1. Maintenance for HV Cables & accessories, practical guidelines
Wim Boone
Frank de Wild

2. Program Introduction Definitions Maintenance strategies
Survey of presently used maintenance programs
Failure modes / Related detection meth. / Maint. actions
Available diagnostic tools
Maintenance case studies
Remaining Life Estimation
Recommendations
Future developments
Conclusions
Evaluation

3. Introduction
CIGRE WG B1-04 completed a report in 2005, dealing with the following aspects of maintenance on HV cables, resulting in replying the following main questions:
What maintenance practices are being applied at present?
What are the causes of failure for cables and accessories?
How can potential failures be detected?
What guidelines can be given to improve?

4. Introduction (2)
The CIGRE report TB 279 contains the following main chapters:
A survey of presently used utility maintenance programs
A list of common failure modes for cable systems, related diagnostic detection methods and maintenance actions
A list of available diagnostic tools
Recommendations for effective and efficient maintenance

5. Introduction (3)
Attention has been focussed on predictive maintenance because:
Predictive maintenance (to avoid failures) may contribute to higher reliability and reduced maintenance costs
The application of predictive maintenance is not yet common utility practice
More tools are now becoming available for a successful application of predictive maintenance

6. Introduction (4)
The economics of a predictive maintenance program has not been considered, because:
Wide variations in operating frameworks between different countries and/or companies
More important to focus on assisting cable users to identify diagnostic tools and methodologies

7. Definitions (IEC 60050)
Maintenance: the combination of all technical and administrative actions, including supervision actions, intended to retain an item in, or restore it to, a state in which it can perform a required function
Predictive Maintenance: an equipment maintenance strategy based on measuring the condition of equipment in order to assess whether it will fail during some future period, and then taking appropriate action to avoid the consequences of that failure

8. Definitions (2)
Preventive maintenance: the maintenance carried out at predetermined intervals or according to prescribed criteria and intended to reduce the probability of failure or the degradation of the functioning item

9. Definitions (3)
Corrective Maintenance: to repair or to replace broken components
Time Based Maintenance: to perform maintenance on a predetermined schedule= preventive maintenance
Condition Based Maintenance: to perform maintenance, based on the results of condition assessment of the component= predictive maintenance

10. Why maintenance? To avoid failures To avoid environmental damage
To avaoid more expensive maintenance later
To extend the life of the equipment
To avoid unsafe situations
To repair failed components
To avoid legal and financial penalties

11. Maintenance strategies (1)
According to a CIGRE questionnaire comparing maintenance on switchgear, transformers, lines and cables, cables have the lowest expenditure on predictive maintenance and the highest expenditure on corrective maintenance
Possible explanation:
Cables are usually invisible
Cables do not have moving parts
Cables have low risk of explosion

12. Maintenance strategy (2)
The available budget is at present obviously not assigned according to the contribution to the overall system reliability
Cables should have a higher maintenance priority as cables and accessories are subject to failure and outage.
A CBM approach appears to offer opportunities to detect potential failures and to reduce the probability of failures in service

13. Survey of presently used maintenance programs
To avoid 3 rd party damage:
Inspections of cable routes
Administrative procedures to provide cable route information to other parties (using a central ofice to coordinate)

14. Survey of maintenance programs (2)
Extruded cables:
Serving test
Visual inspection of terminations
Thermal monitoring
Inspection of alarm equipment (if installed in tunnel)

15. Survey of maintenance programs (3)
FF cables:
Serving test
Inspection of terminations
Inspection of fluid equipment (pressure, tank)
Inspection of fluid alarm equipment
DGA

16. Survey of maintenance programs (4)
Test on specially bonded systems
Inspection of link box
Inspection of surge arrestor
Current measurement
To collect statistical data
Innovative actions
Tan delta measurement
PD measurement
Advanced failure analysis

17. Survey of maintenance programs (5)
Conclusions:
Preventive maintenance rather than predictive maintenance
Frequency of maintenance actions decided by the utility involved
CBM not well applied
Failure analysis
Investigation of “coincidental” samples
Collecting information in data base

18. Failure modes / detection methods / maintenance actions
The following cable types are considered:
Low pressure self contained fluid filled
High pressure fluid filled
Gas compression cables
Extruded cables

19. Failuremodes/detection methods /maintenance actions (2)
A standard list has been compiled according to the following headings:
Item: identifies the component in the system involved
Event/cause: describes the event that has occured to cause failure, damage or degradation
Consequence: describes the effect of the event
Probability of occurence: high, medium low

20. Failure modes/detection modes /maintenance actions (3)
Impact: categorised into: Health & Safety, Environment, System
Diagnostic indicator: the measurable property that changes as a result of the event
Maintenance action: to reduce the probability of the occurrence of the event
Effectiveness: well established/under development
On-line / off-line

21. Failure Mode Analysis Tabel
Table headings Example
Event Damaged outer sheath
Consequence of the event Corrosion sheath
Probability of occurrence (H/L) High
Impact (S, H&S, E) System
Diagnostic indicator Loss of insulation
Maintenance action Serving test+repair
Effectiveness (WE/UD) Well established
On-line/off-line Off line

22. Failure modes of SCFF cables
Damaged cable / oversheath / metal sheath by 3rd party
Damaged metal sheath by corrosion or fatigue
Ingress of water in cable / accessory or link boxes
External mechanical stress due to ground changes, thermal expansion / contraction and improper clamping
Assembly errors
Thermal ageing of insulation
Failure of fluid pressure gauges / transducers or gauge contacts
Movement of cable due to poor clamping

23. Failure modes of HPFF cables
Damaged cable by 3rd party
Leaking or damaged steel pipe due to corrosion
Assembly errors
Leakage of fluid from termination
Failure of fluid feeding and pressurisation system due to fluid leaks in pipework / tank, fluid pump failure, faulty gauges

24. Failure modes of Gas Pressure cables
Damaged cable by 3rd party
Leaking or damaged steel pipe due to corrosion
Assembly errors
Leakage of internal fluid from termination
Failure of gas pressure system due to gas leaks, in associated pip work, gas leak from canister and faulty gauges

25. Failure modes of Extruded cables
Damaged cable, oversheath, metal sheath by 3rd party
Damaged metal sheath due to corrosion or fatigue
Ingress of water in cable accessory of link box
External mechanical stress due to ground changes, thermal expansion / contraction (snaking) and improper clamping
Thermal ageing of insulation
Assembly errors
Movement of cable due to thermal cycling or poor clamping

26. Available diagnostic tools
A list of available diagnostic tools has been compiled under the following headings:
Tool: identifies the available diagnostic tool
Description of method: summarizes basic principles
Event/cause detected: describes the event/cause that has occurred to cause the system failure which are detectable with the diagnostic tool
Comment: regarding the tool
On-line / off-line

27. Available Diagnostic Methods
Table headings Example
Tool Serving test
Description of method DC test 5 min
Application Damaged outer sheath
Effectiveness Effective
On-line/off-line Off-line

28. Available tools for SCFF cables
Cable route inspections
Indications of falling oil pressure
Serving test
Temperature measurement
Thermal backfill survey
PD measurement
Chemical / physical analysis of fluid
X-ray of accessories
Inspection of cable system
Regular gauge maintenance,calibration, alarm
SVL test

29. Available tools for HPFF/GP cables
Cable route inspection
Inspection of falling fluid pressure
Electrical test on pipe coating
Inspection of CP system
Temperature measurement
Thermal backfill survey
Chemical / physical analysis of paper and fluid
X-ray of accessories
Inspection of cable system
Inspection of pumping system
Regular gauge maintenance, calibration, pressure alarm

30. Available tools for Extruded cables
Cable route inspection
Serving test
Tan delta measurement
Temperature measurement
Thermal backfill survey
PD measurement
Chemical / physical analysis of fluid in termination
X-ray of accessories
Inspection of cable system
SVL test
Bonding system test

31. Case studies Administr.method to avoid 3rd party damage in the NL
The serving test on 150 kV cables in the NL
Fluid pressure monitoring in Belgium
Early leak detection in HPFF cable systems in Canada
Locating leaks in SCFF cables in Canada, using tracer technology
Experiences with CBM on MV power cable systems using PD diagnostics in the NL and Italy
Condition assessment with PD measurements
Probalistic optimisation for external cable damage in France

32. Adminisitr. method to avoid 3rd p.d.
Planned work in the soil at a certain site has to be reported to central office KLIC
KLIC registers request and send messages to all underground services, who have underground systems in the vicinity of the site
The owner is asked to provide maps and to give permission to start work
The owner makes a staff member available to attend and to guide work
For each particular activity in the soil specific instructions are given

33. Serving test procedure
Purpose of the oversheath:
To protect the cable against mechanical forces during installation
To protect the metallic sheath against corrosion
To isolate the metallic sheath from earth potential for cross bonding
To protect the insulation against ingress of water

34. Procedure of the serving test (4kV/mm-10kV,5min)
Voltage between sheath and earth
If there is a defect, a high leakage current will flow (>10mA)
The fault is pinpointed using fault localisation techniques
After repair the test will be repeated

35. Fluid pressure monitoring in Belgium
Pressure must be between bar (8 bar for transient pressure)
Pressure loss can indicate defect in metal sheath, causing water ingress+ environmental impact
Preferably loss of fluid pressure should be detected prior to alarm activation (fluid pressure within safe limits)
Visual inspection of termination and fluid tanks
Pressure gauge readings are taken at each injection point and are compared with previous measurements

36. Fluid pressure monitoring (2)
The urgency of leak location is determined by:
Circuit criticality
Leakage rate
Proximity to water
Environmental legislation
Public perception

37. Fluid pressure monitoring (3)
Cost comparison:
Preventive maintenance (without fluid loss)
Hours of operator to check
Corrective maintenance (with fluid loss)
Hours of operator
Localisation
Repair of sheath
Decontamination of the soil

38. Fluid pressure monitoring (4)
Correct maintenance (loss of fluid + electric failure)
Localisation
Repair
Decontamination of soil
All costs related to an electric failure:
Repair of cable
Ingress of water
Damage to other equipment
Power interruption

39. Advanced leak detection methods
Early leak detection using artificial neural networks:
In case of small leaks monitoring of fluid level is interfered by fluid movement because of load fluctuations
Input parameters are: load current, cable fluid pressure, cable fluid temperature, soil temperature
Output from the ELD system, includes the instantaneous leak rate in l/h

40. Advanced leak location methods
Tracer technology:
After detection of leak injection of a harmless tracer gas (perfuorinated chemical) in the fluid reservoir
The leaked tracer evaporates from the fluid, moves up through the soil and can be detected in the air above the cable route
Accuracy 0.001%

41. Probabilistic maintenance optimalisation for 3rd party damage
Application of economic modeling to a maintenance policy
The method expresses the links between the failure and the maintenance tasks
The knowledge of an expert will be combined with statistical data, resulting in the optimal maintenance solution:
32% preventive maintenance (route inspection, serving test
68% corrective maintenance cost
Frequency cable route inspec. 1m; serving test 3-6 y

42. CBM on MV power cables in the NL
Maintenance strategy is developed to detect potential failures by diagnostic testing (lfpdd)
Cost/benefit analysis confirms the economical feasibility:
Cost Benefit
Diagnostics Outage costs
Personnel Follow up failures
Early replacement Claims
Engineering Penalties
Bad reputation

43. CBM on MV power cables in the NL(2)
Selection ctiteria for cable circuits to be diagnosed are based on the following issues:
Expected outage time
Type of region
Current loading
Load pattern
Number of joints
Type of soil
Failure data
Circuits are excluded from testing, if there are no joints and if they meet n-1 criterion

44. CBM on MV power cables in the NL(3)
Each selection criteria is given a value between 1-4
First selection: circuits with total value > 26
Second selection: values 23-26
Circuits with values < 21 are not selected
Conclusions:
Cost / benefit ratio <1
Reliabilty is increased

45. Remaining Life Management
Maintenance can be performed to extend life
Maintenance will not be performed on a component of which the RL is expected to be relatively short
Can the result of diagnostic testing, part of CBM, in terms of “risk of failure” be converted into a RLM?

46. Remaining Life Management (2)
Criteria for end of life:
High risk of failure
High cost of operation
High cost of maintenance

47. An approach for RLM To collect data concerning ageing
To collect data concerning failure statistics
To develop a methodology for RLM
To prepare practical guide lines for RLM
CIGRE WG B1-09 prepares a report on RLM by August
2007.

48. General recommendations
Cary out cable maintenance with a clear strategy
Use a statistical approach in formulating maintenance policy
Keep maintenance strategy under review
Develop and maintain a data base of cable system failures
Fully investigate failures
Pay attention to control 3rd party damage
Implement preventive actions appropriate to the cable type

49. Recommendations for SCFF cables
Basic Maintenance:
Ensure that cable route information is available and procedures are in place to exchange information
Continuous measurement of fluid pressure and pressure alarm
Periodic serving test
Periodic inspection of outdoor terminations
Regular gauge maintenance and calibration

50. Recommendations for SCFF cables (2)
Regular testing of gauge/transducer alarm functionality
Periodic visual inspection of link boxes
Data base including failure events and causes
Additional recommendations:
Continuous measurement of gas pressure in terminations and /or low pressure alarm
Periodic inspection of cooling system

51. Recommendations HPFF/GP cables
Ensure that cable route information is available and procedures are in place to exchange information
Periodic inspections along the cable route
Continuous measurement of pipe fluid pressure and pressure alarms
Periodic inspection of outdoor terminations
Periodic inspection program covering the entire fluid feeding system
Data base including failure events and causes

52. Addition. Recommend. HPFF/GP (2)
Periodic pipe coating survey
Monitoring of pipe to soil potential (where CP is fitted)
Continuous measurement of gas pressure in terminations and/or low pressure alarms

53. Recommendations for extruded cables
Ensure that cable rout information is available and procedures are in place to exchange information
Periodic inspections along the cable route to check 3rd party activities
Periodic serving test
Periodic inspection of out door terminations
Periodic visual inspection of link boxes
Data base including failure events and causes

54. Additonal recommendations for extruded cables
Continuous measurements of pressure in terminations and/or low pressure alarms
Periodic inspection of forced cooling system

55. Future developments Improved hardware and software
Characterisation of extruded insulation
Improved non-destructive tets
Linking field tests to laboratory tests

56. Improved hardware and software
On site PDD with enhanced sensitivity
Noise cancelation
LF dissipation factor measurements
Early oil-leak detection
Mnagement of large data collectionsLarge

57. Characterization of extruded insulation
Acurate determination of the state of thermal ageing
Site tests needed instead of laboratory tests

58. Improved non-destructive tests
Rapid location of cable faults with greater accuracy
Serving test for cable in ducts
Condition assesment tests

59. Linking field tests with laboratory tets
Laboratory tests on “aged” cables do not exhibit the richness of conditions of real life testing
Accelerated ageing test procedures depart from real life ageing
Diagnostic test results of real life samples may help to improve laboratory testing

60. Summary of report
A survey of current utility maintenance plans, presenting preventive maintenace practices, varies widely between utilities
A list of common failure modes for cable systems and related diagnostic dtection methods; failure modes are systematically tabulated and related to diagnostic tools (see following slide)

61. Summary of report (2)
A list of available diagnostic methods; available and commonly used diagnostic tools are discussed in relation to their specific applications
Recommendations for effective and efficient maintenance

62. Conclusions
Cable owners should decide on a clear maintenance strategy, which balances CM, TBM and CBM
Investments costs have to be estimated depending on the type of maintenance

63. Conclusions (2)
Maintenance trends are moving in the direction of predictive maintenance
More cooperation needed between utilities to share knowledge and experience

64. Main References
Maintenance for HV cables and accessories, prepared by CIGRE WG B1-04. Published in August 2005 as CIGRE Technical Report 279
Diagnostic methods for HV paper cables and accessories, CIGREWG Published in February 1998, Technical Report 2001

65. End sheet
Thank you for your attention