Lightning Protection and Impact on Electric System Reliability

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Figure 1: US Lightning Patterns

Lightning Protection Impact on Electric System Reliability

Given the scrutiny applied to utilities citing lightning as a primary cause of a given service interruption without undeniable evidence, it is difficult to establish an accurate assessment of the role lightning plays in reliability performance. As utilities analyze outage incidents, they typically assign cause codes as “line failures” and ‘unknown” and these categories tend to include a significant number of lightning caused outages. This fact has been verified by a number of utilities that have implemented remedial actions to mitigate the impacts of lightning, only to see an immediate decrease in outages related to these other cause codes. Further, many “blue sky” overhead line equipment failures are likely the result of fuse fatigue caused by a lightning flashover. Therefore, whether the data as reported in a utility’s outage management system justifies an investment in lightning protection, it is deemed prudent to do so.

Standard Lightning Protection Practices

Figure 2: Lightning Arrester

The standard (and appropriate) practice is to initially install lightning arresters (a basic staple of lightning protection equipment) fairly uniformly across the system and install additional arresters in areas where there are a preponderance of lightning caused outages (service interruptions). However, without a certain amount of forethought, it is entirely feasible to make a significant investment in the installation of lightning arresters, anticipating a 75 percent impact, but in fact, realizing only a 25 percent (or less) impact due to flaws in the lightning protection scheme. Lightning will find the weakest physical link in its search for a path to ground. It is incumbent on electric utilities to apply strategies designed to “out smart” lightning (i.e. implement a lightning protection system that provides quality lightning protection), for example:

• Scout Arresters address the phenomenon of lightning striking at or near a line (lightning can induce a voltage surge even when it does not directly hit a line) with the over-voltage condition traveling down the line “looking” for a path to ground (which can include another conductor), reaching a bend or double dead-end and reflecting off of the insulators (possible achieving an even higher over-voltage condition). A lightning arrester placed at the point of reflection can prove insufficient, where a scout arrester placed one or two spans before the reflection point can prove effective in mitigating the impact of lightning.
• Root Cause Analysis is a key process in any initiative to reduce lightning-caused outages. Deploying additional lightning arresters is the standard remedy and will be critical in minimizing the impact of lightning on the electrical system. However, a root cause analysis will likely uncover a number of factors that will lead to other tactics to further improve upon the results, namely:
  • Adequacy of grounding-does it aid or detract to the goal of decreasing lightning caused outages?
  • Type of construction (e.g. Basic Insulation Levels and Shared Structures for Transmission and Distribution) can have a direct bearing on the results.
  • Topological factors (natural cover vs. open field) play a role.

To the extent these factors (and others) are identified as root causes to lightning-caused outages, there are obvious measures that can be taken enhance the effectiveness of the lightning protection schemes.

• Use of Informational Tools integrating the capabilities of the National Lightning Detection Network (NLDN) to identify the probable location of a lightning strike with insights gleaned from the Fault Analysis and Lightning Location System (FALLS) are extremely effective in locating lightning-caused transmission line outages and, with acquired expertise, similarly for the distribution feeder backbone. In both instances, this type of analysis can assist in determining the optimal placement of lightning arresters.

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