«Abstract— Plant Operations personnel can avoid a forced shutdown by applying a predictive maintenance program to power cable and equipment systems. ...»
Level 5: The system has a high probability of failure within the next two years. Consider immediate replacement.
Phase resolve studies of signals from several hundred thousand feet of cables (PILC, XLPE, EPR) over the past six years has enabled DTECH to ascertain patterns from the data developed, which, in turn, facilitates estimation of future performance.
Test Methodology - Electrical Equipment The condition of electrical equipment such as transformer, switchgear, and motors connected to power cable can also be assessed with Cable|wise technology. Again, no shutdown of equipment is required while taking readings (Figure 2).
Measurements are taken while the equipment is operating at operating voltage under normal conditions.
Throughout the life of a transformer several operating and physical conditions degrade the transformer cellulose and oil insulation system.
• High temperature and high load currents Figure 2 Data acquisition for a station or unit transformer (above name plate rating)
• Moisture inside the transformer
• Oxygen – oxidation of cellulose and oil
• Voltage surges, lightning
• Physical damage due to moving/re-locating As cellulose and oil ages, their insulating properties degrade, and produce gases that can form voids in the insulating oil. These voids, in turn, are sites for partial discharge.
Cable|wise detects partial discharge in these sites. Aging can progress to a level that will eventually fail the transformer, most commonly in the coil structure.
In addition to insulation aging, the laminated core of a transformer can degrade.
Discharge may take place between laminations at locations where the lamination insulation (usually a metal oxide layer) has broken down. Core discharge can take place over a long time period, but usually does not lead to catastrophic failure of the transformer. Transformers typically fail in the core structure. Cable|wise can discriminate between core and transformer insulation discharge.
Transformers, motors, and switchgear operate with some level of internal discharge, even when new. A single test can indicate the presence or absence of discharges as well as their relative intensity. The discharge intensities and patterns measured in the initial test are unique to that motor only. More than one test within a certain period of time is necessary for an accurate diagnosis, with the period of time between tests depending on the intensity of the discharge. Trend analysis of such a series of tests is a method that provides an accurate assessment of the condition of electrical equipment. This also applies to transformers.
Case Studies Pad mounted switchgear Surface tracking in a pad mounted switchgear was located by the RF sensor located 200 ft away in an manhole (Figure 3). The switchgear was not part of the original condition assessment contract. However, after notifying the client about what was detected, the switchgear was investigated further. The switchgear was opened and a parabolic corona detector was used to confirm that the RF signals appeared to be emitted by surface contact between cable terminations and barriers inside the switchgear. The discharge also registered as heat in a thermal image of the terminations.
Figure 3 Surface discharge detected in a pad mounted switchgear 15-kV Distribution Feeders RF measurements were taken at the base of six terminations of a double cable distribution feeder circuit at the substation end of the cables. The terminations had been in service for 20 years. Analysis of the signals indicated that all six of terminations had an advanced degree of internal tracking (Figure 4). It was recommended that the client replace all six terminations as soon as possible. The client replaced the terminations and the existing cable. The client shipped the terminations, each with a 25 ft length of cable attached, to DETCH.
Laboratory RF measurements confirmed the results of the field measurements. The terminations were then dissected (Figure 5). Heavy tracking was observed in the bore of each elastomer stress cone in all six terminations. Discharge activity eroded some the elastomer at the embedded wire stress cone area. Some of the metal components inside the terminations were heavily corroded, an indication of moisture entry over a long period of time. Silicone grease was found at the termination of the cable semiconducting insulation shield in all six terminations. The grease in some of the terminations had begun to wax, an indication that discharge activity was in progress at the end of the shield. In one termination a double impression in the elastomer stress cone was evidence that the cable had moved ¼” inside the termination.
It was concluded that although these terminations did not fail in service, they certainly were near end of life.
In Figure 6 two motors in a refinery UHV VHF HF LF facility had high levels of RF discharge as measured in 2002. Maintenance was Figure 6 Trend analysis in motors performed on the motors based on the 2002 measurements. The same motors had significantly less discharge when retested in 2003 as shown Figure 6 On -line Discontinuity Locator (ODL) ODL is used to detect direct buried splices and neutral corrosion in cables. The technology used for the tests is DTECH (proprietary) On-line Discontinuity Locator (ODL). A small pulse signal is injected into a cable by a sensor. The signal reflects back to the sensor at changes in the cable impedance. The reflected signals are picked up with the same sensor. The pulse reflection shape indicates impedance changes along a cable. Neutral corrosion, cable faults, both open and short circuit (short circuit, open circuit), and insulation damage, can be detected and located by analyzing the reflected signals.
Figure 7 Neutral corrosion Figure 7 shows an example of severely corroded copper shield tapes located by ODL in a nuclear power plant.
25 kV Rated, Heat Shrink Joints Cable|wise testing was performed on a 24 kV cable system in two utilities. The less than 20 year old, XLPE insulated, direct buried systems exhibited a significant number of sites with elevated levels of degradation. The source of degradation was identified to be the cable splices. The customer elected to remove 12 joints with a 10-meter cable section attached and sent them to the laboratory for visual inspection and tests. The splices exhibited degradation levels 2 through 5. The splices were identified as A through L.
The laboratory evaluation included RF detection, ac breakdown, thermographic and visual examination. The results are given in table 1. The laboratory evaluation confirmed the field data. In addition a correlation was observed between degradation levels and ac breakdown.
15 kV rated Butyl Rubber Cable Partial Discharge measurements were performed in the field on 15 kV rated butyl rubber insulated, armored/PVC jacketed cable installed at a production facility. Visual observation of the cable revealed various degrees of jacket cracking from light to severe cracking (In some places there was no outer jacket at all). Subsequently, the 250 meter section of cable, which showed discharges of high level distributed uniformly throughout the length of the cable, was removed from service. This section was then cut into ten meter long samples and brought to DTECH for further examination. The samples were subjected to various electrical and chemical tests in the laboratory. The samples exhibited a breakdown strength in the range of 8.0 to 16.00 volt/mm. Chemical testing also indicated a uniform thermal degradation both radially and longitudinally. This indicates that the cable had degraded along its entire length, independent of the condition of the jacket.
15 kV rated Submarine Cables Condition assessment was performed on two circuits of submarine cables (rated at 24 kV and operated at 13.2 kV). Each circuit consisted of three single conductor, 4/0, EPR insulated, armor cables. The circuits were in service 12 years. A small portion of the circuits was installed on land, while the majority was in 150-meter deep water. A system overload condition lead to thermal degradation of the cables. Both circuits suffered several failures within one week. As a result the circuits were tested by the DTECH system. Data from testing revealed that the land portion of the cables at both shores had degraded completely. The data also showed that the water portion of the cables was still usable due to the cooling effect of the water. The land portions of the cables were replaced, and the circuit returned to service. This action saved the remainder of the tourist season for the island. The cables were completely replaced the following year prior to the tourist season.
Conclusion Cable|wise online condition assessment has a proven track record of being a cost effective, timely, and versatile diagnostic tool that assesses the condition of each component of an energized electrical system, including all sections of cable, cable accessories, transformers, motors, and switchgear. It can increase electrical system reliability, and avoid unplanned outages and power plant electrical problems.
Acknowledgement The authors thank Dr. Nezar Ahmed, Principal Technical Consultant, for providing much of the information presented in this paper, and to David Bogden, Engineering Manager, for compiling data for this paper.
References  S. A. Boggs, “The Case for Frequency Domain PD Testing in the Context of Distribution Cable”, IEEE Electrical Insulation Magazine, Vol. 19, No. 4, pp. 13N. Srinivas, T. Nishioka, K. Sanford and B. Bernstein, “Non-Destructive Condition Assessment of Energized Cable Systems”, IEEE T&D Conference, Indianapolis (2005) to be published.
 N. Ahmed, N. Srinivas, “Insitu partial Discharge Detection in Power Plant Cable”, Proceedings for the Twenty-Sixth Water Reactor Safety Information Meeting, Vol. 3, U.S. Nuclear Regulatory Commission Office of Nuclear Regulatory research, pp. 289-301  N. Srinivas and N. Ahmed, “On-Line Versus Off-Line Partial Discharge Testing in Power Cables”, IEEE T&D Conference (2001).