Security Or Reliability?
"What Is ?"
Over the last few years, the terms Security and Reliability have increasingly been used interchangeably and lately the word Security appears to have been dropped all together in favour of Reliability. In fact, these are two fundamentally different concepts and, I believe, it is important to remember the difference especially for system planners, sytem operators, and the writers of operating and planning standards. I have not found in the literature definitions of these two terms that cover the relevant factors. The definitions I would like to offer are as follows.
Definition of Reliability
Reliability is the probability that the power system capability to continue to operate within prescribed limits of performance for a specified period of time in a given environment of intrinsic contingency hazards and of prevailing operating and maintainance practices.
Definition of Security
Security is the capability of the system to continue to operate within prescribed limits of performance when subjected to contingency events of pre-specified severity.
The evaluation of system Reliability requires: Massive amounts of equipment failure data; Very clever algorithms to enumerate the system states that need to be analyzed; Complex calculations models to determine whether each state is a success or a failure; And large amounts of computer time. To the best of my knowledge, to date there is no practical computer program in existence that can do Reliability evaluation of a practical size power system and produce results of practical value.
On the other end, evaluation of Security is very doable. It requires: Choosing bases cases to cover seasonal, scheduling, and maintenance limiting operating conditions; Testing these base cases with a handful of contingencies of predefined severity using simulation models such as Load Flow, Short Circuit, Transient Stability, and Dynamic Stability computer programs; And analyzing the results from each test to determine whether the system remained within the prescribed limits of performance.
Because of the above, the electrical power systems are being planned and operated based on Security. Accordingly when referring to the capability of a power system being able to operate within acceptable performance limits, the term to be used is Security and not Reliability.