By Greg Stark, Enerdynamics Instructor
In the wake of the devastating tornadoes that recently ravaged the Midwestern U.S., a question arose that I think warrants an explanation: “When tornadoes take out transmission lines, why aren’t there widespread blackouts?”
It’s a great question.
First, today’s transmission lines are engineered to withstand extremely high winds in some cases. But even when they don’t, widespread blackouts caused by tornadoes aren’t as common as one might think. Here’s why: The transmission grid is a big interconnected network. When a transmission has an unplanned trip – which can happen for various reasons – and it goes off, there are a few things that can happen:
- Some customers served by that line may have blackout conditions in a local area and that load goes away from the system until power can be rerouted. This tends to be more localized.
- Power that was moving through the tripped line automatically looks for other paths to get where it is trying to go through multiple other network paths. As long as that power can find a different transmission line to flow over to get where it is ultimately trying to go, the lights will stay on AS LONG AS THESE OTHER LINES ARE NOT OVERLOADED. If the other lines are overloaded, they would trip off too and could result in a blackout over a much wider area.
- System operators have backup generation units available on reserve status. These are called spinning reserves. If a line trips out, the power that is lost can sometimes be replaced by quickly ramping up the output of one of the reserve units and delivering the power through a different transmission path or delivering the power locally in the area where there is unserved demand. System operators may also have the ability to reduce loads quickly through implementation of curtailment contracts with customers willing to reduce loads under emergency conditions.
Today, such scenarios are all modeled in advance through the Power Flow Models and computer simulation programs running at the control centers. The transmission system model looks at the current system configuration and then runs through a series of “what if” scenarios that consider what would happen if any line would trip. This computer-generated modeling helps ensure the transmission system has adequate contingencies for the loss of a line. If the current configuration has some issues regarding reliability from loss of any given line, the computer program may suggest a reconfiguration of the transmission substation switches so as to provide better contingencies (i.e. alternate paths to what exists right now) or may suggest ramping of reserve units or implementation of customer curtailments.
Having said that, there still can be areas of the grid where there aren’t as many contingencies as the operator would like. I’ll use Colorado as an example since the person who asked this question lives in northern Colorado: Some of the areas north and east of Fort Collins, Colorado, (the state’s northern-most city) have much lower population densities and less transmission infrastructure. Because of the reduced transmission infrastructure, there may not be enough network inter-connections to provide the level of contingencies that metro areas like Denver, Colorado Springs or even Fort Collins have.