Russia could have collapsed Ukraine's grid much sooner, they simply chose not to, because of the commands of their commander in chief when the conflict started to cause minimum disturbance to civilians. When the order came to attack the secondary transformers Ukraine's grid collapsed quite quickly. The fact is Russia could do even more damage to Ukraine's grid if they wanted to, they never hit the main transformers in the grid thus far, the only reason they have not done that is to prevent Ukraine's nuclear reactors from going into meltdown from lack of power.
Ukraine’s grid did not collapse. The only one’s feeling the effects of the reduction in capacity are civilians. For the military to be impacted, Russia would need to destroy close to 100% of the grid, like the US did in Iraq. Even in the short campaign against Yugoslavia, the US led NATO force took out a greater share of the generating capacity than Russia did to Ukraine and did it in a focused 3 day span:
"
Three consecutive nights of air attacks caused extensive blackouts in Belgrade, Novi Sad, and Nis, the three largest cities in Serbia, Yugoslavia's dominant republic. In contrast with previous attacks on the power supply – in which allied warplanes triggered temporary outages by dropping carbon-fiber filaments that shorted out electrical lines – NATO forces this time struck at Serbia's five major power-transmission stations with high-explosive munitions, causing damage that could take weeks to repair.
Officials at the Pentagon and at NATO headquarters in Belgium said allied jets deliberately attacked the power grid, aiming to shut it down more completely and for longer periods than at any time previously in the two-month-old air campaign. U.S. officials estimated the attacks had shut off power
to about 80 percent of Serbia."
Source:
It is also a matter of doctrine, since Russia does not use their air force in the same way the US does. They never did. Their air force is mostly expected to be used to provide air cover and CAS for their army, not go into perilous adventures deep into enemy territory. Russia has their experience during WW2 as a formative event where they were on the back foot in terms of air power most of the time. What few deep strikes with strategic bombers they made in WW2 were basically a disaster despite heavy investment into strategic bombers in the 1930s.
That’s pretty much the point: Russia only has itself to blame for having invested in an airforce that plays a marginal role in the war.
And even the much lauded Western allied strategic bombing campaigns of WW2 were considered to be a failure for the most part when they were analyzed in the post WW2 era. Just try reading about Freeman Dyson someday.
US bombers dropped 43kt of conventional bombs in July 1945 and were ramping up to drop a staggering 115kt a month by September with the planned arrival of the 8th Air Force on Okinawa. After the war, USSBS concluded that conventional strategic bombing alone would have shattered Japan’s will to fight:
“Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey's opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated."
