Page 313 - Decision Making Applications in Modern Power Systems

274 Decision Making Applications in Modern Power Systems

3. Emergency state
For a system operating in the alert state, if in case disturbance opera-
tional limits are violated, the system automatically enters the emergency
state mode. In this case instead of preventive actions, corrective or emer-
gency control actions are necessary to bring the power system back to
the normal or alert state. Corrective control approaches may involve
exciter control, fast generation reduction or increase, generation unit trip-
ping, high voltage direct current (HVDC) modulation, system protection
devices, load curtailment, etc.
4. Extreme state
In this state the corrective action would have failed to operate satis-
factorily, and as a result, a series of events occurs, and parts of the sys-
tem may be disconnected. To prevent total system collapse or extensive
blackouts, severe actions such as load shedding and controlled islanding
should be implemented.
5. Restorative state
When the electric system is rendered stable and the fault is cleared,
the power system finally enters the restorative state. In this state, control
actions are taken in steps to reconnect the lost parts of the system until
the normal state in which the system was before a disturbance is
achieved.

11.2.1 Real-time operation
Interconnected power systems are usually subdivided into different control
areas, where each area may represent one country or part of a system for big-
ger countries. The transmission system operator (TSO) is the responsible
entity for the security of a single control area. Nowadays, each area is moni-
tored and controlled by the TSO through an IT infrastructure, commonly
known as the supervisory control and data acquisition (SCADA). Recently,
SCADA systems, however, are replaced by wide-area measurement system
(WAMS) and control due to its superiorities.
The WAMS measures data using remote devices, which are installed at
strategic points throughout the grid, and the information is gathered at one
control center through communication channels. This data is processed by
computer systems, and it gives the system’s operating state in real time.
Control commands, which are to be sent from the center back to the system,
are determined based on the system state. The system is also equipped with
local control devices, which helps to protect the equipment and to provide
system-wide services after specific commands have been sent. Generally,
voltage and frequency control and the security level assessment are the main
tasks so as to keep the system in the normal state.