Reliability and Resiliency: Application to Transmission and Distribution Systems

Recent studies show that climate change is the biggest driver of extreme weather events and constantly increasing wind speeds [1, 2]. As climate conditions become worse, an increase in the frequency of Category 4 and 5 hurricanes is expected. Electric utilities within 50 miles [80 km] of a coastline could potentially face severe storm damage and require significant structure upgrades and/or replacement.

Table 1.  Failure Rates of Wood Transmission and Distribution Poles [Ref. 2]

Table 1 gives some typical recorded data on wood pole failures in a Southern coastal area from 2009. The average annual probability of occurrence of a Category 1 storm in that area is calculated as 0.0356 compared to 0.00054 and 0.00018 for Category 4 and 5 storms, respectively. Still, the amount of damage sustained is disproportionately higher for Category 4 and 5 events.

In addition to hurricanes, winter ice/snowstorms also carry the potential of larger ice thickness on wires as witnessed in the 1998 Quebec ice storm where radial ice on conductors was almost 4 in.(102 mm).

Code Mandates 

Wind speeds associated with catastrophic storms were consistently observed to be much larger than what the applicable design codes and standards suggest. ASCE Manual 74 [3] recently increased the minimum MRI from 50 years to 100 years. This change from ASCE is the result of recognizing that climate change is pushing average wind speeds up everywhere. But is shifting the MRI/RP to 100 years adequate? Should we design for larger RPs?

Prior to 2019, the previous 3rd Edition of Manual 74 provided adjustment factors for choosing a specific design wind speed or ice thickness for a given MRI/RP. For example, selecting a 200-year RP requires adjusting the wind load (not pressure) by a factor of 1.30. Similarly, using a 200-year RP for ice requires adjusting the ice thickness by a factor of 1.50. Such adjustments will no doubt serve to decrease the probability of line or structure failure although the economic ramifications or costs have to be determined as they pertain to that specific line. These costs usually depend on line length, terrain, number of circuits, bundling of wires, voltage, and structure materials.

Reliability

Structural Reliability is mathematically defined in terms of a Reliability Index β as:  

where R = Resistance (moment capacity, axial capacity etc. based on material strength)

            Q = Load Effect (applied bending moment, axial load etc., function of wind, ice)

σR, σQ = Standard Deviation of R and Q (assumed normally distributed variables)

Equation (1) makes it clear that high values of resistance R associated with low coefficient of variation of material properties will help enhance the reliability of a structure. This will counteract the large variations in climactic variables such as wind speeds and ice thickness. A typical target β value is usually 3.0 which translates to a probability of failure of 0.00136 [4] although engineers are free to design for other higher values of β. Ensuring reliability of transmission structures’ performance (including poles, towers, frames, components) is the first step towards improving grid resiliency.    

Resiliency

Figure 1 shows the elements that comprise grid resiliency [6]. From an engineering viewpoint of electric transmission and distribution grids, resiliency means:

• Ability to withstand external forces or pressures without excessive deformations or collapse;
• Ability to recover, restore or bounce back to the initial configuration in the shortest possible time; and,
• Ability to survive and continue some level of normal function even after subjected to severe events   

As illustrated in Figure 2 [1], on a time scale, optimum grid resiliency means lowest time consumed to restore system performance to its initial level.

Techniques to improve electric grid resiliency are discussed in detail in the EPRI Report [6]. Targeted grid hardening can help improve grid resiliency significantly. This means strengthening of selected high-priority or critical lines and circuits, structures and their components, by means of increased structural reliability and employing advanced materials (see below). Use of Fail-Safe structures (i.e., mechanical fuses) can also help confine any damage to a smaller area thereby reducing the restoration demands.

Advanced Materials 

Steel, concrete, and composite structures are all engineered materials and all are viable material systems for grid repair and restoration having demonstrated their performance in severe storms. From practical and constructability perspectives, tubular steel and composites are preferred while replacing damaged wood structures. However, use of steel is subject to availability (custom orders) and shipping considerations to storm-damaged areas. Lattice towers are labor- and material- intensive and are deployed only as a like-for-like replacement for damaged towers.    

A recent study [4] showed that modular, composite (FRP or fiber-reinforced polymer) transmission and distribution poles provide a reliability index (4.84) which is three times that of wood poles (1.55) of the same height and class. The failure probabilities associated with these poles are 0.000001 (FRP) and 0.085 (wood). In plain words, this means that for every 1000 poles considered, wood poles would experience 85 failures whereas FRP composite poles would have virtually no failures at all.

How does the use of composites help in grid reliability? From statistical and planning perspectives, utilities seek lower values for two types of indices, namely System Average Interruption Frequency Index (SAIFI) and System Average Interruption Duration Index (SAIDI). These are defined as: 

When composite poles are considered, failure-related interruptions would be virtually non-existent; which in turn means that the above two indices would be significantly reduced. The report by Brown [2] also supports this observation: composites are recommended due to their exceptional strength-to-weight ratio, no susceptibility to corrosion, good electrical insulation, protection against woodpecker and insect damage, requiring minimal maintenance and a projected service life of 80 years. 

Risk Management

Enhancing individual pole reliability by using composite materials explicitly translates to managing and reducing the risks associated with retaining older materials and components. In other words, eliminating structural failure is equal to eliminating a substantial portion of risk for a utility system. If this is combined with higher design MRIs or RPs, then the result will be a robust, reliable and resilient system.  

Conclusions                                                                                                                                                               

In this article, we briefly discussed the concepts of electric grid structural reliability and resiliency with specific reference to climate change, hurricane wind speeds, code provisions, as well as composite pole materials.

Regulating agencies now recognize the dangers posed by climate change and this is reflected in updated design RPs for wind and ice. Adopting larger RPs will always have a positive hardening effect on grids although a cost-benefit analysis must be performed to determine the appropriate solution. FRP composite poles have been proven to be more than three times as reliable when compared to wood poles and these offer engineers an attractive option to consider during storm recovery and restoration operations, as well as grid hardening efforts to mitigate threats to the system.    

References

1.  ANL, National Electricity Emergency Response Capabilities, Argonne National Labs Report to DOE, 2016.

2. Brown, R., Cost-Benefit Analysis of the Deployment of Utility Upgrades and Storm Hardening Programs, Quanta Tech Report, Project 36375, Raleigh, NC, 2009.

3.  ASCE Manual 74, Guidelines for Transmission Line Structural Loading, 4th Edition, 2019.

4. Kalaga, S., Reliability Assessment of Transmission Poles, European Journal of Engineering and Technology Research (under review), 2022.

5.  NESC C2, National Electric Safety Code, IEEE, 2017.

6.  EPRI, Electric Power System Resiliency: Challenges and Opportunities, 2022.