Predicting Medium-Voltage Underground-Distribution Cable Failures John P. Ainscough P. E., Member IEEE Ian W. Forrest P. E.

Transcription

1 Predicting Medium-Voltage Underground-Distribution Cable Failures Nov. 11, 2009, Scottsdale, AZ

2 Introduction

3 The Birth of Suburbia - Starting in the 1950 s

4 White House Conference on Natural Beauty Discussion Topics May 25-26, 1965 A solution to the problems of automobile junkyards The possibility of underground installation of utility transmission lines Policies of taxation which would not penalize or discourage conservation and the preservation of beauty Areas in which the Federal Government could help communities develop their own programs of natural beauty The possibility of a tree-planting program

5 History of MV-UD Cable Installations

6 Millions of Housing Units The Driver in the Installation of Underground Primary Cable is New Housing Housing Starts History

7 Other Factors Impacting the Amount of Underground Primary Cable Installed by Electrical Utilities Percentage of new housing units served by underground (a number that has steadily grown over the last five decades, and is now over 90% nationally). Commercial expansion, with underground distribution, associated with new housing developments. Increased construction of underground feeder lines. An increasing amount of replacement activity as cable failure became an issue.

8 Correlation Between Housing Starts and Primary Cable Electrical World Construction Surveys (1972 to 1986). The Aluminum Association data on primary URD cable manufactured in the United States from 1971 to AEIC supplied survey data on primary UD cable installed by year, with recorded failures (1964 to 1983). NEMA Product Statistical Bulletins reporting on pad-mounted and sub-surface type transformers. Insulated Wire and Cable, periodic Current Industrial Reports, latest issue June 2007 by the US Census Bureau.

9 MILLIONS OF FEET Use of Regression Analysis Applied to the Data Sources To Estimate Medium-Voltage, Underground Cable installed annually by US electrical utilities US UTILITY MV-UD CABLE ESTIMATED INSTALLATIONS HISTORY Replacement EPR & TRXLP EPR HMWPE XLPE TRXLP YEAR INSTALLED

10 Existing Installed Population of Primary Underground Cable Insulation Type Millions of Feet Thousands of Miles HMWPE XLPE 2, TRXLPE 2, EPR 2, Replacement 1, Total* 8,833 1,672 *Not including Replacement Assuming a $30/ft replacement cost, and further assuming that only the HMWPE and XLPE populations are subject to early failure, the unavoidable pending replacement cost to US electrical utilities is around $80 billion.

11 MV-UD Cable Aging

12 Aging Phenomenon Insulation deterioration due to a combination of electro-chemical and partial discharge processes Because of the geometry of the deterioration from these processes, it was described as treeing (electrical treeing and water treeing) Without a mechanism to resist insulation deterioration, the process continues to the point where the size and density of the deterioration causes an overall reduction in the ability of the insulation to withstand voltage, and causing spontaneous electrical failures to occur Understanding that this proneness to fail is an aging phenomenon is key to developing a failures predicting model

13 Electrical Treeing

14 Actions Taken to Increase the Life of MV-UD Cable Year* Action 1967 HMWPE began to be replaced with XLPE 1978 Increasing installation of cable in conduit 1981 Beginning of wide-spread use of PE jackets 1982 Introduction of tree-retardant XLPE 1983 Shift to increased use of EPR 1984 Triple-extruded cross-linked insulation shields 1985 Increasing application of elbow arresters at 15kV 1986 Focus on XLPE/TRXLP compound cleanliness 1988 Introduction of strand-fill technology 1989 Silicone injection cable-cure technology 1990 Increased use of 133% insulation level (RUS) 1991 Clean-shield technology introduced vpm discharge-free requirement in standard 2004 New pellet inspection technology promoted *Approximate year when the practice became significant

15 Statistical Methodology and Assumptions

16 Single Year of Installation Curve Fitting of Failure Data to an Exponential Curve Single Year Curve Fitting h(x,t) = Xa(t-g) b Where h(x,t) = number of failures in population X in year t. X = population in miles. a = constant characteristic of the population. g = grace period prior to the initial failures. b = exponent characteristic of the population. X x 0 g x x x h(x,t) x x x x x x

17 Multiple-Years of Installation Curve Fitting of Failure Data to a Composite Exponential Curve

18 Making the Connection to the Shape and Scale Parameters of the Weibull Distribution h(x,t) = Xa(t-g) b or h(t) = a(t-g) b for a cable length of one mile. h(t) = t - g is the above relationship in the Weibull hazard function format. Where b and b + 1 b + 1 a

19 The Weibull Probability Density Function f(t) = t - g exp[-( t - g ) ], t > g Where is refered to as the shape parameterof the Weibull distribution and influences the spread of the distribution, and is the scale parameter and denotes the 63.2 percentile of the distribution for any value of the shape parameter.

20 Weibull Hazard Function and Probability Density Function Superimposed Wiebull Hazard and Probability Density Functions Probability Density Function Hazard Function Years

21 Modifying the Weibull Probability Density Function for Varying Unit Length and Allowable Failures f(t) = t - g exp[-( t - g ) ], t > g Where = b + n and = ( b + 1 b + 1 xa/n ), x in miles 1 x = section length n = failures allowed before section replacement (basically adjusting the model for repairable cable)

22 Sources of Data and Verifying the Model

23 San Diego Gas and Electric Duke Power Iowa Lakes Electric Public Service of New Hampshire Jersey Central Power and Light Public Service Company of Colorado Florida Power and Light Houston Lighting and Power Baltimore Gas and Electric RUS 1989 survey Organizations Contributing Data in the Early 1990 s

24 Verifying Goodness of the Data and Model For the same construction, XLPE has longer life than HMWPE 220 mil insulations outperform 175 mil insulations of the same type A cable with an overall jacket (whether PVC or PE) has a longer life than the same construction without a jacket Cable of the same construction installed in regions of higher rainfall and lightning occurrence had shorter life than in a dry, lightning free area (e.g. North Carolina versus southern California) MV-UD cable with a solid center conductor outperforms the same construction cable with an unfilled stranded conductor

25 P e rc enta ge of Fai l ure s pe r Y e a r Model Generated Life Expectancy of Early Vintage Cable Types Estimated MV-UD Cable Life Expectancy Based on replacing a 2,500 ft half-loop following the third failure mil HMWPE, stranded conductor, no jacket mil HMWPE, stranded conductor, no jacket 175 mil XLPE, stranded conductor, no jacket 175 mil XLPE, solid conductor, no jacket 175 mil XLPE, stranded conductor, jacket Y e a rs In s ta lle d

26 Application of Model

27 History of Representative MV-UD 175 mil XLPE Cable Installations, Failures and Replacement Year Failures Actual Miles Replaced and Retired Miles Installed Year Failures Actual Miles Replaced and Retired Miles Installed Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown Unknown

28 Number of Failures Curve Fitting Results Using the Model Where the a, b, and g are selected based on minimizing the sum of the squares of the differences between the actual and model failures Actual Failures Versus Model with 90% Confidence Limits Year Tap 175mil XLPE a= b= 1.28 g=

29 Selecting the Model Parameters Based on Minimizing the Sum of the Squares Selecting a and b for g = 15 years 4e+5 4e+5 3e+5 3e+5 2e+5 2e+5 1e+5 5e Exponent b Constant a

30 Number of Failures % Confidence Limits How the Spread Increases With Poorer Fit of the Data to the Model 4000 Actual Failures Versus Model with 90% Confidence Limits Year Tap 175mil XLPE a= b= 1.1 g=

31 Replacement Rules-of-Thumb Reported by Utility Respondents in 1996 R e p la c e a fte r O n e F a ilu re T w o F a ilu re s T h re e F a ilu re s F o u r F a ilu re s F iv e F a ilu re s B a s e d o n E v a lu a tio n R e s p o n s e s T ra n s m is sio n & D is trib u tio n W o rld, Ju ly

32 Number of Failure s Impact on Annual Failures of Selecting a Retirement Scenario of Replacing an Average 350 ft. Section Following the Third Failure Actual Failures Versus Model with 90% Confidence Limits Year Tap 175mil XLPE a= b= 1.28 g= Identify the length of cable to be removed in feet 350 (If a feeder remember to multiply the trench feet by three.) Allowable Failures per half-loop or feeder: 3

33 Number of Failures Impact on Annual Failures of Selecting a Retirement Scenario of Replacing an Average 2,500 ft. Half-loop Following the Fourth Failure 2500 Actual Failures Versus Model with 90% Confidence Limits Year Tap 175mil XLPE a= b= 1.28 g= Identify the length of cable to be removed in feet 2500 (If a feeder remember to multiply the trench feet by three.) Allowable Failures per half-loop or feeder: 4

34 es Annual Failure Rate Weibull Hazard Function for 2500 ft. Half-loop 175 mil XLPE Tap Cable 2,500 Ft Half-Loop Harzard Function Failures per Year per 100 miles (Point at which utilities became aware there was a problem with HMWPE Year of Service 175 mil XLPE Tap Cable 2,500 Ft. Half-Loop Cumulative Failures Replace 350 ft. Section following 3rd Failure (21 Failures total in the 2,500 ft Half-Loop

35 ICC Reported MV-UD Cable Failures (HMWPE and XLPE) from 1984

36 Cumulative Failures Annual Fa ilure (Point at which utilities became aware there was a problem with HMWPE Comparison of the Two Retirement Scenarios: Forcing Life by Repair Year of Service 175 mil XLPE Tap Cable 2,500 Ft. Half-Loop Cumulative Failures Replace 350 ft. Section following 3rd Failure (21 Failures total in the 2,500 ft Half-Loop Replace Half-Loop following 4th Failure 5 0 Year of Service

37 Proportion of Population Replaced per Year Life Distributions Resulting from Two Retirement Scenarios of the Same Repairable Population Comparison of Two Retirement Scenarios For the Same Repairable Population Replace an average 2,500 ft Half-loop follow ing the fourth failure - average section life 43 years Replace an average 350 ft section follow ing the third failure - average section life 73 years Years Installed

38 Number of Failures Using the Model to Determine the Applied Retirement Strategy Based on Actual Replacements Actual Failures Versus Model with 90% Confidence Limits Year Tap 175mil XLPE a= b= 1.28 g= Identify the length of cable to be removed in feet 2500 (If a feeder remember to multiply the trench feet by three.) Allowable Failures per half-loop or feeder: 3.23 To accept the Optimal Rehabilitation Scenario (ORS) type 'yes': yes To accept the ORS with catch-up type 'yes': yes Amount* 0 (Catch-up works only if ORS is selected) *Negative amt. Means actual is lagging the ORS positive means actual is ahead of the ORS

39 Miles Installed Replacement Scenario: Replacing One Mile After Seven Failures Original and Replacement Populations Actual Planned Year Installed

40 Guidelines for Using the Model Annual results will occur over a range and, unlike a deterministic model, one years worth of results does not justify a change of plan Updating the model with the latest consistently collected annual failure and replacement results is essential When dealing with a small population a non-statistical approach is in order It is not possible to predict monthly improvements to a problem that is going to take years to resolve Solutions require a systematic approach to retiring the failing population If the population under investigation is feeder cable, failures may be bi-modal

41 Number of Failures Data Input Form Name of Company: Example Description of Cable: Tap 175mil XLPE Note to Users: There are three hidden sheets that are part of this statistical model. If it becomes necessary to make changes on these sheets it is strongly recommended that the user seek assistance from the author, Year Only enter data in shaded areas. First Year of Installations 1972 Most recent year of data 2008 Failures Actual Miles Replaced and Retired Miles Installed Predicted Failures Predicted Retirements Only make changes in shaded areas. Range for curve fitting: 1999 to Actual Failures Versus Model with 90% Confidence Limits Year Tap 175mil XLPE a= b= 1.28 g= Identify the length of cable to be removed in feet (If a feeder remember to multiply the trench feet by three.) Allowable Failures per half-loop or feeder: To accept the Optimal Rehabilitation Scenario (ORS) type 'yes': yes To accept the ORS with catch-up type 'yes': yes Amount* (Catch-up works only if ORS is selected) *Negative amt. Means actual is lagging the ORS positive means actual is ahead of the ORS Selecting the best combination of a, b, and g to achieve the minimum sum-of-squares is an iterative process Since the exponent, b, should be greater than 1, and the grace period, g, is in a range that can be surmised from the data, this is a good starting point. A best a can then be selected followed by iterations that vary each of the values slightly until a best fit is achieved

42 Conclusions

43 MV-UD cable failures result from accelerating insulation deterioration due to electrical aging Conclusions Different types of MV-UD cable have different aging characteristics MV-UD cable aging failures follow an exponential proneness-to-failure curve Since MV-UD cable is repairable, the life expectancy of installed cable is a function of both its proneness to fail in the installed environment, and the retirement policy of the utility

44 Conclusions (continued) The expense to repair, versus the expense to replace, makes repair cost effective beyond the point of acceptable customer service Retirement policy at a utility will evolve from section repair and replacement to more wholesale replacement once failures reach a level that have too much negative impact on service The national scope of the MV-UD cable failure and replacement issue is huge, growing and requires a systemic approach by utilities

45 Conclusions (continued) It is assumed that MV-UD cable types installed in the last twenty-five years have significantly better life expectancy Gaining knowledge of expected performance of the younger generations of installed MV-UD cable (those that have not been installed long enough to have failure history), as well as cable with long service life and no aging failures, would be a worthy industry effort

46 Contact Information John Ainscough is with Xcel Energy, Denver, CO Ian W. (Bill) Forrest is with Forrest Associates, Peterborough, NH