FIELD AGING OF ETHYLENE-PROPYLENE-RUBBER (EPR) CABLES UNDER NORMAL AND ACCELERATED VOLTAGE STRESS AT MEMPHIS LIGHT, GAS AND WATER DIVISION

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1 FIELD AGING OF ETHYLENE-PROPYLENE-RUBBER (EPR) CABLES UNDER NORMAL AND ACCELERATED VOLTAGE STRESS AT MEMPHIS LIGHT, GAS AND WATER DIVISION Philip E. Cox, PE Underground Electric Systems Engineer, Member IEEE Memphis Light, Gas And Water Division Memphis, Tennessee - USA Abstract This document describes the project criteria and alternating-current breakdown test results from medium-term (9-14 years) field aged EPR URD type cables. These cables have been field aged under normal and accelerated voltage stress. This is intended to be an interim report, with additional testing as the field aged cable reaches 21 years of service. Keywords Distribution cable, field aging, EPR. I. Introduction In 1980, Memphis Light, Gas and Water Division (MLGW) changed it s underground cable specifications to require ethylene-propylene rubber (EPR) type insulation and a LLDPE cable jacket. This was in response to a high failure rate of the existing cable system consisting of unjacketed high-molecular weight polyethylene (HMWPE) insulated URD type cables. 1 In late 1981 or early 1982, MLGW was approached by The Texas Instruments Corporation to participate in field aging studies of different types of concentric neutral constructions of unjacketed URD type cables. The types of concentric neutral wires to be studied consisted of galvanized-coated steel-clad copper, mild-steel coated copper and tin-coated copper #14 awg wires. These cables were to be removed at one, two, three, five, seven and ten years to determine corrosion rates of the neutral wires. The (then) supervisor of Materials Management at MLGW, Ed Rogers, saw this as an excellent opportunity to electrically test field-aged cables of various types of EPR compounds in conjunction with neutral corrosion studies. The intent behind the EPR field aging portion of the study was to provide a correlation between laboratory aging and in-service field aging. This was to be accomplished by well documented cable manufacturing and installation; with a statistically significant number of tests for each type of cable and test interval. Laboratory aging of the same manufacturing runs of cable used in field aging were to be used to provide comparative data. Because there has always been a greater amount of laboratory aging data, this project was to be biased toward collection of field aged data. The specifications for the cables to be tested were written in such a manner to have three types of neutral construction and two types of insulation construction. 2 Six different companies bid on the project, which included providing the cables specified and provide alternating-current break-down (ACBD) testing at the intervals noted in the preceding paragraph. The types are noted as follows: Type I - #2 awg Cu Conductor, 260 mils EPR insulation, 10#14 Galvanized Coated Steel Clad Cu Concentric Neutral. Type II - #2 awg Cu Conductor, 260 mils EPR insulation, 10#14 Mild Steel Coated Cu Concentric Neutral. Type III - #2 awg Cu Conductor, 175 mils EPR insulation, 10#14 Sn-Coated Cu Concentric Neutral. Three cable companies provided bids which were within the budget constraints of the project. Table I, Bid Award Cable Types, indicates the types and specifications of the cables awarded to the three manufacturers. Approximately five (5), three-thousand (3000) foot reels of each type of cable, shown in Table I, were supplied for installation. Each cable company utilizes a different EPR compound. Although the insulation/cable classifications listed in ICEA S , Standard For Concentric Neutral Cables Rated 5 through 46 kv, did not exist in 1982, the insulations/cable constructions used in this project, would fall into the current classifications shown in Table I. Preliminary Report Page 1 of 10

2 Table I Bid Award Cable Types Manufacturer Neutral Type Neutral 175 Mil Wall 260 Mil Wall ICEA Classification Mfr A I Zn-Stl-Cu X I Mfr A III Sn-Cu X I Mfr B II Stl-Cu X IV Mfr B III Sn-Cu X IV Mfr C II Stl-Cu X III Mfr C III Sn-Cu X III II. Cable Installation Over a period of approximately one and one-half (1-1/2) years, in 1983 and 1984, these cables were installed in nine (9) subdivisions/apartment complexes in the Memphis area. These installations were retro-fit installations to replace direct buried unjacketed highmolecular weight polyethylene (HMWPE) #2 awg Cu cables. 1 The cables supply 23 kv system voltage (13.2 kv-to-ground) to single-phase 50 to 100 kva (50 kva typical) grounded-front pad-mounted transformers in a URD loop, normally-open-point design configuration. The retro-fit installation provided a conduit(ed) system with two parallel conduit runs between pad mounted transformers. One conduit contains a MLGW standard #2 EPR cable, and the other contains the cable under test. The test cables were not installed from the source to the first transformer. The MLGW standard cable was used to energize the subdivision/apartment complex initially; and the test cable was terminated with a capped separable-insulated-connector elbow. After all test cable installations were complete, each span of test cable was placed into service, with the parallel standard cable elbow capped for future use. All of the test cables were energized within five calendar days. The test cables were marked with tags indicating the manufacturer and type of cable, the MLGW standard cable was marked with operating instructions in the event of a test cable failure and each transformer was marked with a sign inside the hood of the transformer to call engineering in the event of a test cable failure. Table II, Cable Types - Quantity Installed & Stored, indicates the quantity of cable installed by type and manufacturer. 3 A quantity of each type of cable was held in reserve for future laboratory testing. This cable was stored indoors in a heated, but not cooled, water pumping station building. Because of the quantity of water moving through the building, in addition to air flow from basement areas, the summer temperature usually did not exceed 90 F (32 C). Note: With-in several months of installation, The Texas Instruments Corporation discontinued sponsoring the project. All remaining funding, paid by TI for administration of the project, was refunded by MLGW. Table II Cable Types - Quantity Installed & Stored Manufacturer Type Quantity Installed Quantity Stored Mfr A I 9,113 Ft 925 Ft Mfr A III 9,287 Ft 2,630 Ft Mfr B II 9,403 Ft 3,117 Ft Mfr B III 9,903 Ft 3,027 Ft Mfr C II 5,983 Ft 3,090 Ft Mfr C III 8,110 Ft 3,130 Ft Total= 51,799 Ft 15,919 Ft Preliminary Report Page 2 of 10

3 III. Aging Conditions Because all of the test cables are subjected to the same service voltage, and the cables are designed with 175 and 260 mil wall thicknesses, the electrical stresses are different. Because a 260 mil wall is considered to be a normal wall thickness for 25 kv Class cables, the 175 mil wall cables can be considered to be under a moderately accelerated aging condition. It should also be noted that a #2 awg conductor size is a nonstandard diameter for 25 kv Class cables. Because the conductors used in the test cable are smaller in diameter than 1/0 awg, which is the minimum standard conductor diameter for 25 kv class cables, the maximum insulation stress is greater in all of the test cables compared to typical installations of 15 kv and 25 kv class cables. Table III, Typical Test Cable Electrical Aging Stress, indicates these comparisons. All of the test cables were installed in 2 PVC conduit at a depth of ~42, with an estimated ambient temperature of F (7-21 C). The conduits containing the test cable were typically filled with water through-out the year. 4 The typical loop loading was ~400 kva (7 to 12, 50 kva transformers, typical). Table III Test Cable Electrical Aging Stress, V/Mil Conductor Wall Thickness Max Stress Percent Typical Min Stress Percent Typical Average Stress Percent Typical # # #1/ Based upon MLGW Phase-To-Ground Voltage, kv, in V/Mil 1/0 awg, Class B Compressed Conductor with 260 Mils Insulation IV. Testing Of Field Aged Cables The specifications of the project required ACBD testing of 25 samples of each type of cable at the intervals noted in the Introduction, in addition to testing cables shortly after manufacturing. The actual test intervals (thus far) have been zero, two, five and nine years. The number of samples tested for each set of data has raged from 19 to 33 samples, with only two sets of data being less than 25 samples (19 and 23). Generally, enough cable has been removed from service to allow for rejected tests because of termination failures, which are not included in the test data. The ACBD test protocol used in this project requires an active test length of 25 feet. Each test begins at 13 kv phase-to-ground (approximately service voltage), with 5 kv increments every five (5) minutes until insulation breakdown. All of the testing has been performed with AC resonant test sets and utilized de-ionized water type terminations. All field aged ACBD tests have been performed at the company which manufactured the cable. Each sample is dissected in the area of the test failure and documented on the test data sheet designed for this project. 5 In addition to testing cable originally installed in this project, two additional sets of data are shown in this document. These cables are MLGW specification cables which were installed during routine construction. A span of ten (10) year service aged cable was removed because of an impulse failure which occurred in service. 6 A span of cable from the first shipment of EPR cable delivered to MLGW was removed after 14 years of service and tested just for curiosity. Each of these cable falls into Class IV of the current ICEA S specification. Year zero cables were tested after typical offgassing time under factory storage conditions. Samples were taken from the beginning, middle and end of the manufacturing run. All other cables were removed from service, at the intervals noted. The cable was reeled onto a metal reel for transport to the MLGW materials testing laboratory. At the MLGW-MTL the cable was cut into thirty-nine (39) foot lengths, each cable end was sealed, the samples were then placed in individual 1-1/2 PVC conduits; the conduits were then partially filled with tap water and capped. The cables were then shipped direct to the cable manufacturer s HV testing laboratory and remained in the water-filled conduits until within several hours of ACBD testing. The data is presented graphically in two formats. The first two graphs indicate Preliminary Report Page 3 of 10

4 average, minimum, and maximum ACBD values versus time in a linear format. Graphs 3-8 indicate each cable type in a Weibull format, showing all data points. 7 Over 645 ACBD tests were performed to develop this data. This represents over 16,125 feet of active length of test cable and over 25,155 feet of cable removed, shipped and prepared for testing. 8 AC Breakdown Voltage Vs Time 260 Mil Wall Thickness AC Breakdown Values, kv (b Avg) (b Min) (Field Impulse MLGW) (1980 MLGW) "A" - (Red) - Center "B" - (Blue) - Right "C" - (Black) - Left (1984) Time,Years (2005) AC Breakdown Values, kv AC Breakdown Voltage Vs Time 175 Mil Wall Thickness "A" - (Red) - Center "B" - (Blue) - Right "C" - (Black) - Left (1984) (2005) Time,Years Preliminary Report Page 4 of 10

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8 Table IV Average AC Breakdown, V/Mil 260 Mil Wall Cables Year Mfr A Mfr B Mfr C Mil Wall Cables Year Mfr A Mfr B Mfr C IV. Testing Of Laboratory Aged Cables As noted in Section I, this project has been biased toward the collection of field aged data, with cables held in reserve for supplemental laboratory aged data. Because of MLGW s involvement in the cable industry (primarily through the efforts of Ed Rogers, at that time) this project did not go unnoticed. The Electric Research Power Institute (EPRI) modeled Project 2713 after this project at MLGW. The major differences were: (1) the data collection in the EPRI project was biased toward laboratory aged data, supplemented with field aged data and (2) the insulation types under test included both cross-linked polyethylene (XLPE) and EPR. Because of the differences in EPR compounds available for medium-voltage utility application, engineers at MLGW strongly encouraged EPRI project managers to include more than one EPR compound. Once the project was well underway, EPRI indicated that it would have been wise to have included additional EPR compounds in the test, and EPRI would be interested in acquiring the field aged data generated in the MLGW project thus far (Mid-1992). MLGW agreed to provide this data in exchange for laboratory testing of shelfaged MLGW project cable, with the test data to be mutually beneficial to both parties. MLGW supplied the teat data to EPRI, and shipped some of the shelf-aged cable to an independent laboratory under contract with EPRI. This laboratory aged the cable for 9+ months and performed some testing as shown in Table V, Shelf- Aged/Laboratory Aged Test Results. It was intended to collect significantly more data, however EPRI ceased funding of this testing. The independent laboratory removed the cables and scrapped them before notification to MLGW. MLGW contacted this laboratory and they could provide no explanation as to why this had been done, and could not produce the raw data from the testing. It is currently under consideration to perform additional laboratory testing on the remaining cable in storage, at the cable manufacturer s HV laboratories. Preliminary Report Page 8 of 10

9 ACBD after 13 Years of Storage All ACBD in V/MIL Table V Shelf-Aged/Laboratory Aged Test Results Voltage Class Manufacturer ACBD New ACBD Shelf Aged % of New 15 Mfr A Mfr B Mfr C Mfr A Mfr B Mfr C Lab aging, 30 C±3, 19 kv to Ground, 9 months, 80kV impulse 3 times/week Voltage Class Manufacturer ACBD No Imp ACBD w/ Imp % New No Imp % New w/ Imp % Shelf Aged % Shelf aged, Impulse 15 Mfr A Mfr B Mfr C Lab aging, 30 C±3, 30 kv to Ground, 9 months, 80kV impulse 3 times/week Voltage Class Manufacturer ACBD No Imp ACBD w/ Imp % New No Imp % New w/ Imp % Shelf Aged % Shelf aged, Impulse 25 Mfr A Mfr B Mfr C V. Conclusions This is intended to be an interim report and will supply the utility and research industry with statistically significant, well documented, in-service field aged test data. It appears that all of the cable insulations under moderately accelerated aging have stabilized at an average AC breakdown value greater than eight (8) times the normal in-service voltage. 9 And at least two of the cable insulations under normal aging have stabilized at an average AC breakdown value almost seven (7) times the normal in-service voltage. 10 This rather simplistic approach is not intended to be a rigorous, or final, analysis of the data in this project. 1 See Service Life And Field Aging Of Medium Voltage Cables In Memphis Light, Gas Water Division s Underground Electric Distribution System By Ed Rogers, Presented IEEE Chicago, Illinois See Appendix 1 For Ordering Description Used In The Bids. 3 See Appendix 2 For Detailed Listing Of Locations And Quantity. 4 Only Two Conduits Were Found To Be Dry During Cable Removal In Preparation For Testing. Preliminary Report Page 9 of 10

10 5 See Appendix 3 For Example Of Typical Test Data Sheet. 6 The Impulse Which Caused The Cable Failure Also Caused The Failure Two Pad-Mounted Transformers, The Riser Pole Arrester And The Termination/Arrester Bracket. 7 See Appendix 4 For Raw Data In Tabular Form. 8 Cable Quantities Do Not Include Amounts Which Were Removed, But When Divided Into Test Lengths Did Not Meet The Required 39 Foot Test Length; Also, This Does Not Include Cable From Invalid Tests, Typically Termination Failures. 9 Based upon 175 mil wall cables being operated on a 12 kv system voltage, 7.2 kv-to-ground, and an average ACBD value of ~60 kv, >340 V/Mil. 10 Based upon 260 mil wall cables being operated on a 23 kv system voltage, 13.2 kv-to-ground, and an average ACBD value of ~90 kv, >340 V/Mil. Preliminary Report Page 10 of 10

11 Appendix 1 Type I 2/C, 260 Mil Wall, Hot Dip galvanized Coated Steel Clad Concentric Neutral URD Cable 1/C #2 awg 7-Strand, soft-annealed copper wires Class B concentric lay stranding conforming to the latest edition of ANSI/ASTM B-33, Standard Specification For Tinned Soft or Annealed Copper Wire For Electrical Purposed, (one and one-half (1-1/2) percent minimum and three (3) percent maximum compressed strands required)with an average minimum wall of of extruded semiconducting thermosetting, minimum average wall of of ethylene propylene rubber insulation, outside diameter range over the insulations shall be minimum and maximum, minimum average wall of of freestripping semiconducting thermosetting insulation shielding material, outside diameter range shall be minimum to maximum and 1/C consisting of ten (10) strands of #14 awg hot dip galvanized coated steel clad copper wires, applied helically and evenly spaced over the insulation shielding. The copper core shall have a nominal diameter of to which is metallurgically bonded a layer of low carbon steel with a thickness of , the wire shall be hot dip galvanized with a nominal 0.5 ounce of zinc per square foot (neutral wire available through Texas Instruments). Rated cable voltage 25 kv. Type II 2/C, 260 Mil Wall, Rust Inhibited Coated Concentric Neutral URD Cable 1/C #2 awg 7-Strand, soft-annealed copper wires Class B concentric lay stranding conforming to the latest edition of ANSI/ASTM B-33, Standard Specification For Tinned Soft or Annealed Copper Wire For Electrical Purposes, (one and one-half (1-1/2) percent minimum and three (3) percent maximum compressed strands required)with an average minimum wall of of extruded semiconducting thermosetting, minimum average wall of of ethylene propylene rubber insulation, outside diameter range over the insulations shall be minimum and maximum, minimum average wall of of freestripping semiconducting thermosetting insulation shielding material, outside diameter range shall be minimum to maximum and 1/C consisting of ten (10) strands of #14 awg rust inhibited coated steel clad copper wires, applied helically and evenly spaced over the insulation shielding. The copper core shall have a nominal diameter of to which is metallurgically bonded a layer of low carbon steel with a thickness of , the wire shall be hot dip galvanized with a nominal 0.5 ounce of zinc per square foot (neutral wire available through Texas Instruments). Rated cable voltage 25 kv. Type II 2/C, 175 Mil Wall, Coated Copper Concentric Neutral URD Cable 1/C #2 awg 7-Strand, softannealed copper wires Class B concentric lay stranding conforming to the latest edition of ANSI/ASTM B- 33, Standard Specification For Tinned Soft or Annealed Copper Wire For Electrical Purposes, (one and one-half (1-1/2) percent minimum and three (3) percent maximum compressed strands required)with an average minimum wall of of extruded semiconducting thermosetting, minimum average wall of of ethylene propylene rubber insulation, outside diameter range over the insulations shall be minimum and maximum, minimum average wall of of freestripping semiconducting thermosetting insulation shielding material, outside diameter range shall be minimum to maximum and 1/C consisting of ten (10) strands of #14 awg soft annealed copper wires, conforming to ANSI/ASTM B-33, Standard Specification For Tinned Soft or Annealed Copper Wire For Electrical Purposes, applied helically and evenly spaced over the insulation shielding. Rated cable voltage 15 kv.

12 Appendix 2 Date Sketch Location Mfr A - I Mfr A - III Mfr B - II Mfr B - III Mfr C - II Mfr C - III 11.Feb B Summer Trace Apartments Mar A Releigh Woods Aprtments Mar A Willow Wyck Townhomes Oct A Bennington SD Sep Bavarian Village 02.Sep A Northwood Hills SD Aug A Ridgeway Estates SD Oct A Willaimsburg park SD - Sec I Apr A Wildwood Manor - Sec A Totals >>

13 Appendix 3

14 Appendix 4A 260 Mil Wall Cable Raw Test Data A I-0 B II-0 C II-0 A I-2 A I - 2b* B II-2 C II-2 A I-5 B II-5 C II-5 A I-9 B II-9 C II-9 B - 10 B Does not include first Values in A I

15 Appendix 4B 175 Mil Wall Cables Raw Test Data A III-0 B III-0 C III-0 A III-2 B III-2 C III-2 A III-5 B III-5 C III-5 A III-9 B III-9 C III