T&D World Magazine

What is the Condition of Your Insulator?

EPRI and utilities develop a hot-stick tool to identify high-risk composite insulators prior to live work.

An essential requirement for ensuring worker safety when performing live work with transmission composite insulators (also called polymer or nonceramic insulators) is to confirm both the electrical and mechanical integrity of installed polymer units as well as any replacement units.

Unlike porcelain or glass insulators, electrically defective composite insulators are difficult to identify because there are no generally accepted and easily applied procedures to accomplish this, and because instruments for testing composite insulators prior to live work are not readily available. As a result, some utilities have opted not to use composite insulators, and even some of the utilities that do use composite insulators avoid live work on structures with these insulators.

For years, there have been no portable tools available for use by line workers to detect defective composite insulators. However, in 2003, EPRI initiated a project to develop a simple detector for assessing the integrity of polymer insulators as they are installed.

Collaborative Development

The primary focus of EPRI's initial project was on double-circuit 230-kV steel-lattice structures; however, other types of 230-kV structures also were considered. For the configurations tested, the project determined the percentage of a composite insulator that could be electrically conductive before it would reduce the switching surge flashover levels below acceptable levels. The testing also confirmed the use of a ladder meter was not a viable option to assess the electrical condition of composite insulators in service.

One of the significant contributions the project made was to determine the critical length of defects an inspection tool would have to identify in service for the specific configurations tested. Based on switching impulse tests, the critical defect was defined as a defect that bridges, or shorts out, approximately 18% of the insulator length. This was the first step in the development effort.

Motivated by the results of the initial effort, an early concept for a technology to address the issue was identified and hardware development was initiated. In parallel with this effort, a separate EPRI project collected detailed information of composite insulator failures in the field and evaluated field-aged units. The North American failures were categorized in a database as a function of failure mode. Some of the field-aged units removed from service also were dissected.

Suitable Characteristics

During the project, the characteristics of a suitable insulator fault detector were developed. The detector should provide a clear indication when a critical or more-severe-than-critical defect is present and should be insensitive to less-than-critical defects. The detector should detect major defects in energized polymer insulators and ignore minor defects; use an approach currently familiar to the line worker's craft; be field-rugged and simple to operate, yet small and light for ease of handling; and minimize distortion of insulator voltage distribution.

The Solution

The EPRI research team developed a hot-stick tool that has two spring-loaded electrodes separated by approximately 6 inches (152 mm). The electrodes are pushed up against the sheath of the insulator, and a high frequency, high voltage is applied between the electrodes. The unit knows to apply the voltage and start a measurement because of the force applied back through the spring-loaded electrodes.

A sensor embedded in the grounded electrode receives a signal during the measurement process. This signal is analyzed, and the unit informs the user whether the test section has any conductive or semiconductive properties. This helps determine whether the insulator is electrically compromised. The results are communicated to the user through tones and a red or green light. More detailed information also can be read, and stored wirelessly, in real time using an iPad or similar device.

The operator tests the insulator at 6-inch increments starting at the energized end. If more than a predetermined length is identified as being electrically compromised, the operator stops taking measurements and makes an assessment on whether the work site should be worked under energized conditions.

Development Challenges

As the EPRI research team created a plan to develop the insulator defect detector, many challenges had to be overcome.

To be effective in its measurements, the detector would need to bond to the end fittings when evaluating the adjacent section of the insulator. One challenge was if the floating tool were close to an energized insulator end fitting, an arc would form from the tool to the end fitting. An electrical connection to the insulator end fitting was needed to prevent this arc from developing while the adjacent section of insulator is evaluated. But with the technology under development, an electrical connection to the end fitting could result in misleading readings.

To resolve this, the team devised a solution that involved creating a floating guard electrode, or a Faraday cage, that would surround the probes and electronics housing and could be safely bonded to energized or grounding components. After a few iterations, the floating guard design for the detector was successful in performing measurements while bonded onto an end fitting. Tests also revealed it performed with increased robustness in high fields and when bonding onto the insulator end fitting.

Another challenge was weight. This is important because field personnel have to operate the tool at the end of a hot stick. By 2010, the team was able to get the weight of the detector unit down to 4.7 lb (2.13 kg), and in 2011, further decreases in weight were realized.

After years of development and small-scale testing, the first full-scale outdoor testing at 345 kV was completed by utility field personnel in June 2010 at the EPRI laboratory in Lenox, Massachusetts, U.S. Several conclusions resulted from those tests:

  • The testing tool was able to conduct measurements that are repeatable and consistent on both good and defective long-rod composite insulators over a range of voltages from an aerial device.

  • Later Development Stages

    The tool showed good defect sensitivity; there was a 50% to 350% increase above baseline measurements, depending on the type of defect.

  • The tool was able to use a smaller internal power supply, enabling a further reduction in weight, which was needed.

Another challenge identified was the reliability of results when a measurement was made while an arc was terminating on test device electrodes. The researchers initially thought the bonding electrodes used to ensure the unit was electrically connected to the insulator metal end fitting would overcome this, but it was shown inadvertent arcing may exist during the time of measurement due to motion of the hot stick and the presence of a defect. Further development was needed to address these issues.

Some process questions also remained from the testing. On the topic of repeatability, could the same operator conduct multiple tests of the same object and achieve the same results? Could different operators perform the same testing and achieve the same results? To confirm the answers to these questions required future tests to be performed in a blind manner, where participants were unaware of the condition of the insulator, to ensure the results were not manipulated.

Getting Close

The team continued to develop the device. Significant effort was spent to reduce the weight to just below 4 lb (1.8 kg) in 2011; there still may be additional ways to reduce the weight and size. In addition, the measurement method, algorithms and electronics were modified to ensure the device was more resistant to arcing activity terminating on the electrodes. The team also improved the interface of the testing device based on feedback from the utility personnel who participated in the testing.

Another series of tests were performed by seven utility personnel at 138 kV in June 2011 at the EPRI lab in Charlotte, North Carolina, U.S. Those tests revealed there was effective repeatability with different testers using the device. Out of 185 measurements taken during the tests, two readings resulted in false positives. Analysis of the results has allowed the researchers to adjust the algorithms to mitigate this issue.

The Charlotte lab tests led to a full-scale live working test at the EPRI high-voltage lab in Lenox in September 2011. A team of utility field personnel and experts were on site to participate in these tests. The focus of the tests was to perform full-scale 345-kV testing with multiple line workers using configurations identical to what a utility field worker would experience. Testing was performed on I-string, V-string and deadend configurations.

Advancing to Robotics

Full-scale units were used for de-energized testing. The testing device successfully identified defects when it was bonded and unbonded to the end fittings. The tool also effectively communicated the results to the operator. The tool was easy to operate at the 138-kV and 230-kV hot-stick lengths and challenging, but not impossible, at the 345-kV length. Although the unit was more resistant to arcing, it was not perfect. Watching the testers in action and reviewing the results, the team identified numerous areas of improvement that would make the tool easier to operate.

Changes were made and the utility personnel and EPRI research team returned to the EPRI Lenox laboratory in November 2011 for evaluation. The testing went flawlessly with all of the recent modifications working as intended. The unit was resistant to arcing, allowing measurements to be made while a significant arc was terminated on the end fitting. The testing also verified the use of the tool with a corona ring in place.

EPRI is currently working with a commercializer to manufacture, supply and support the live working NCI tool to the utility market. Ten units are presently being constructed and will be provided to utilities to evaluate in the field and provide feedback to the development team. The units will be ready for deployment in the first quarter of 2012. Some development and testing are still necessary, including adjusting the unit to address post-type insulators, refining algorithms and refining test procedures to ensure reliability.

One outcome of the full-scale testing was that using the unit at the end of a 500-kV hot stick with no assistance would be challenging. There are work procedures that could possibly address this, but in 2010, EPRI researchers identified this as an appropriate application for robotics. EPRI initiated the development of an insulator crawler, which would take the detector technology as a payload.

In 2010, a feasibility study and detailed design of the insulator crawler were completed, and in 2011, a technology demonstrator was constructed and tested on I-string, V-string and deadend de-energized insulators with success. The hot-stick composite insulator test tool was then integrated into the insulator crawler and tested, showing very promising results with improved repeatability over measurements made by an operator using a hot stick. Although there is a long way to go on this challenging development, the project is showing the use of robotics in the future has significant advantages, including more repeatable measurements, addressing ergonomic issues and removing personnel from energized situations.

Project Participants

Andrew Phillips ([email protected]) is technical director of transmission and substations area in the power delivery and utilization sector at EPRI. His current research activities focus on the overhead transmission, underground transmission, substations and high-voltage direct-current programs, where he manages more than US$25 million in research and development funding on behalf of EPRI members. Phillips' special areas of interest are robotics, nonceramic insulators, lightning and grounding, inspection and assessment of components, sensor development and daytime corona inspection.

Ed Hunt ([email protected]) has been in the electric trade for almost 30 years and with the Western Area Power Administration since 1992. He participates on the EPRI Live Working Task Force and has been a key promoter of finding answers for polymer insulator and live working issues. Hunt is on National Electrical Safety Code Subcommittee 8 and has been an associate member of IEEE for 15 years, helping to review, write and update technical standards, guides and papers for the Electrical, Safety and Maintenance of Lines Group of the IEEE. He is a certified utility safety professional.

Alan Holloman ([email protected]) has worked in the transmission area with Georgia Power Co. and Southern Company for more than 33 years. Starting in line construction as a groundman, he continued his progression as lineman, bare-hand crew leader, bare-hand foreman and transmission line supervisor to his present position as transmission maintenance and support team leader. Holloman represents Southern Company on numerous technical and standards groups, including the EPRI Live Working Task Force as chair, the EPRI Inspection, Assessment & Maintenance Task Force, the Electrical Safety and Maintenance of Lines Committee of the IEEE, and the National Association of Corrosion Engineers.

Over the years, several utilities collaboratively funded the hardware development project, and a strong team of advisors guided EPRI researchers. Several people were integral to this process:

Alan Holloman, Southern Company

Ed Hunt, Western Area Power Administration

Jude Awiylika, San Diego Gas & Electric

Alf Bonanno, Mike Mclean and Dave Tuttuci, Powerlink

Robert Gordon and Wyn Weaver, CenterPoint Energy

Cal Hoppe and John Podnar, FirstEnergy

John Kile and Marty Delashmitt, Tennessee Valley Authority

CK Ng, Hydro One

Ken Brown, Bonneville Power Administration

Companies mentioned:

Tim Olson, Manitoba Hydro

Ron Lund and Scott Walz, Nebraska Public Power District.

Linwood Blacksmith and Tyson Lies, Tri-State Generation and Transmission

Bonneville Power Administration | www.bpa.gov

CenterPoint Energy | www.centerpointenergy.com

EPRI | www.epri.com

FirstEnergy | www.firstenergycorp.com

Hydro One | www.hydroone.com

Manitoba Hydro | www.hydro.mb.ca

Nebraska Public Power District | www.nppd.com

Powerlink | www.powerlink.com

San Diego Gas & Electric | www.sdge.com

Southern Company | www.southerncompany.com

Tennessee Valley Authority | www.tva.gov

Tri-State Generation and Transmission Association www.tristategt.org

Western Area Power Administration | www.wapa.gov

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