The delay in construction of new generation- transmission systems due to various causes forces the power utilities to operate the existing facilities closer to their designed limits. This makes the system vulnerable to electrical disturbances. Therefore, greater attention should be paid to keep the system always in a secured state. It, however, requires new tools and techniques to enhance the security of a power system. This book provides new load flow models for security analysis and also hybrid expert system for security control. A new approach has been adopted to extract off-line knowledge of a power system. This knowledge is then expressed in terms of rules. These rules constitute the symbolic logic part of the hybrid expert system. In an on-line environment the hybrid expert system receives on-line data and after few calculations and comparisons with stored data suggests the appropriate control actions required to alleviate violations and thus ensures security of the system. This work would be helpful to the researchers in the area of power system analysis and control, and also to the operating personnel in a power system control center.