Understanding How Our Grid Operates
Physics and Basic Operation
The US has three “grids.” They are the Eastern Interconnect, Western Interconnect, and ERCOT. In technical terms, they are AC wide-area synchronous grids. Let’s unpack that.
AC is alternating current. AC is what comes into our home and powers many of our appliances. In contrast to direct current (DC), AC “changes the direction and magnitude” of its voltage and current. How fast AC changes direction is its frequency. Synchronous grids share frequency but also the timing (phase). Wide-area grids can span whole continents.
US Interconnects; Source
Virtually all grids are AC. 100+ years ago, it was easy to make AC by spinning a generator, and analog transformers could change the voltage. Houses need low voltage, but low voltage requires large wires to transport electricity efficiently. The solution was to generate electricity in large power plants and transport it to users at high voltage, then step it down outside your house.
A wide-area grid like the Eastern Interconnect is similar to 20 water tanks connected with hoses. Each water tank is a vertically integrated monopoly utility like Duke Energy or an independent system operator (ISO) like SPP or PJM.
ISOs in the US; Source: SustainableFERC
Each ISO or utility is responsible for balancing the water level in its tank as power plants add water and customers remove it. If there is a problem in SPP - say a cold snap spikes demand and trips power plants offline, the water level starts to decrease. The lower level creates a potential energy difference between SPPs tank and next door MISO. So water starts flowing from MISO’s tank to SPP. MISO plants will try to add more water to maintain their level. More water will start flowing in from other tanks like PJM if MISO can’t keep up. Hoses cost money, so there is only so much water that can flow between the tanks.
I’m simplifying concepts around voltage, frequency, and current, but the grid generally works this way.
There are many benefits to wide area synchronous grids. If one territory has problems, they automatically start getting help without any permission or human in the loop. Physics makes it happen instantly. The connections for this help are just wires.
The negative is that tail consequences go up. Water levels do not fall gracefully. If they fall below a line, the bottom drops out. Every other tank is attached to a black hole, pushing them towards the critical threshold. The reason is that out-of-specification frequency will damage equipment so that equipment shuts off to protect itself. Cascading failures can lead to wide outages from relatively benign beginnings. Operators and utilities across the grid must have tight cooperation to prevent these events.
Grids Favor Big, Traditional Power Plants
Economies of scale, maintenance requirements, and demand patterns lead to wide-area grids.
Several factors explain why power plants benefit from scale. First, they are complex construction projects. It takes years to build traditional power plants. Increasing the plant size tends to lower construction management costs as a share of the cost. Second, these plants are full of pipes and vessels. For cylinder-like shapes, the volume increases faster than the surface area. Bigger pipes, boilers, and turbines use less steel per watt and have similar maintenance requirements.
Thermal power plants (nuclear, geothermal, gas, coal, etc.) require weeks-long maintenance outages every year or two. Large grids spread output loss over many plants. France’s electricity system would be untenable if they weren’t in the European grid because their nuclear fleet can have issues that take many plants offline at once. Luckily they can lean on their neighbors to help supply electricity in years like 2022, when the nuclear fleet may only have a 60% capacity factor due to unplanned maintenance outages.[1] Extended maintenance outages can cause issues when unexpected cold or hot weather happens. During winter storm Uri, Texas’s ERCOT had almost 10 GW out of its 90 GW capacity offline for maintenance and repairs.
Electricity demand is highly variable by location and time. Distributing load over a wider region reduces the required power plant capacity by sharing resources.
Extensive grids minimize the investment in power plants.
The Grid and Emerging Technologies
The math for wind turbines, solar farms, and batteries differs from traditional power plants.
Wind sees even more benefit from wide-area grids than traditional power plants, even though an individual turbine is <1% of the output of a large power plant. The wind is likely to be blowing somewhere, but that somewhere isn’t likely to be where the load is! Wind developers dream of mega transmission projects to take wind electricity from the Great Plains to population centers in the east.
Solar cells and battery cells are inherently low voltage electrical devices. They are flat and limited in size to minimize resistance. Even cylinder car batteries have flat electrodes rolled up and put into the can. The scaling effects are mild. Construction overhead is much smaller than traditional power plants because they are modular and easy to install. Solar insolation is much more consistent than wind speed, so the economics favor local generation. Solar farms require less maintenance than thermal power plants and rarely need to go offline. The grid is less beneficial for solar than other technologies.
The Difficulty of Change