## 2 The Nature of Stress and Its Relation to Contingencies  
The concept of stress in the context of the paper could be defined as \"operating a system under a reduced level of security [2].\" While most power systems are operating close to their normal limits, such a state is not considered as a stress in this context. Rather, stress has been defined in terms of the  
Figure 1: Concept of the ASI. (N = Normal, A = Alert, E = Emergency condition, X = eXtreme condition. There would be a local display of Levels and at the System Office there would simply be the four lights plus a curser click to bring up only the important parameters on the main screen. contingencies that are suffered by the power system. The degree of severity of the contingency has likewise been defined as the corresponding operating state and has also been defined as a performance level and denoted by the symbols NAEX, as shown in Figure 1. The objective of the ASI is to select the appropriate performance level and forward this to the System Operator for action. For a stress-alert device to be effective it must be able to predict the stresses that occur in the system (at its location) and their degree of severity. The principal characteristics of this device are therefore its predictive and analytical features and to describe these one should begin with the third and last module of ASI, namely the Output Module, as shown in Figure 1.  
While the Output Module is the object of the ASI design and its alert system, it is probably necessary to review the process whereby this alert system was conceived. To predict a possible stress in the system one must first accumulate a repository of patterns related to system disturbance. They are obtained by simulating these contingencies and recording the results. This is the area where data mining is useful. From these patterns (referred to as ASI Predictors) the ASI can predict the result of an actual system disturbance and its degree of severity from its repository of coincidental events, a process termed \"pattern recognition.\"  
Thus the ASI encompasses the following unique and distinctive features, namely:  
- A data-mining system to select the simulated contingencies off-line and record their patterns. - A pattern-recognition system to predict the degree of severity of an actual contingency. - The built-in algorithms necessary to carry out these complex calculations. By these means, the objective of the ASI as a stress-alert device can be achieved. It is important to realize that the signals sent to the System Operator by the ASI are the single most important pieces of information that can be delivered. They describe the state of the system at the ASI location. These signals are defined as follows based on NERC Planning Standards. N: denotes a normal condition that does not involve a contingency and simply alerts the System Operator to the fact that ASI is operational. A: denotes alert in a degraded system (N-1) that requires the System Operator to correct this situation so that it does not deteriorate into a more severe condition. E: denotes an emergency condition (a system on the verge of collapse) that allows the System Operator to reject load or generator (or both) to maintain the integrity of the system. X: denotes an extreme condition that will require the System Operator to create 'islands' that are self-sustaining so as to maintain at least some degree of service. Since the states of the system (the Performance levels) are related to the contingencies, there must be an algorithm to relate the Performance levels to the contingencies to assure the successful operation of the ASI. The relation between the states of a system and the contingencies has been covered extensively in the paper on the \"Process of System Collapse\" [3]. Four possible states of the system have been defined, namely: secure, degraded, verge of collapse and extreme.