Risk-based Power Systems Security (RISIKO)
The determination of security levels, for given operating conditions, has been traditionally done using deterministic criteria, meaning that an operating condition is identified as secure if it can withstand the effects of every contingency in a pre-specified contingency set. In this context withstanding the effects means that the given contingencies will not violate loading or voltage criteria nor make the system unstable. If one or more contingencies are in violation, the operating state is often called an alert state. In such a case actions are taken to move the system into the secure region. If no disturbances are in violation, then no action is necessary, but actions can be taken to enhance the economic efficiency of the energy delivered to the end-users.
Although such a deterministic approach is conceptually straightforward, it does not directly and explicitly mirror the level of operational risk that the power system actually faces. An adequate measure of risk must consider both the consequences of an undesirable outcome as well
as its probability of occurrence. The traditional deterministic contingency analysis accounts for the probability of occurrence of various events, but only in a somewhat implicit and simplified manner. Only the contingencies that are included in the predefined list are thought to have a sufficiently high probability to deserve consideration. This list normally includes all contingencies that arise when each line, transformer or generator in the system is individually taken out of service. A system state that meets this requirement is said to be N - 1 secure, where N represents the number of system components. Classical deterministic contingency analysis does not handle the consequences of undesirable events any more rigorously than their probability. If any operating constraint is violated for any of the predefined contingencies, the operating state of the power system is deemed insecure. The extent of the violation is not taken into account. In other words, with the deterministic approach to security assessment, there is either an unacceptable risk or no risk at all. One could argue that this bipolar assessment is moderated in practice by the judgment of the human operators who decide, based on their experience, whether more severe contingencies should be taken into consideration and whether remedial action is truly needed. While the value of human expertise should not be discounted, it is unlikely that the associated subjectivity and liability will remain acceptable in a competitive environment.