The ICC Website is changing. As this transition is made, our new format can be found at www.pesicc.org/ICCWP. Please send any comments or questions to mvh@voncorp.com.
John Tanaka, Chairman, University of Connecticut, reported that the Insulated Subcommittee met at 10:00 am on Wednesday, November 6, 1996 with 143 members and guests in attendance.
Chairman Tanaka opened the subcommittee 5 meeting with the announcements of the dates of the forthcoming ICC meetings. He also said that a copy of the current membership list of subcommittee 5 was included with the sign-up sheets on each of the clipboards being circulated. Attendees were asked to check this list; and, if membership in subcommittee 5 is desired, an application form to this effect should be filled out and submitted to Rick Hartlein. Tanaka also announced that commercialism in the ICC presentations is not allowed, and that if such were to occur, they would not appear in the minutes.
Jean-Marie Braun of Ontario Hydro Technologies gave a presentation on the use of X-ray irradiation in partial discharge testing. (See Appendices 5-L-1 to 5-L-4.) He pointed out that people have used irradiation since the 1960s to trigger partial discharge. The X-rays being used are not for radiography but for measuring electrical properties.
One of the needs for a new partial discharge testing is in the R&D effort currently being carried out on 400 to 500 kV XLPE underground cables. Because of the consequences of failure, more searching tests are needed for commissioning after laying and during operation. This is because, unlike oil-paper systems, the PE cables are unforgiving of partial discharge. Whereas the cables and prefabricated accessories are factory tested, site assembly is difficult to assess. In this process, the accessories are the weakest point.
The current issues of partial discharge testing are detection sensitivity, noises at the test level, and interpretation of the signals. This report concerns a novel X-ray induced technique as well as technology to provide a sensitive partial discharge measurement in the laboratory demonstrated at up to 350 kV on a 230 kV cable. Dedicated hardware has been assembled using ultrawide band technology by a development team of BICC, National Grid, and Ontario Hydro Technologies. The data acquisition unit involves fiberoptic links, selectable bandwidth from 30 k to 200 Mhz, bandwidth magnitude of 0.2 V - 5 V in four ranges, and distributed couplers. X-rays enhance void discharges. The discharges occur in a void when the field exceeds the critical field and free electrons are present. In actual practice, intermittent partial discharge is not recognized because the discharge inception voltage (DIVa) is much higher than discharge extinction voltage (DEV). The true DIV is close to the DEV. By providing initiatory electrons, the DIVa is reduced to DEV. This makes the measurement of intermittent of PD possible. The narrow X-ray beam makes possible the location of the partial discharge. Research and Development is being done to try to assign failure mechanisms from the data gathered.
The results performed were shown in 3D and 2D graphs with and without X-rays. The technology of sorting the pulses was borrowed from GIS technology developed at OHT. A 300V discharge inception voltage was found to be detectable at 50 kV with X-ray irradiation.
The system is applicable to any insulation system. Medium voltage cables can be studied before and after wet aging. Because radiography is practiced in Japan, the use of the X-ray induced PD testing is do-able in the field. Accessories are a problem as shown by recent CESI tests. The accessories lend themselves to this technology. The outlook is that on-line tests at lower voltage can be done with sensitivity. The current issue is that of interpretation. A final overhead gave credit to the collaborators.
Ray Bartnikas commented that PD detection in XLPE cables and SF6 show that cables act as coupling devices. Broad band detection provides little advantage because signal deteriorates along the cable. He wondered whether there was any advantage to increasing the bandwidth from 33 MHz to 200 Mhz.
Braun said that they are changing the disadvantage to an advantage in that they are looking at the accessories rather than at the cable itself. Since they are not as worried about the cable as they are of the accessories, the attenuation along the cable will help in looking at the accessories.
Bartnikas then asked with the X-ray technique, what is the minimum size of the voids which can be detected?
Braun said that they had detected voids down to 100 m. There is a difficulty in artificially forming a void of small sizes to test the detectability. The best which has been done is to find naturally occurring voids. Very small naturally occurring voids are difficult to study since they collapse on cutting/manufacturing..
Bartinikas then said that the most important question is "What does this mean with respect to remaining life?" Also as a final question, Bartnikas observed that when there is impulse voltage across a void, the rise time is shorter than when X-rays are used. He asked whether any differences in rise times were observed?
Braun said that there is a tightening of the pulse distribution. In the absence of X-rays, one gets a high space charge build up.
Because of the time with respect to the schedule, Tanaka cut off questions suggesting that there could be some if the session ended early. Otherwise, individual questions would have to be addressed to Braun privately.
Steve Szanizlo of Union Carbide gave an update on the test program for 105o to 140o C temperature exposure for TR-XLPE. In the mid sixties, 130oC was established as the emergency overload for XLPE. A 1980s EPRI project has shown that both XLPE and EPR can withstand an emergency overload of 145o C. As shown in Appendix 5-M-1, the time to 40% retention of elongation at 150o C was the greatest for TR-XLPE. The data for EPR A was taken on a semicrystalline material. The data for EPR B was taken from the literature on measurements taken on 0.1143 cm (45 mil) wall #14 insulated wire. The other data were obtained on 0.1905 cm (75 mil) molded plaques.
Currently, tests are being done on wet electrical aging at higher temperatures. Work at NEETRAC, previously reported by Hartlein, includes complete cable tests on dry conductor with jacket and cables aged in tanks at 90oC conductor temperature. (Appendix 5-M-2). The aging failures on 0.6604 cm (260 mil) wall jacketed cable designs without moisture barrier have shown that the TR-XLPE have no failures after 1500 days of aging (Appendix 5-M-2).
Also reported by Hartlein at a previous ICC meeting was the thermomechanical testing simulating duct bank installation. There was found to be dimensional stability after the 140o C test for both TR-XLPE and EPR. On testing to 78740 V/cm (200 V/mil), there was less than 5 pC of PD before and after load cycling for all of the cables tested. (Appendices 5-M-2 to 5-M-3).
The second phase of thermomechanical testing is a joint project being carried out with BICC to demonstrate high temperature performance (105/140o) of TR-XLPE cables. The cable designs being studied are 380 mm2 (750 kcmil) compressed aluminum (61w), 0.8763 cm (345 mil) wall with various metallic shield/jacket combinations. These are 24 x 14 AWG bare copper neutral with LLDPE jacket, longitudinally applied corrugated copper tape shield/LLDPE jacket, 14 x 24 AWG plain copper wires with Mylar tape/PVC jacket, and copper tape shield with PVC jacket. Thus far, the deformation of all designs at 140o C are comparable to performance at 130o C which has been successfully used for the past 25 years. At 69 kV, the partial discharge was less than 5 pC before and after load cycling for all cables. The dissipation factor at 140o conductor temperature was less than 0.2% before and after load cycling for all cables. The shield volume resistivity at 105 and 140o C conductor temperature for all cables fell within AEIC CS 5-94 limits.
In conclusion, Szaniszlo stated that the thermal stability of TR-XLPE is superior to other insulations, the wet electrical aging of TR-XLPE at high temperatures is superior to other insulations, and the deformation of TR-XLPE at 140o C is similar to that observed at 130oC which has been used successfully for other insulations for decades. These observations would indicate the suitability for higher temperature operation of feeder cables and PILC replacement.
There were no questions.
Mark Walton of BICC Marshall Technology Center gave an update on EPRI project WO2713-15. Walton reminded the audience that the tasks for this project are to maintain cables installed at four utilities for an additional three years or until failures occur, to keep simulated field aging facility fully operational for an additional three years or until failures occur, and to make the simulated field aging facility available for development of novel "In Situ" methods for evaluating the integrity of cable insulation systems. The objectives are to correlate prior laboratory aging results from RP2713-02 with the service aged cables at the four utilities as well as to correlate the field experience with cables aged in a simulated field aging facility in Marshall, Texas. With this data, it is hoped to modify the XLPE aging model with data from the simulated and actual field aging failures. Ultimately, the object is to provide information to specification writing groups. The specifics on the installation at the four utilities and that of the Marshall Technology Center are shown in the slides in Appendices 5-N-1 to 5-N-2. The 2286 m (7500 feet) of direct buried cable, both jacketed and unjacketed, at the Marshall site have recently been tested by Dr. Mashikian using his partial discharge detector. No partial discharge could be detected. There were six splices. Some PD was detected at two of the six splices at voltages above the operating voltage.
Walton concluded that no news is good news.
Bob Smith of Kerite asked about the likelihood of a failure under simulated field aging tests. Based on the results of accelerated aging tests, when would a failure in these selectively short cable lengths be expected?
Walton said that their aging model does take into account the cable length. With field installations, the temperature of the cable in the field are not known.
Smith then asked that if a 15 year life was predicted and the cable fails at 6 year, what does this do to the model? Walton said that the model had been checked at 4,1 condition and a good correlation was obtained. Walton, however, cautioned that field conditions were being encountered.
Steve Szaniszlo pointed out that the models on ACLT have water in the conductor whereas the cables in the field do not have water introduced into the conductor. Walton said that this was a very good point.
Bruce Bernstein commented that the cables aged in the laboratory under the test conditions failed over a period of approximately two months to over four years; this shows the significance of temperature on the loss of life under wet aging. The cables in the field at the four utilities, that Mark Walton discussed, have about eight years of service aging to date, with no failures. This allows one to make a qualitative estimation at this time, of the value of the aging procedure relative to service aging. As the service aging continues, one will be able to "zero in" on the correlation of the lab with the field aging.
Also, the cables under study were manufactured in the mid-1980s; improvements in shield materials since then have allowed increases in GMTFs for 4 4 conditions, from about 60 days to much longer times, e.g. months. Hence, the ability to achieve even longer service life seems achievable today.
Frank Kuchta and Yingli Wen of Pirelli Cable then gave a presentation of reduced wall EPR cable. For the first part of the presentation, Frank Kuchta described the electrical characteristics of the reduced wall cables. The motivation for reducing the wall thickness of the cables is to enable replacement of PILC cables by replacing the paper/oil cables with extruded cables which can fit into the existing duct. Part I of the presentation discusses the theory of insulation wall thickness and the test program carried out to evaluate the electrical performance of reduced wall EPR cables. The full paper describing the work is shown in Appendices 5-O-1 to 5-O-8. The second part of the presentation was given by Yingli Wen and addressed the mechanical performance on both conventional and reduced wall EPR insulated cables. This full paper is found as Appendices 5-P-1 to 5-P-6. The conclusion was that the reduced wall EPR cables can safely withstand the same pulling forces as recommended for conventional wall cables. These two papers are the same as those presented at the Transmission and Distribution Conference in Los Angeles, September 15-20, 1996.
Hans Gnerlich observed that when cables are pulled the TDR image has changed. The propagation velocity and the attenuation constant seem to be affected. (See Appendix V-Q-1). He asked how pulling changes the dielectric constant or other cable properties that would affect propagation velocity and attenuation. He noted hat the industry standard requires dc acceptance tests, which are of no use in determining the insulation's ac reliability.
Wen said that Gnerlich had brought up a good point and that it may be considered in future studies. She was not sure whether the change in TDR time was due to the increase in length of the cable or due to the change in electrical properties.
Jack Lasky of Okonite asked whether the wall thicknesses on the chart were minimum point or minimum average. Kuchta said that they were the minimum average value and for minimum point thickness is 95% of the minimum average thickness.
Nigel Hampton, BICC, commented that it is important to remember that as the insulation thickness is reduced there is an associated increase in the insulation screen stress. This increase in stress is likely to lead to a reduction in the reliability of the splices and terminations unless, of course, their performance is improved to compensate.
Hampton went on to ask whether, in the calculations presented, the reduction in the thickness is driven by the value of the Weibull shape parameter "b". In the text "a conservative value of 12" was referred to. How was this conservative value defined? Was it, in fact, the lower 95% confidence limit or based on some other confidence level?
The answer was that it was derived from extensive testing in the USA and Italy. Additionally, the "b" parameter utilized was based on wet aged cables which provides a conservative value. Dry aged cables will provide a high "b" parameter.
Carl Watkins AT Plastics asked whether any morphological changes occurred that could explain the changes in attenuation or TDR times.
Kuchta did not notice any significant differences in the morphology that could be related to break down levels. Watkins asked about the crystallinity. There were no comments by the presenters.
Because 11:45 am, the time for adjournment had come and gone, Tanaka declared the
Subcommittee 5 meeting to be adjourned.
Return to the SC-5 Homepage
Return to the ICC Homepage
