Page 312 - Decision Making Applications in Modern Power Systems

Decision-making-based optimal generation-side Chapter | 11 273

and industrial uses. Any disturbance to the power supply usually leads to det-
rimental effects both to the society and economical activities. It is key and
important issue that the safe and reliable operation of power systems be
guaranteed at all times. Therefore the primary function of a power system is
to supply its customers with electrical energy as economically as possible
with acceptable reliability and quality levels.
In general, reliability can be defined as the ability of the power system to
provide the desired level of service continuously over an extended time hori-
zon apart from only a few instances where this service is interrupted
[28 30]. A power system is said to be reliable if the customer power
requirements can be met on demand and to be secure if it can withstand
unforeseen disturbances. Based on the definitions given to guarantee that the
power system is reliable, it is a prerequisite that power system must be
secured for the greatest part of its operation time. Power systems can be clas-
sified as dynamic time-varying systems; hence, to achieve a secure and reli-
able performance, certain conditions need to be met to guarantee that.
Mainly frequency and voltage should be maintained within power system
operation limits, equipment overloading should be avoided at all times, for
example, if generators are overloaded, life span of the equipment can be sig-
nificantly reduced.
In Ref. [30], five different states of power system operation are defined
based on the ability of the system to withstand disturbances. These states of
operation are as follows:
1. Normal operation
Generally, the power system is said to be in a normal state if all oper-
ational limits are satisfied and is operating in a secure mode. Therefore
during a disturbance, no system limits must be violated. In the first stage,
when a disturbance occurs, which can be loss of generation or sudden
load increase, the automatic voltage and frequency controllers are acti-
vated to keep the frequency and voltage within acceptable levels. The
automatic control loops are also present in the other states of operation of
the power system. In this state the economic operation of the power sys-
tem is also of paramount importance. Therefore to minimize the opera-
tional costs, changes in generation of the power units are inevitable after
a power disturbance, known as economic dispatch approach.
2. Alert operation
A power system is said to be alert if the system that was previously
operating with no limit violations fail to operate within its limits after a
disturbance. In this condition, preventive control actions are employed to
return the power system to the normal state since system security is at
stake. Preventive control approaches try to balance the generation and
load after a disturbance and the process may involve in increased system
reserves, topological changes, etc.