Impact of Large Power Transformer Bushing Seismic Vulnerability on the Electrical Grid

Numerous studies suggest that the Pacific Northwest Subduction and the San Andreas Fault systems are past due for a severe seismic event. In the time that has passed since the last highly destructive event near an urban area (Northridge, 1994, M6.7), major changes to our electrical grid have introduced uncertainty in the impact such an event would have on critical infrastructure. While design practices have improved since 1994, the seismic vulnerability of large power transformers remains in question. These critical substation components are essential for power delivery but are extremely expensive and becoming difficult to procure with lead times that can be as long as five years. Thus, it is essential to understand the seismic risk transformers represent to the bulk electrical grid where bushing failure was of primary concern. This work presents a parametric study of the commonly used high voltage transformer-bushing systems on the 10,000 synthetic bus system (138kV and 345kV), from which the probability of a given transformer bushing to exceed design amplification standards was used to approximate whether the transformer would fail during a seismic event. These risk values are then applied to the 10,000 synthetic bus system of the western interconnect through Monte Carlo sampling. Depending on line protection settings, the resulting showed 7.2–11.7% and 2.4–10.7% of cases did not exhibit load loss for San Andre’s and Seattle Washington, respectively. However, 80.5–89.8% and 88.6–91.1% of cases exhibit losses greater than 100 MW of lost load for the San Andrase and Seattle Washington areas, respectively.