Maintenance for HV Cables and Accessories. December 2014 Tutorial, WG B1.40

Transcription

1 Maintenance for HV Cables and Accessories December 2014 Tutorial, WG B1.40 1

2 Table of Contents Introduction Definitions Maintenance strategies Survey of presently used maintenance programs Failure modes/related detection meth./maint. actions Available diagnostic tools Maintenance case studies Recommendations Future developments Conclusions

3 Introduction CIGRE WG B1-04 completed a report in 2005, dealing with the following aspects of maintenance on HV cables: - 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: Maintenance strategies A survey of presently used maintenance programs A list of common failure modes for cable systems, related detection methods and maintenance actions A list of diagnostic tools Recommendations for effective and efficient maintenance Maintenance case studies Future developments

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, 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 Practical definitions (1) - 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 Practical definitions (2) Maintenance before a failure: - Time based (Preventive) maintenance - Condition based maintenance (Predictive maintenance) Maintenance after a failure - Corrective maintenance

11 Why Maintenance? To avoid failures To avoid environmental damage To avoid more expensive maintenance later To extend cable life To avoid unsafe situations To repair failed components To avoid penalties

12 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

13 Maintenance strategy (2) The available budget is at present obviously not assigned to improve the overall system reliability Cables should have a higher maintenance priority as cable and accessory 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

14 Questionnaire Utility members of WG B1-04 were approached informally: - What is present maintenance policy? - What maintenance actions are taken?

15 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)

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

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

18 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

19 Survey of maintenance programs (5) Conclusions: - Preventive maintenance rather than only corrective maintenance - Frequency of maintenance actions decided by the utility involved - Predictive maintenance (CBM) under development - Failure analysis - Investigation of coincidental samples - Collecting information in data base

20 Failuremodes/detection methods/maintenance actions Identification of the main cause of failure Which diagnostic method is available to detect? What are the following maintenance actions?

21 Different type of cables - Low pressure self contained fluid filled - High pressure fluid filled - Gas compression cables - Extruded cables

22 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, desccribes the effect of the event - Probability of occurence,high, medium low

23 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

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

25 Failure modes of SCFF cables Damaged cable/oversheath/metal sheath by 3 rd 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

26 Failure modes of HPFF cables Damaged cable by 3 rd 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

27 Failure modes of Gas pressure cables Damged cable by 3 rd 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

28 Failure modes of Extruded cables Damaged cable, oversheath, metal sheath by 3 rd 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

29 Diagnostic tools What major diagnostic tools are available? Why and how are they applied? What is the status of acceptance?

30 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

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

32 Available diagnostic tools Cable route inspections Cable system inspection Serving test Temperature measurement Tan delta test Thermal backfill survey PD measurement Chemical/physical analysis of fluid X-ray of accessories Sheath voltage limiter test Bonding system test

33 Additional diagnostic tools for FF cable Indication of falling pressure Regular gauge maintenance and calibration Regular testing of gauge/transducer alarm functionality Electrical test on pipe coating Inspection of CP system

34 Maintenance case studies Practical examples of present maintenance actions The cases demonstrate benefits of mostly preventive maintenance actions

35 Case studies Administr.method to avoid 3 rd 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

36 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 sends 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

37 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 for cross bonding - To protect the insulation against ingress of water

38 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

39 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

40 Fluid pressure monitoring (2) Urgency leak location: - Circuit criticality - Leakage rate - Proximity to water - Environmental legislation - Public perception

41 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

42 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

43 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

44 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%

45 Probabilistic maintenance optimalisation for 3 rd party damage Application of econmic 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

46 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 Diagnostics Personnel Early replacement Engineering Benefit Outage costs Follow up failures Claims Penalties Bad reputation

47 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

48 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 Circuits with values < 21 are not selected Conclusions: Cost/benefit ratio <1 Reliabilty is increased

49 Remaining Life Management Maintenance and RL are related: - Life extension requires maintenance action - No maintenance if RL=0 RLM will not be dealt with in this tutorial, but in following tutorial, by Frank de Wild: Remaining Life Management and Replacement Planning

50 Recommendations for performing maintenance Recommendations to be separated in: The general recommendations, summarizing the key findings of the report (the 7 commandments ) The recommendations are specified per cable type

51 General Recommendations Carry out maintenance with a clear strategy A statistical approach should be used in formulating maintenance policy Keep maintenance strategy under review Develop and maintain a data base of cable system failures

52 General Recommendations (2) Fully investigate failures or material that comes available, due to cable replacement Pay particular attention to minimising 3rd party damage Implement preventive actions, as part of basic maintenance, appropriate to the cable type

53 Basic Maintenance Ensure that cable route information is available and procedures are in place to exchange information Periodic inspections along the route Periodic serving test Periodic inspection of outdoor terminations Periodic inspection of link boxes Data base including failure events

54 Additional maintenance FF cables Continuous measurement of fluid pressure and pressure alarm Periodic inspection program covering the entire fluid feeding system Periodic pipe caoting survey/ monitoring pipe to soil potential

55 Future developments Improved hardware and software (PDD on line) Characterisation of extruded insulation (Ageing mechanisms) Improved non-destructive tets (Diagnostics) Linking field tests to laboratory tests (Accelerated ageing)

56 Conclusions Cable owners should decide on a clear maintenance strategy which balances predictive, preventive and corrective maintenance Maintenance strategy should be related to a clear budget: - Investments: Predictive& Preventive - Costs: Corrective

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

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

59 Final page Thank you for your attention.