The power distribution system plays an essential role in our modern society in North America. Hanging on the back of hundreds of millions of utility poles, it delivers electric power, perhaps the defining utility of the 21st century, to homes and businesses around the world.
Yet because of the pole-based nature of its construction, the distribution system is vulnerable to natural disasters and calamities such as tornadoes and hurricanes. These natural calamities and changes in climatic conditions have huge effects on the distribution system and the thousands that depend on the power it supplies.
Pole owners need to analyze their network to create system resiliency to combat nature and build a strong distribution grid. This article will look at system resiliency and how accurate pole load analysis using software like PoleForeman helps create it.
What is System Resiliency?
The utility distribution grid is designed as a self-protecting system composed of protective devices throughout the grid to isolate faults. Simply put, a fault created anywhere on the feeder, like that of a pole failure, will cause power outages from the first upstream protective device to the end of the circuit. For example, just one pole failure on a mainline feeder can knock out power to neighborhoods, businesses, and entire communities. Thus, to minimize the frequency of power outages, power companies strive to ensure their distribution grid is resilient to weather-related events. The term “system resiliency” is used to describe how well the utility poles making up the distribution network can withstand loadings from extreme weather and climate change events. As explained below, pole load analysis plays a significant part in identifying system resiliency.
Is it the same as storm hardening?
Storm hardening is the process of replacing and retrofitting the existing poles. Thus ensuring the utility poles are better suited to endure loadings from extreme weather conditions. In short, storm hardening is one of the processes of attaining system resiliency.
Hardening initiatives have become standard practices for utilities serving the areas prone to extreme weather and environmental factors. The process mainly focuses on sustaining the utility poles against hurricanes, tornadoes, and ice storms. In 2017 alone, two hurricanes cost a Florida utility 2.5 billion dollars. As the frequency increases, so does the price tag. Mitigating these hazards has quickly become a critical piece of system resiliency backed by regulation. Regulations build resiliency by setting a common standard by which a pole owner can judge resiliency and compliance across their entire network.
How do we create a resilient grid?
Due in part to the risk of pole failure caused by storms and other natural events, utilities are designing their electric grid above and beyond the minimum standards required in the National Electric Safety Code (NESC). The NESC offers a set of minimum guidelines for designing and constructing electric infrastructure that increases safety and creates system resilience.
The strength and loading section of the NESC gives several major design factors to consider when performing a loading analysis on a distribution network.
NESC Grade of Construction
Under NESC, utility poles can fall under three different grades of construction. The grade generally determines the margin of safety. Generally, higher grades of construction mean a higher degree of system resiliency. The three construction grades are:
- High margin of safety
- Required when the pole supports spans that cross limited access highways, railroads, and navigable waterways.
- Most common and provides a basic margin of safety
- Utilized for the typical power and joint-use distribution pole
- Two separate grades for crossing and elsewhere
- The lowest grade of construction
- Mostly used for emergency and temporary construction
- Not allowed for poles supporting electric facilities.
For each NESC construction grade, load and material strength factors are applied during the analysis process. According to electricalengineeringresources.com:
“The Load Factor (LF) accounts for the uncertainty of the given load and/or simplifying assumptions made in the analysis. This factor increases the applied load on the structure based on the required construction grade.
The strength factor (SF) decreases the efficient strength of the structure. The Strength Factor accounts for the variability of the resistance property.”
NESC Weather Loadings (Rule 250)
Given the exposure of poles to weather events, NESC gives three weather loading requirements. Of the three, the load that has the most significant effect on the pole rules the design. The rules are:
General Ice and Wind (NESC Rule 250B)
Extreme Wind (NESC Rule 250C)
- Required for poles 60+ feet above ground to withstand winds up to 150 mph.
- Commonly used on poles less than 60ft for hardening feeders to improve system resiliency, restoration times, and reliability indices.
Extreme Ice with Concurrent Wind (NESC Rule 250D)
- Provides geographic ice and wind loadings based on historical meteorological data
- Ice loads can reach 1.25 Inches with wind speeds up to 60 mph.
Performing pole loading analysis using NESC load cases will provide an idea about the percent loading on the pole. Poles with utilization percentages at or above 100% do not meet NESC minimum requirements. Therefore, these poles should be replaced, re-enforced, or re-engineered.
For example, if a pole is loaded to 110% capacity, owners would know that in case of extreme climatic conditions, the pole will not necessarily withstand that weather event and could be a point of failure on the grid.
So, what do all these requirements mean for a more resilient network? They set the bar for mitigation against all of these environmental factors and provide the baseline for safety and resilience. The question then becomes, where do you start?
Accurate Pole Load Analysis creates a more resilient system.
A more resilient network starts with data. Information about the pole’s condition allows the power companies to take proactive measures to minimize the consequences of extreme climatic events. A study by the National Institute of Building Science determined that every $1 spent on hazard mitigation saves society an average of $4. By extension, the money spent by pole owners on resilience pays off handsomely as the frequency of major climate events increases.
How could pole owners apply their spending to resilience and compliance? Simply put, an accurate Pole Loading Analysis can determine a network’s resiliency.
To complete pole loading analysis, pole owners need to go to the pole to collect data physically. The data includes information about pole properties, type and size of equipment, line angle for the wires, attachment heights, and pole condition. A fielder can collect this information in various ways, but not all of them are created equal in terms of collection speed and accuracy. The IKE device is among the most efficient in capturing accurate field measurements and photographic records, but we are not here to cover collection methods. That’s another article altogether.
Once the information is obtained from the field, a back-office engineer can perform pole loading analysis to evaluate the pole’s loading to capacity ratio. A traditional loading capacity is the ratio of the applied load to the pole strength for a given load case (specific wind, ice, and temp). This ratio gives an idea of any excess strength capacity of the poles and determines if the poles comply with the NESC requirement. In addition, it also allows a utility company to evaluate if a storm hardening will be required.
In addition to understanding the need for storm hardening, the pole load analysis makes owners aware of the weaker points in the grid. Knowledge of these weak points allows owners to identify the possible failure points in an outage event and allocate resources to mitigate the hazard.
Leverage pole load analysis software
Anyone who has completed pole loading analysis by hand can tell you that it is by no means fast. Pole Loading analysis software reduces the human effort involved in designing the power grid. Software, such as PoleForeman, SpidaCALC, O-Calc Pro, or QuickPole allows users to quickly build a model of poles as they exist in the field. The model is then used to ensure the NESC standards are met. In addition, users can quickly alter attachment height, line angles, etc., and get the analysis report for a high number of poles in a short period.
IKE Structural, known more commonly as PoleForeman, is the IKE pole load analysis software. Used by some of the largest electric utilities in the country, it has been a trusted partner in creating system resilience for over 20 years. As a part of the IKE suite of products, it fully integrates with the IKE Platform. Users can import the field data measurements directly from IKE Office for fast PLA for high volumes of poles. Additionally, users can export PLA data into the formats and systems most commonly used across the industry.
Software like PoleForeman allows pole owners to process pole loading analysis at scale. By increasing the volume and efficiency of pole load analysis, pole owners can understand the system resiliency and NESC compliance of their piece of the distribution grid. System resiliency through accurate pole load analysis protects the distribution networks that keep our lights on nationwide in an ever-changing world where environmental hazards never seem to let up.