General Review on Aging Electricity Distribution Systems andMaintenance Strategies

Raviraj v. Kadam", Gopal S. Patil2, Rajesh M. Holmukhe"

lSection Engineer, Pune Rural, Maharashtra State Electricity Distribution Company Ltd.,

Email: ravLmsedcl@gmail.com

2Assistant Engineer, Pune Rural, Maharastra State Electridty Distribution Company ltd.,

3Assistant Professor, Dept. of Electrical Engineering, Bharti vidyapeeth COE,Pune-46,

ABSTRACT

The deregulation of electricity markets has increased the consciousnessof economicalfactors in electricity network utifities.To get a maximum return on investments, the assetmanagement has become more important. Assystem equipment continue to age andgradually deteriOrate the probability of service interruption due to component failureincreases. An effective maintenance strategy is essential in delivering safe and reliableelectrical power to customers economically. Thispaper discussesaging infrastructure ofpower industry. economics of aging coupled with well developed process of optimizingequipment life cycle. Thispaper isan attempt to discussan effort made to maximize lifetimes of electricity network components through proper maintenance strategy and tominimize maintenance cost in the same time without decreasing the reliability of supply.In many countries the lifetimes of distribution networks are coming to their end. In the "near future the decisions on renovation or repairing of networks will be unavoidable.

Keywords: Equipment Aging. life cycles. Condition monitoring, deterioration.

INTRODUCTION

As the deregulation of electricity market has increased the consciousness andresponsibility of economical factors in electricity network utilities.working methods areand will be changed radically. To get maximum return on investment, the assetmanagement has become more important. In principle, improving systemreliability andreducing operational and maintenance cost are top priority of electricity utilities [13].One of the main components of electric power delivery asset management is capitalbudgeting and O&M of existing facilities. Since in many cases the cost of constructionand equipment purchase are fixed, O&M expenditures is the primary candidate forcost cutting and potential savings as the equipment continue to age and graduallydeteriorate, the probability of service interruption due to component failure increases.

Electric utilities are confronted with many challenges in this new era of competition:RisingO&M cost, growing demand on system, maintaining high level of reliability andpower quality, and managing equipment aging. Therefore, the health of equipment isof at most importance to the industry because revenues are affected by condition ofequipment [12]. When demand is high and equipment is in working order, substantialrevenues can be realized. On the contrary, unhealthy equipment can result in serviceinterruption, customer dissatisfaction, lossof goodwill, and eventual lossof customer. Aneffecfive maintenance strategy isessential to deliver safe and reliable elecfrical powerto customers economically.

Equipment Aging

Today, preserving and or enhancing system reliability and reducing operation andmaintenance cost are top prioriTIesfor elecmc UTIlities[8]. A UTIlitybuys a transmissionand disfribution components- a PSC/RSJpoles, service transformers, indoor mediumvoltage brealcer, or underground cable section. And puts it in to operation soon thereoperofion. leaving it in service while tending to it with maintenance and repair asnecessary until it fails. at which point it is replaced. Becfrical equipment deteriorates(wear out) while in service for a host of causes. Theseinclude sustained heating due toelectrical current flow, insulation degradation due to voltage tress,wear and materialfatigue of mechanical parts. corrosion from chemicals in the soil,air. or from byproductsof material deterioration, damage by vegetation, insects, animals and man as well astornadoes, hurricanes, earthqualces and floods, and a slow but steady deteriora1iondue to the effect of sun,wind, rain, ice and snow.

As system equipment conTInues to age and gradually deteriorates the probability ofservice interruption due to component failure increases [12].

Concept of life lime

Equipment performs both technical and economic functions in power systems.Thereare three different concepts of life time for power systemequipment:

o. Physical e limeA piece of equipment start to operate from itsbrand new condition to a status in whichit can no longer be used in normal operating state and must be retired. Preventivemaintenance can prolong itsphysical life time.

b. Technical life timeA piece of equipment may have to be replaced due to technical reason although itmay still be physically used. Forexample. A new technology isdeveloped for a type ofequipment and manufacturers no longer produce spare parts. Thismay result in a

situation in which utilitiescannot obtain necessary parts or parts become too expensivefor maintenance.

c. Economic life timeA piece of equipment no longer valuable economically, although it stillmay be usablephysically.Thereare two methods for estimating the economic life time:• Thecapital value of any power equipment each depreciated every year. Once the

remaining capital value approaches zero, the equipment reaches the end of itseconomic life time.

• In addition to depreciating the capital cost of equipment. operating andmaintenance cost are considered. O&M cost usually increases over time asequipment ages and may become excessive. They may even exceed thedepreciated value of the equipment. It could be cost effective to retire andreplace the equipment before itscapital value reacheszerorather than continue toface highO&Mcosts[7].

Tackling of aging infrastructure

Thespecific approaches to aging infrastructureare different for each utility.And utilitiesare focusing on developing data ware housesand information management skillstohelp track the age and the condition of the equipment [1]. Industrymustdevelop thecondition monitoring and tracking databases as well as the planning andmanagement mechanism to utilize that information. If it is to manage aging and iteffects well appficable generalization that can be applied to aging eleciric utilityinfrastructurewhich are listedbelow:

• Average age isnot as important asthe amount of very old equipment.• Most important is the amount of equipment that will become very old in the next

decade.• Inspection, maintenance and service priorities should be different for older

equipment.• Information on age, condition, and service and maintenance history is a key to

effectively targeting older equipment.• Changes in operational procedures can improve performance and lower cost

significantly.

Optimizing equipment lifecycle

There are a variety of issues that must be considered when actively managingequipment over its entire lifecycle. Some of the more important include conditionmonitoring, life extension, repair vs. replace and optimize lifecycle management.

o, Condition monitoring:Some prerequisite for a condition monitoring system are illustrated in a fig.lbelow

EQUIPMENTDEFECT

Sensorresponsive to leading defect

Noise reductiJn and data transmission

Data interpretation and decision making

~

Figure 1: lil'rroc:r-r-.:"""":>'ft""'O:.....,...."T"<7'r~~"""'......,...,rrtakingby a condition monitoringsystem

CORRECTDECISIONON PLANTSTATUS

There are three major links in the chain in making a correct and timely decision inresponse to an equipment defect. whether this is incremental such as prematureinsulation degradation or sudden such as a defect threatening plant failure and lossofsupply [1]. The more a utility can distinguish be1ween the equipment that needattention and that which does not. The more it can cut costs while managing agingand its effect well. Condition assessmentat all level of an elecfric systemischallenging.Many utilities are placing much more emphasis on retaining and using data oncondition and its trends to drive changes in when and how they care for equipment.Often these efforts focused on more aggressive inspection and testing.Techniques that are becoming more popular include infrared inspection; dissolve gas-in - oil analysis. frequency response signatures and many others. Online conditioning isalso becoming more popular especially for substation equipment that can be costeffectively monitor through SCADA systems. Online techniques range from simplealarms (e.g. temperature and pressure) to continuous monitoring of dissolve gas in oil[6].

b. LifeextensionEquipment replacement is often expensive, resource intensive, and operationallydisruptive. For this reason utilities are increasingly looking at life extension strategies as away to defer replacement. Often an equipment life extension program is coupled witha condition monitoring program. When the condition of a piece of equipment reaches

a certain degree of deterioration, life extension options are examine for economicattractiveness [5].

c. Repair vs. replaceUtilitypractices stillvary widely in this area with some utilitiesmaking explicit repair vs.replace decisions. While others only replace equipment if it fails or becomes

overloaded.

d. Optimized lifecycle managementThe uHimate goal of a utility isto provide reliable elecfridty for the lowest rates.Inherent in this goal is life cycle cost minimizafion of equipment and the systemstheyconstitute. Ideally the utility determines the policies of utilization. operation. inspection.maintenance-repair. retirement and replacement in an integrated manner aimed atminimizingtotallevelized cost. In effect. the utility is optimizing expected rlfetime vs.cost. A strategy of 'over specify it' (Keep loading low. care it well) will result inequipment that lastslong time but might prove expensive to own. A strategy of 'starveit' (cut all possible costs out of the initial purchase cost. load it highly. inspect andservice it infrequently) will result in equipment that cost much lessto own but might failfar sooner. Somewhere in the middle there is a optimum point for every type ofequipment and application [5]. Thisis a simple concept. but making it work is only justbecoming feasible in power industry. Optimized lifecycle management requires gooddata. effective conditions and remaining life analysis and evaluation, soundimplementation. In the middle item isa particular challenge for some equipment.

Maintenance philosophy

Since a gool of any electric utility company is to supply reliable power tocustomers at low cost, prevention of power system failures is of paramountimportance during the design and operation of the system. Distribution systemshave the greatest impact on customer outage frequency and duration. Onoverhead distribution systems, a fault can be classified as temporary orpermanent. Approximately 75 to 90 percent of the faults are of a temporarynature caused by trees, animals, lightning, high winds, flashovers, and so on [11].If the temporary fault cannot be cleared by the automatic interrupting devices,it will become a permanent fault that requires repairs by a Crew. Note that otherutility companies may experience different conditions of distribution faults. Someof the failures cannot be avoided, such as lightning, whereas others can beprevented by proper maintenance plans. Maintenance is an important part of thelife-cycle of embedded systems, and must be considered from the design stagethrough the end-of-life stage of the system.Maintenance covers two aspects of systems- operation and performance. Maintenance is generally performed in anticipation of,or in reaction to, a failure. Maintenance is performed to ensure or restore systemperformance to specified levels [10]. Once the equipment has been purchased it must

be maintained. Maintenance is defined as the restoring of an item to its originalcondition or to working order. Maintenance isdefined in IEEEpower engineering society(PES)task force report as an activity "wherein an un failed device has, from time totime, itsdeterioration arrested, reduced or eliminated". Thiscan be achieved by repair,replacement of parts or total replacement of the item. The management makes thechoice between these alternative measures based on practical and economicgrounds. The decision made at the purchasing will have an input into the type of themaintenance to be carried out. In fact the manufactures /suppliers provide the detailsof regular maintenance schedule. This may be rigorously followed for the betterperformance as well as the increasing the life of equipment. Rg.2 below showsevolution processof maintenance strategies.

ReiabiHycenteredrnnintPnOI'V"

Maintenance strategy

~~~~q~q~qRg.2Evolutionof maintenance strategies

Maintenance strategy

When the maintenance is being planned, the first decision is to select the appropriatemaintenance strategy. The maintenance strategies consist of operations and means,which are chosen to achieve the set general goals of the utility within a generalstrategic plan. An effective maintenance isnot synonym for low-cost maintenance. Themost important is to see the influence of maintenance actions on the Processandoperation results.For example, in the case of electricity network, the resultscan bedecreased number of outages and shortened outage times or better quality ofelectricity [7]. Themaintenance strategy can be divide into two classes.The first one iscorrective maintenance and the second one is the preventive maintenance.Corrective maintenance (CM) includes faults occurred in the electricity network.Nowadays, well known strategies applied to preventive maintenance are time-basedmaintenance (TBM),condition based maintenance (CBM) and reliability centeredmaintenance (RCM) [5]. A time based maintenance strategy features predefined

intervals rooted in empirical feedback, where components are checked or replacedafter a specified period of use. Thisapproach generally produces satisfactory results.However, it will not be the most cost-effective option in all cases, since the equipmentwill not usually remain in operation up to the end of the possible life time. Conditionbased maintenance is driven by technical condition of the equipment. Under thisapproach, all major parameters are considered in order to determine the technicalcondition with maximized accuracy. Forthis reason, detailed information via diagnosticmethods or monitoring systemsshould be available. The condition based strategy canbe considered a kind of basic s1rategy of developed methods. Reliabili1ycenteredmethods isa strategy which additionally includes a reliabili1y-basedpart [2]. Aim of thisapproach is to combine the importance of the equipment in the network and theactual condition of the equipment. The selection of maintenance strategy has to bemade for every component 1ype and in some in some cases for every component.There is not only right solution for certain component. but some guidelines for theselection can be given as in figure 3.

figure 3: Selection of maintenance strategy for components or component groups.

General concluding remark

Equipment aging is a fact of rife in power systems.Dealing with aged equipment hasbeen a challenge in the utili1yindustry for year. Aging infrastructure will have higher costto operate and maintain and, more importantly, lower reliability. The lower reliability ofaging infrastructure makes the physical system lessreliable and the electricity marketmore risky. In this paper discusses the issues of power system equipment aging,concepts of life time and optimizing equipment life cycle. Maintenance activity canextend the life of equipment but could be very costly for equipment at their end-of-lifestage. A compromise between and replacement must be carefully considered.

Acknowledgements

We are thankful to MSEDCL (Pune Rural Circle) & Bharati Vidhyapeeth CaE, Pune forpromoting us for this project. This study is carried out at O&M Division Manchar &SubDivision Junnar; writers especially express their appreciation to the Department ofElectrical Engineering, Pune. We especially acknowledge support from library staff ofBVCOE. The author would like to thank Executive Engineer Mr.D.V.Padalkar,MSEDCl,Manchar Division

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