing transfer reactance by parallel lines, bundled conductors and series capacitor compensation, raising $P_{max} = EV/X$.
3. Single-pole switching and high-speed auto-reclosing, so healthy phases keep transferring power.
4. Fast-acting excitation systems with power system stabilisers, raising the internal emf during the disturbance.
5. Fast valving of steam turbines to cut mechanical input quickly.
6. Braking resistors at the generator bus to absorb excess power during the fault.
7. Generator tripping or load shedding to restore the power balance.
8. Higher system voltage, intermediate switching stations, and FACTS devices (SVC, STATCOM, TCSC).