Pole Testing Ensures High Reliability and Long Life
Byline: Lee A. Renforth, IPEC Ltd., and Andre Taras, Hydro-Quebec
Electric utilities are placing increasing emphasis on cost-effectively extending the life of existing facilities while maintaining adequate levels of safety and reliability. In the case of overhead lines, utilities face the problem of aging wood poles (most will reach the end of their "design life" over the next 10 to 20 years) combined with tight capital and maintenance budgetary requirements.
Furthermore, since the mid-1990s, more telecommunications, cable television, internet and wireless carriers in the United States and Canada are seeking to expand their markets by attaching their equipment and cables to utility poles. Most in-service wood pole lines were not designed to carry this additional hardware and are overloaded beyond their original safety factors.
To compound the issue, extreme weather has increased liability concerns due to these additional loads. Overloading also can be an issue with newer poles if their fiber strength is lower then the accepted nominal value. Previous research on new poles and a sample test confirmed this is often the case with newer growth timber.
One solution to these concerns is to apply cost-effective, reliability-based inspection and pole management programs that provide quantifiable data on asset condition. With such data in hand, utilities can render appropriate judgments on preventative and corrective maintenance to maintain line availability at minimal cost and provide reliable life-extension.
A six-month pilot project carried out by Hydro-Quebec (Montreal, Quebec, Canada) between September 2001 and February 2002 incorporated new non-destructive testing (NDT) wood pole inspection tools with a reliability-based wood pole management program.
Hydro-Quebec made a presentation on this project and this technique at the CEA Technologies conference "Workshop on Utility Pole Structures" held in Winnipeg, Canada, in June 2002. The concepts, inspection techniques and management methods described here are based on reliability-centered power line management, as pioneered in the United States in the 1980s and adapted to United Kingdom and Canadian practices. This approach to life-extension of overhead lines through reliability-based inspection and management is an alternative to rebuild approaches. It provides significant cost-benefit ratios when applied in the United States, Canada and the United Kingdom.
An overhead line network consists of large numbers of relatively simple structures. The reliability of any one line segment is equal to that of the structure with the lowest reliability level (the structure that is the weakest link in the chain).
The pole structures can vary in strength because of the different fiber strengths of various wood species and the variations within the same species. Variations in strength around the mean value of any species have been measured at around 20% to 40% for new poles and up to 80% for in-service poles in the United Kingdom, United States and Canada. The strength variation follows a normal distribution about the mean, which is typical for natural products such as wood. The variation in strength of in-service wood poles has been assessed through several destructive test programs.
The results of the destructive testing of a sample of 150 U.K. Scots pine poles (Fig. 1) shows that while there is a wide variation in pole strength of a particular age, the average strength of wood poles decreases with time in service. Such results are typical of most species of wood used in transmission and distribution lines.
Hydro-Quebec conducted a comprehensive destructive testing program from 1996 to 2000 to evaluate all commercially available wood pole non-destructive evaluation (NDE) technologies and their capability to accurately measure pole strength. Close to 500 poles were measured for strength with the NDE instruments and then subjected to full-scale destructive tests. Hydro-Quebec used the POLUX [superscript][TM] Strength Tester (distributed by Pole + Management Inc. of Montreal) to measure the remaining strength of its poles for this pilot project.
Figure 2 shows an example of the estimate to actual strength measured during the test program. Note that in almost all cases, the actual strength of the in-service poles tested was significantly lower than the mean strength rating given in the Canadian standard for wood poles for overhead lines (CSA 015 or ANSI 05.1). This result suggests that using the mean values available from the CSA or ANSI standards alone is inadequate for the purpose of assessing the strength of in-service wood poles.
The results of the destructive and non-destructive pole tests can be used to provide an average strength degradation rate for wood poles in a particular climatic region. Figure 3 illustrates the average strength degradation rate for wood poles in Hydro-Quebec from both destructive tests and NDE assessments in the field. Applying this degradation rate to in-service poles is highly useful in asset management and maintenance planning.
Variation in wood pole strengths and the degradation of in-service poles have led to a review of safety margins in overhead line designs. Such margins are reduced if the pole does not continue to meet class requirement and if additional hardware is subsequently added. Safety margins for line designs must also be continually evaluated because of the increase in the range of applied loads (wind-on-ice loading and wind-only loading).
POLUX makes it possible to accurately measure the wood fiber strength of in-service poles. The instrument integrates visually rated wood pole parameters (knots, age, size) with its measurement of wood density and moisture content to compute the remaining strength of the pole.
The additional measurement of the Resistograph [superscript][TM] Constant Force Drill enables the user to identify decay cavities beyond the 2-inch (5-cm) reach of the POLUX probes but does not compute strength. Such voids are then factored into the calculation of the remaining section modulus. This integrated approach provides quantifiable strength data, which is characterized by a best strength estimate (mean value) along with a standard error of estimate (SEE), from which a normal distribution for the strength prediction can be generated.
Integrated vs. Conventional
Traditionally, the maintenance decision to repair, treat or replace a wood pole is made by the lineman in the field based primarily on visual sounding (with a 2-lb [0.9-kg] hammer) and boring/auger drilling inspection. These conventional line inspection schemes rely on the experience and knowledge of the linemen in the field and the line engineer in the office to provide a qualitative assessment of line condition.
The new integrated approach represents the combination of several tools, devices and methods, which provide repeatable, highly resolved and quantifiable data on material condition and remaining structural strength.
The new approach supplements standard inspection procedures with non-destructive pole strength evaluation and quantified component condition ratings. The results from the field inspection are combined with worst-case loading analysis (wind-on-ice loading) and detailed structural analysis to provide a factor of safety (FOS) value for any one structure. This assessment methodology is followed by a probabilistic approach enabling structures to be compared on a like-by-like basis. The approach also provides the utility asset manager with reliability, life expectancy, strength degradation and risk calculation data to assess the integrity and long-term performance of a line.
The pilot project objectives were to:
Measure the remaining strength of the poles with the POLUX NDE instrument.
Calculate the transverse and buckling loads on the pole due to wind and ice by taking inventory of the pole attachments and line hardware (including telecom and cable TV hardware).
Use the Resistograph NDE instrument to look inside the pole for decay pockets to be factored into the calculation of the remaining section modulus of the pole.
Determine the FOS by comparing the remaining strength with measured strength.
Calculate the probability of failure (POF) by using risk analysis techniques.