The Electrical Grid

The grid is the network of power plants, wires, transformers, substations and control rooms that carries electricity from where it is made to where it is used. In the United States it is not one system but thousands of pieces of equipment owned by thousands of separate companies, cooperatives and public agencies, operated together under common rules so that the whole thing behaves as a single machine.

What makes that machine unusual is that it stores almost nothing. Gas can sit in a pipeline and water can sit in a reservoir, but electricity on the grid is consumed at the moment it is produced. Every time a furnace switches on or a factory shift ends, some generator somewhere has to change its output within seconds to match. Operators do this continuously, and the measure of whether they are succeeding is frequency: hold the system at 60 cycles per second and supply and demand are in balance.

The continental United States is covered by three interconnections. Within an interconnection every generator spins in step with every other one, so a plant tripping offline in one state is felt instantly across the whole region. Between interconnections, the alternating current is not synchronized. Power crosses only through a small number of direct-current ties that convert it on the way through.

Voltage is the organizing principle of the system. Power is pushed up to very high voltage to travel long distances with low losses, then stepped down in stages as it approaches the customer. Each step down happens in a substation, and each layer is planned, paid for and regulated differently.

Balancing is done on three overlapping clocks. Years ahead, planners decide what to build. A day ahead, operators schedule which plants will run in each hour. Within the hour, they adjust output every few minutes, and automatic controls correct the remainder second by second.

When balance fails, the consequences spread fast. Protective equipment disconnects lines and plants to save them from damage, which shifts the imbalance onto whatever is left. The 2003 Northeast blackout began with a handful of overloaded lines in Ohio and ended with roughly 50 million people in the dark across eight states and Ontario. It led directly to mandatory, enforceable reliability standards administered by the North American Electric Reliability Corporation.

These pressures interact. A grid serving more load, with more of its supply dependent on weather, needs more transmission to move power between regions and more firm capacity that can run when it is called. Both take a decade to plan and build under current processes, which is why grid policy has become as consequential to decarbonization as the cost of any individual generating technology.