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The Online Journal on Power and Energy Engineering (OJPEE) Vol. (1) – No. (3)

Reference Number: W09-0022 99

Predictive Maintenance StrategyBased on Disturbance Recorders

Dr. Mohamed A. EL-HADIDY Dr. Dalal H. [email protected] [email protected]

Egyptian Electricity Transmission Company (EETC), Egypt

Abstract-Electric utilities have always relied onmaintenance programs to keep their equipment in goodworking condition as long as it is feasible. In the past,maintenance routines consisted mostly of pre-definedactivities carried out at regular intervals (scheduledmaintenance). However, such a maintenance policy maybe quite inefficient: it may be too costly (in the long run)and may not extend component lifetime as much aspossible. In the last ten years, many utilities replaced theirfixed-interval maintenance schedules with more flexibleprograms based on an analysis of needs and priorities, oron a study of information obtained through periodic orcontinuous condition monitoring (predictivemaintenance).

This paper presents, in some value event examples, theimportance of using Disturbance Recorders as an effectivetool for predictive and condition-based maintenance.These cases include actual records from the EgyptianNetwork that helped, after deep analysis, to determine theneed for station equipment maintenance, together with acase from Belgium. The cases presented in this paper are:

Circuit breaker mechanism problem Improper operation of pre-insertion resistor in

EHV transmission system Circuit Breaker restriking Intermittent fault on a distribution level Fault location on high voltage transmission line Generator shaft tortional stresses

Keywords: Condition based maintenance, Disturbancerecorders,

1. INTRODUCTION

Electric Utilities in the Arab world are undergoing a periodof fundamental changes with respect to interconnection,deregulation, and competition to improve their economicefficiency. In addition to the existing interconnections,namely those of the Maghreb, Algeria is alreadyinterconnected to Tunisia (4 connections) and to Morocco (2connections), Spain to Morocco, Libya to Egypt, Egypt toJordan, and Syria to Jordan and Lebanon, several newinterconnection projects are under way linking Morocco toAlgeria (third connection), Algeria to Tunisia (fifthconnection), Tunisia to Libya, Libya to Egypt, and Syria toTurkey. Concerning the interconnections presently plannedbetween Egypt and Saudi Arabia and that between Egypt andLibya at 220 kV that its voltage is planned to be increased to400 kV at a later stage.

The de-regulated electricity market causes rather quick anddrastic changes in the operational conditions. New, unknown

load flow patterns show up more frequently for the systemoperator. Also, the inter–connectors are becoming heavilyloaded to cross-border power exchanges. Consequently, eachoutage of an asset (equipment) can have a direct impact onthe benefit margins of the revenues as the supply of energymay be interrupted.

The purpose of maintenance is to extend equipmentlifetime, or at least the mean time to the next failure whoserepair may be costly. Furthermore, it is expected that effectivemaintenance policies can reduce the frequency of serviceinterruptions and many undesirable consequences of suchinterruptions. Maintenance clearly affects component andsystem reliability: if too little is done, this may result in anexcessive number of costly failures and poor systemperformance and, therefore, reliability is degraded; if done toooften, reliability may improve but the cost of maintenancewill sharply increase. In a cost-effective scheme, the twoexpenditures of maintenance cost and increase reliability dueto maintenance must be balanced.

Maintenance is just one of the tools for ensuringsatisfactory component and system reliability. Others includeincreasing system capacity, reinforcing redundancy andemploying more reliable components. At a time, however,when these approaches are heavily constrained, electricutilities are forced to get the most out of the devices theyalready own through more effective operating policies,including improved maintenance programs. In fact,maintenance is becoming an important part of what is oftencalled asset management.

From a review of present maintenance policies in electricutilities it is concluded that maintenance at fixed intervals isthe most frequently used approach, often augmented byadditional corrections. Newer “as needed” methods, such asReliability Centered Maintenance (RCM), are increasinglyconsidered for application in North America[1].

Nowadays, devices are becoming ever more intelligent asmanufacturers embed processing capability for optimizinglocal performance. Coordination with other devices isspreading as communication networks link greater numbersof equipment and systems together. Demand response,substation automation, wide-area measurement & control andpredictive maintenance are just some of the application areasthat are driving smart operations advancements into theelectric system.

Disturbance Monitoring Systems (DMS), also known asDigital Fault Recorders (DFR) provide valuable informationfor the study of electric power system performance and faultanalysis, preventing later coming failures or interruptions inelectrical circuits. Nowadays, in world-wide electricityutilities, monitoring systems provide detailed information,

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alerting employees not just that something is wrong, butexactly what is wrong.

This paper present, in some actual study cases, DisturbanceRecorders as an available tool that can help the utility todetermine the need for station equipment maintenance basedon the records that obtained from DMS, and also preventpossible cascaded failures.

2. MAINTENANCE PRACTICES

When we discuss maintenance, we consider several typesof maintenance as follows[2]:Replacement: Restoration wherein a device is removed and

one in better condition is put in its place; ifthe device is failed, it is replaced by aworking one. It is often assumed that thedevice so installed is new.

Repair: Restoration wherein a failed device isreturned to operable condition. (Note: It iscommon to use the term correctivemaintenance for both replacement and repair).

Maintenance: Restoration wherein an unfailed device has,from time to time, its deterioration arrested,reduced or eliminated.

Scheduled Maintenance carried out at regularMaintenance: intervals (rigid schedule). (Note: Another

term often used for this activity is preventivemaintenance).

Predictive A maintenance carried out when it isMaintenance: deemed necessary, based on means of

condition monitoring.Emergency A predictive maintenance that must beMaintenance: carried out immediately, or with the shortest

delay possible, after condition monitoringdetects a danger of imminent failure.

Corrective Maintenance actions carried out toMaintenance: restore a defective item to a specified

condition using tests, measurements, andadjustments made to remove or correct a faultor faulty equipment.

As mentioned above, definitions are given of a fewfundamental concepts discussed in this paper. In the past,maintenance was done based on pre-defined schedules, i.e, onfixed periods of time "scheduled maintenance". Significantchanges in the power utility industry are driving therequirement for change in this previously common practice.Another factor is that there are many real life examples inwhich errors that occur during this preventive maintenancecaused significant system or equipment problems.Consequently, the concept saying "If it is not broken, don't fixit" is raised up. But it was found that the delay formaintenance for long time following this concept may lead tocatastrophic failures.

Now, the new concept for improving the traditionalmaintenance is based on the idea" If it doesn't tell you, don'tfix it". What this means is that different monitoring and

analysis tools inside the disturbance monitoring devices can"predict" the need for maintenance of monitored equipment,and also, after detailed analysis, help to "correct" the faultypart. By this way we may get the point where maintenance isnot scheduled, it is done when monitoring says it needs to bedone.

Predictive Maintenance has so many advantages: Increased component operational life/availability Allows for on-time corrective actions Decrease in equipment or process downtime Decrease in costs for parts and labor Improved worker and environmental safety Improved worker morale Energy Savings Substantial savings over preventive maintenance

programs. Eliminates most tear-down events. Provides information to perform a root cause

analysis.

Predicative Maintenance leads to costs savings as follows: Reduction in maintenance costs. Elimination of breakdowns. Reduction in downtime. Decreased spare parts inventory.

3. DISTURBANCE MONITORING SYSTEMS AS ATOOL FOR CONDITION-BASED MAINTENANCE

MANAGEMENT

Disturbance Recorders can play vital role in themaintenance management of any utility. They are scatteredand permanently connected through out the power system.The main goal of using these recorders was traditionally asfollows [3]: Determining the performance of system components Analysis of the nature and cause of a disturbance. Identifying equipment misoperations Identifying causes of power system oscillations Analysis and correction of protection system and control

deficiencies Reducing the risk of recurring misoperations Gathering of modelling data to verify the relevant

characteristics of the models and para-meters used insimulation programs,

It is obvious, from the above mentioned objectives that datacaptured by these recorders enable full analysis of the systemcomponent performance and identification of equipment missoperation. This means that they represent a very useful toolfor predictive maintenance analysis.

The recorded values and waveforms of voltages andcurrents can identify the behaviour of the system components.Consequently their operative status and performance can befully analyzed. Two main requirements have to be fulfilled inorder to fully run the required analysis:



1 Additional Software capable of handling the captured datafor the purpose of identifying the incorrect performance ofthe specified equipment.

2 Accumulated expertise capable of reading the hiddeninformation given by this data and analysis softwareoutcomes.

Data analysis and diagnostics may be completely farmedout. In the next few years [1] utilities may not be willing toregularly upgrade their diagnostics software, and willprobably hire monitoring vendors to process the data fromtheir substations. As concluded from this, we have to getstarted to train a qualified staff that will be responsible toanalyze and correlate the valuable data obtained from thesedevices.

In the following sections, we will give some actualexamples that prove the importance of DisturbanceMonitoring Devices as a predictive and correctivemaintenance management tool. Also, some cases that use theobtained records as a basic input for analysis will bediscussed. These include local Egyptian and world wide casescovering the majority of power system components, e.g.circuit breaker, insulators and generator shaft.

4. CIRCUIT BREAKER CONDITION MONITORING

Periodic maintenance of circuit breaker is done to ensurethat the trip circuit and mechanism operate correctly and alsothat the interrupting capability has not been compromised dueto previous fault interruptions. Generally, such maintenance isbased on a fixed time interval, or a fixed number of faultcurrent interruptions. These methods of monitoring circuitbreaker condition give a rough guide only, and can lead toexcessive maintenance.

Using the monitoring systems, the CB maintenance can beconditionally bases on the following two criteria: The calculation of the number of operations in circuit

breaker for three phases or for each phase under faultcondition can be done and recorded in the database ofsuch breaker. Also elongation of time till the secondmaintenance by using the per-phase consideration fordecision of breaker maintenance may be achieved.Suitable setting of maintenance may be chosen to startmaintain the breaker at due time based on thisinformation.

∑I2t can be calculated via the information of the faultrecords, for each breaker, and stored in the data bases ofsuch breaker. A condition-based maintenance could bestarted when this value exceeds the manufacturer specifiedsetting.

4.1 Cases of Improper Operation of CB MechanismThe need of CB operating mechanism to be maintained can

be predicted as shown in the following three recorded casesfor the Egyptian 500kV circuit breakers.

Figure 1 shows a DFR record for a CB switching-on toenergize a 500 kV transmission line [4]. The result of this

process shows that the CB switching-on is fulfilledsuccessfully, as it could be noticed from the end of the recordsince it shows three phase balanced voltages and currents butthe first 4.75 cycles shows that the CB operating mechanismneeds to be maintained as quickly as possible because theclosing of phase A has delayed for about 95ms.Fig. 2 shows another non satisfactory performance of the pre-insertion resistor operation of phase B lasts about 7cycles(140ms), which leads to subjecting the TL to hightransient voltage values, and this, of course, has its bad effecton the TL insulators which may lead to decreasing theirlifetime or performance. The first 6 cycles (120 ms) at thebeginning of the record in Fig. 3 include very importantinformation which could never be noticed without seeing thisrecord. This record gather the phenomena found in the abovetwo cases. The first few cycles show that the CB operatingmechanism need to be maintained as quickly as possiblebecause the closing of phase B has delayed for about 1 cycle(20ms), while phase C is delayed for about 2.5 cycles (50ms). The same record indicates non satisfactory performanceof the pre-insertion resistor operation for phases A and C. Asa result, the TL is subjected to transient voltage values whichreach about double the rated voltage.

Figure (1) Case of Improper Operation of CB Mechanism ofphase

Figure (2) Case of improper operation of pre-insertionresistor of phase B

Figure (3) A Case of Improper Operation of CB Mechanism



In the above mentioned three cases, correctivemaintenance was done as soon as possible based on thepredictive information obtained from the DisturbanceRecorders.

4.2 Circuit Breaker RestrikingCircuit breaker is designed to interrupt the fault current by

mechanical opening of its contacts inside sufficient isolatingmedium. For some reasons, this insulation may become weakand insufficient resulting in restriking of the electric arc of theshort circuit current.

Although the breaker is mechanically totally open, it doesno longer achieve the interruption of the fault current. This isclear in Figure (4-a), where phases R and S are successfullyopened and interrupted the load currents, but the fault currentin phase T still flowing and restriked after interruption asshown inside the highlighted circles. This damage is shown inthe next photographs of the damaged pole as shown in Figs(4-b) and (4-c) . The two photographs give an overview ofhow the moving and the fixed contacts are seriously damagedas burning occurred due to the internal arcing on the movingand fixed contact [5].

From the fault described above, it can clearly be seenprotection plan can be a basic input for maintenance thatan in-depth fault analysis, making use of records and soimprove the overall reliability of the network coming fromDFR’s, together with a well-known

(a) Case of breaker restrike on phase T

(b) Burned Fixed contact (c) Burned Moving contactFigure (4)

5. INSULATION MAINTENANCE

5.1 Intermittent FaultThe phenomenon of intermittent faults, indicate insulation

deterioration. It causes high current spikes as shown in Fig. 5.This record was captured at one of the Egyptian 66/22kVsubstation. The duration of these current spikes and the pause

between their occurrences do not permit the appropriateprotection to operate, or even to sense them. However,disturbance recorders can capture these types of faults asshown. Once this phenomenon has discovered, themaintenance crew can make some investigations and/or teststo find the place where the insulation is deteriorated and takethe appropriate corrective maintenance action before a shortcircuit occurs causing insulation or equipment damage.

This case study proves the capabilities of DisturbanceRecorders as a maintenance tool since it gives earlydetection before serious consequences occur due toinsulation deterioration

Figure (5) Intermittent fault on a 22kV feeder – Phase B

5.2 Fault LocationFault location has two issues[6]. The first is an operating

issue, in which the system should be isolated to preventfurther occurrences. The other is a maintenance issue, as towhat equipment needs checking or repaired in order to returnit to service as soon as possible.

Modern Disturbance Recorders have an integral faultlocator that uses information from the high scanned voltageand current inputs to provide a distance to fault calculation atthe cursor location. Figure 6 shows an actual record that usedfor calculating the fault location on a high voltagetransmission line with high accuracy (one span only).

This accuracy in fault location saves maintenance timeand money since the crew can arrive at the specifiedlocation to take the corrective maintenance actions whichlead to improve the reliability of the system by reducing theoutage time.

Figure (6) Fault Record that used in calculation of faultlocation



6. MONITORING SYSTEM FOR GENERATOR SHAFTTORSIONAL STRESSES

A growing study phenomenon is the failure of generatorshafts and their auxiliary parts caused by high currents atfrequencies below system frequency [7]. Auxiliary parts caninclude the retaining rings, the shaft itself, and the connectionof the blades to the shaft.

These failures are often catastrophic to date, can occur withlittle or no warning. They are often transient and short induration and are mostly caused by fast changes in the load,causing a sub or interharmonic high current. Generallyspeaking, the subharmonic current causes a “twist” in thegenerator shaft which affects the integrity of the shaft and itsrigid components.

The sources of torsional current can be any number ofmechanisms to be found in a typical load profile of anelectricity customer: motor starts high current switching interaction with HVDC interaction with series capacitor compensated

transmission interaction with arc furnaces

These are just a few examples of the sources of thetorsional current. These mechanisms can cause phaseimbalance, negative sequence currents and power swings. Inaddition, some individual transients also lead to mechanicalstresses which are short in duration but very great in strength.Examples of this include: incorrect synchronization when the generator is

brought onto load short-circuits close to power stations

The number of occurrences, the length of time, and theproduced transients are all functions that can cause thedeterioration of the generator faster than anticipated.

To date, most solutions have taken a physicalapproach to the problem, e.g. the installation ofvibration dampers, feedback circuits, etc. Little has beendone in the area of monitoring the electricalcomponents.

The purpose of the study is to introduce a disturbancemonitoring system that measures the subharmonic current andprovides the data used to make decisions about themaintenance of the generator with the aim of preventingcatastrophic damage.

High sample rate of up to 30 kHz can be used for thesecases to show the transient stresses, as shown in figure 7. Bychronologically correlating the data saved using the long-termrecording method to action on the network, it is possible toidentify at which stage in the production process the greateststrain is placed on the generator, or, to ascertain which loadresults in the greatest strain on the generator. Using themaximum values, it is also possible to find out if and whenpermissible limit values have been exceeded.

Figure (7) Transient fault record with 30 kHz sample rate

Figure 8 shows the current flow during a normal Steel Millactivity as rms-envelope. Figs 9, 10 show the results whichcould be obtained from the harmonic analysis of the currentwaveforms. Analysis of the transient faults and the maximumvalues of current are performed. This data provides a high-resolution record of network activity, making it possible tocarry out accurate fault analysis. The shown 6th and 20thsubharmonics, help in taking decisions about themaintenance before catastrophic damage occurrence.

Figure (8) RMS value for current flow during normal steel mill activity



Figure (9) The duration distribution graph of the positive sequence component for 20Hz. The x-axis shows the time (100%means total generator on time) and the y –axis shows the fraction of the pos. sequence component.

Figure (10) The slow fluctuation of the 6Hz component and its average and peak values within one hour.

7. OTHER FIELDS OF APPLICATIONS

A wide horizon for the applications using DisturbanceMonitoring Devices exists. Electric Utilities make efforts tobe proactive, rather than reactive, and, PredictiveMaintenance is considered as the best solution trying toprevent cascading and/or catastrophic failure of equipment.

Large power transformers are a class of very expensive andvital components in electric power systems, so it is veryimportant to minimize the duration of unwanted outages.There are some other fields of applications that DMS can adda value in the predictive maintenance operation of thetransformer. Monitoring of the transformer on-load tap changer's

motor current Correlate Dissolved Gas Analysis (DGA) with fault

information (fault energy) can avoid transformerfailure by scheduling proper predictive maintenance.

And much more

8. CONCLUSIONS

Today, the electric power industry's long-term visioncomes to end the traditional periodic maintenance, and threethings are contributing to breakup: The expansion of condition monitoring

The need to cut costs, and The desire to get the most out of existing equipment

This paper has introduced local and world-wide value eventexamples that prove the capabilities of DisturbanceMonitoring Systems as a tool for improving the overallperformance of the power grid by using them as an essentialtool for maintenance management operations. The advancedmonitoring, recording and analysis gives the user and electricutility valuable tools for improving the efficiency, reliabilityand reducing the cost of maintenance in electric powersystem.

Another important factor for improving the reliability andreducing the cost is the predictive capability of suchequipment, which gives early alarming for a problem to besolved correctively before serious consequences.

Actual examples for: Improper operation of CB mechanism and/or pre-

insertion resistor in EHV transmission system Circuit Breaker restriking Intermittent fault on a distribution level Fault location on high voltage transmission line

Tortional stresses in Generator shaft have proved thecapabilities of DMS as a maintenance management tool fromall its directions: predictive and emergency. Also, prove howthese devices increase the reliability by helping to take



corrective actions (corrective maintenance).It worth mentioned that the predictive maintenance is

considered as main topic for increasing the reliability,availability and optimising the assets in the new trend of theSmart power grid by minimizing the operational andmaintenance cost.

9. REFERENCES

[1] Electrical World, November/December 2000.[2] A Report of the IEEE/PES Task Force on Impact of

Maintenance Strategy on Reliability of the Reliability,Risk and Probability Applications Subcommittee, "ThePresent Status of Maintenance Strategies and the Impactof Maintenance on Reliability," IEEE Transactions onPower Systems, Vol. 16, No. 4, November, 2001

[3] Alexander Bykhovsky, Joe H. Chow, "Dynamic DataRecording in The New England Power System and anEvent Analyzer For The Northfield Monitor," IXSEPOPE symposium, May-23 To 27-2004, Rio DeJaneiro, Brazil.

[4] Samir E. El-Arab, Mohamed A. El-Hadidy, Dalal H.Mostafa,” "Role of The Monitoring Systems InImproving the Performance and Capabilities ofInterconnected Electrical Networks”, Arab LeagueSeminar, April 2005, Cairo, Egypt.

[5] Patrick De Cuyper, "Fault Analysis As A Basic Input forMaintenance Management to Improve the OverallReliability of the Belgian Transmission Grid", 2004Fault And Disturbance Conference Atlanta, April 27 –28, 2004 Georgia, USA

[6] Alexander Apostolov, Rick Taylor, "PredictiveMaintenance Based on Protective Relays Data", 2000Fault And Disturbance Conference Atlanta, Georgia,USA

[7] Michael Jesinghausen , Guy Wasfy, "TorsionalMonitoring System to Detect Torsional Currents forCorrective Action", 2006 Fault And DisturbanceConference Atlanta, Georgia, USA

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