I’ve lived nearly all my life in the South Eastern Region Interconnection (SERC) and my family has been in the region since before the dams were constructed which now feed power systems in the region. I have an extremely high level of respect for the people who operate out national and local grids and welcome any of them to comment their thoughts the article.
Setting the Stage
I went back to my hometown for Christmas 2022. During this time of record cold temperatures I was staying in a rental cabin where I didn’t have access to the typical Winter preps people in the region have been making for years (wood fire place, propane or kerosene heaters, non-electric heat). I was prepared for potentially breaking down during my drive and being stranded overnight or for several hours but I wasn’t prepared for what the weather might bring after I arrived at my lodging for the next several days.
Unbeknownst to me the cabin I rented did not have any auxiliary heat to support the central heat, only two split units which were running at full blast when we arrived and still couldn’t heat the cabin to the set point. Theoretically the heating of this cabin should’ve been a non-issue but several factors led to the Split Unit’s inability to adequately heat the space: 1) Too little, or no, insulation in the floor 2) Proximity to a creek behind the property 3) Still installed ducting that ran under the cabin conducted cold air inside 4) Generally poor insulation – all contributed to reducing the split unit’s effectiveness. These poor building factors along with the near 0 Fahrenheit temperatures combined became too much for the system to overcome.
Thankfully the cabin did have an electric space heater. If it weren’t for the space heater I don’t know what we would’ve done. Likely I would’ve wound up at another location for the night. We hauled the oversized space heater into the bedroom and closed up the doors for the night. I set the space heater to 68 or 70 degrees put on some warm clothes and crawled into bed for the night.
Outage?
At around 4:00 AM I woke up and noticed that the power seemed to be out. I figured a tree had fallen or someone had run off the road and taken out a power pole. This type of thing was quite common when I was growing up in the region (power going out when it’s cold). I didn’t think much of it and figured the power would come back on soon. I think the power was probably out for 40 minutes this time. I was pleasantly surprised when the power came back on quicker than expected. I jumped out of bed and turned back on the space heater to try and recover the several degree drop that occurred during the blackout. I got back in bed but failed to fall asleep due to the lack of heat in the room. I was only in bed for maybe 15 minutes before I gave up and went to the other room to let my partner sleep.
At this point it was probably around 5:50 or so. I decided to start making some coffee in the hopes that might warm me up since the space heater and the split units could not do the job. I turned on the coffee pot and it was almost as if I had tripped out a breaker as the power went out. I guessed based on intuition that the local utility was being forced to implement power cuts at the direction of TVA.
As expected, about 20 minutes later my power came back on. I made my coffee and tried to re-warm the house. The space heater was keeping my partner warm but I had to put on a couple more layers of clothes to keep warm in the probably 58 degree living room. The split unit simply couldn’t recover the temperature in the cabin.
After getting some hot coffee in my system I started looking for confirmation that my assumptions were correct. I found that confirmation in the form of a post from our local Utility provider. They mentioned that in accordance to the ELCP step 50 local power providers were being asked to reduce their system load by 10%. To most people that doesn’t seem like much and ideally wouldn’t be very hard to achieve. However, this does ignore a lot about our aged infrastructure and the level of granular control a local operator typically has over the distribution of electricity. All substations are three-phase powered. This means that from the sub-station out to the low voltage system there is limited control of the loads. Each station can only turn on or off each of the output phases from the medium voltage transmission lines. These single phase lines are what would typically power the end users of a local grid. What’s interesting about these types of rolling blackouts for the local population is the oddity that occurs closer to the substations.
“[My neighbors across the road have power but I don’t]” was something I read several times while following the situation on social media. The local power companies typically cycle through each phase turning off one phase at a time per substation. So the main circuits typically within the city limits and where the critical infrastructure exists are prioritized, as they must be, to ensure the continued effectiveness of essential services like water, sewer, sheriff and emergency services. As the substations get further and further from the main feedpoint of a system the loads under control get less and less dense. So the main substation turning off a single phase would be about 33% of the system load for the utility. Where as the substations further out from the main feedpoint control a smaller part of the load cutting power to a phase from an edge feedpoint is still about 33% of the power for a given substation but for the overall system it’s much lower. I’m going to assume that outside the city limits accounts for only 60% of the system load whereas the inner city limits account for about 40% of the load. If we assume a single substation feeds that remaining 60% load cutting a single phase would be about (2/3 * 2/5) or about 13% of system load. typically there are several substations that operate as high voltage feed points and probably about 5:2 medium voltage substations for each feedpoint substation. So for every 2 high voltage feedpoint stations there would be 5 medium voltage substations. This depends on local population changes and topography as much as electrical demand.
So for 3 of the medium voltage substations a different phase would be cut. It might look something like this:
| Station ID | Phase1 | Phase 2 | Phase3 |
| 1 | off | on | on |
| 2 | on | off | on |
| 3 | on | on | off |
| 4 | on | on | on |
| 5 | on | on | on |
If you look above there are two substations which remain completely powered. This might be due to critical infrastructure requirements or simply that curtailment at only 3 stations in the system is sufficient to meet the requirements for a given curtailment step/phase. This is really up to the emergency planning and ground situation for the local and regional power distributers.
As I was passing time early Christmas morning I was curious what the situation looked like for the hardworking individuals actually trying to prevent a system collapse. I found a post from TVA indicating that their power demand was at a record high, over a 100 megawatts higher instantaneous power demand from the previous record set just hours prior. The instantaneous demand was only half the story though. The amount of power delivered was measured in Gigawatt hours of more energy delivered over a 2-day period than the previous records. For those unfamiliar with power delivery, that’s a shitload of power. Imagine a large city of about 700,000 people just popping into existence and you’d get an average load of about 1100 mega-watts (assumed a ~1500 average load). TVA announced new record energy delivery in the days following Christmas.