Page 315 - Decision Making Applications in Modern Power Systems

276 Decision Making Applications in Modern Power Systems

outage of a line, a transformer or a generator, and, in some cases, simul-
taneous component outages. As earlier indicated, the definition of secu-
rity implies that both during and after a disturbance no operational limits
violation should occur. Therefore security can be further classified as
static security assessment and dynamic security assessment. The security
of the system through the SCADA can either be continuously assessed or
only when it is necessary. However, in practice, a static security assess-
ment is performed where contingency screening is done. Here only the
most severe contingencies are used for the dynamic security assessment,
which involves dynamic simulations and evaluation of the transient state
trajectory.
When the system is rendered insecure, preventive control actions, such as
adjustment of the generation schedule to restore the system to a secure oper-
ation, are employed by the TSO. If a disturbance occurs before the security
is restored, the state of the system may enter the emergency state, and addi-
tional actions should be taken.

11.2.2 Decision-making-based planning and economic operation

The actions that the TOS is performing when approaching a secure real-time
operation can be divided into three main categories: day-ahead operation
planning, short-term planning, and long-term planning [31].
Examples of long-term planning tasks are load forecast and identification
of the new system conditions, investigation of system extensions, and control
actions planning (preventive, corrective). Short-term planning has the follow-
ing tasks: procurement of reserve power, approval of maintenance decisions,
etc. Security assessment is not only limited to real-time operation but is also
performed in the rest of the planning phases. In the latter case the evaluation
of security is done over different possible scenarios of the system conditions.
In day-ahead operation planning the responsibilities of the TSO are to sched-
ule a day-ahead unit commitment, a generation dispatch, and make decisions
about the reserve procurement in certain control areas, while minimizing
the operational costs and satisfying the security requirements. The process of
identifying the optimal generation dispatch while satisfying the network
constraints is traditionally referred to as optimal power flow (OPF). The
OPF problem has many variables that, in addition to generation dispatch,
may also include set points for tap changers, phase shifters, generation
terminal voltage, all of which can be adjusted to give a more optimal cost
performance.
If static security constraints are included in OPF, it is called the security-
constrained OPF (SC-OPF). In the SC-OPF the control variables that corre-
spond to a disturbance-free scenario represent preventive control actions. If
the optimal values of control variables are available, the system will result in