maintenance management of electrical equipment 2

8/10/2019 Maintenance Management of Electrical Equipment 2

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Maintenance Management OfElectrical Equipment (Condition

Monitoring Based) Part 2

Posted AU G 27 20 14byASH OK PA RI KH in ENERGY AND POWER,MAINTENANCEwith 4 COMMENTS

Maintenance Management Of Electrical Equipment (Condition Monitoring Based) Part 2 (photo credit: fluke.com)

Continued from the first part of this technical articleRead i t here.
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Predictive Maintenance Management (PDM)

The concept of Predictive Maintenance Management (PDM) is not new. PDM program is

the maintenance program drawn out for the electrical equipment based on regular monitoring

of the actual physical condition, operating parameters, operating efficiency and otherindicators.

Predictive maintenance is a condition-drivenpreventive maintenance program.

The common premise of PDM is that the data gathered by regular monitoring is technically

analysed. Based on analysis, maximum time interval, which could be kept between repairs,

would be predicted so as to minimize number and cost ofunscheduled outages due to

machine failuresas well as reduce uncalled for scheduled outages for preventive

maintenance and the cost thereof.

The concept of PDM was evolved in order to overcomenumerous difficulties experienced bythe industries while implementing Breakdown Maintenance Management (BMM) and PMM

programs as discussed briefly in foregoing points.

Besides overcoming technical problems, the maintenance practice was required to be made

highly cost effective without jeopardizing the equipment performance, so as to remain

most competitive in the market.

In fact, PDM can be considered as further extension of PMM program. Monitoring of

operating parameters in critical high value equipment has been carried out since long. For

example, monitoring of oil and winding temperatures in the transformers has been a normalpractice under PMM program. Under PDM program, in case of abnormal trend of

temperatures, the maintenance group would estimate when to take shutdown for necessary

maintenance to remove the problem and inform the production group probable date.

The transformer would be attended accordingly so as to avoid major breakdown.
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Transformer predictive maintenance (photo credit: turnkeypower.biz)

If breakdown takes place in between, therepairing job would be carried out under Breakdown

Maintenance Management (BMM).

Similarly, critical machines such as turbines and generators are always provided with highly

reliable control and instrumentation system to continuously monitor operating parameters

such as vibrations, speed, voltage, current, winding temperature, exciter circuits, etc. to

assess the condition of such rotating machines.

However, the condition monitoring systems considered as a part of PDM have their own

limitations in the sense that they are unable to monitor few other vital operating parameters

like internal core temperatures, hot spots on windings, condition of insulating oil, etc.

It is necessary to open the machine and inspectto find out probable damage inside. In

case of transformer, the oil samples are drawn out periodically and got tested at recognisedlaboratory for number of tests to assess the condition of oil.

The short-comings of PMM and PDM programs have led the plant engineers to carry out

further research to upgrade the systems to reduce various short-comings by further

optimisation and make it cost-effective. The improved system has been named as Condition

Monitoring based Maintenance Management so as to differentiate with predictive

maintenance.


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Condition Monitoring based Maintenance Management(CDM)

As discussed elsewhere, condition monitoring based maintenance management (CDM) is

further evolution of preventive and predictive maintenance management.

CDM program comprises of methods which attempt to predict or diagnose problemsin

the electrical equipment based on analysis of findings, which is more or less similar to the

method adopted under PDM. Predictions are usually based on the trending of parameters

and test results.

CDM adds two enormously imp ortant d imensions to PDMas fo l lows.

CDM deals with the entire system as an entity. A major shift from the piecemeal

methodologies of PDM can be seen in this holistic approach to maintenance. The system

can still be implemented one step at a time, and it realizes its greatest potential when

applied consistently and evenly across the entire range of system maintenance concepts.


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Partial discharge test performed on site (photo credit: epowerplus.com)

Another added dimension in CDM program is the concept of ignoring or extending

maintenance intervals. Evaluation and trending techniques under predictive maintenancehave been used historically to confirm maintenance decisions which were taken previously

based on the expert opinions.While CDM system may often find defects under development

not otherwise identifiable, as seen from Figure 1.

One may feel apparently that CDM does little toward reducing the cost of preventive

maintenance, as CDM program may actually increase the maintenance costs marginally for


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some installations due to additional analysis. But it is not so as discussed elsewhere in the

paper.

Consider the simple example discussed hereunder

An insulation resistance test is performed on a 220 kV substation bus. The one (1) minute

test result of 6/1996, corrected for temperature, is 5,225 Meg-ohms. The minimum value

of 5,000 Meg-ohmsis acceptable as per applicable Indian Standards.

The ini t ia l evaluat ion mig ht be to accept the equ ipment and con tinu e to keep in

service.

However, the decision to keep the equipment in service should not be taken based only on

the current value of insulation resistance strength. Any such decision should be based on the

review of trend of insulation resistance over a period of last 3-4 years. The trend is plotted inthe form of graph given in Figure 1.

Reviewing t he trend, however, would un doub tedly lead one to a total ly di f ferent

conc lus ion.

Figure 1 Insulation Resistance Trending Chart

The trend of IR values over a period of time clearly shows a serious deterioration of bus

insulation level. The last two readings, taken six months apart, show a drop from

approximately10,500 Meg-ohms to 5,225 Meg-ohms. This clearly shows that the failure

may take place any time.

Assuming continuance of this trend, the insulation resistance of this bus bar is likely to reach

zero within next one year.


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This simple example clearly establishes the value of plotting the trend and, by extrapolation,

the value of applying condition monitoring techniques to the test results.

Monitoring of operating parameters

As discussed above, this technique is implemented to assess the condition of various parts

of all the electrical equipment. The condition of the equipment is assessed based on

monitoring of various operating parameters and by conducting different condition monitoring

tests.

The operating parameters are monitored generally on-line, whereas, some of the condition

monitoring tests are performed on on-line and some are performed on off-line. Based on the

evaluation of operating parameters and/or test results and plotting their trend, the decision is

taken as to when the maintenance of that equipment should be taken up.

However, th is type of m aintenance would n eed sophis t i ca ted test ing equipment and

ski l ls for analyzing the test resul ts.

maintenance management of electrical equipment 2

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