Insulators Removed from Service

There were 200 115-kV and 138-kV insulators removed from service for evaluation, of which 74 were subjected to a detailed examination comprised of a visual inspection, hydrophobicity measurement, dye penetration test, dissection and, in some cases, mechanical testing. The remaining units were evaluated only by performing a visual inspection.

In all cases, it was found that the most severe degradation was observed in the same areas where dry corona activity was seen during the daylight discharge inspections. On some units, it was found that the degradation of the sheath and end-fitting seal progressed so far that the rod was exposed to the environment. These latter units are considered high-risk units; failure is considered inevitable.

Mechanical tests were also performed on the majority of units. It was found that the units still retained their mechanical strength. Therefore, the degradation had not progressed so far as to affect their mechanical strength.

The evaluation of the units removed from service showed these findings:

• The degradation observed could be directly attributed to dry corona activity or wetting discharge activity.

• Varying levels of degradation were observed on more than 80% of the units evaluated.

• Degradation varied from initial degradation to severe damage that would result in a high risk of failure of the unit.

• The older the units, the more significant the degradation.

• Severe damage was only observed on units that had been installed more than seven years, with units exceeding 12 years in service having the most significant damage.

Information into Action

Based on the approach taken, four recommendations were developed. First, for the insulator designs and configurations evaluated, it was determined that corona rings were needed. Second, before installing a new design, 3-D E-field modeling or full-scale three-phase testing is necessary to determine whether corona rings are needed. Third, units in service need to be retrofitted with a corona ring or replacement units need to be installed with corona rings. And finally, other transmission lines with 115-kV and 138-kV insulators without corona rings need to be evaluated using both discharge inspections and E-field modeling to determine whether accelerated aging is occurring.

Companies mentioned in this article:

As soon as the presence of electrical discharge was verified with the daytime corona camera, EPRI worked closely with PSE&G engineers to develop an approach to assess the level of risk for the installed population of insulators. It also helped formulate a suitable remediation plan and instruct the PSE&G workforce on the condition assessment of field units, which included a customized field guide and training video.

The results from this project allowed the PSE&G re-conductoring project to continue, prevented a greater population of polymer insulators from being installed without corona rings, avoided the need to replace a significant number of in-service polymer insulators and established an approach for future assessments. One negative consequence from a business perspective was the additional time and cost incurred to revisit each structure in order to address the issue.

PNM has replaced all deadend insulators on the 14-mile (23-km) 115-kV transmission line on which the failures occurred. All of its other 115-kV lines have been inspected with daytime corona cameras and units with corona activity were addressed. For new designs, PNM is specifying corona rings for certain applications, including braced-post designs.

Early Detection

Deterioration due to corona discharge activity takes time to develop. Condition assessments made on an existing in-service population of polymer insulators are a vital element for utilities to assess the degree of deterioration and their individual levels of risk.

At PSE&G, research findings have underscored the need to ensure polymer insulators at all transmission voltage levels are free of dry weather corona by evaluating the E-fields in the design and procurement process. Having a tool like the EPIC software package allows utilities to assess E-field levels in the design phase. Since many, if not most, of the major polymer manufacturers have supported the development of the EPIC software program, they can also run the analysis for the utility either before or during the insulator procurement process. EPRI criteria for electric field levels on polymer end fittings will be incorporated into polymer specifications.

This also points out the need for utilities to provide polymer manufacturers with specific structure information so they can better assess the need for corona rings. Findings also support the value of performing as-build benchmarking after any line construction project with a DayCor camera to ensure the line and associated hardware are corona free. Detecting problems early enough enables a less costly intervention and avoids a potential failure of the polymer string.

EPIC Software

To address the application of corona rings on transmission line composite insulators, the Electric Power Research Institute (EPRI) has developed 3-D electric field (E-field) modeling software for use by utilities and manufacturers. The EPRI Polymer Insulator Computation (EPIC) software has been specially designed for composite insulators and calculates the E-field distribution, comparing the results against EPRI or custom threshold levels. This helps the user determine whether a corona ring is necessary or is of the correct dimensions. In order to minimize the effort and expertise required to use the software, it has been designed using a database where structures, conductors, line hardware and insulators are selected from manufacturer or standard databases. It has a graphical interface to make selections easy.

The EPIC software currently models suspension, angle and deadend structures. Post and braced-post configurations are being incorporated into the system in 2010 with initial work on hardware rings for porcelain and glass insulators. The software has been designed with several unique features that users will recognize in functionality and capability:

• Enables utilities to make informed decisions on the application of corona rings

• Enables use by non-experts in CAD and electromagnetic field modeling

• Reduces model creation time due to the use of industry standard databases

• Significantly reduces the time needed to determine 3-D E-field distribution, enabling the evaluation of different scenarios

• Enables a standard modeling approach using data from different manufacturers and utilities

• Allows member utilities to review and adjust models provided by participating manufacturers as part of a procurement process.

The EPIC software was developed by EPRI with the aid of 32 participating utilities and four insulator manufacturers.

DayCor www.daycor.com

EPRI www.epri.com

PNM www.pnm.com

PSE&G www.pseg.com

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Raymond Ferraro (raymond.ferraro@pseg.com) has been employed at Public Service Enterprise Group for 35 years, where he has held technical and management positions in the areas of electric overhead and underground transmission, information technologies, metrology, and telemetering and controls. He is currently a technology development and transfer consultant in the emergent technology department of PSEG, responsible for seeking out, evaluating and transferring diverse new technologies and products to electric delivery.

Emilie Dohleman (edohlem@pnm.com) serves as manager of stations and lines design ream and land services team, Public Service Company of New Mexico. She has worked on design and construction of high-voltage transmission lines for more than 20 years. Dohleman has been active in EPRI insulator committee and other EPRI tailored collaboration projects for several years. She is a civil engineer with BS degree from Montana State University and a MS degree from the University of Minnesota, and is a licensed engineer in New Mexico and Oregon.

Dr. Andrew Phillips (aphillip@epri.com) is the director of the transmission lines and increased power flow research area of EPRI's power delivery and utilization sector. His current research activities focus on the overhead transmission, underground transmission, increased power flow and high-voltage dc programs. His special areas of interest are line insulation, lightning and grounding, inspection and assessment of components, sensor development and daytime corona inspection. Phillips holds three U.S. patents and is the author of more than 60 journal and conference publications.