AUTOMATIC GENERATION CONTROLS (AGC)

Disturbances may also change the voltage at the generator’s terminals. In response, the generator’s automatic voltage regulating system will sense the change and adjust the generator’s field excitation, either up or down, to compensate.

Transient stability or instability considers that period immediately after a disturbance, usually before the generator’s governor and other control systems have a chance to operate. In all cases, the disturbance causes the generator angles to change automatically as they adjust to find a new stable operating point with respect to one another. In an unstable case, the angular separation between one generator or group of generators and another group keeps increasing. This type of instability happens so quickly, in a few seconds, that operator corrective action is impossible.

If stable conditions exist, the generator’s speed governor system, sensing the beginning of change in speed, will then react to either admit more mechanical energy into the rotor to regain its speed or to reduce the energy input to reduce the speed. Directives may also be received by the generator from the company or balancing area control center to adjust its scheduled output.

In addition to the measures noted to improve steady-state stability, other design measures available for selected disturbances to mitigate this type instability are:

Increasing the speed by which relays detect the fault and the speed by which circuit breakers operate to disconnect the faulted equipment sooner
Using dynamic braking resistors which, in the event of a fault, are automatically connected to the system near generators to reduce export from the generators
Fast valving systems on turbines allowing rapid reduction in the mechanical energy input to the turbine generator
Automatic generator tripping
Automatic load disconnection using under frequency or under voltage protective systems
Special transmission line tripping schemes

Dynamic instability refers to a condition in which the control systems of generators interact in such a way as to produce oscillations between generators or groups of generators that increase in magnitude and result in instability, that is, there is insufficient damping of the oscillations. These conditions can occur either in normal operation or after a disturbance.

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