6 1 Power System Control: An Overview
The discontinuous controls generally stabilize the system after severe distur-
bances and are usually applicable for highly stressed operating conditions. They
perform actions such as generator/load tripping, capacitor/reactor switching and
other protection plans. These power system controls may be local at power plants
and substations, or over a wide area. These kinds of controls usually ensure a
post-disturbance equilibrium with sufficient region of attraction [21]. Discontinuous
controls evolve discrete supplementary controls [38], special stability controls [39]
and emergency control/protection schemes [40–42].
Furthermore, there are many controls and protections systems on transmission
and distribution sides, such as switching capacitor/reactors, tap-changing/phase
shifting transformers, HVDC controls, synchronous condensers and static VAR
compensators. Despite numerous existing nested control loops that control differ-
ent quantities in the system, working in a secure attraction region with a desired
performance is the objective of an overall power system control strategy. It means
generating and delivering power in an interconnected system is as economical and
reliable manner as possible while maintaining the frequency and the voltage within
permissible limits.
1.4 Controls at Different Operating States
Power system controls attempt to return the system in off-normal operating states
to a normal state. Classifying the power system operating states to normal, alert,
emergency, in extremis and restorative is conceptually useful to designing appropri-
ate control systems [1,43]. In the normal state, all system variables (such as voltage
and frequency) are within the normal range. In the alert state, all system variables
are still within the acceptable range. However, the system may be ready to move
into the emergency state following disturbance. In the emergency state, some sys-
tem variables are outside of the acceptable range and the system is ready to fall into
the in extremis state. Partial or system wide blackout could occur in the in extremis
state. Finally, energizing of the system or its parts and reconnecting/resynchronizing
of system parts occurs during the restorative state.
Based on the above classification, power system controls can be divided into
the main two different categories (1) normal/preventive controls, which are applied
in the normal and alert states to stay in or return into normal condition and (2)
emergency controls, which are applied in emergency or in extremis state to stop the
further progress of the failure and return the system to a normal or alert state.
Automatic frequency and voltage controls are part of the normal and the preven-
tive controls, while some of the other control schemes such as under-frequency load
shedding, under-voltage load shedding and special system protection plans can be
considered under emergency controls.
Control command signals for normal/preventive controls usually include active
power generation set points, flow controlling reference points (FACTS), voltage set
point of generators, SVC, reactor/capacitor switching, etc. Emergency control mea-
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