Transmission outage

A key consideration in operating transmission is to avoid outages. Transmission outages can be caused by a variety of reasons including weather events, lightning, fires, malicious acts including vandalism and terrorism, failure of substation or circuit equipment, failure of protection systems, vegetation coming in contact with lines, unstable power system conditions, and human error. Cascading outages are outages that propagate across a wide area of the grid. They occur when an initial disturbance causes further disturbances such as interruptions of supply or further transmission outages. In this case, large portions of the grid can go out of service before operators are able to correct the disturbance. 

An example of what causes transmission outages in the U.S. can be found in data collected by the North American Electric Reliability Corporation (NERC). For 2017, NERC reported that the vast majority of transmission outage hours were due to equipment failure or weather. Protection equipment on the transmission system is designed to quickly isolate disturbances so that outages are limited to a small part of the grid.

Key Causes of Transmission Outages in 2019

% of hours

Failed AC circuit equipment: 43.26

Failed AC substation equipment: 18.06

Weather, excluding lightning: 12.38

Vegetation: 5.67

Foreign interference: 5.55

Unknown: 4.35

Other: 3.40

Failed protection system equipment: 2.85

Human error: 1.11

Power system condition: 0.88

Lightning: 0.70

Vandalism, terrorism, or malicious acts: 0.48

Contamination: 0.45

Fire: 0.42

Failed AC/DC terminal equipment: 0.31

Environmental: 0.07

Source: NERC

In recent years, a rising concern is the system’s vulnerability to physical or cyber attacks. NERC Critical Infrastructure Protection (CIP) standards outline security protocols that must be followed by all transmission owners in the U.S. and Canada to mitigate those risks. Similar protocols are applied in other countries.