Unit Derating Levels in Spinning Reserve Studies

The application of probability methods in power system reliability evaluation started over thirty years ago. The bulk of the work has been in the static capacity area and the first major paper on spinning reserve evaluation appeared in 1962(1). A second paper(2) illustrated the application of this approach in a medium size utility. Both these papers illustrated the development of a risk index which includes the probabilities of capacity deficiencies and unforeseen load conditions. In a more recent paper(3), Markov processes were used to determine the transient probabilities associated with a generating unit containing a single derated state. The general expressions are rather complicated and it is difficult to extend this approach to include units with more than one derated state. The discussion to Reference 3 proposes a state space model which assumes that the lead times are small relative to the unit repair times and therefore it is reasonable to assume that no repairs will be made during this lead time. This method though approximate can be applied to units with several derated states.