Leave the Shovels at Home: Subgrade Inspection of T&D Structures without Excavation

By Tip Goodwin, EPRIsolutions

Business consultants will tell you that you nurture and maintain the assets you own because it is by far the cheapest way to manage your investment for continued success. Those assets may be intellectual property, human resources, material resources or plant in service. Electric utilities are faced with this reality every day as they try to maximize the value of their plant in service for generating and delivering electricity to their customers.

Today, that pressure is increasing due to the need for increased reliability, performance expectations by customers and performance expectations by financial institutions—the “Street” and the stockholder. Asset management is a balancing act of reactive, preventive and proactive maintenance. All of these practices benefit from information gained about asset conditions.

Routine inspections and diagnostic monitoring of asset condition provide the basis for making appropriate maintenance budget decisions. EPRIsolutions, of Palo Alto, Calif., provides several technologies that perform non-destructive evaluation (NDE) type tests to assess the corrosion activity attacking transmission, distribution and substation structure, and grounding components buried in the ground. These technologies provide condition data at 15 percent to 25 percent the cost of traditional excavation methods. The corrosion activity reducing the service life and performance capability of tower stub angles, steel poles, anchor rods, ground mat risers and equipment foundations are quickly assessed and measured with the Below Grade Corrosion Meter (BGCM). Applying the BGCM protocol to a population of towers or anchor rods provides condition data that can alert the maintenance organization to assets being attacked by high corrosion rates or identify areas of limited corrosion. This information enables the O&M department to prioritize future inspection cycles to match criticality, thus maximizing O&M budget dollars.

The BGCM protocol measures the polarization resistance between the component in question and the surrounding soil. Corrosion rates of materials are quickly characterized from the proven electrochemical measurement of the polarization current. In the laboratory, the polarization resistance can be applied to accurately measure the component’s corrosion rate in units like mm/unit time. In the field, however, absolute quantification of the corrosion rate is difficult, because several parameters in the driving mechanism are unknown. Most notable of these is the engagement area experiencing corrosion.

For example, we know the surface area of an anchor rod per unit of length, but we do not know the exact length of rod experiencing corrosion. Traditionally, the area most often associated with subgrade corrosion is the first 18 to 24 inches of depth. In this area, moisture and oxygen are readily available for the oxidation process. Corrosion is traditionally assumed to be less active at greater depths. Inspections, where grillage foundations or anchor rods have been fully exposed by excavation, however, show that corrosion can and does take place the full length of the component—to depths exceeding 10 feet.

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Inability to measure an accurate corrosion rate does not, however, deter from the ability of the BGCM protocol to isolate components and areas where corrosion rates are very high, moderate or nearly inert. Having this information, users can concentrate their budgets on critical areas for remediation. Figure 1 (next page) illustrates a typical BGCM inspection report assessing the condition of components on a transmission line. The three color-coded data points reflect conditional classification based on BGCM inspection data. They range from “minimal activity” (green) to “moderate” (yellow) to “severe corrosion” (red). Following an inspection protocol for a group of components will develop a similar pattern which identifies the components requiring most attention to sustain desirable reliability and service life goals.

With the application of these subgrade inspection protocols, the owner can evaluate his asset’s condition quickly and efficiently with a crew of no more than two workers. BGCM technologies can assess 50 to 60 components in a day, much more efficiently than traditional excavation procedures which would at best inspect 12 to 20 components in a day. Direct cost comparisons of the excavated inspection method vs. application of the BGCM protocol at several utilities have demonstrated an 85 percent cost savings to perform the inspection by leaving the shovel at home and applying the BGCM protocol.

Additional savings are realized by adjusting future inspection cycles, based on corrosion activity. Components that exhibit minimal levels of corrosion activity are shifted to longer inspection cycles. Moderate activity also warrants appropriate inspection intervals to monitor whether the corrosion rate changes. Using the BGCM protocol, asset owners can prioritize remediation and inspection efforts to make best use of maintenance dollars, while sustaining component performance at the desired levels and extending the component’s service life.

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Studies have illustrated that performing maintenance activities at a steady, ongoing rate is much more effective than irregular expenditures of larger sums of money. Figure 2 illustrates the patterns obtained in managing the degradation of a system asset. All components have a natural degradation curve for their characteristics. To maintain a very limited level of degradation, frequent replacement is required. A successful asset management plan that consistently assesses an asset’s condition and takes appropriate action to address critical degradation levels will maintain the next best degradation management curve.

Unfortunately, many maintenance programs currently are driven by a more reactive pattern where less frequent expenditures of larger sums are applied to maintain an asset. While the larger financial expenditures may provide a false sense of appropriate care, the diagram illustrates that the overall condition of the asset experiences a faster degradation rate than the other maintenance strategies. T&D assets are significantly different than other systems where there is an eventual end of service life and the system is automatically replaced. Historically, T&D assets are only replaced when they are no longer capable of carrying the electrical load that is required on a given corridor. Even then, our first intuition is to evaluate the potential of upgrading or uprating the asset to accommodate the desired electrical capacity. Ongoing maintenance is critical to the management of these assets. The BGCM protocol is an efficient diagnostic tool to allow utilities to properly optimize the management of their T&D assets.

Tip Goodwin is manager of technology delivery at EPRIsolutions. He is located at the Haslet Engineering and Test Center in Haslet, Texas.

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