Probabilistic Short-Circuit Uprating of Station Strain Bus System-Overview and Application

Increasing levels of fault currents in the Ontario Hydro system have made the designs of some existing 230 kV strain bus station Installations Inadequate if conventional deterministic design techniques are employed. Uprating costs could be reduced by several million dollars per station, if a greater short-circuit capability for the existing strain bus and structures could be established using new probabilistic techniques. For this purpose, a multi-discipline task force from a variety of divisions within Ontario Hydro was created to evaluate 230 kV Insulator string failure probabilities at Middleport Transformer Station (TS) and assess the impact of possible failures in terms of personnel safety, repair costs, outage costs, and system reliability. This paper and the three companion papers present an overview of the probabilistic uprating process as applied at Middleport TS. The study indicated that only marginal work (If any) Is required to maintain adequate safety, reliability and cost criteria, assuming that currents are limited to within the existing 230 kV circuit breaker ratings.