For years, electric utilities in the United States have relied on cutouts in the power distribution grid and on the overhead distribution system to provide overcurrent protection. Utilities using distribution cutouts on overhead lines traditionally have implemented cutouts made of porcelain, the material of choice for most every utility throughout the 20th century.
PPL Electric Utilities is no exception. A large utility system based primarily in eastern Pennsylvania, PPL is dedicated to serving its 1.36 million electric customers with the most reliable, stable, safe and secure distribution system possible.
Distribution cutout insulators made from porcelain were a key component in PPL's protection, security and operation of the distribution system for many years. In the 1980s, however, PPL noticed the porcelain cutouts it had used for so many years were increasingly becoming damaged or affected by the cold-weather climate of the Northeast. Cutouts have long been considered a commodity: If the cutout becomes damaged, it is simply discarded and replaced with a new unit.
The Downside of Porcelain
Historically, porcelain insulators have caused issues for cutouts. The brittle nature of the porcelain insulator makes it susceptible to breakage from handling in transit to the warehouse, in the warehouse, on trucks en route to the job and during installation.
There were often challenges with the potting material and the process used in setting the metal mounting brackets and live component materials into the porcelain. This potting material may have had a different coefficient of expansion compared to the metal and porcelain. During extreme temperature changes — including subzero freezing conditions — the natural expansion and contraction process, if not well compensated for in the design or during the manufacturing process, would allow moisture ingress. During extreme cold periods, this moisture would freeze, expand and crack the insulator, which ultimately would lead to mechanical failure.
Cutout Options Investigation
Not long after these issues were discovered, PPL began an extensive analysis and evaluation of all its distribution cutouts and began to notice similar porcelain-related issues across the distribution system. PPL proactively sought an alternative plan to porcelain cutouts by using a more reliable material.
These findings recently led PPL to make an executive decision to no longer install porcelain cutouts on the distribution system and replace them with another insulator material. Once this decision was made, the utility immediately launched a new program to evaluate all available cutout technologies and material options outside of porcelain.
The program led PPL to discover that polymer concrete cutouts offered many benefits. As porcelain cutouts failed, the utility replaced them with polymer concrete cutouts. Over the years of replacing porcelain cutouts with polymer concrete cutouts, PPL realized that polymer concrete has many advantages over porcelain, including an outstanding service record. Over a 20-year period, PPL installed more than 250,000 units. Over this same period, there were no polymer concrete insulator failures. Overall safety is another key advantage.
When electrical line workers would reach up to the porcelain cutout with the hot stick, they would never know if the porcelain cutout was damaged and cracked. The concern was whether the porcelain would come apart when force was applied to it. If the porcelain had been damaged by freeze-thaw cycling (causing cracks to propagate throughout the insulator body), the result could be porcelain fragments dropping on the workers. The polymer concrete prevents moisture penetration, thus resisting damage from freeze-thaw cycles. This resistance results in no crack propagation through the insulator and, therefore, fewer opportunities for shattering or falling apart.
Also, the porcelain cutouts had a history in the PPL service area of being objects for people wanting to practice target shooting with a rifle. The result was a shattered porcelain cutout. With the polymer concrete insulator, a typical result would be a chip flying off the cutout with no cracking or crack propagation. The insulator remains intact.
In 2009, the excellent track record of the polymer concrete insulator led PPL to make the executive decision to remove all porcelain cutouts from its distribution systems systematically and replace them with polymer concrete cutouts from ABB.
Polymer Concrete Search
After unacceptable performance from another supplier, and a research and development comparison search project, PPL approached ABB in 1988 about creating a polymer cutout.
In 1990, PPL elected to move forward with its program to install all new cutouts using a polymer concrete-designed cutout product from ABB. The ABB polymer concrete cutout provided all of the standard features common with cutout vendors: voltage class, basic impulse level, fuse, 300-A disconnect blade, seacoast design, cutout-arrester combo and kick-out spring. However, it also provided additional benefits:
Cold-weather reliability (resists damage from freeze-thaw cycles, no rod-to-insulator potting process required and metal directly molded into the insulator)
Mold-in metal parts that prevent moisture penetration
Excellent electrical properties and dielectric strengths
Superior mechanical toughness
Service history, dating back to 1991
Durability that reduces typical breakage from shipping, storage and installation
Non-brittle material to withstand the rough handling often associated with this type of equipment and shatter proof
Cracks not propagated through insulator like porcelain.
PPL also began using polymer concrete cutout insulators on all of its single insulator 600 and 900 disconnect and load-breaking distribution switches. These switches also are provided by ABB using the same polymer concrete insulators as the cutouts.
Polymer concrete insulators are a better alternative to potted porcelain for cutouts installed in predominately cold-weather climates. Its resistance to water ingress makes it preferred over porcelain, which has a tendency to crack or shatter in extreme cold-weather climates. The polymer concrete insulator cutout is a one-of-a-kind technology within the industry, but one that is growing in popularity among utilities.
Proprietary manufacturing materials make the polymer concrete cutout cold-weather resistant. Its unique molding process makes it more resistant to moisture ingress and, therefore, more resistant to damage from freeze-thaw cycles. Safety also has been enhanced by a shatter-proof design, eliminating any possibility of shattered porcelain falling on a utility worker.
Polymer concrete provides excellent electrical properties and dielectric strengths as well as superior mechanical toughness. The unique manufacturing process also provided a smooth exterior surface, which provides excellent hydrophobicity to expel water quickly. Most of ABB's cutout and switch designs are available with a polymer concrete insulator.
PPL's change-out program involved the removal of approximately 500,000 cutouts. At the beginning of the new decade in 2010, nearly 290,000 of all porcelain cutouts have been removed and changed out to polymer concrete cutouts, with approximately another 210,000 to go.
Testing the Polymer Concrete Cutout Design
To further substantiate the belief that polymer concrete cutouts have performed without an insulator failure, PPL decided to test a small sample of the cutouts installed on its system. It selected 30 units that had been in service for 12 to 14 years on the PPL distribution system. The sample units were tested by ABB, an independent testing lab and another utility's testing lab. The results from all of these tests verified the units performed without failure while exposed to the typical weather conditions of the northeastern United States. Further, they were still in good condition and capable of passing the original design test, per IEEE standard.
ABB had additional tests performed per IEEE C37.41 on two cutouts that were originally installed on the PPL system in the 1991-1992 time frame. The following tests were performed at PowerTech Labs with the following results:
Test 1 (IEEE C37.41 clause 6 and table 7): One cutout was tested at series 1 with a 6K fuse link (three shots at 0-degree, 90-degree and 140-degree closing angles, respectively). The cutout passed the test and cleared all shots with no damage to the device.
Test 2 (IEEE C37.41 clause 5): One aged cutout was tested for power frequency dry and wet withstand and impulse withstand voltage. The cutout passed the test with no damage to the device.
Excellent Operational Experience
PPL can now report that, with more than 250,000 units installed over the last 20 years, there has not been a single known cracked polymer concrete cutout in its network.
PPL found the durability of the polymer concrete design, with less risk of brittle fracturing, met its expectations with no breakage in transit, in the warehouse, on line trucks and in handling during the entire installation process. Also, the weight of the polymer concrete cutout was the same as the porcelain cutout. The material's consistency has created a better-quality cutout and has maintained that quality over a significant manufacturing period.
The quality and reliability are greatly improved over the porcelain design based on the repeatability of the manufacturing process. PPL currently uses the polymer concrete cutout for transformer and capacitor fuses, tap switches and bypass switches on reclosers.
Mark Berner ([email protected]) is the manager of engineering work strategy for PPL Electric Utilities in Allentown, Pennsylvania, U.S. He has a BSEE degree, is a registered professional engineer and has more than 25 years of engineering experience focused on the planning, design and maintenance of electrical power systems. In addition, he is currently leading the effort to replace the aging infrastructure for all transmission, substation and distribution equipment on PPL's system.