Sounds like transformers have a narrow operating temperature range.
Excerpt:
Transformers can be particularly vulnerable, especially the equipment located in cooler areas that typically aren’t prone to vicious heat waves that cause temperatures to soar to 100 degrees or even higher.
“When the temperatures are too hot, transformers in areas that are usually cooler can’t always handle unusually hot weather,” Smith said.
The transformer situation underscores the challenges that confront a utility whose service territories include foggy coastal areas, some of the nation’s largest cities, farmlands, mountains, deserts, foothills and interior valleys whose temperatures can gyrate over the course of a few months.
In the long term, the limit is the degradation of the insulation on the wires. Like most chemical reactions, its rate increases exponentially with temperature. [0] gives a relative damage rate of
9.8e-18 * exp(15e3/T)
with T in Kelvin. Around the usual operating temperature of 100C, every 10C increase in temperature reduces the transformer life by 3x. So transformers that should last 40 years last 13 years.
Excerpt:
Transformers can be particularly vulnerable, especially the equipment located in cooler areas that typically aren’t prone to vicious heat waves that cause temperatures to soar to 100 degrees or even higher. “When the temperatures are too hot, transformers in areas that are usually cooler can’t always handle unusually hot weather,” Smith said.
The transformer situation underscores the challenges that confront a utility whose service territories include foggy coastal areas, some of the nation’s largest cities, farmlands, mountains, deserts, foothills and interior valleys whose temperatures can gyrate over the course of a few months.
http://archive.is/p9Giy