In our previous examples with the three-node network, we had treated the transmission lines as if they could carry as much power as we wanted them to. In reality, engineers place limits on the transfer of power across a given transmission line. There are several reasons for this, but one of the most important reasons is shown in the following two short videos:
The first video shows what happens when a tree contacts a power line. The video will start at 2:17 and you should let it run through 4:35 (approximate run time 2:08).
The second video shows a 500,000 volt line opened under load (run time 9 seconds).
You might remember from a basic science class that when things heat up, they expand, and remember from Lesson 2 that when current is pushed through a material with some resistance, that material gets hot, and expands. When power lines heat up, the expansion causes the power line to "sag." If the line sags too much, it could touch a nearby tree and cause a short circuit, whereby a large amount of current is transferred from the power line to the tree. The tree then heats up and…well…watch the videos if you haven't already!
To prevent "sag," limits on transmission line power flow are established. These are sometimes called path ratings. These are limits on the number of MW that the line can carry at any given moment. Path ratings can change by season, time of day, or with system conditions, although for our purposes we'll treat them as being constant under all conditions. A transmission line that is operating at or above its path rating is said to be "congested."
Path ratings on transmission lines are constraints that system operators aren't supposed to violate. But the job of the transmission system operator isn't that easy. Unlike with pipelines, a transmission system operator can't just open or close a valve to keep additional flow from reaching a specific branch or part of the network. Remember what we learned from Ohm's Law in Lesson2: power flows along all parallel paths. So if a transmission grid operator wants to change the pattern of flows through the power grid, they need to adjust the outputs of the generators.
The key here is that in the presence of transmission congestion the grid operator can't use a perfect economic dispatch. The system operator must tell some generators to back off, and tell others to increase power. This is called out-of-merit dispatch and it winds up increasing the total cost to serve electricity demand.