Why some unit trips result in dropping customers

Events on the electrical system happen fairly regularly and for the most part no one notices them unless the event results in customers being dropped from the system. A trip of Cat Arm a few days ago resulted in customers being dropped in what's called an under frequency load shed. This is an automatic protection on the system which dumps load when generation and load get too far out of balance. Dropping load helps stabilize the system thus avoiding a total collapse. A trip at Granite Canal yesterday didn't trip anyone. Let's look at why.

On March 26th, 2016 at 11:02 hours, the Cat Arm plant tripped offline when the transmission line into that facility tripped off during severe weather in the area. Cat Arm is at the end of the line so when the line goes so to does the plant. Cat Arm is rated for 127 MW and when it tripped was supplying approximately 100 MW.

For whatever reason, Granite Canal tripped off line on March 29th at 17:41 hours. Granite Canal is rated for 40 MW and was supplying something less than that at the time of the trip.

Granite Canal was carrying less load than Cat Arm and that's why one resulted in an under frequency load shed and the other didn't. In the figure below  I have overlaid frequency trends of both events. It's clear that when Cat Arm tripped, taking the 100 MW with it, a greater imbalance was created in the system resulting in load shedding being activated at 58.8 Hz. The relative magnitude of each imbalance can be seen in the fact that the frequency dropped further for the Cat Arm event.

To finish up, whether a unit trip will result in a customer outage or not will depend on the amount of load it is carrying at the time of the trip but also the total amount of load and generation available on the system at that time. The system is typically more stable in winter as opposed to summer as most all generators are on line during the winter.