TNB Cable Maintenance Manual

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | i ___________________________________________________________________________

TNB Distribution Division

Maintenance Manual :

Underground Cable System

2007

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Acknowledgement

We would like to express our deepest gratitude to TNB Distribution Division especially to Mr. Halim Osman, Chief Engineer, TNB Distribution Engineering Services for giving us the opportunity to develop TNB Distribution Division Maintenance Manual: Underground Cable System. Acknowledgement also goes to Muhammad Azizi Abdul Rahman and Jazimah Abd Majeed for their valuable contribution and assistance in developing this manual.

The project team would also like to express its highest gratitude to the TNBR Management team for their supports starting from the initiation until its completion as well as the various groups/units in TNBR especially to IT for their support in developing this manual.

Our deepest expression also to Dr. Prodipto Sankar Ghosh, RUP Consultant Plus Inc. (M) Sdn. Bhd for his guidance, patience, support and encouragement towards the successful completion of this manual development.

Special thanks to Huzainie Shafi Abd Halim, Radzlan Hisham Mohd Arifin and Zairul Aida Abu Zarim for their valuable contribution and support towards the smooth execution of this manual development.

Lastly we would like to extent our indebtedness to ILSAS whose valuable input has been a great help for the successful development of this manual.

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | iii ___________________________________________________________________________

I. Purpose of the Manual

This manual outlines sets of recommended maintenance practices for underground cable system and to be used as a reference in the execution of related maintenance tasks by in-house of external service providers.

Analysis of test results or interpretation, decision criteria and recommendations are generally based on available industry standards and experiences of subject matter experts (SME) in TNB Distribution. However, owing to unique equipment system design and characteristics, failure modes and performance as experienced by TNB Distribution, expert judgments must be exercised when finally applying these recommendations. In this respect, there is also a need to refer to other related documents namely manufacturers recommendations and other documented evidences related to operational historical performance of specific equipment as additional inputs to the decision-making process.

II. Scope and Validity

This manual covers full scope of maintenance, testing and diagnostics tasks for MV and LV underground cable system at three critical stages of the asset life namely: commissioning, in –service and re-commissioning after failure. Although the motivation in the development of this manual is more for the standardization of advanced diagnostic testing related to condition-based maintenance, the more routine inspections and maintenance tasks are also included for completeness and to ensure further standardization of these maintenance tasks. The contents, or parts thereof, of the manual shall remain valid until such time further revision is made. The custodian of this Manual is Engineering Services, Engineering Department, and TNB Distribution.

III. Relevant Standards and References

Users of this Manual are advised to refer to the following set of standards and references so as to acquire more in-depth understanding of the relevant standards being quoted in this Manual and related subject matter.

1. IEEE 400-1991 – IEEE Guide for Making High Direct Voltage Tests on Owner Cable Systems in the Field

2. IEEE 400.2 – 2004 – IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)

3. IEEE 400 – 2001 – IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems

4. IEEE STD 1425 – 2001 IEEE Guide for the Evaluation of the Remaining Life of Impregnated Paper Insulated Transmission Cable Systems

5. IEEE NO.83 -1963 – Radial Power Factor Tests on Insulating Tapes in Paper Insulated Power Cable

6. Condition Assessment of Power Cables using Partial Discharge Diagnostic at Damped AC Voltages – Frank Westler, SEBA KMT

7. Electric Cables Handbook, third edition – G.F. Moore BICC Cables

8. Electrical Power Equipment Maintenance and Testing - Paul Gill, Marcel Dekker Inc. 9. Tan Delta Cable Testing: Overview and Frequently Asked Questions – High Voltage Inc. 10. Condition Monitoring using Partial Discharge Method on Cable Mapping – Final Report

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List of Abbreviations

A Ampere AC Alternating Current

AM Ampere Meter

ASTM American Society for Testing and Materials

BS British Standard

CBM Condition Based Maintenance

CM Condition Monitoring

CMMS Computerised Maintenance Management System

CRO Cathode Ray Oscilloscope

CTC Critical Technology Challenges CTCs Critical Technological Challenges

DC Direct Current

DGA Dissolve Gas Analysis

DS Dielectric Spectroscopy

emf Electro Magnetic Field

EPDM Ethylene Propylene Diane Monomer ERMS Enterprise Resource Management System F Farad

FMECA Failure Mode Effect And Criticality Analysis GIS Geographical Information System

HV High Voltage

HVDC High Voltage Direct Current Hz Hertz

IEC International Electrotechnical Commission IEEE Institute of Electrical Electronic Engineer

IR Insulation Resistance

km kilometre

kV Kilo Volt

LGB Laporan Gangguan Bekalan

LV Low Voltage

MIL-STD United States America Military Standard MTBF Mean Time Between Failures

MV Medium Voltage

NASA National Aeronautics and Space Administration

nC Nano Coulomb

O&M Operation and Maintenance

O/C Open Circuit

OWTS Oscillating Wave Testing System

pC Pico Coulomb

PD Partial Discharge

PDEV Partial Discharge Extinction Voltage PDIV Partial Discharge Inception Voltage PE Polyethylene PILC Paper Insulated Lead Cable

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | v ___________________________________________________________________________

PM Preventive Maintenance

PRN Probability Risk Number

PT&I Predictive Testing And Inspection PVC Poly Vinyl Chloride

RCM Reliability Centred Maintenance

RPN Risk Priority Number

SCADA Supervisory Control And Data Acquisition SF6 Sulphuric Hexafluoride

TDR Time Domain Reflectrometry UG Underground

VLF Very Low Frequency

VM Volt Meter

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List of Figures and Tables

Figure 2.1 Growing Expectation of Maintenance...24

Figure 2.2 Probability of Failure with Age...25

Figure 2.3 Failure Patterns...26

Figure 2.4 Schematic Representation of Proactive Maintenance ...29

Figure 2.5 P-F Curve...33

Figure 2.6 P-F Interval...33

Figure 2.7 Periodicity of Condition Based Maintenance...34

Figure 3.1 Construction of single core XLPE cable ...39

Figure 3.2 Construction of three core XLPE cable...39

Figure 3.3 Construction of triplex XLPE cable ...40

Figure 3.4 Construction of PILC cable...41

Figure 3.5 Construction of PVC LV cable...42

Figure 3.6 Construction of XLPE LV cable ...42

Figure 3.7 Construction of Premoulded type Joint ...43

Figure 3.8 Types of connectors (a) mechanical (b) crimped ...43

Figure 3.9 Construction of Termination ...44

Figure 3.10 Electrical Stress at End of Cable Semi-Conductive Screen ...45

Figure 3.11 Geometric Stress Control ...45

Figure 3.12 High Dielectric Constant Stress Control ...45

Figure 4.1 Electric circuit of insulation under dc voltage test ...57

Figure 4.2 Insulation Current Characteristics ...59

Figure 4.3 Representation of Cable ...61

Figure 4.4 Comparison between new and old cable ...62

Figure 4.5 Tangent delta in frequency sweep ...63

Figure 4.6 Comparison between new and old cable under dielectric spectroscopy ...63

Figure 4.7 Equivalent circuit diagram of cable insulation with voids ...65

Figure 4.8 Occurrences of internal discharges...66

Figure 4.9 PD Test Setup ...67

Figure 4.10 PD Pulse Generation in Cables...67

Figure 4.11 PD Pulse Characteristic in Cables ...68

Figure 4.12 Contact Resistance Test Setup...69

Figure 5.1 Connection between Analyzer and PILC Cable...83

Figure 5.2 Connection between HV Unit, Analyzer and XLPE Cable...85

Figure 5.3 Schematic of Test Circuit ...87

Figure 5.4 Schematic of Test Circuit ...89

Figure 5.5 Connection between Analyzer and PILC Cable...90

Figure 5.6 Connection between Analyzer, HV Unit and XLPE Cable...91

Figure 5.7 Connection of IR Testing Equipment...93

Figure 5.8 Test set up for Sheath Integrity Test...94

Figure 5.9 Connection of Test Leads to Cable Joint...95

Figure 5.10 Shock wave discharge ...98

Figure 5.11 Fault Location Procedure Flowchart ...99

Figure 5.12 Sheath fault pre-location by the voltage drop method...100

Figure 5.13 Sheath fault location with DC voltage...101

Figure 6.1 Flowchart for Calculating Cable Condition Index ...116

Figure 7.1 Flow Chart of Raw Waveform and Processed Data...130

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | xi ___________________________________________________________________________

Figure 7.3 Processed Data (PD Mapping) of PD VLF ...132

Figure 7.4 Raw Waveform of OWTS PD...133

Figure 7.5 Raw Waveform of OWTS PD...134

Figure 7.6 Processed Data (PD Mapping) of OWTS PD...135

Figure 7.7 Processed Data (Histogram) of OWTS PD ...135

Figure 7.8 Raw Waveform (Good Response) of XLPE DS ...136

Figure 7.9 Raw Waveform (Non Deteriorated Response) of XLPE DS ...137

Figure 7.10 Raw Waveform (Voltage Dependent Response) of XLPE DS ...137

Figure 7.11 Raw Waveform (Leakage Current Response) of XLPE DS ...138

Figure 7.12 Raw Waveform of PILC DS...139

Figure 7.13 Processed Data (Moisture Content) of PILC DS...140

Figure 7.14 Raw Waveform of Thermography...141

Figure 7.15 Processed Data of Thermography ...141

Table 1.1 Criticality Ranking...17

Table 1.2 Sample Maintenance Approach Table...18

Table 3.1 Types of Underground Cables and its Accessories ...36

Table 3.2 Cable components and their function ...38

Table 3.3 Failure Severity Ranking and Definition ...46

Table 3.4 Failure Probability Ranking and Definition...46

Table 3.5 Failure Detectability Ranking and Definition...47

Table 3.6 FMECA for MV Cables...48

Table 3.7 FMECA for MV Joints ...51

Table 3.8 FMECA for MV Terminations ...52

Table 4.1 Cable Maintenance Matrix...56

Table 5.1 MV XLPE Cable for Insulation Integrity Test ...72

Table 5.2 MV PILC Cable for Insulation Integrity Test...73

Table 5.3 LV Cable for Insulation Integrity Test ...73

Table 5.4 MV XLPE Cable for Sheath Integrity Test ...73

Table 5.5 MV XLPE, MV PILC & LV for Integrity of Connections...73

Table 6.1 Contact Resistance...117

Table 6.2 Thermography...117

Table 6.3 Tan delta ...118

Table 6.4 Insulation resistance...119

Table 6.5 Operation and Maintenance Performance Scoring ...120

Table 6.6 Age Scoring ...121

Table 6.7 Tier 1 Cable Condition Index ...121

Table 6.8 Cable Data Quality Indicator Scoring...122

Table 6.9 Final Tier 1 Cable Condition Index Value...122

Table 6.10 Cable Tier 1 Condition-Based Alternatives...123

Table 6.11 Partial Discharge Test Score Adjustment ...124

Table 6.12 Dielectric Spectroscopy Test Score Adjustment ...125

Table 6.13 Total Cable Condition Index Value...126

Table 6.14 Cable Condition Based Alternatives...127

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Chapter 1

Introduction

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1 Introduction

1.1 Background

Electrical distribution equipment is generally designed for a certain economic service life. Equipment life is dependent on operating environment, maintenance program and the quality of the original manufacture and installation. Beyond this service life period they are not expected to render their services up to expectation with desired efficiency. However, certain equipments are found to operate satisfactorily even after the expected economic life span which may be attributed to good site conditions and good maintenance.

However, generally due to poor quality of raw material, workmanship and manufacturing techniques or due to frequent system faults, over loading, environmental effect, unexpected voltage swings and over voltage stresses on the system during the operation, many equipment fail much earlier than their expected economic life span. Moreover, due to the above cited reasons, the failure of vital equipment has become a regular feature and the high rate of failure has become a cause of concern for electrical utilities. The concept of simple replacement of power equipments in the system either before or after their economic service life, considering it as weak or a potential source of trouble, is no more valid in the present scenario of financial constraints.

Today the paradigm has changed and efforts are being directed to explore new approaches/techniques of monitoring, diagnosis, life assessment and condition evaluation, and possibility of extending the life of existing assets (i.e. circuit breaker, cables, oil filled equipment like transformers, load tap changer etc., which constitute a significant portion of assets for distribution system). Minimization of the service life cycle cost is one of the stated tasks of the electrical power system engineers. For electrical utilities this implies for example to fulfill requirements from customers and authorities on reliability in power supply at a minimal total cost.

The main goal is therefore to reach a cost effective solution using available resources which is captured by the concept of Asset Management. Maintenance is one of the areas where higher effectiveness is sought for, and utilities are implementing new strategies for maintenance and management of assets. The pressure to reduce operational and maintenance costs is already being felt and the concept of Preventive Maintenance is undergoing change. In practice, the traditional understanding of maintenance is to "fix it when it breaks". This is a good definition for repair, but not maintenance. This style of maintenance is reactive. In modern and forward thinking utilities, it has been realized that proactive, rather than reactive maintenance management brings the best results. Adopting a proactive approach to maintenance will improve maintenance effectiveness dramatically within the confines of the

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organizational and cultural environment of an existing, predominantly reactive maintenance program.

Most equipment require regular and effective maintenance to operate correctly and meet their design specifications. The consequences of ineffective equipment maintenance can be huge in terms of system reliability indices, revenue loss and organizational image. Therefore, the importance of effective maintenance through condition monitoring of electrical equipment in the system is gaining importance to reduce the occurrence of such incidents. Assessing the condition and thereby reducing failures of equipment is a key to improving reliability and also effectively extending the life of equipment. Hence utilities are continuously in search of best maintenance practices other than traditional methods/techniques to assess the condition of equipment in service so that remedial measures can be taken in advance to avoid disastrous consequences thereby saving lot of valuable resources.

The potential cost savings of Best Maintenance Practices can often be beyond the understanding or comprehension of management. Unfortunately, in some people's minds, the words "Best Practices" evoke some difficulty to understand, ever-changing and unachievable goal towards which they are supposed to focus without hope of ever attaining. "Best Maintenance Practices in Power Utilities" can be benchmarking standards, which are real, specific, achievable and proven standards for maintenance management and by adopting this will make any maintenance department more efficient to reduce operating and maintenance costs, improve reliability, and increase morale. Best Maintenance Practices comprise of standards and methods. Standards are the measurable performance levels of maintenance execution and methods and strategies are procedures that must be practiced in order to meet the standards. Overall, the combination of standards, and methods and strategies are elements of a Planned Maintenance Management system.

This manual will introduce you to "Best Maintenance Practices in Power Utilities", define the standards and show you how to set target and reach the performance levels of Best Maintenance Practices. It will also provide you with detailed study on failure modes, criticality assessment, strategies and actions to be taken, maintenance procedures and analyses needed to execute Best Maintenance Practices. It has been shown that when maintenance is planned and scheduled, a twenty-five person maintenance force operating with proactive planning and maintenance scheduling can deliver the equivalent amount of work of a maintenance team of forty persons working in a reactive maintenance organization. A CMMS is critical to an organized, efficient transition to a proactive maintenance approach. The types of reports and data tracking that can be obtained from CMMS are work orders and all kinds of reports. A final item to consider when incorporating Best Maintenance Practices is integrating the use of contractors into the maintenance activity of the organization and following the same format of information/data to be collected and entered into CMMS.

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 15 ___________________________________________________________________________ The process of transition from a reactive maintenance organization to a totally proactive structure is not an overnight project. It will take time, effort and planning to accomplish. The transition requires commitment from all levels of the organization.

1.2 Maintenance Practice in United States Bureau of

Reclamation, Denver, Colorado

Bureau of Reclamation has developed a document on their maintenance practices on electrical equipment owned and operated by them. Maintenance recommendations are based on industry standards and experience in Reclamation facilities. However, equipment and situations vary greatly, so Reclamation suggests other sources of information to be consulted (e.g., manufacturer’s recommendations, unusual operating conditions, personal experience with the equipment, etc.) in conjunction with these maintenance recommendations.

Reclamation follows Preventive Maintenance (PM) practice of maintaining equipment on a regular schedule based on elapsed time or meter readings. The intent of PM is to “prevent” maintenance problems or failures before they take place by following routine and comprehensive maintenance procedures. The goal of Reclamation is to achieve fewer, shorter, and more predictable outages focused on the most important equipment. Reclamation categorized electrical maintenance activities into three types:

Routine Maintenance – Activities that are conducted while equipment and systems are in service. These activities are predictable and can be scheduled and budgeted. Generally, these are the activities scheduled on a time-based or meter-based schedule derived from preventive or predictive maintenance strategies. Some examples are visual inspections, infrared scans, cleaning, functional tests, measurement of operating quantities, lubrication, oil tests, governor, and excitation system alignments.

Maintenance Testing – Activities that involve the use of testing equipment to assess condition in an off-line state. These activities are predictable and can be scheduled and budgeted. They may be scheduled on a time or meter basis but may be planned to coincide with scheduled equipment outages. Since these activities are predictable, some offices consider them “routine maintenance” or “preventive maintenance.” Some examples are Doble testing, insulation resitance testing, relay testing, circuit breaker trip testing, alternating current (AC) hipot tests, high-voltage direct current (HVDC) ramp tests, battery load tests.

Diagnostic Testing -Activities that involve use of testing equipment to assess condition of equipment after unusual events such as faults, fires, or equipment failure/repair/replacement or when equipment deterioration is suspected. These activities are not predictable and cannot be scheduled because they are required after a forced outage. Each office must budget for these events. Some examples are Doble testing, AC hipot tests, HVDC ramp tests, partial

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discharge measurement, wedge tightness, core magnetization tests, pole drop tests, turns ratio, and core earth.

Failure analysis studies complemented by industry standards and the preventive maintenance schedule practiced by Reclamation on their primaryequipment are presented below.

1.2.1 Power Cables

The cables used are either solid dielectric or oil-filled. In the case of critical circuits, periodic maintenance tests are justified during the life of the cable to determine whether or not there has been significant insulation deterioration due to operational or environmental conditions. Cables are tested in accordance with manufacturer’s recommendations and industry standards. When done properly, maintenance tests can detect cables that are approaching failure without accelerating the deterioration process. Direct current (DC) high potential tests effectively reduce in-service failures from faults of the cable or its accessories. Periodic direct-current maintenance tests are not practiced for XLPE cables. Except for infrared scanning, the cable circuit is de-energized before maintenance. For Oil filled cables oil analysis including DGA are done annually. Refer Appendix 1.1 for details of maintenance schedule for power cables.

1.2.2 Circuit Breakers

Most breaker maintenance except infrared scanning are performed with equipment de-energized. Breakers are tested in accordance with manufacturer’s recommendations and industry standards. Contact resistance and motion analyzer tests are highly recommended for in-service breakers on a regular basis to monitor condition of the operating mechanism. Power factor and ac high potential tests with contacts open are also practiced but with lesser frequency. Moisture tests on gas in SF6 gas breakers are also done periodically. Meters and gauges are calibrated annually. Manufacturer’s instructions are strictly followed in performing ac high potential test on vacuum bottle to avoid X-radiation. Overhauling of breakers with new seals and contacts are done based on number of operations, load and timing analyzers information and/or guidelines. Refer Appendix 1.1 for details of maintenance schedule for vacuum and SF6 breaker.

1.2.3 Transformers

Transformers are tested in accordance with manufacturer’s recommendations and industry standards. Bushings are tested based on Doble Guideline and the periodicity of test is adjusted depending on the condition. Annual infrared scanning for bushing is also practiced. Based on DGA results several electrical tests for the main windings and core earthing are recommended as per industry standards. Cooling accessories are also tested periodically for condition assessment. Pressure relief device and gas relays are also included in their

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 17 ___________________________________________________________________________ preventive maintenance schedule. Tap changer failure has been identified as one of the dominant failure modes and its maintenance schedule has also received prime importance. Refer Appendix 1.1 for details of maintenance schedule for oil-filled power transformers.

1.3 Maintenance Practice in NASA

NASA Center has developed a guide to perform preventive maintenance tasks for facilities systems and sets initial Predictive Testing and Inspection (PT&I) alarm limits. In their journey of RCM they felt the necessity of the understanding of the selected machine's failure modes and the consequences of that failure. The maintenance approach followed by NASA is based upon identifying, mitigating, and/or preventing failure. For each equipment category the most common (the dominant) failure modes of the item with the highest probability of occurring are being identified. In addition to the failure mode NASA has also considered the consequence of failure.

Table 1.1 provides the method used to rank system criticality based upon the consequences of failure. For the lowest ranked systems (identified as Rank Number 1 on Table 1.1), a run-to-failure approach is often used. And in the highest ranked systems (Ranking Number 5), a redesign effort is usually undertaken to shift the consequence of failure to a lower rank. The recommended strategy identified in the table is adjusted based upon stressful operating conditions and system redundancies.

Table 1.1 Criticality Ranking

Ranking Effect Consequence

1 Negligible The loss of function will be so minor that it would have no discernible effect on the facility or its operations.

2 Minimal

The loss will cause minimal curtailment of operations or may require minimal monetary investment to restore full operations. Normal contingency planning would cover the loss.

3 Marginal

The loss will have noticeable impact on the facility. It may have to suspend some operations briefly. Some monetary investments may be necessary to restore full operations. May cause minor personal injury.

4 Critical

Will cause personal injury or substantial economic damage. Loss would not be disastrous, but the facility would have to suspend at least part of its operations immediately and temporarily. Reopening the facility would require significant monetary investments.

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Ranking Effect Consequence

5 Catastrophic

Will produce death or multiple death or injuries, or the impact on operations will be disastrous, resulting in long-term or permanent closing of the facility. The facility would cease to operate immediately after the event occurred.

NASA Reliability-Centered Maintenance Guide provides Predictive Testing and Inspection (PT&I) schedule including sample procedures. To determine the most effective intervals for maintenance tasks NASA follows age related maintenance actions in order to reduce the cost of unnecessary and/or ineffective maintenance. PT&I monitoring intervals are set in order to determine the onset of failure and to take an action before the failure occurs. According to NASA like all time/cycle tasks, if the interval is too short, there will be wasted effort (labor and material) and if the interval is too long, failures will occur. For each equipment category NASA has developed a table that identifies the maintenance approach for the Equipment Items within the category. The table includes the Equipment Item, the applicable procedures, and three Periodicity Codes. The Periodicity Codes are provided to assist the NASA Centers in determining how often to perform the maintenance task based upon the consequences of failure.

Table 1.2 Sample Maintenance Approach Table

Procedure Periodicity By Criticality Rank Equipment Item Number Description 2 3 4

Brkr-02 Inspect and Test Vacuum or Oil Filled Circuit Breaker

3A 3A A

PT&I-05 Test Insulation 3A 3A A

Medium Voltage Circuit Breaker, Vacuum

PT&I-08 Power Factor Test 3A 3A A

Brkr-03 Inspect and Test SF6 Circuit Breaker

3A 3A A

Medium Voltage Circuit Breaker, SF6

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 19 ___________________________________________________________________________ Periodicity Codes

The Periodicity Codes used by NASA are described below: D = Daily W = Weekly M = Monthly Q = Quarterly S = Semi-Annually A = Annually

OC = On Condition: usually based upon results of a Predictive Testing and Inspection (PT&I) test

Multiples of the above are sometimes used and are identified by a number followed by a letter. For example, 5A indicates a procedure is scheduled every 5 years. Maintenance schedule for transformers and circuit breakers practiced by NASA is presented here as sample.

1.3.1 Transformers

Transformer dominant failure modes identified by NASA are deterioration of the electrical insulation, deterioration of the electrical connections, and exterior corrosion. Over time, heat generated internally slowly breaks down the paper insulation in all types of transformers. For oil filled transformers, the oil insulation system also deteriorates, also due to heat. In dry type units, moisture contamination contributes to the insulation deterioration. Repeated heating and cooling cycling can loosen connections, both internal (tap connections, winding termination points) and external (bushing connections). Harsh ambient conditions can corrode transformer tanks, cooling fins, and attached accessories such as control panels and conservator tanks. Most of the above failure modes progress slowly over time. Consequently go/no-go tests such as turns-ratio testing are ineffective at finding failure patterns. Trending test data is necessary to identify these failure patterns. The maintenance approach for transformers therefore focuses on using applicable PT&I technologies such as infrared thermography, oil testing and insulation power factor testing. The periodicity of condition monitoring tasks according to criticality ranking for all types of distribution transformers are detailed in Appendix 1.2.

1.3.2 Circuit Breakers and Switchgear

Circuit breakers used in NASA are as follows:

• Moulded Case – a sealed breaker with self-contained tripping and overload mechanisms. • Oil Filled – mineral oil is the primary insulating medium. Normally medium and high

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• Vacuum – a ceramic cylinder contains the operating contacts. The insulating medium is a lack of air in the bottle, which allows for close contacts. This type of breaker is normally only used for medium voltage systems.

• Sulfur Hexafluoride (SF6) – SF6 is used as the insulating medium. Operating voltage can be as high as 500 kV rated.

Dominant failure modes for circuit breakers identified by NASA are binding in the operating mechanism, control circuitry failure, development of high resistance in the power connections, exterior corrosion, and deterioration of the electrical insulation. Of these failure modes, binding operating mechanism and control circuitry failure are the most common, resulting in a circuit breaker that will not open or close as required. For oil filled breakers the oil system also deteriorates due to repeated operations, and for SF6 breakers (SF6 gas is the insulating medium) leaks in the SF6 containment is a dominant failure mode.

It should be noted in the periodicity section of the table in Appendix 1.2 that some breakers have recommended maintenance frequencies of no longer than three years, and only low voltage molded case breakers should be run to failure. The limiting factors for these determinations are both cost and reliability. Medium and high voltage units (especially SF6 and air breakers) also benefit from maintenance cycles of three years or less. The periodicity of condition monitoring tasks according to criticality ranking for all types of breakers is detailed in Appendix 1.2.

Dominant failure modes for switchgear identified by NASA are high resistance at mechanical connections, control relay failure, and corrosion for units installed outdoors or in harsh environments. Additional failure modes that cause operational difficulties include racking mechanism failure (not allowing a breaker to be racked in/out) and shutter assembly/insulation barrier failure (which would not allow a breaker to be racked in or leave energized bus connection uncovered). Typically the bar made from copper bar stock is bent into specific angles and various lengths to fit the configuration of the switchgear. A failure at one of the mechanical connections normally results in a bus bar that becomes greatly distorted and not able to be reused. Replacement times depend on availability of the proper copper bar stock and then manufacturing it into the proper configuration.

As a result the use of PT&I technologies, Infrared Thermography and Ultrasonic testing, become very important for long term reliability. The periodicity of condition monitoring tasks according to criticality ranking for all types of switchgears is detailed in Appendix 1.2.

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 21 ___________________________________________________________________________

1.4 TNB Distribution Division’s journey toward Best

Maintenance Practice

The main themes for the TNB Electricity Technology Roadmap have been postulated as follows:

• Reliable and efficient delivery system • Intelligent power delivery systems

• Value added electricity products and services • Enhance environmental management

Technology will play an important role to enable the improvements in reliability and operational efficiency on the existing electricity delivery infrastructure. The critical technological challenges (CTCs) during this period are described below:

• Improvement in operational efficiency

• Application of modern maintenance techniques • Enhancement of grid system reliability

• Improvement in quality of equipment, components, fuel, infrastructure and systems design

It therefore envisions that the following technologies can provide significant improvements to the operational efficiency of the power delivery systems:

• Condition-based monitoring and Risk-based Inspection of critical components • Basic SCADA for distribution systems

• GIS-based network information systems and applications

The drive to enhance the utilization of utility assets requires significant improvements in maintenance techniques. In a highly competitive business environment, utilities are required to utilize their assets for longer periods, while reducing downtime or outages. One way in which this can be achieved is through the optimization of maintenance strategies.

In the past, maintenance strategies have usually been dictated by the original equipment manufacturers to be time-based. These strategies are usually rather conservative. New sensing technologies have now enabled condition based monitoring and opened new dimensions in maintenance techniques and strategies. Data and information obtained from condition based monitoring can be analyzed for anomalies and trends. These analyses form the basis for predicting potential failures and scheduling maintenance strategies that would maximize on the operating hours, while minimizing failure.

Therefore, the combination of sensing technologies together with information analyses and statistics provide the opportunity for what is called reliability centered maintenance. This technology allows for flexibility in maintenance strategies and allows utilities the ability to maximize the potential of their assets, while reducing unplanned outages and down times.

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TNB Distribution Condition Based Maintenance (CBM) Program has been divided into following tasks:

I. Task 1: Maintenance assessment

i. Review existing asset maintenance processes and work practices

ii. Identify operation and maintenance cost improvement opportunities through CBM application

iii. Cite relevant industry’s best practices in CBM application

II. Task 2: Review existing asset management tools and systems

i. Assess sufficiency of data elements ii. Assess integration possibilities

of existing asset management tools (i.e.: ERMS, LGB, GIS)

III. Task 3: Develop CBM processes, methodologies and models

i. Develop the CBM processes and methodologies ii. Conduct network risk and criticality analysis

iii. Conduct equipment and network Failure Modes, Effects, and Criticality

Analysis (FMECA)

IV. Task 4: Define CBM implementation objectives, strategies and measures

i. How to identify critical/high risk equipment to be prioritized

ii. What CM technologies are relevant as identified through economic and risk assessment

iii. How to measure the cost effectiveness of the CBM strategies

V. Task 5: CBM network architecture, hardware and software

i. Define the architecture and functional specification of the computerized CBM system (CMMS)

ii. Identify interfacing possibilities with existing asset management system

VI. Task 6: Identify tangible benefits and evaluation measures related to CBM

i. Define the tangible benefits of the CBM program (in terms of improved system/component reliability and reduced/maintained operation and maintenance expenses)

ii. Specify the methodology to evaluate the effectiveness of the CBM program

VII.Task 7: CBM implementation master plan

i. Conduct a pilot implementation of the master plan

VIII.Task 8: Propose implementation approach and work plan

i. Identify key activities, tasks, schedule and manpower requirements for implementing the TNB Distribution Division CBM Master Plan

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 23 ___________________________________________________________________________ TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 23 ___________________________________________________________________________

Chapter 2

Maintenance

Management

Maintenance

Management

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2 Maintenance Management

2.1 Background

With the increasing age of the population of assets, complex designs and changing expectations, organizations are making efforts to assess the internal condition of the equipment while in service before catastrophic failures can take place to ensure higher availability and reliability. Key challenges faced by maintenance engineers are as follows:

• To select the most appropriate techniques to deal with each type of failure process in order to fulfil all the expectations of the owners of the assets, the users of the assets and of society as a whole.

• In the most cost-effective and enduring fashion.

• With the active support and co-operation of all the people involved.

At the wake of this avalanche of change, maintenance engineers are continuously in search for a new approach to maintenance that can be adopted to ensure that the physical asset will continue to do whatever its users want it to do in its present operating context and also strategies to maximise the life of the equipment at a minimal cost.

Maintenance management is also responding to changing expectations. Since the 1930’s, the evolution of maintenance can be traced through three generations (shown in Figure 1) to capture growing expectations of the industries and more importantly maintenance engineers.

First Generation • Fix it when it is broken

Second Generation • Higher plant availability • Longer equipment life • Lower costs

Third Generation

• Higher plant availability and reliability • Greater safety

• Better product quality

• No damage to the environment • Longer equipment life

• Greater cost effectiveness

1940 1950 1960 1970 1980 1990 2000

Figure 2.1 Growing Expectation of Maintenance

2.2 Failure Patterns

Traditional perception recommends that the best way to maximize the performance of assets is to overhaul or replace them at fixed intervals. This is based on the premise that there is a direct relationship between the amount of time equipment spends in service and the likelihood that it will fail, as shown in Figure 2.2, which suggests that most assets are

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 25 ___________________________________________________________________________ expected to operate reliably for a period "X", and then wear out. Traditional thinking suggests that X could be determined from historical failure records and manufacturer’s guidelines. This relationship between age and failure relationship is applicable for some failure modes that are typically associated with fatigue and corrosion.

Figure 2.2 Probability of Failure with Age

Today’s equipment is much more complex causing remarkable changes in equipment failure patterns. Figure 2.3 shows failure probability against age for a wide variety of assets. Pattern A is the well-known bathtub curve, and pattern B is the same as Figure 2.2. Pattern C shows slowly increasing failure probability with no specific wear out age. Pattern D shows low failure probability at start then a rapid increase to a constant level, while Pattern E shows a constant failure probability at all ages. Pattern F starts with high infant mortality and then drops to a constant or very slowly increasing failure probability.

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Figure 2.3 Failure Patterns

2.3 Maintenance Techniques

There has been tremendous growth in new maintenance concepts and techniques. They are broadly classified into following categories:

• Reactive maintenance

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• Predictive or Condition based maintenance • Proactive maintenance

2.3.1 Reactive Maintenance

Corrective maintenance means fixing things either when they are found to be failing or when they have failed. It includes:

• Breakdown maintenance • Repair-when-fail

• Run-to-failure

Strategy of reactive maintenance assumes that failure is equally likely to occur. Major downside of reactive maintenance is unexpected and unscheduled equipment downtime if failed or repair parts are not available. Both labour and materials are used inefficiently. Replacement parts are stocked at high levels which incurs high inventory cost.

2.3.2 Preventive or Calendar Based Maintenance

Preventive maintenance usually means overhauling items or replacing components at fixed intervals. It includes activities like:

• scheduled inspection • adjusting alignments

• cleaning and lubrication parts • replacement

• calibration • repair of parts

The above tasks are performed at pre-defined intervals without regard to equipment condition or degree of use. It will reduce serious unplanned machine failure. The scheduled maintenance is based on MTBF (or failure rate). The major weakness is that in reality failures are equally likely to occur at random times and with a frequency unrelated to the average failure rate. Thus calendar-based maintenance can be costly and ineffective when it is the sole type of maintenance practiced. Although many ways have been proposed for determining the correct frequency of scheduled maintenance tasks, none are valid unless the in-service age-reliability (i.e. failure rate versus age) characteristics of the systems are known. To determine periodicity, the following techniques are recommended:

• Anticipating failure from experience • Failure distribution statistics

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2.3.3 Predictive or Condition Based Maintenance

Predictive or condition based tasks entail checking if something is failing. It includes: • Non-intrusive testing

• Visual inspection

• Operational data to assess machinery condition

To check whether something is failing the "failure-finding tasks" are carried out using various on-site testing methods. The data collected from on-site testing are called Condition Monitoring (CM) data. A few examples of CM data are:

• Flow rates • Temperature • Pressure • Electrical parameter • Ultrasonic testing • Vibration monitoring • Oil analysis • Optical sensing • Thermography

Usually FMECA is being practiced to identify condition monitoring techniques appropriate for different failure modes of equipment to assess their condition.

The CM data are analysed using the following techniques to identify the precursors of failure: • Trend analysis

• Pattern recognition

• Comparing tests results against specified limits • Statistical process analysis

Through trending or other predictive analysis methods, the maintenance interval is decided. For trending purposes a minimum of 3 monitoring points will be required before failure. CM does not give all types of equipment failure modes and therefore should not be the sole type of maintenance practiced.

2.3.4 Proactive Maintenance

This is the capstone of Reliability Centred Maintenance philosophy. It improves maintenance through better:

• Design • Installation

• Maintenance procedures • Workmanship

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 29 ___________________________________________________________________________ It is characterised by an effective feedback system between the maintenance technician and design engineer. One must ensure that design mistakes made in the past are not repeated in future design. The equipment is viewed from life-cycle perspective. Constantly maintenance procedures are re-evaluated to find optimal mix. Its main objective is to extend machinery life and to obtain zero breakdown. The activities undertaken are schematically represented in Figure 2.4.

Figure 2.4 Schematic Representation of Proactive Maintenance

2.4 Failure Modes, Effects and Criticality Analysis (FMECA)

Initially, the FMECA was called FMEA (Failure modes and effects analysis). The C in FMECA indicates that the criticality (or severity) of the various failure effects are considered and ranked. Today, FMEA is often used as a synonym for FMECA. These are methodologies designed to identify potential failure modes for an equipment or system, to assess the risk associated with those failure modes, to rank the issues in terms of importance and to identify and carry out corrective actions to address the most serious concerns.

Failure modes, effects, and criticality analysis (FMECA) is a methodology to identify and analyze:

• All potential failure modes of the various components of a system • The effects these failures may have on the system

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FMECA is a very structured and reliable technique for failure analysis developed by the U.S. Military. FMECA is used during the early design phases to assist in selecting design alternatives with high reliability and high safety potential to ensure that all conceivable failure modes and their effects on operational success of the system have been considered. It also provides a basis for maintenance planning and provides a basis for quantitative reliability and availability analyses.

2.4.1 Types of FMECA

FMECA are of three types:

• Design FMECA is carried out during equipment design phases to eliminate all conceivable failures that can happen during the whole life-span of the equipment. • Process FMECA is focused on problems stemming from how the equipment is

manufactured, maintained or operated.

• System FMECA looks for potential problems and bottlenecks in larger processes, such as entire production lines.

2.4.2 Standards Related to FMECA

FMECA standards are:

• MIL-STD 1629 “Procedures for performing a failure mode and effect analysis” • IEC 60812 “Procedures for failure mode and effect analysis (FMEA)”

• BS 5760-5 “Guide to failure modes, effects and criticality analysis (FMEA and FMECA)”

2.4.3 Prerequisites of FMECA

Prerequisites for FMECA studies are:

1. Defining the system to be analyzed and dividing it into manageable units called functional elements.

2. Collecting available information that describes the system to be analyzed; including drawings, specifications, schematics, component lists, interface information and functional descriptions.

3. Collecting information about previous and similar designs through interviews with design personnel, operations and maintenance personnel and component suppliers.

2.4.4 Preparation of FMECA

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 31 ___________________________________________________________________________ A suitable FMECA worksheet for the analysis has to be designed which can easily fit into the maintenance management system. A typical FMECA worksheet covering the most relevant columns is discussed below.

Task No. Task Description

1. In the first column a unique reference is assigned to a functional element.

2. The functions of the element are listed in the second column. Functions are also categorized according to various operational modes for the element.

3. For each function of the element the potential failure modes are then identified and listed in column three. Failure mode is defined as a functional failure or in other words a non fulfillment of the functional requirements of the functions specified in column 2.

4. The failure modes identified in column three are studied one-by-one. The failure mechanisms or causes that may contribute to a failure mode are identified and listed. Some failure modes are evident, others are hidden.

5. The effects each failure mode may have on other elements in the same subsystem (local effects) and/or on the system (global effects) are listed in column four. The resulting operational status of the system after the failure can be recorded, that is, whether the system is functioning or not, or is switched over to another operational mode.

6. In some cases consequences such as safety consequences, environmental consequences, operational consequences and economic consequences are also listed in separate columns in the worksheet.

7. The severity index rank corresponding to the failure mode is then assigned in column

five. The severity classes and the ranks can be described in various ways. A typical

example is shown below:

Rank Description

10 Catastrophic Failure results in major injury or death of personnel. 7-9 Critical Failure results in minor injury to personnel.

4-6 Major Failure results in a low level of exposure to personnel, or activates alarm system.

1-3 Minor Failure results in minor system damage but does not cause injury to personnel.

8. The likelihood that the failure will be detected is then listed in column six. An example of detectability ranking is given below:

Rank Description

1-2 Very high probability that the defect will be detected 3-4 High probability that the defect will be detected 5-7 Moderate probability that the defect will be detected 8-9 Low probability that the defect will be detected

10 Very low (or zero) probability that the defect will be detected

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An example of a classification is shown below:

Rank Description

Very unlikely Once in 1000 years Remote Once in 100 years Occasional Once in 10 years Probable Once per year

Frequent Once per month

More sophisticated numerical value of probability of failure can also be calculated and assigned using past record of failure data and using appropriate statistical modeling technique.

10. The various possibilities for detection of the identified failure modes are listed in column

eight. These may involve on-line and off-line diagnostic testing and proof testing.

11. Possible actions to correct the failure and restore the function or prevent serious consequences are listed in column nine. Actions that are likely to reduce the frequency of the failure modes should also be recorded.

12. The risk related to the various failure modes is presented in column ten by Probability/Risk number (PRN). Sometimes it is called criticality assessment. A PRN is derived by assigning a numerical value to the frequency/probability of the failure mode and another value to the severity of the failure mode. More sophisticated PRN can be calculated by attaching different numerical weightings to different categories of failure consequences (safety, environmental, operational and economic). If historical failure rates and costs are available, these rankings can be refined using Pareto analysis.

Some methodology recommends computation of Risk Priority Numbers as shown below:

XXXXXRPN (risk priority number) = Fr x Cr x DetXXXXX

where, Fr = probability of occurrence, Cr = criticality or severity and Det = detectability

2.4.5 Limitations of FMECA

In spite of being so popular FMECA also has downsides and they are: • It is a tedious, time-consuming and expensive process

• It is not suitable for multiple failures

2.5 Frequency or Periodicity of Condition Based

Maintenance Task

Traditionally, the periodicity of condition based maintenance tasks used to be decided based on two factors; the frequency of the failure and/or severity of the failure. Sometimes these two are combined together and expressed as the criticality of the equipment. Recent studies

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TNB Distribution Division Maintenance Manual : Underground Cable System P a g e | 33 ___________________________________________________________________________ have shown that periodicity of condition based maintenance tasks should be based on more appropriate factor called failure period also known as the "P-F interval". Figure 2.5 illustrates this in the form of P-F curve, which shows how a failure starts and deteriorates to the point at which it can be detected (the potential failure point "P"). Thereafter, if it is not detected and suitable action taken, it continues to deteriorate - usually at an accelerating rate - until it reaches the point of functional failure ("F").

Figure 2.5 P-F Curve

The amount of time which elapses between the point where a potential failure occurs and the point where it deteriorates into a functional failure is known as the P-F interval, as shown in Figure 2.6. The P-F interval will vary with the failure modes.

Figure 2.6 P-F Interval

The P-F interval governs the periodicity with which the condition based maintenance tasks should be undertaken. The periodicity must be significantly less than the P-F interval if we

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wish to detect the potential failure before it becomes a functional failure. Unless there is a good reason to do otherwise, it is usually sufficient to select a periodicity equal to half the P-F interval. If the P-P-F interval is too short for it to be practical to monitor for the potential failure, or if the nett P-F interval is too short for any sensible action to be taken once a potential failure is discovered, then the condition based task is not appropriate for the failure mode under consideration. For instance, Figure 2.7 shows how a P-F interval of 9 months and a periodicity of 1 month give a nett P-F interval of 8 months.

TNB Cable Maintenance Manual

TNB Distribution Division

Maintenance Manual :

Underground Cable System

2007

Acknowledgement

I. Purpose of the Manual

II. Scope and Validity

III. Relevant Standards and References

List of Abbreviations

Table of Contents

List of Figures and Tables

Chapter 1

Chapter 1

Introduction

Introduction

1 Introduction

1.1 Background

1.2 Maintenance Practice in United States Bureau of

Reclamation, Denver, Colorado

1.2.1 Power Cables

1.2.2 Circuit Breakers

1.2.3 Transformers

1.3 Maintenance Practice in NASA

1.3.1 Transformers

1.3.2 Circuit Breakers and Switchgear

1.4 TNB Distribution Division’s journey toward Best

Maintenance Practice

Chapter 2

Chapter 2

Maintenance

Management

Maintenance

Management

2 Maintenance Management

2.1 Background

2.2 Failure Patterns

2.3 Maintenance Techniques

2.3.1 Reactive Maintenance

2.3.2 Preventive or Calendar Based Maintenance

2.3.3 Predictive or Condition Based Maintenance

2.3.4 Proactive Maintenance

2.4 Failure Modes, Effects and Criticality Analysis (FMECA)

2.4.1 Types of FMECA

2.4.2 Standards Related to FMECA

2.4.3 Prerequisites of FMECA

2.4.4 Preparation of FMECA

2.4.5 Limitations of FMECA

2.5 Frequency or Periodicity of Condition Based

Maintenance Task

Chapter 3

Chapter 3

Cable Asset

Category

Cable Asset

Category

3 Cable Asset Category

3.1 Categorization of Underground Cable and its

Accessories in TNB Distribution Division System

3.2 Construction of Cables and its Accessories

3.2.1 XLPE Cable

3.2.1.1 Single Core Cable

3.2.1.2 Three Core Cable

3.2.1.3 Triplex Cable

3.2.2 PILC Cable

3.2.3 LV Cable

3.2.3.1 PVC LV Cable