Line Loading and Clearance Calculations Improved

Georgia Power Co., a subsidiary of Southern Compnay, has seen its workforce change as a result of downsizing and retirements; therefore, the background experience of the distribution employees varies widely. They may have an engineering, marketing, accounting, customer service or line construction background. For a company that installs thousands of distribution poles each year, and strings miles and miles of overhead power lines, the diverse makeup of Georgia Power's (Atlanta, Georgia, U.S.) workforce prompted the utility to take a closer look at its engineering training and tools.

As an electric utility, Georgia Power has a responsibility to meet basic National Electrical Safety Code (NESC) requirements when installing and maintaining its power distribution facilities. In the past, the utility relied on rules of thumb, experience or hand calculations to determine pole classes, guy wire tensions and conductor clearances. The only pole loading guide we had was an old specification sheet that showed the pole class required for different sizes of banked overhead transformers, regardless of other facilities on the pole. Furthermore, there was no engineering documentation for this specification sheet.

Because making hand calculations was a time-consuming process and required an extensive engineering background, along with knowledge of the NESC, most engineering field personnel did not perform these calculations. As a result, we were generally using past experience or rules of thumb to determine pole classes and guying requirements. Since most distribution pole line specifications have always been fairly standard, this method of applying rules of thumb and past experience was probably acceptable because if it worked then, it will still work now.

Guying is similar to pole loading in that the manual calculations are cumbersome, time consuming and sometimes difficult for field engineering personnel, especially those with little engineering or math background. As with pole loading, rules of thumb and past experience for guying were used most of the time.

But times have changed. The Telecommunications Act and Georgia's Territorial Act caused a great increase in the number of joint-use pole lines, not only with multiple communications attachments but also with multiple joint attachments with supply companies. Insurance companies are starting to question damage claims or not pay them because of various incidents, including storm-caused damage. Utilities had allowed communications companies to install cables on their poles without determining if the poles met the NESC strength and loading requirements. It is not uncommon around Atlanta to have a distribution pole with two or three supply companies along with several telephone and CATV companies attached. Because it is getting more difficult to obtain guying easements, the use of self-supporting poles has increased dramatically. Rules of thumb and past experience are simply not good enough anymore.

Like pole loading and guying, an understanding of conductor dynamics and the NESC is required to accurately determine pole heights. Georgia Power's conductor sag-tension tables show how the conductors respond to varying temperatures and how mechanical loading conditions are used to determine the maximum final sag of conductors. To manually determine conductor final sags, engineers would locate the conductor in the sag tables, find the appropriate ruling span and length, and then find the worst-case final sag based on NESC requirements. Using this final sag data, along with conductor/cable and equipment attachment points per Georgia Power's specifications and NESC clearance requirements, would enable us to determine pole heights. We want to make sure our people have adequate training and the tools they need to get their jobs done right. To help achieve this goal, we have deployed the PoleForeman and SagLine engineering software products developed by PowerLine Technology Inc. (www.powerlinetech.com).

PoleForeman allows Georgia Power's field workers to quickly and easily analyze a structure with easy editing features to make changes in the design criteria that will not only check NESC requirements, but will aid in determining the most economical installation.

Our goal when implementing the PoleForeman and SagLine software products was to provide an efficient and accurate method of performing engineering-related calculations. These tools allow us to achieve consistency among our engineering departments across the state. For example, when the extreme wind loading (NESC Rule 250C) was revised in the 2002 NESC, we were able to implement that change immediately using the PoleForeman software. Also, the recent change in ANSI 05.1 that requires a reduction of a wood pole's fiber stress with increased height will automatically be taken into account using the software. The capability to seamlessly implement changes from the NESC or our own standards is a great benefit to the company.

We've found the graphical user interface to be straightforward to use. Our field personnel draw the power line layout just as they would in a work order. They specify inputs, including wire size, span length and joint-use attachments, and then run the analysis. The results let the designer know if the pole meets basic NESC strength requirements. The program features a solid model view that provides a 3D representation of the pole, which is beneficial for verifying attachments. With the PoleForeman software, we can run pole loading and guying calculations in 10 minutes or less.

SagLine helps determine conductor ground clearances and vertical clearances between supply and communications facilities to ensure NESC compliance. Like PoleForeman, the user interface is simple. We specify inputs, including wire size, pole height and span lengths, and the program plots the sag profile for the span. The program has a terrain-modeling tool that allows the user to model the ground line topology under the span. The measuring stick calculates the conductor clearance at any point within the span. We no longer have to search through sag tables or make manual sag plots as the software provides this output.

Georgia Power deployed the Pole-Foreman and SagLine software programs as part of its SOCKET initiative. SOCKET is a conglomeration of engineering software programs used by Southern Company's distribution field personnel. The applications within SOCKET include transformer loading, voltage drop, flicker, cable pull, pole loading and clearances. We believe this software platform has many advantages over past methods of performing distribution engineering calculations. Programs like PoleForeman and SagLine help take subjectivity, guess work, and generalized assumptions out of the equation. These programs can perform calculations with tremendous speed and accuracy, which allows us to look at “what-if” scenarios and optimize our designs.

Another benefit is the training aspect. We can take complicated subject matter like pole loading and train someone without a technical background to perform that task. This makes us more efficient and productive as a company.

Mickey Gunter has extensive experience in distribution engineering design, standards and training. He recently retired after a 38-year career with Georgia Power Co., but is still actively involved in teaching National Electrical Safety Code Schools for Georgia Power Co. and Southern Co. engineering and line personnel. Gunter serves on ANSI C-2 NESC Subcommittees (SC4, SC7 and Interpretations), and the NESC committees of the Southeastern Electric Exchange and the Edison Electric Institute. [email protected]

Three Examples of the PoleForeman Software in Use

Example 1: Choose a 50-ft pole, Grade C (noncrossing), Medium Loading District, 3-795AAC primary conductors with #4/0 ACSR neutral, tangent construction, vertical spacing, 250-ft ruling span, 3-100 kVA transformers and three communications attachments.

According to our spec page, a Class 3 wood pole will work. But is this OK? With the PoleForeman software, we can now use the above data to determine if this 50-ft, 3 pole is indeed adequate for both height and strength for this type installation. After analyzing the structure with PoleForeman and using the Sag Profile option, we find that a minimum 50-ft pole is required for the height. However, the vertical loading was 111%, which exceeded the NESC strength requirements. By simply changing the 50-ft Class 3 to a 50-ft Class 2, we now find that the vertical loading is 89%. So, a 50-ft Class 2 wood pole will be adequate for this installation.

Example 2: Choose a 50-ft pole, Grade C (noncrossing), Medium Loading District, 3-336 ACSR primary conductors with #4/0 ACSR neutral, horizontal dead-end construction, 250-ft ruling span, 3-100 kVA transformers and 2-11.5M anchor guys (25- and 22-ft leads, respectively).

According to our spec sheet, a Class 3 pole will work. After analyzing the structure with PoleForeman and using the Sag Profile option, we found that a 45-ft pole will work for the height. However, a 50-ft Class 3 pole has a vertical loading of 105% and the lower anchor guy with the 22-ft lead has a loading of 104%. What are our options? Install a 50-ft Class 2 pole that meets both NESC clearance and strength requirements and increase the lead lengths of the anchor guys to 25-ft and 28-ft, respectively. Or, install a 45-ft Class 3 pole with 22-ft and 25-ft lead lengths, which also meets basic NESC clearance and strength requirements. Of course, the 45-ft Class 3 is the most economical pole to choose and still provides basic NESC safety requirements.

Example 3: Choose a 50-ft pole, Grade C (Non-crossing), Medium Loading District, 3-2/0 ACSR primary conductors with #2/0 ACSR neutral, horizontal dead-end construction, 250-ft ruling span, 3-100 kVA transformers, self-supporting structure with no guys.

It is difficult to use rules of thumb and past experience to determine the pole class of self-supporting poles because we have not had that many in the past. Also hand calculations can be labor intensive, time consuming and difficult. As a result, we would generally give it our best guess, maybe even using a steel or concrete pole. Using PoleForeman to analyze this type installation, we find that a 45-ft Class H-6 wood pole is adequate for the height requirement, but has horizontal loading of 101%. A 50-ft (15-m) Class H-6 wood pole has a horizontal loading of 103%. Since a Class H-6 wood pole is the largest standard pole purchased by Georgia Power Co., what do we do now? We can now use the Moment, Shear, Axial and Deflection data created by PoleForeman to give to a pole manufacturer to customize a class pole that will work, which is generally what we do.

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