1RESIDUAL LIFE ASSESSMENT

AND LIFE EXTENSION OF HV TRANSMISSION

EQUIPMENT IN SAUDI ARABIA

by:

ATUL SEHGALEngineer III

Engineering & Design Services DepartmentElectrical Engineering Division

SCECO-EastP.O. Box 5190, Dammam 31422

Saudi Arabia

2ABSTRACT- The Transmission and DistributionNetwork in the Kingdom of Saudi Arabia is poisedfor rapid growth in the short term and medium termtime perspective. Rapid network proliferation helpsto meet gross power demands of industrial andconsumer sectors but brings in fresh problems relatedto reliability and operational economics. In ascenario of substantial investment needs but at thesame time serious resource constraints, operationaleconomics assumes added importance. Residual lifeassessment and life extension techniques of HVtransmission equipment are phased programs ofdiagnostics, condition monitoring and preventivemaintenance of equipment aimed at reducingequipment outage and deferring replacement orrenewal. The techniques are essentially based on theadage, A stitch in time saves nine. Theyincorporate professional tools for effecting a smoothtransition from time-based maintenance to condition-based maintenance yielding life extension of agingequipment with resultant cost savings of substantialmagnitude. This paper throws light on the need andmodalities of adopting these techniques in thetransmission system of Saudi Arabia and describesthe various relevant tools which can be applied topower system networks in this country.

Note: Authors views expressed in this paper are notnecessarily of the organization he represents.

Introduction

With rapidly growing power system network in SaudiArabia, attention is required to be given to effectinggreater economy in operations so that the moneysaved could be utilized for expanding powergeneration and evacuation capacity. Technologies atthe present point of time and in the short to mediumterm future shall largely be geared to achieving betterperformance out of the existing equipment. Thepresent tight economic situation in a period ofgeneral global economic recession also justifiesactivities for conservation and optimal utilization ofexisting resources, which too are not unlimited.Residual Life Assessment and Life ExtensionTechniques of HV Transmission Equipment havebeen applied and successfully implemented in USA,Italy and New Zealand. These technologies haveafforded great techno-economic benefits to theelectricity utilities.

Predictive maintenance is a new term coined aftersuccessful R&D in the area of HV transmission lineand substation equipment diagnostics for assessment

of residual life of the aging equipment and achievinglife extension. Predictive maintenance, in contrast toconventional time-based maintenance, makes use ofthe present condition of the equipment to definerequired maintenance activities and stipulates whenthese activities should be performed. In a typicalpredictive maintenance program, the primaryobjectives are:

Reduction of Operation & Maintenance costs Enhancement of system reliability Lowering of long-term capital requirements

Because aging equipment can have higher failurerates, todays utilities have to increasingly considerthe efficiency of their maintenance and replacementpolicies. In many utilities, most of the equipment aremaintained by following a corrective (rectification,repair or replacement on failure) and an overall time-based strategy. In the latter case, the time interval isusually chosen conservatively to ensure highreliability. A smooth changeover from time-basedmaintenance to condition-based maintenance isaccomplished for the desired cost savings andreliability improvement. Condition-basedmaintenance can provide more effective managementtool to tackle the terminal behavior of the equipment.

Basic Steps in the Approach

The basic steps in the implementation of the optimalmodel incorporating condition-based maintenance inan overall, broader, time-based maintenanceperspective can be listed below:

1st. Making a priority list of equipment to be coveredunder the model

Return on Investment Analysis

2nd. System Functional Analysis

Collection and compilation of failurestatistics

Failure mode prioritization and criticalityanalysis

3rd. Aging Model

Identification of early indicators ofdeterioration

Performing verification experiments Economic interpretation and analysis

3Determination of Equipment Residual Life is basedon Steps B and C above.

4th. Maintenance Programming

Maintenance Planning Inspection Program Diagnostics

5th. Preparation of Expert Support System

Object Structure Fixed Data/Inspection Data Knowledge Rules/Maintenance Activities

F. Feedback to improve the model

Life Extension is based on Steps D, E and F.

In Step A, a cost benefit analysis is performed torank the need of developing condition-basedmaintenance systems. A breakdown of the costsavings for substation equipment is given in Figure1. Priority has to be given to transformers in highvoltage substations because they constitute bulk ofthe realizable cost savings.

In effect, the condition-based maintenance of powersystems equipment focuses on reliabilityenhancement because equipment outage is expensive.In some utilities, this approach has been incorporatedthrough an application software for optimizingsubstation, transmission line and power systemcontrol and also system protection and control.

A quality substation maintenance program mustaddress both reliability and cost issues. In SaudiArabia, an overall emphasis on reliability aspect has,hitherto, been greater than on cost. The economy hasbeen historically cash rich and, therefore, most of theT&D network has been engineered quiteconservatively with large safety and stabilitymargins. With the present pressure on oil revenue inthe wake of an overall global recession and anincreasingly commercial environment in general, it isonly too logical that technologies and engineeringpractices effecting cost savings should be adopted ina big way. Taking cue from the experience ofcountries like USA, New Zealand, Netherlands andItaly that have successfully implemented ResidualLife Assessment and Life Extension programs ontheir T&D networks, in the following paragraphs is

outlined a strategy that can be brought into practicein Saudi Arabia.

Determination of Residual Life of Equipment

The determination of useful residual life oftransmission and distribution equipment is not asimple task. First of all, it calls for accuratehistorical data on individual equipment on itscondition and maintenance. More often than not,such data is not available with many utilities. InSCECO-East as also in other SCECO systems inSaudi Arabia, fortunately, the maintenance practicesare well-organized and the equipment are also nottoo old, the oldest equipment being hardly 25 yearsold. However, getting past maintenance data datingback to 20 years is often difficult because pastmaintenance in most utilities has been piece mealand poorly documented. Another reason foruncertainty in the availability of accurate data isaggressive coastal and geothermal environmentswhich cause accelerated aging of equipment. This isquite true of many parts of SCECO-East system inthe Eastern Province of Saudi Arabia where highlyhumid and corrosive environment has dictated designpractices which were conservative and aimed toachieve maximum reliability. Resource constraintwithin the utility company for undertaking detailedplanning work required for data collection andintegration is also a problem. A furthercomplementary requirement in assessing equipmentcondition is that the data collection and integrationprocess needs to be repeatable, requiring minimaladministrative effort and accurately reflective of thediverse physical conditions of all the systemequipment besides being capable of automaticallyincorporating recent maintenance work.

The only way to meet the above requirements andtake care of the related problems is to develop asoftware program that accurately reflects theequipment aging process. This, in turn calls forbuilding an information system that would providethe required information in a manner suitable forcomputer processing. Five major data inputs can beselected for the program.

1. A detailed and accurate data base of the utilityspresent transmission line and substation assetsincluding all critical and keyequipment/components.

2. The condition of all the critical and key

components at a known point in time. This

4condition information can be stored in a database in the form of numerical condition codes.

3. Parameters indicating the nature and character

of environment where components of T&Dnetwork are located.

4. Scientific assumptions about how long each of

the various components should last in eachenvironment.

5. The cost of replacing/repairing each component

at the end of its useful life and thereplacement/repair item to be used.

Two basic assumptions underlie this model. The firstis that each network component deteriorates in alinear fashion from new to replacement stage at aspeed which depends on the inherent design life(normal life) of the component and the severity of theenvironment in which it is located (see Figure 2).The second assumption is that routine maintenanceshall be performed in an organized and predictablemanner.

As an illustrative example, condition coding forunpainted galvanized transmission tower made ofsteel is given in Table I.

The condition coding of all equipment needs to bedone in such a way as would facilitate computerprocessing. Residual life allows for reaction time toplan and carry out equipment replacement beforetransmission line reliability is undermined. Thecondition of individual transmission line componentscan be determined by comparing an observedcondition in the field against predetermined keycondition indicators. The condition assessment workcan be conveniently awarded to dedicated contractorsworking under specific lump sum contract.

Environment coding can be done by formulatingenvironment parameters. These parametric codes areused to precisely document the appropriate speed ofdeterioration of individual equipment. For theexample under discussion, five such parameters needto be formulated. These are as follows:

1. Zinc corrosion rate 2. Wind run 3. Resistivity of soil 4. Annual rainfall 5. Aluminum corrosion rate

The life assessment assumptions are the key data thatdrive the modeling program. For every componentin service, an expected life of that component can bedetermined within its particular environment. Themaximum life is determined based on calculationsusing the least aggressive environment category.These assumptions are then used to calculate theresidual life of the equipment in question. Themodel calculates a Replacement Date (RD) for theequipment or component by applying a simpleformula;

RD = Assessment Date + [Maximum Life x(Condition Code - R/L) xEnvironment Code]

were R/L = Selected Point of Replacement

To illustrate with an example, a spacer has anallocated maximum life (life in the best environment)of 55 years. It was last inspected in 1995 andassessed a condition code of 40%. Assuming theenvironment at the site was determined to merit anenvironment code of 0.60, it has been decided toreplace this type of spacer when it reaches a residuallife of 20% (i.e. an R/L of 0.2). Therefore,

RD = 1995 + [55 x (0.4-0.2) x 0.60]= 2001.6 or (2001 and 7 months)

Based on the calculation, a new spacer will be neededin the year beginning July 2001.

Then Residual Life= RD-Present year= 2001.6 - 1998= 3.6 years

This modeling and calculation can likewise beapplied to every other component or equipment ofT&D network, i.e. substation equipment like powertransformers, CBs, instrument transformers,transmission line conductors, shunt reactors,series/shunt capacitors and also completetransmission lines and substations, the latter throughparameter summations.

Diagnostics, Inspections and Condition Monitoring

Determination and subsequent revision of equipmentcondition codes shall call for professionaldiagnostics, inspection and condition monitoring.Some practical examples of dynamic diagnosticsare given below.

5 1. On-Load Tap Changer (OLTC) ConditionMonitoring. Failure statistics show that morethan 50% of the failure risk of powertransformers is due to OLTC. Conditionmonitoring codes can be developed for theresistance of the selector switch contacts, whichcan become carbonized in the long term. Thecorrelation between dissolved gases in oil andthe OLTC condition has been investigated,showing that trends for specific gases provide awarning only in severe cases because quantity ofgases produced in low level degradation is verysmall. Another important failure mode relates tothe OLTC mechanical part. Therefore, motorcharacteristics can be used for diagnosis.

2. Circuit Breaker (CB) Condition Monitoring.

Presently, CB condition monitoring systemshave been developed which can capturecondition of the CB every time it operates-tripand close coil currents, open and close conditionof contacts, fault current values and stationbattery voltage dips. The systems provide outputalarms for operate times, coil currents andcontact duty and can accept inputs from varioustypes of transducers. Systems which effectivelymonitor SF6 gas density and SF6 gas pressurethrough microprocessors are also available andcan be used for predictive maintenance of CBs.

3. Power Transformer Diagnostics

A state-of-the-art implement for power transformerdiagnostics is the transformer fault gas analyzer.This microelectronic device provides real timemeasurement of the four key gases associated withfault currents in transformers-CO2, H2, C2H2, andC2H4. The analyzer uses a metal-insulator semi-conductor sensor which is inserted into thetransformer oil through a small valve opening. Thesensor provides on-line, in-situ data that indicatesnot only present condition but also critical trends,such as an increase in individual gases.

Another predictive maintenance technology that hasrecently found use in substations is a transformerwinding hot spot detector. This device features asmall temperature sensitive probe which is placed inclose contact with the windings and is connected tooutside monitoring equipment by an optical fiber.Several utilities are now using this detector tomanage transformer loading and avert overheatingproblems.

Today, microprocessor-based systems are availablewhich can capture, store and format data for futuredata retrieval via a laptop PC, a built-in data retrievalcard or modem. Analog input channels in suchsystems can monitor drive motor current of OLTC,load current, tap position, control voltage, top andbottom oil temperatures and dissolved gas acquiredthrough a force-oil flow space loop. On bushingCTs, measurements are made for leakage current andpower factor. Even relay voltages can be monitoredwithout altering relay circuitry. Systems cancontinuously measure key performance indicatorsand use a self learning expert sub-system to detectchanges in operating conditions that indicateincipient malfunctions. If a trend towardsmalfunction in indicated, the system uses aprioritized maintenance alert indicating the likelycause and urgency of response.

4. Current Transformer Bushing diagnostics

Modern technology application has led to thedevelopment of monitoring systems to track thequality of oil-paper insulation of in-service highvoltage equipment such as CT bushings. Thesystems are based on conventional testing techniquesand software control to calculate tan delta andcapacitance. They are capable of continuouslymonitoring the deterioration of insulation based uponon-line operating conditions of the equipment.

Based on the above considerations, the followingbroad observations and recommendations are madefor SCECO-East in Saudi Arabia:

1. Existing maintenance practices areprofessionally well-organized and systematizedbut could be improved for meeting in a betterway the requirements of medium term future onachieving maximum economy and cost effectivesolution to operational problems.

2. Latest technologies on on-line diagnostics of

equipment as well as professional computersoftware-based maintenance practices need to beembraced considering that these are increasinglybeing adopted by utilities internationally.Preventive maintenance should become adynamic, technology-centered practice ratherthan a stereo-typed, ritualistic activity.

3. A working group may be constituted for studying

latest methodologies and technologies the worldover for Residual Life Assessment and Life

6Extension of HV Transmission Equipment andidentifying which set of methodologies andtechnologies could be appropriately adapted intothe SCECO-East system with best results.

4. After the working group as aforesaid has

performed its task, a complete Residual LifeAssessment and Life Extension Program may bedevised, if necessary, with the help of outsideconsultants at reasonable cost. KEMA(Netherlands), CESI (Italy) and EPRI (USA) aresome of the agencies which have practicalexperience in formulation and participativeimplementation of such projects and arecompetent to provide professional consultancy.

5. Finally, recommendations cited above also in

general apply to other SCECO systems in SaudiArabia considering the fact SCECOorganizations in the various provinces of thekingdom are going to be merged into a singleutility, Saudi Electricity Company, in the nearfuture.

Money saved is money earned. In Saudi Arabia,following the constitution of single nationwide utilitycompany, the Saudi Electricity Company, increasingfinancial participation by foreign companies and

other Saudi companies is envisaged which meansthat the reconstituted utility shall have to work withgreater economic discipline. Further, it is only toological to expect that greater economic interactionamong the GCC countries shall witness bulk importand export of power across the boundaries of SaudiArabia and the establishment of a nationwide grid inthis country. This is in addition to the expectation ofa far more commercial environment in the future, asalready discussed before in this paper. Based on theexperience of successful implementation of ResidualLife Assessment and Life Extension projects not onlyin power transmission network but also nuclear andfossil power plants besides as diverse an industry asairlines, a gross approximation will put the net costsavings resulting from such implementation to thetune of 15% of the transmission system O&M cost.This is stupendous savings. In SCECO-East, suchcost savings as per 1996-97 figures work out to aboutSR 75 million on an annual basis. Therefore,adoption of modern techniques of Residual LifeAssessment and Life Extension of Power NetworkEquipment and Projects is, doubtlessly, the need ofthe hour.

References:

1. EPRI Journal, May/June 1995 2. Conference Proceeding : Management of Power Systems Growth, Aging, Obsolescence and Safety; The 1998 Electrical

Engineering Technical Exchange Meeting & Exhibit-Dhahran (KSA) 3. Economic Maintenance Strategies for the Future: Transmission & Distribution World; February 1997 4. A Practical Approach to Maintenance: Transmission & Distribution World, April 1996 5. Annual Report: SCECO-East 1996-97 6. Substation Maintenance Manual: Vol. 1, No. 47; SCECO-East

7Comparison of Attainable Savings for High-Voltage Equipment

50%

20% 16%29%

71%84%80%

50%

0102030405060708090

100

Transformer CircuitBreaker

Disconnect InstrumentTransformer

Figure 1:Shaded Portions Show Savings

Net

Pre

sent

Val

ueSavingsCost

0102030405060708090

100

Years from Present Time

Fai

led

Con

diti

on C

ode

(%) Slope Depends on Environment

(Steeper for More Aggressiveor Harsh Environment)

Figure 2:

Present Time

Imminent Failure

Tr

Residual Life

Safety Factor Nil

Increasing Failure Risk

New Equipment

Table 1: Condition Coding for Unpainted Galvanized Steel Tower.Condition

CodeGuidance Notes for Physical Assessment in the Field

100% Steel new, bright new surface90% Surface dulled to light gray coloring80% Roughening of Zinc surface70% Start of crusty build-up in areas of salt-exposure60% Threads and heads on nuts/bolts have rust, some steel members darkened by rust50% Bolts rusty, bracing steel shows spots on undersides, some members red/brown40% Bolts show corrosive erosion, bracing steel and members showing rust30% Many members appear red or brown, most bolts need replacement20% Severe flaking rust on bracing, rust also on main steel members10% Evidence of severe corrosion, loss of metal and loss of compressive strength0% Structural failure under normal wind loads is a possibility

Table 2: Environment Coding for Loss of Galvanic Coating.Years to GalvanizingFailure from 915 g/m2

Zinc LossRate

(g/m2/year)

EnvironmentCode

60 9.0 1.050 11 0.840 14 0.630 20 0.420 45 0.210 90 0.1

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