A Generalized Power System Reliability and Security Index

This paper develops a risk-based integrated index that reflects power system reliability and security both directly and indirectly. Direct characteristics may express the risk to not fully supply load at various load levels and contingencies. Indirect characteristics address such undesirable conditions as circuit overloads, voltage problems, low stability margins, area interchange violations, insufficient generation reserves, unfeasible power flows, etc. Although indirect characteristics do not necessarily cause load losses, they nevertheless signal about a weakened performance of the system (reduced “security/reliability margin”). This reduced margin may lead to sometimes hardly predictable and quantifiable load losses (via remedial actions, islanding, and instability), unforeseen events (cascading outages), severe system failures (voltage collapse), etc. The new index gives a more comprehensive answer regarding the general degree of both reliability and security in the system by combining diverse contributing factors using a fuzzy logic like approach. It is designed to flexibly accommodate various priorities and admittance of power utilities regarding particular characteristics integrated in the index. The index is conveniently scaled: it expresses reliability as a per cent value, so that 100% corresponds to a “fully reliable system”, and 0% to a “completely unreliable system”. It allows comparing power systems of different size. The index is applicable in both deterministic and probabilistic approaches, for the past and planned time periods. The existing reliability indices such as expected unserved energy, system minutes of interruption, and others can be linked to and derived from the general index. The index has been developed in Southern Company, one of the largest power utilities in America. Although this index can be used with any reliability analysis software, this particular implementation is oriented to the reliability assessment program TRELSS (Transmission Reliability Evaluation of Large-Scale Systems) distributed by Electric Power Research Institute (EPRI). Numerical examples are provided for a large multi-area power system model derived from but not identical to the real Southern System. It contains more than 4400 busses and 6400 branches.