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Wim BooneFrank de Wild
Maintenance for HV Cables & accessories, practical guidelines
1. Introduction
2. Definitions
3. Maintenance strategies
4. Survey of presently used maintenance programs
5. Failure modes / Related detection meth. / Maint. actions
6. Available diagnostic tools
7. Maintenance case studies
8. Remaining Life Estimation
9. Recommendations
10. Future developments
11. Conclusions
12. Evaluation
Program
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?
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
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
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
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
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
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
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
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
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
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)
Survey of maintenance programs (2)
Extruded cables: Serving test Visual inspection of terminations Thermal monitoring Inspection of alarm equipment (if installed in tunnel)
Survey of maintenance programs (3)
FF cables: Serving test Inspection of terminations Inspection of fluid equipment (pressure, tank) Inspection of fluid alarm equipment DGA
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
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
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
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
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
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
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
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
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
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
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
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
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 Serving test
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
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
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
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
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
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
Fluid pressure monitoring in Belgium
Pressure must be between 0.2-5.25 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
Fluid pressure monitoring (2)
The urgency of leak location is determined by: Circuit criticality Leakage rate Proximity to water Environmental legislation Public perception
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
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
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
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%
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
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 BenefitDiagnostics Outage costsPersonnel Follow up failuresEarly replacement ClaimsEngineering Penalties
Bad reputation
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
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
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?
Remaining Life Management (2)
Criteria for end of life: High risk of failure High cost of operation High cost of maintenance
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 August2007.
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
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
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
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
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
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
Additonal recommendations for extruded cables
Continuous measurements of pressure in terminations and/or low pressure alarms
Periodic inspection of forced cooling system
Future developments
Improved hardware and software
Characterisation of extruded insulation
Improved non-destructive tets
Linking field tests to laboratory tests
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
Characterization of extruded insulation
Acurate determination of the state of thermal ageing
Site tests needed instead of laboratory tests
Improved non-destructive tests
Rapid location of cable faults with greater accuracy
Serving test for cable in ducts
Condition assesment tests
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
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)
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
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
Conclusions (2)
• Maintenance trends are moving in the direction of predictive maintenance
• More cooperation needed between utilities to share knowledge and experience
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 21-05. Published in February 1998, Technical Report 2001
End sheet
Thank you for your attention