For the purpose of explaining the security monitoring function, it is expedient to classify the infinite number of possible operating conditions of a power system in terms of security. For this purpose observe that the power system should satisfy the following requirements:

(1) Operating constraints such as: limits on system frequency, limits on bus voltage magnitude, limits on circuit loading, etc. Any attribute of system component, such as loading of a circuit, reactive power generated by a unit, etc. can be expressed as a function of the system state and controls,