Thanks Wayne,I can give a longer technical explanation if desired, but the basic answer is that it has stayed hot a long time and hasn't cooled off at night. And the extent to which the current temperature pattern is a deviation from local historical norms is greater in some place than others. If prior to the current event most transformers were being run near the boundary of their thermal envelopes, then the locations where the current temperature pattern is a bigger deviation from the past will have more transformer failures.
Cheers, Wayne
I saw on the news Saturday or so that PG&E was shipping transformers to various locations to pre-position them. So, PG&E must know they are a likely failure point and wanted to have replacements close to where they might be needed. Your explanation helps fill in the gaps on why this was a good move.
PS. Are these things filled with oil of some type? If so, I can't imagine working with hot oil in this heat.
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