Page 316 - Decision Making Applications in Modern Power Systems

Decision-making-based optimal generation-side Chapter | 11 277

a secure steady-state point, which depends only on the existing automatic
control loops. Different control variables corresponding to each contingency
represent corrective control actions that need to be taken for that contin-
gency. Here, after a contingency, the system will result in a secure steady-
state point of operation depending on both the existing automatic control
loops and the new set points that result from specific corrective actions. Fast
and slow corrective control actions must be clearly distinguished and are
applied appropriately taking into account the time duration for which the
components are allowed to be overloaded. For instance, power flow line lim-
its can be separated into two levels. First, the steady-state limits should be
met for a continuous operation. These limits may be violated for some min-
utes as long as they do not exceed the emergency limits, which, if violated,
will result in catastrophic effects, such as line tripping. Devices with differ-
ent time constants could be scheduled to offer corrective control dealing
with different component limits. Generally, TSOs do not include security
requirements in the day-ahead schedule optimization problem but only per-
form an a posteriori security analysis. In the event in which the security
assessment shows that the system security will be compromised, control
actions are employed until a secure dispatch is obtained. A priori-
contingency power flow analysis can incorporate also new postcontingency
set points for the devices that offer corrective control.
To satisfy the security requirements a sufficient reserve power must be
available to balance the system after a contingency. Ideally, the reserves
must be sufficient enough to supply power in the event that the largest gener-
ator in the system trips or must correspond to a percentage of the peak load.
It can be seen that optimal day-ahead planning is of great importance to
ensure that the system is secure. Due to the complexity of the problem, the
different underlying market mechanisms and the level of system uncertainty,
different alternative implementations have been already proposed; however,
obtaining a satisfactory solution is still subject of ongoing research.

11.2.3 Operation and planning problems to be addressed
In planning for the day-ahead operation of power systems, a secure and eco-
nomic schedule for the generating units and the reserves must be designed.
This, however, comes at the expense of additional investment and opera-
tional costs, thus revealing the trade-off between a secure and an economic
system operation. In an ideal setup, where the system is considered determin-
istic, there are ways to satisfy the minimum cost operating point while at the
same time satisfying the desired security level. Power systems, however, are
essentially stochastic since they are subjected to stochastic power flows, load
uncertainty, unpredictable component outages, etc.
The operation of a power system under uncertainty has been a subject of
key research. Regardless of the wide research, there is still no specific