Prior to 1980, PPL Electric Utilities (PPL; Allentown, Pennsylvania, U.S.) installed more than 8 million ft (2.4 million m) of bare concentric neutral cable, which is about 25% of PPL's population of underground cables. PPL began experiencing an exponential growth of in-service cable failures as the cable approached the end of its design life. PPL researched the bare concentric neutral cable and found that other companies also were starting to see the same type of trends, revealing an industry trend. If the failures continued to grow at the same rate, the in-service cable failures would reach an operationally unmanageable level. Also, the costs associated with replacing the cable would climb to unacceptable levels. In 1999, PPL began developing a cable-testing program to address the rising number of in-service cable failures and reduce the rising costs due to in-service cable failures.
Cable Replacement History
In 1970, PPL began installing bare concentric neutral cross-linked polyethylene (XLPE) 15-kV-rated cables directly buried in the ground. Today, PPL is having the most problems with this type of cable installation. In 1981, PPL switched to covered neutral cable, which has proven to have much better performance compared to the bare concentric neutral. In many of the cables, PPL has found deteriorated neutrals that can cause stray voltage problems and lead to above-normal stress on the insulation. Insulation failures also are a leading cause of cable problems. PPL has experienced water ingress into splices, terminations or the cable insulation, as well as water and electrical trees in the insulation.
In the mid-1990s, PPL began experiencing failures in the bare concentric neutral XLPE 15-kV-rated cables directly buried in the ground. At first, the failures did not seem like a serious problem, but in five years time, the failures had nearly doubled. PPL realized something needed to be done to address the excessive amounts of cable failures.
In 1999, PPL embarked on an underground cable-testing program in an effort to economically reduce the increasing rate of cable failures. The idea of testing the cables was to replace the ones in the worst condition first. Prior to purchasing the test set, PPL was replacing all of the underground cables in poor-performing underground residential development (URD). The business case for the first test set was based on the savings realized by deferring the cost for replacement of the cables that were found to have remaining life. A typical URD on PPL's system had, on average, about 25 cable sections (about 500 ft [152 m] per cable section) per URD, but some URDs could have up to 50 cable sections. Therefore, the typical URD had about 125,000 ft (38,100 m) of cable with replacement costs at about US$35 per foot. At $500,000 to $1 million per development, PPL could only budget one or two developments per year. This was an unacceptable means to deal with the growing trend. The concept of the testing program was that 50% of the cable sections in any given URD would have remaining life. By testing, PPL could defer the costs to replace the 50% with remaining life and use that money to replace the worst cables in the next URD.
By the end of 2002, PPL hired an additional crew to help minimize schedule conflicts and optimize the use of the test set. The initial process and coordination issues experienced during the inception of the program had diminished and the program was beginning to run smoothly. During this time, PPL evaluated the business need for purchasing an additional test set and increasing the number of cable sections tested per year, which would also increase the number of cable sections that would need to be replaced each year. PPL determined that in order to keep up with the cable failure trend from a reliability standpoint, the utility needed to test more cables per year. So in late 2002, PPL purchased an additional test set, and by the end of 2003, an additional crew was hired to ensure optimal use of both test sets.
PPL's underground cable-testing program has three main objectives. Its first is to maintain or even reduce the number of in-service underground cable failures at an operationally acceptable level. This is important from both a customer satisfaction standpoint as well as an economic standpoint.
Currently, PPL experiences about 400 in-service underground cable failures per year. In the five years prior to the inception of the cable-testing program, in-service underground cable failures had shown a geometrically increasing trend that had the potential to lead to operationally unacceptable levels. Typically, underground cable failures result in long restoration times. PPL has both radial-fed URDs and loop systems. In the radial-fed systems, customers remain out of service until the fault is located and repairs are made to the cable. In both radial and loop systems, the most time-consuming part of restoration is locating the fault. In addition to the long restoration times, the costs associated with locating and repairing the cable become very expensive.
The second objective of the program is to minimize the overall cost of cable maintenance. The program provides diagnostic information to determine the best maintenance alternative. Limited resources make it difficult to maintain an aging infrastructure. The cable-testing program provides a means of prioritizing cable replacements to prevent failure. The basic concept is to replace the worst cables first and leave the cables with remaining life in service. PPL also has used cable-curing methods to remediate cables that do not need replacement but show signs of water trees through the insulation. From PPL's experience, cable curing is about one-third of the cost of replacement.
The third objective is to minimize the impact to customer satisfaction by attempting to avoid the third in-service underground cable failure. Customer-satisfaction surveys have indicated that customer satisfaction declines rapidly after the second customer outage occurs. At this point, the program has not reached a proactive level, but each year PPL is getting closer and closer to meeting this objective. Outside of the cable-testing program, PPL replaces any underground cable that has failed twice in six months or has failed three times within any time frame.
Currently, PPL tests about 750 to 850 cable sections per year. On average, 45% of the cables need to be replaced and 15% need remedial treatment. At first glance, one may ask, why even test? If almost 50% of the cable sections need to be replaced, why not have a wholesale replacement program? Testing provides a means to defer the cost of replacement for the 40% of the cables that test “good,” which means there is still remaining life in them. This also allows PPL to replace cables in another URD that are in much worse condition.
In terms of the program's impact on the cable failure trend, it appears that the trend is still increasing, but not as drastically as it once was. PPL does not expect to understand the full impact on reliability for a few more years. In terms of costs, the program has definitely provided savings in deferred replacement costs in the amount of nearly $14 million as of June 2005.
Jill Sinkiewicz, a support engineer in Distribution Asset Management, is responsible for planning distribution system facility upgrades and distribution maintenance for PPL Electric Utilities. Sinkiewicz graduated from Villanova University in 2002 with a BSEE degree and also passed the Fundamentals of Engineering Exam. In 2002, Sinkiewicz joined PPL Electric Utilities Corp. in 2002 in the Reliability & Maintenance Department working on various transmission and distribution maintenance programs. firstname.lastname@example.org
John L. Hinkle has worked for PPL Electric Utilities Corp. for 45 years in various test and supervisory positions. He is presently supervisor of Insulation Test, where he supervises all PPL's high-voltage insulation testing of transformers, generators, circuit breakers, cables, motors, lightning arresters and electrical protective equipment for workers, including switch sticks and hot line tools. Hinkle graduated from Villanova University in 1960 with a BSEE degree, and in 1966, he obtained a Pennsylvania Professional Engineer's License. email@example.com
Three off-line tests are used to assess a cable's condition. First, a Time-Domain Reflectometer Test (TDR) is performed to check for neutral corrosion. The second test conducted is either a Power Factor or Tan Delta Test to check the overall insulation losses and condition. The third test, a Partial Discharge Test (PD), is conducted to check for localized defects in the cable, splices or terminations.
The TDR sends a low-voltage pulse in the order of a few nanoseconds duration between the cable conductor and neutral down the cable, which reflects back from the other end or any open or partially open location in the cable or neutral. Increases of impedance between the cable conductors cause a positive reflection and decreases of impedance cause a negative reflection. Splices cause a positive and a negative reflection, because as a pulse enters a splice, the conductors separate causing an increase of impedance and as the pulse leaves the splice the conductors become closer together again causing a decrease of impedance. Broken strands cause an increase of impedance and a positive reflection. The more broken strands that exist at a point, the larger the reflection becomes.
The height of the anomaly determines the severity of the neutral damage. If less than 25% of the strands are broken, the anomaly is too small to be recognized. If 25% to 50% of the strands are broken, the anomaly is smaller than a splice. If 50% to 75% of the strands are broken, the anomaly is larger than a splice but is smaller than the cable end reflection. If 75% to 100% of the strands are broken, the anomaly is larger than the cable end reflection. The TDR calculates the distances from the end where the test is being applied to each anomaly and to the cable end.
The Power Factor or Tan Delta Test is performed on the cable insulation at a voltage about one-fourth of rated cable voltage, operating voltage, and about one and one-half times the rated cable voltage. Power factor equals watts divided by volt-amperes (IR/IT) and tan delta equals watts divided by reactive power (IR/IC). Good insulation has a low power factor or tan delta and very little increase of power factor or tan delta with increased voltage applied. This test does not locate a bad spot, but it does measure the overall quality of the entire cable insulation.
The PD test is performed on the cable insulation at voltages below operating voltage, operating voltage, one and one-half times the cable rated voltage, and two times the cable-rated voltage using an alternating voltage (ac) source. Partial discharges are small electric sparks or discharges in the order of Pico-coulombs that occur in the cable insulation, splices, terminations or on the surfaces of terminations. The discharges do not completely bridge the insulation, but emit measurable pulses of various frequencies from about 100 kHz to 400 kHz. The pulses are very short, typically 1 ns to 5 ns long. This test detects the location, severity and repetition rate of the PD that occurs at that test voltage. Based on the results of these three tests, it is recommended that the cable is replaced, injected with silicone, or to remain in service as is.