Power Transformer 4

8/14/2019 Power Transformer 4

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Power Transformer

By

Dr. Tarek Saad Abdel-Salam


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Definitions

acceptable

conditions:

The conditions in which an

item is able to perform itsrequired function and/ormeet the relevant

specification.


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Definitions electrical station: building, rooms, or

designated space thathouses the electricalequipment in an

installation.


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Definitions electrical

equipment:

A general term for the

equipment (e.g.,materials, fittings,devices, appliances,

fixtures, apparatus,machines) used as apart of, or in

connection with, anelectric installation.


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Definitions emergency action: Action that should be

taken immediately toavoid seriousconsequence.


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Definitions examination: An inspection with the

addition of partialdismantling as required,supplemented by meanssuch as measurements

and nondestructive testsor high-potential tests, inorder to arrive at areliable conclusion about

the condition of an item.


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Definitions failure (or

breakdown):

The termination of the

ability of an item toperform its requiredfunction.


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Definitions inspection: Maintenance action

comprising a carefulscrutiny of an itemcarried out withoutsignificant dismantling

and using all thesenses as required todetect anything that

causes the item to failto meet an acceptablecondition.


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Definitions item: Any part of

equipment, includinga composite, whichcan be individually

considered.


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Definitions maintenance: A combination of any

actions carried out toretain an item in, orrestore it to, an

acceptable condition.


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Definitions electrical preventive

maintenance:

A system of planned

inspection, testing,cleaning, drying,monitoring, adjusting,corrective modification,

and minor repair ofelectric equipment tominimize or forestallfuture equipment

operating problems orfailures.


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Definitions nonroutine

maintenance:

Unplanned

maintenance that isnot the result of abreakdown.


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Definitions post-fault

maintenance:

Maintenance

necessary onswitchgear after aspecified number of

fault clearanceoperations.


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Definitions preventive

maintenance:

Maintenance carried

out with the objectiveof preventingbreakdown. It may

include routine ornonroutinemaintenance.


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Definitions repair or corrective

maintenance:

Maintenance

necessary to restoreto an acceptablecondition an item that

has ceased to meetan acceptablecondition.


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Definitions routine

maintenance:

Maintenance

organized and carriedout in accordancewith a predetermined

policy or plan toprevent breakdown orreduce the likelihood

that an item will fail tomeet an acceptablecondition.


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Definitions operational check: An action carried out

to determine whetheran item functionscorrectly.


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Definitions test: A measurement

carried out todetermine thecondition of an item.


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Definitions diagnostic testing: A technique involving

the establishing ofcomparative data formonitoring and

checking thecondition ofequipment.


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Definitions overhaul: Maintenance of an

item includingexamination andreplacement or

rebuilding asrequired.


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Definitions major overhaul: An overhaul that

includes majordismantling and/orreplacement of items

to complete themaintenance.


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Definitions minor overhaul or

servicing:

An overhaul that is

limited to lubricationand/or replacement ofconsumables.


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MAINTENANCE TESTING OVERVIEW

Introduction

Maintenance testing is an importantprocedure that is used to detect deficienciesin electrical equipment before the equipment

fails in service


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Introduction

Maintenance testing needs may vary witheach of these categories. Keep in mind thatthe purpose of maintenance testing is to

determine if the equipment will continue toproperly perform its function. In many cases,the testing consists of simulating different

operating conditions and evaluating how theequipment responds.


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Insulation tests

One characteristic that all types of electricalequipment have in common is the use of some formof insulation. At its most basic level, all electricalequipment have some part or parts that conduct

electricity and other parts that do not. A bareoverhead distribution line is held up by insulators andalso utilizes the air around it for insulation.Transformer windings have insulation around eachturn of the conductors and, in some cases, use oil, inaddition, to raise the insulation value between theconductor and the grounded components.



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Insulation tests

The primary factor that determines the level ofinsulation that is required is the operating voltage.Other factors, such as current and frequency, alsoplay a part; however, they are secondary to voltage.

Therefore, the first consideration in testing insulationis whether it can support the required voltage withoutbreakdown. This is accomplished by measuring theleakage current that flows through the insulationmedium when a voltage is applied.



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Insulation tests

There are almost as many types of insulation as thereare different applications. There are several things,however, that they all have in common. Moisture andcontamination decrease an insulators ability to

withstand voltage and increase the amount of leakagecurrent that will flow. The insulation will alsodeteriorate with age. Overheating causesdeterioration to be greatly accelerated. A commonrule of thumb is that the life expectancy of theinsulation is cut in half for every 10 C above itsrating that the equipment operates.



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Insulation tests

Another characteristic that some types ofinsulation have is that, as the voltagerises, the insulation will maintain its

integrity until it reaches the point offailure. Then, near the point of failure, theinsulating capability drops very rapidly,

often accompanied by an arc or puncturein the insulation.



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DC tests

The most common method of testinginsulation integrity is to apply a dc voltage andmeasure the leakage current. The insulationresistance is then determined by dividing thevoltage by the current. There are manycommercially available test instruments thathave specific voltage outputs and provide the

readings directly in ohms. This is referred toas insulation resistance testing, andfrequently as Megger testing or Meggering.



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DC tests For low-voltage equipment, common test voltages are

100 V, 250 V, 500 V, and 1000 V. Test instrumentsare also available that have test voltages of 2500 V,5000 V, and 10 000 V for use on medium-voltageequipment.

For most testing beyond 10 000 V, the test equipment

no longer has a fixed output voltage. This isconsidered high-potential testing, commonly called hi-pot testing; and the voltage is continuously adjustableso that it can be ramped up slowly.

The leakage current is usually measured directlywhen hi-pot testing is being performed, since thevoltage is no longer fixed and getting a direct readoutin ohms would be more difficult.



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Insulation resistance tests

Insulation resistance tests are typicallyperformed on motors, circuit breakers,transformers, low-voltage (unshielded)cables, switchboards, and panel boards todetermine if degradation due to aging,environmental, or other factors hasaffected the integrity of the insulation. This

test is normally conducted for 1 min, andthe insulation resistance value is thenrecorded.


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High-potential testing

High-potential testing, as its name implies, utilizeshigher levels of voltage in performing the tests. It isgenerally utilized on medium-voltage (1000V-69 000V) and on high-voltage (above 69 000 V) equipment

The leakage current is usually measured. In somecases, such as in cable hi-potting, the value ofleakage current is significant and can be usedanalytically. In other applications, such as switchgear

hi-potting, it is a pass/fail type of test, in whichsustaining the voltage level for the appropriate time(usually 1 min) is considered passing.


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High-potential testing

Circuit breaker high-potential testing


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AC tests The most common ac insulation test is ac hi-pot testing at

60 Hz. The capacitance of the insulation is a large factor in ac

testing. With dc insulation testing, the capacitance of theinsulation charges up over time and the residual leakagecurrent is an indication of the resistance of the insulation.

This is not true of ac insulation testing. Since the voltage ischanging at 60 Hz, the leakage current may bepredominantly the capacitive charging current of theinsulation under test.

AC hi-pot testing is usually a pass/fail type test in whichpassing means that the insulation was capable of holdingthe required test voltage, usually for 1 min.


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Power-factor testing dielectric loss angle

(DLA) testingPower-factor testing is a special type of ac

insulation testing. In power-factor testing,

the phase relationship between the appliedtest voltage and the resulting leakagecurrent is determined.


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Power-factor testing dielectric loss angle

(DLA) testingA special test set is used that supplies

voltage and current at 60 Hz .

The volt amperes and watts are measuredwith the test set and the power factor isdetermined.


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Insulation power-factor testing


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Transformer turns ratio (TTR) testing

The voltage across the primary of a transformer isdirectly proportional to the voltage across thesecondary, multiplied by the ratio of primarywinding turns to secondary winding turns.

In order to ensure that the transformer was woundproperly when it was new, and to help locatesubsequent turn-to-turn faults in the winding, it is

common practice to perform a TTR test.


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Infrared scanning

Infrared scanning is a method that is utilized tolocate high-resistance connections (hot spots) byusing a camera that turns infrared radiation into avisible image

This test is performed with the equipment inservice carrying normal load current, which is amajor advantage because it does not interrupt

normal production. Exposure to energizedequipment, of course, carries the possibility ofexposure to electrical hazards.


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Infrared scanning

Infrared inspection


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Oil testing

Many medium- and high-voltage transformers andcircuit breakers utilize different types of oils forinsulation.

Chemical testing of the oil has proven to be a very

dependable method of locating existing orpotential problems

This can be tested on-site by measuring the voltage

at which dielectric breakdown occurs with a specialtest set that is designed for this purpose.


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Grounding testsPower system groundingis for the purpose of

minimizing the electrical hazard that the powerdistribution system poses to people.

Signal reference groundingrefers to the use ofground as a reference for electronic controls and

communication. In many instances, electronicequipment uses its own metal frame or case asthe signal reference.

Lightning protectionis intended primarily todissipate the energy from a lightning strike in amanner that is safe for personnel and that causesthe least amount of equipment damage


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Grounding electrode test

The NEC [B53] uses the term groundingelectrode for the electrical conductor orground rod that is buried in the earth. The

test utilized to determine the resistancebetween a grounding electrode and theearth is called a fall-of-potential test


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Ground resistance testing


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Two-point resistance tests

Two-point resistance tests are used tomeasure the resistance of the equipmentgrounding conductor and its bonding of

the electrical equipment


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Soil resistivity test

Soil resistivity is a measure of resistanceper unit length of a uniform cross-sectionof earth, usually expressed in ohm-

centimetersIt is performed with a four-terminal test set

that uses four equally spaced electrodes,

driven into the ground


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Functional testing

Functional testing consists of simulatingvarious normal and abnormal conditions,and monitoring the system performance

for proper operationThis can be as simple as opening and

closing a circuit breaker electrically


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Testing procedures and specifications

There are many sources of information ontesting and maintenance of electricalequipment.


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MAINTENANCE TESTING OVERVIEW Sources of electrical equipment testing and

maintenance information

a) American National Standards Institute (ANSI);

b) American Society for Testing and Materials(ASTM);

c) Association of Edison Illuminating Companies(AEIC);

d) Institute of Electrical and Electronics Engineers(IEEE);


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Sources of electrical equipment testing andmaintenance information

e) Insulated Cable Engineers Association (ICEA);

f) InterNational Electrical Testing Association (NETA);

g) National Electrical Manufacturers Association(NEMA);

h) National Fire Protection Association (NFPA);

i) Occupational Safety and Health Administration(OSHA).


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NFPA 70B-1998, Electrical Equipment Maintenance

[B54]

The development of NFPA 70B began in

1968 with the Board of Directors of theNational Fire Protection Association whoestablished a committee to develop

suitable texts relating to preventivemaintenance of electrical systems andequipment used in industrial-type

applications with the view of reducing lossof life and property


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NFPA 70B-1998, Electrical Equipment Maintenance

[B54]

The purpose is to correlate generally

applicable procedures for preventivemaintenance that have broad applicationto the more common classes of industrial

electric systems and equipment withoutduplicating or superseding instructions thatmanufacturers normally provide.

Power Transformer 4

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