Power Blinks Mean the Grid Protection System Is Working

NLI lineman Carl Moore installs a new fuse. Photo by Dan Wanous

Have you ever wondered why your power blinked on and off? This is the work of the protection system on the Northern Lights, Inc. (NLI) power grid at work. The blink you experienced is likely caused by one of NLI’s reclosers momentarily opening after detecting a brief fault on the distribution system.

A recloser is a programmable, high-voltage electric switch. Similar to the circuit breakers in your home, if there is trouble on the distribution power line, the recloser will open to allow the fault to clear. If the fault is a sustained fault— such as a tree laying on a line—the recloser remains open. If it was a momentary fault such as a tree briefly brushing against a line, it would open and then close. A recloser may open and close two to three times depending on how it is programmed.

Reclosers are used throughout the NLI distribution system. NLI has approximately 350 reclosers installed at substations and on power lines throughout its service area. The other important system protection devices that are even more common are fused cutouts. There are thousands of fused cutouts on NLI’s system.

The fuse within the fused cutout assembly has a wire that melts and breaks the circuit when an electric current exceeds its rated current value. A fuse needs to be replaced by a lineman when it blows, after the fault condition has been corrected.

With reclosers and fused cutouts, the level of current and how quickly it trips or blows must be coordinated with all the devices from the substation to the end of a line. Engineers at NLI evaluate time and current curves to program reclosers and determine fuse timing and type to install at a particular location. This helps with isolating faults to the closest protection device so an entire line does not experience an outage for a fault at the very end of a line.

As always, NLI strives to keep the power on, but when the distribution system needs protecting from further damage, the reclosers and fuses are there to do their job.

Kristin MettkeNorthern Lights Engineering & Operations Manager Kristin Mettke is an electrical engineer and has worked in the electric utility industry most of her career.