Improvement of System Stability
Apart from the use of fast-acting AVRs the following techniques are in use:
- 1. Reduction of fault clearance times, 80 ms is now the norm with SF6 circuit breakers and high-speed protection (see Chapter 11).
- 2. Turbine fast-valving by bypass valving-this controls the accelerating power by closing steam valves. Valves which can close or open in 0.2 s are available. CCGTs can reduce power by fuel control within 0.2 to 0.5 s.
- 3. Dynamic braking by the use of shunt resistors across the generator terminals; this limits rotor swings. The switching of such resistors can be achieved by thyristors.
- 4. High-speed reclosure or independent pole tripping in long (point-to-point) lines. In highly interconnected systems the increase in overall clearance times on unsuccessful reclosure makes this technique of dubious value. Delayed autoreclose (DAR) schemes with delays of 12-15 s are preferable if the voltage sag can be tolerated.
- 5. Increased use of H.V. direct-current links using thyristors and GTOs also alleviates stability problems.
- 6. Semiconductor-controlled static compensators enabling oscillations following a disturbance to be damped out.
7. Energy-storage devices, for example batteries, superconducting magnetic energy stores (SMES) with fast control, providing the equivalent of a UPS.