Or on a smaller scale, hybrid and open warfare, domestic terrorism [1] or just rednecks shooting at power lines [2] and transformers [3] already cause a lot of damage to power infrastructure.

An AR-15 or Kalashnikov is enough to cause serious damage, a hit squad or two can take out unprotected infrastructure on a wide scale in a very short time - and most substations consist of barely more than a fence and barbed wire to deter wildlife from entering, not a concrete wall that would thwart attackers.

[1] https://www.schneier.com/blog/archives/2014/01/military_styl...

[2] https://newtoncountytimes.com/stories/shooting-power-and-fib...

[3] https://www.sasktoday.ca/southeast/local-news/carlyle-rcmp-s...