Maintenance for HV Cables & accessories, practical guidelines

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

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

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 Benefit Diagnostics Outage costs Personnel Follow up failures Early replacement Claims Engineering 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 August 2007.

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

Maintenance for HV Cables & accessories, practical guidelines