power transformer 3

8/14/2019 Power Transformer 3

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Power TransformerPower Transformer

ByByDr. Tarek Saad AbdelDr. Tarek Saad Abdel--SalamSalam


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PROTECTION OF TRANSFORMERSPROTECTION OF TRANSFORMERS

LowLow--voltage circuit breakers can be used for thevoltage circuit breakers can be used for thetransformer protection required by NECtransformer protection required by NEC

Section 450Section 450--3. Refer to this section for the specific3. Refer to this section for the specificprimary and secondary protection requirements forprimary and secondary protection requirements for

transformers over 600 V nominal and equal to or lesstransformers over 600 V nominal and equal to or lessthan 600 V nominal. The protection discussed in thisthan 600 V nominal. The protection discussed in thissection of the NEC is intended to protect the transformersection of the NEC is intended to protect the transformeronly.only.

Protection of the primary and secondary conductors mayProtection of the primary and secondary conductors maybe obtained by proper selection of cables.be obtained by proper selection of cables.


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Application considerationsApplication considerations

Considerations for the application of lowConsiderations for the application of low--voltagevoltagecircuit breakers for transformer protectioncircuit breakers for transformer protectioninclude the following:include the following:

a) Will they clear the system for short circuitsa) Will they clear the system for short circuits

within the transformer?within the transformer? b) Will they prevent the transformer fromb) Will they prevent the transformer from

becoming overloaded beyond its ability?becoming overloaded beyond its ability?

c) Will they protect the transformer fromc) Will they protect the transformer fromdamage during a throughdamage during a through--fault condition on thefault condition on theload side?load side?


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d) Do they have adequate interrupting ratingsd) Do they have adequate interrupting ratingsfor faults at their loadfor faults at their load--side terminals?side terminals?

e) Will they handle the transformer inrushe) Will they handle the transformer inrush

current without nuisance tripping?current without nuisance tripping? f) Can they tolerate the current transients duringf) Can they tolerate the current transients during

inrush and during other operating conditions?inrush and during other operating conditions?

g) Do they provide conductor protection?g) Do they provide conductor protection?

h) Is groundh) Is ground--fault protection provided (iffault protection provided (ifrequired)?required)?

Application considerationsApplication considerations


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Transformer w ith a primary rated over 600 VTransformer w ith a primary rated over 600 V

When the transformer primary is over 600 V andWhen the transformer primary is over 600 V andthe secondary is 600 V or less, lowthe secondary is 600 V or less, low--voltagevoltagecircuit breakers might be used as the secondarycircuit breakers might be used as the secondarytransformer protection. The rating of thistransformer protection. The rating of this

secondary protection must not exceed 125% ofsecondary protection must not exceed 125% ofthe transformer rated secondary current, or thethe transformer rated secondary current, or thenext higher standard rating or setting fornext higher standard rating or setting forunsupervised transformer applications per NECunsupervised transformer applications per NEC

Section 450Section 450--3(a)(1); NEC Section 4503(a)(1); NEC Section 450--3(a)(2)3(a)(2)allows 250% of the transformer rated secondaryallows 250% of the transformer rated secondarycurrent forcurrent forsupervisedsupervisedinstallations.installations.


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Transformer primary and secondary rated 600 V orTransformer primary and secondary rated 600 V orbelowbelow

Primary protection onlyPrimary protection only The overload ratings or settings determined by theThe overload ratings or settings determined by the

following paragraph do not necessarily providefollowing paragraph do not necessarily provide

conductor protection. For example, NEC Section 240conductor protection. For example, NEC Section 240--

3(i) states that transformer secondary conductors3(i) states that transformer secondary conductors

(other than two(other than two--wire) are not considered to bewire) are not considered to be

protected by the primary over current protection.protected by the primary over current protection.

Before making the final selection of the circuitBefore making the final selection of the circuit--breaker rating, conductor protection must be verifiedbreaker rating, conductor protection must be verified


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Primary protection onlyPrimary protection only NEC Section 450NEC Section 450--3(b)(1) states that if only primary protection is3(b)(1) states that if only primary protection is

to be used for a transformer of 600 V or less, that protectionto be used for a transformer of 600 V or less, that protectionshall be an individual over current device on the primary side,shall be an individual over current device on the primary side,rated or set at not more than 125% of the rated primary currentrated or set at not more than 125% of the rated primary current

of the transformer as shown in Figure 4of the transformer as shown in Figure 4--20. If the primary20. If the primarycurrent rating of the transformer is less than 9 A, the exceptiocurrent rating of the transformer is less than 9 A, the exceptionsnsallow the over current device to be rated up to, but no moreallow the over current device to be rated up to, but no morethan, 167% of the transformer primary current rating. If thethan, 167% of the transformer primary current rating. If theprimary current rating of the transformer is less than 2 A, theprimary current rating of the transformer is less than 2 A, the

exceptions allow the over current device to be rated up to, butexceptions allow the over current device to be rated up to, butno more than, 300% of the transformer primary current rating.no more than, 300% of the transformer primary current rating.



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Transformers w ith secondary protectionTransformers w ith secondary protection When the transformer has secondary protection, anWhen the transformer has secondary protection, an

individual over current device is not required on theindividual over current device is not required on the

primary side if:primary side if:

a) The over current device on the secondary side isa) The over current device on the secondary side is

rated or set at not more than 125% of therated or set at not more than 125% of the

transformer secondary rating, andtransformer secondary rating, and

b) The primary feeder over current device is rated orb) The primary feeder over current device is rated orset at not more than 250% of the transformerset at not more than 250% of the transformer

primary current ratingprimary current rating



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Other considerations for protecting transformersOther considerations for protecting transformers Selecting the current ratings is only part of theSelecting the current ratings is only part of the

job of protecting the transformer. Transformerjob of protecting the transformer. Transformerdamage curves, current inrush data, overloaddamage curves, current inrush data, overload

capabilities, and information on transientcapabilities, and information on transienttolerances can be obtained from thetolerances can be obtained from themanufacturers of the transformers and IEEEmanufacturers of the transformers and IEEEstandards. Refer to the IEEE C57 Collection andstandards. Refer to the IEEE C57 Collection and

IEEE Std 242IEEE Std 242 --1986. This type of information1986. This type of informationw ill help the designer determine the proper tripw ill help the designer determine the proper tripunit settings.unit settings.

PROTECTION OF TRANSFORMERSPROTECTION OF TRANSFORMERS


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Need for protectionNeed for protection

Transformer failure may result in loss of service.Transformer failure may result in loss of service.However, prompt fault clearing, in addition toHowever, prompt fault clearing, in addition tominimizing the damage and cost of repairs,minimizing the damage and cost of repairs,usually minimizes system disturbance, theusually minimizes system disturbance, the

magnitude of the service outage, and themagnitude of the service outage, and theduration of the outage. Prompt fault clearingduration of the outage. Prompt fault clearingusually prevents catastrophic damage. Properusually prevents catastrophic damage. Properprotection is, therefore, important forprotection is, therefore, important for

transformers of all sizes, even though they aretransformers of all sizes, even though they areamong the simplest and most reliableamong the simplest and most reliablecomponents in the plantcomponents in the plants electrical system.s electrical system.


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Need for protectionNeed for protection


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Causes of FailureCauses of Failure

a)a)WindingWindingbreakdownbreakdown, the most frequent cause, the most frequent causeof transformer failure. Reasons for this type ofof transformer failure. Reasons for this type of

failure include insulation deterioration or defectsfailure include insulation deterioration or defects

in manufacturing, overheating, mechanicalin manufacturing, overheating, mechanicalstress, vibration, and voltage surgesstress, vibration, and voltage surges

b)b)TerminalTerminalboards and noboards and no--load tap changersload tap changers..

Failures are attributed to improper assembly,Failures are attributed to improper assembly,damage during transportation, excessivedamage during transportation, excessive

vibration, or inadequate design.vibration, or inadequate design.


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c)c)BushingBushingfailures.failures.CausesCauses include vandalism,include vandalism,contamination, aging, cracking, and animals.contamination, aging, cracking, and animals.

d)d)LoadLoad--taptap--changerchangerfailures.failures.CausesCauses includeinclude

mechanism malfunction, contact problems,mechanism malfunction, contact problems,insulating liquid contamination, vibration,insulating liquid contamination, vibration,

improper assembly, and excessive stressesimproper assembly, and excessive stresses

within the unit. Loadwithin the unit. Load--taptap--changing units arechanging units arenormally applied on utility systems rather thannormally applied on utility systems rather than

on industrial systems.on industrial systems.



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e)e)MiscellaneousMiscellaneousfailures.failures.CausesCauses includeincludecore insulation breakdown, bushingcore insulation breakdown, bushing

current transformer (CT) failure, liquidcurrent transformer (CT) failure, liquid

leakage due to poor welds or tankleakage due to poor welds or tankdamage, shipping damage, and foreigndamage, shipping damage, and foreign

materials left within the tank.materials left within the tank.



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Failure of other equipment within theFailure of other equipment within thetransformer protective devicetransformer protective devices zone ofs zone of

protection could cause the loss of theprotection could cause the loss of the

transformer to the system. This type of failuretransformer to the system. This type of failureincludes any equipment (e.g., cables, bus ducts,includes any equipment (e.g., cables, bus ducts,

switches, instrument transformers, surgeswitches, instrument transformers, surge

arresters, neutral grounding devices) betweenarresters, neutral grounding devices) between

the next upstream protective device and thethe next upstream protective device and the

next downstream device.next downstream device.



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Objectives in transformer protectionObjectives in transformer protection

Protection is achieved by the proper combination ofProtection is achieved by the proper combination ofsystem design, physical layout, and protective devices assystem design, physical layout, and protective devices as

required to:required to:

a) Economically meet the requirements of thea) Economically meet the requirements of the

application,application,

) Protect the electrical system from the effects of) Protect the electrical system from the effects of

transformer failure,transformer failure,

c) Protect the transformer from disturbances occurringc) Protect the transformer from disturbances occurringon the electrical system to which it is connected,on the electrical system to which it is connected,


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Objectives in transformer protectionObjectives in transformer protection

d) Protect the transformer as much as possible fromd) Protect the transformer as much as possible fromincipient malfunction within the transformer itself, andincipient malfunction within the transformer itself, and

e) Protect the transformer from physical conditions ine) Protect the transformer from physical conditions in

the environment that may affect reliable performance.the environment that may affect reliable performance.


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Protection of different Types of transformersProtection of different Types of transformers

Under the broad category of transformers, two types areUnder the broad category of transformers, two types arewidely used in industrial and commercial power systems:widely used in industrial and commercial power systems:

liquid and dry.liquid and dry. Liquid transformers are constructed to have theLiquid transformers are constructed to have the

essential element, the core and coils of the transformer,essential element, the core and coils of the transformer,contained in the liquidcontained in the liquid--filled enclosure. This liquid servesfilled enclosure. This liquid servesboth as an insulating medium and as a heatboth as an insulating medium and as a heat--transfertransfermedium.medium.

The dry transformers are constructed to have the coreThe dry transformers are constructed to have the coreand coils surrounded by an atmosphere, which may beand coils surrounded by an atmosphere, which may bethe surrounding air, free to circulate from the outside tothe surrounding air, free to circulate from the outside tothe inside of the transformer enclosure. The dry coilsthe inside of the transformer enclosure. The dry coilsmay be conventional (with exposed, insulatedmay be conventional (with exposed, insulatedconductors) or encapsulated (with the coils completelyconductors) or encapsulated (with the coils completely

vacuumvacuum--cast in an epoxy resin).cast in an epoxy resin).


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Preservation systemsPreservation systems

Dry preservation systemsDry preservation systems

Liquid preservation systemsLiquid preservation systems


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Dry preservation systems are used to ensure anDry preservation systems are used to ensure anadequate supply of clean ventilating air at an acceptableadequate supply of clean ventilating air at an acceptableambient temperature. Contamination of the insulatingambient temperature. Contamination of the insulatingducts within the transformer can lead to reducedducts within the transformer can lead to reducedinsulation strength and severe overheating. Theinsulation strength and severe overheating. Theprotection method most commonly used in commercialprotection method most commonly used in commercialapplications consists of a temperatureapplications consists of a temperature--indicating deviceindicating devicewith probes installed in the transformer winding ductswith probes installed in the transformer winding ductsand contacts to signal dangerously high temperature byand contacts to signal dangerously high temperature by

visual and audible alarm. Figure 11visual and audible alarm. Figure 11--1 illustrates this1 illustrates thisfeature.feature.


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The following types of dry systems areThe following types of dry systems arecommonly used:commonly used:

Open ventilatedOpen ventilated

Filtered ventilatedFiltered ventilated

Totally enclosed, nonTotally enclosed, non--ventilatedventilated

Sealed airSealed air-- or gasor gas--filledfilled



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Liquid preservation systems are used to preserve theLiquid preservation systems are used to preserve theamount of liquid and to prevent its contamination by theamount of liquid and to prevent its contamination by thesurrounding atmosphere that may introduce moisturesurrounding atmosphere that may introduce moistureand oxygen leading to reduced insulation strength andand oxygen leading to reduced insulation strength andto sludge formation in cooling ducts.to sludge formation in cooling ducts.

The importance of maintaining the purity of insulating oilThe importance of maintaining the purity of insulating oilbecomes increasingly critical at higher voltages becausebecomes increasingly critical at higher voltages becauseof increased electrical stress on the insulating oil.of increased electrical stress on the insulating oil.


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The sealed tank system is now used almost toThe sealed tank system is now used almost tothe total exclusion of other types in industrialthe total exclusion of other types in industrial

and commercial applications. The followingand commercial applications. The following

types of systems are commonly used:types of systems are commonly used: Sealed tankSealed tank

PositivePositive--pressure inert gaspressure inert gas

GasGas--oil sealoil seal

Conservator tankConservator tank


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Sealed tankSealed tank

The sealedThe sealed--tank design is most commonly used and istank design is most commonly used and is

standard on most substation transformers. As the namestandard on most substation transformers. As the nameimplies, the transformer tank is sealed to isolate it fromimplies, the transformer tank is sealed to isolate it fromthe outside atmosphere.the outside atmosphere.

A gas space equal to about oneA gas space equal to about one--tenth of the liquidtenth of the liquid

volume is maintained above the liquid to allow forvolume is maintained above the liquid to allow forthermal expansion. This space may be purged of air andthermal expansion. This space may be purged of air andfilled with nitrogen.filled with nitrogen.

A pressureA pressure--vacuum gauge and bleeder device may bevacuum gauge and bleeder device may befurnished on the tank to allow the internal pressure orfurnished on the tank to allow the internal pressure orvacuum to be monitored and any excessive staticvacuum to be monitored and any excessive staticpressure buildup to be relieved to avoid damage to thepressure buildup to be relieved to avoid damage to theenclosure and operation of the pressureenclosure and operation of the pressure--relief device.relief device.This system is the simplest and most maintenanceThis system is the simplest and most maintenance--freefree

of all of the preservation systems.of all of the preservation systems.


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PositivePositive--pressure inert gaspressure inert gas

The positiveThe positive--pressure inertpressure inertgas design shown in Figuregas design shown in Figure

is similar to the sealedis similar to the sealed--tanktankdesign with the addition ofdesign with the addition ofa gas (usually nitrogen)a gas (usually nitrogen)pressurizing the assembly.pressurizing the assembly.This assembly provides aThis assembly provides a

slight positive pressure inslight positive pressure inthe gas supply line tothe gas supply line toprevent air from enteringprevent air from enteringthe transformer duringthe transformer duringoperating mode oroperating mode or

temperature changes.temperature changes.Transformers with primaryTransformers with primarywindings rated 69 kV andwindings rated 69 kV andabove and rated 7500above and rated 7500 kVAkVA

and above typically areand above typically areequipped with this device.equipped with this device.


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GasGas--oil sealoil seal

The gasThe gas--oil seal designoil seal design

incorporates a captive gasincorporates a captive gasspace that isolates a secondspace that isolates a secondauxiliary oil tank from theauxiliary oil tank from themain transformer oil, asmain transformer oil, asshown in Figure. Theshown in Figure. The

auxiliary oil tank is open toauxiliary oil tank is open tothe atmosphere andthe atmosphere andprovides room for thermalprovides room for thermalexpansion of the mainexpansion of the main

transformer oil volume.transformer oil volume.


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Conservator tankConservator tank

The conservator tank designThe conservator tank designshown in Figure does notshown in Figure does nothave a gas space above thehave a gas space above theoil in the main tank. Itoil in the main tank. Itincludes a second oil tankincludes a second oil tank

above the main tank coverabove the main tank coverwith a gas space adequatewith a gas space adequateto absorb the thermalto absorb the thermalexpansion of the main tankexpansion of the main tankoil volume. The second tankoil volume. The second tank

is connected to the mainis connected to the maintank by an oiltank by an oil--filled tube orfilled tube orpipe.pipe.


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Protective devices for liquid preservation systemsProtective devices for liquid preservation systems

LiquidLiquid-- level gaugelevel gauge PressurePressure--vacuum gaugevacuum gauge

PressurePressure--vacuum bleeder valvevacuum bleeder valve

PressurePressure--relief devicerelief device


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Liquid-level gauge


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PressurePressure--vacuum gaugevacuum gaugePressurePressure--vacuum bleeder valvevacuum bleeder valve


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PressurePressure--relief devicerelief device


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Mechanical detection of faultsMechanical detection of faults

Two methods of detecting transformer faultsTwo methods of detecting transformer faultsexist other than by electric measurements:exist other than by electric measurements:

a) Accumulation of gases due to slowa) Accumulation of gases due to slow

decomposition of the transformer insulation ordecomposition of the transformer insulation oroil. These relays can also detect heating due tooil. These relays can also detect heating due to

highhigh--resistance joints or due to high eddyresistance joints or due to high eddy

currents between laminations.currents between laminations.

b) Increases in tank oil or gas pressures causedb) Increases in tank oil or gas pressures caused

by internal transformer faults.by internal transformer faults.


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GasGas--accumulator relayaccumulator relay

A gasA gas--accumulator relay, commonlyaccumulator relay, commonlyknown as the Buchholz relay, is applicableknown as the Buchholz relay, is applicable

only to transformers equipped withonly to transformers equipped with

conservator tanks and with no gas spaceconservator tanks and with no gas spaceinside the transformer tank.inside the transformer tank.


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GasGas--detector relaydetector relay

The gasThe gas--detector relaydetector relay

shown in Figure is ashown in Figure is a

special device used tospecial device used to

detect and indicate andetect and indicate an

accumulation of gasaccumulation of gasfrom a transformerfrom a transformer

with a conservatorwith a conservator

tank, eithertank, either

conventional or sealed.conventional or sealed.


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Static pressure relayStatic pressure relay

The static pressure relay can be used on allThe static pressure relay can be used on alltypes of oiltypes of oil--immersed transformers. They areimmersed transformers. They are

mounted on the tank wall under oil and respondmounted on the tank wall under oil and respond

to the static or total pressure.to the static or total pressure.


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Sudden pressure relaysSudden pressure relays

Sudden pressure relays areSudden pressure relays arenormally used to initiatenormally used to initiateisolation of the transformerisolation of the transformerfrom the electrical system andfrom the electrical system andto limit damage to the unitto limit damage to the unit

when the transformer internalwhen the transformer internalpressure abruptly rises. Thepressure abruptly rises. Theabrupt pressure rise is due toabrupt pressure rise is due tothe vaporization of thethe vaporization of theinsulating liquid by an internalinsulating liquid by an internal

fault, such as internal shortedfault, such as internal shortedturns, ground faults, orturns, ground faults, orwindingwinding--toto--winding faults.winding faults.


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Sudden oilSudden oil --pressure relaypressure relay

The sudden oilThe sudden oil--pressure relay is applicable to allpressure relay is applicable to alloiloil--immersed transformers and is mounted onimmersed transformers and is mounted on

the transformer tank wall below the minimumthe transformer tank wall below the minimum

liquid level.liquid level.


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Sudden oilSudden oil --pressure relaypressure relay


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Sudden gasSudden gas--pressure relaypressure relay

The sudden gasThe sudden gas--pressure relay is applicable topressure relay is applicable toall gasall gas--cushioned oilcushioned oil--immersed transformers andimmersed transformers and

is mounted in the region of the gas space. Itis mounted in the region of the gas space. It

consists of a pressureconsists of a pressure--actuated switch, housedactuated switch, housed

in a hermetically sealed case and isolated fromin a hermetically sealed case and isolated from

the transformer gas space except for a pressurethe transformer gas space except for a pressure--

equalizing orificeequalizing orifice


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Sudden gas/ oilSudden gas/ oil --pressure relaypressure relay

A more recent design of the relays is the suddenA more recent design of the relays is the suddengas/oilgas/oil--pressure relay, which utilizes twopressure relay, which utilizes two

chambers, two control bellows, and a singlechambers, two control bellows, and a single

sensing bellows. All three bellows have asensing bellows. All three bellows have a

common interconnecting siliconecommon interconnecting silicone--oil passageoil passage

with an orifice, and an ambientwith an orifice, and an ambient--temperaturetemperature--

compensating assembly is inserted at thecompensating assembly is inserted at the

entrance to one of the two control bellows.entrance to one of the two control bellows.


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Dissolved faultDissolved fault --gases detection devicegases detection device

The dissolved faultThe dissolved fault--

gases detectiongases detection

device can be useddevice can be usedfor continuousfor continuous

monitoring ofmonitoring of

hydrogenhydrogen


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Thermal detection of abnormalitiesThermal detection of abnormalities

Causes of transformer overheatingCauses of transformer overheating

High ambient temperatureHigh ambient temperature

Failure of cooling systemFailure of cooling system

External fault not cleared promptlyExternal fault not cleared promptly

OverloadOverload Abnormal system conditions, such as lowAbnormal system conditions, such as low

frequency, high voltage, nonfrequency, high voltage, non --sinusoidal loadsinusoidal loadcurrent , or phasecurrent, or phase --voltage unbalance.voltage unbalance.


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Undesirable results of overheatingUndesirable results of overheating

The consequences of overheating include theThe consequences of overheating include thefollowing:following: Overheating shortens the life of the transformerOverheating shortens the life of the transformer

insulation in proportion to the duration of the highinsulation in proportion to the duration of the high

temperature and in proportion to the degree of thetemperature and in proportion to the degree of thehigh temperature.high temperature.

Severe over temperature may result in anSevere over temperature may result in animmediate insulation failure.immediate insulation failure.

Severe over temperature may cause theSevere over temperature may cause thetransformer coolant to heat above its flashtransformer coolant to heat above its flashtemperature and result in fire.temperature and result in fire.


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Liquid temperature indicator (top oil)Liquid temperature indicator (top oil)

The liquid temperature indicator measures theThe liquid temperature indicator measures the

temperature of the insulating liquid at the top of thetemperature of the insulating liquid at the top of the

transformer. Because the hottest liquid is less densetransformer. Because the hottest liquid is less dense

and rises to the top of the tank, the temperature ofand rises to the top of the tank, the temperature of

the liquid at the top partially reflects the temperaturethe liquid at the top partially reflects the temperature

of the transformer windings and is related to theof the transformer windings and is related to theloading of the transformer.loading of the transformer.


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Thermal relaysThermal relays

Thermal relays are used to give a more direct indicationThermal relays are used to give a more direct indicationof winding temperatures of either liquid or dryof winding temperatures of either liquid or drytransformers. A CT is mounted on one of the threetransformers. A CT is mounted on one of the threephases of the transformer bushing. It supplies currentphases of the transformer bushing. It supplies currentto the thermometer bulb heater coil, which contributesto the thermometer bulb heater coil, which contributesthe proper heat to closely simulate the transformer hotthe proper heat to closely simulate the transformer hot--spot temperaturespot temperature


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HotHot --spot temperature thermometersspot temperature thermometers HotHot--spot temperature equipment is similar to the thermal relayspot temperature equipment is similar to the thermal relay

equipment on a transformer because it indicates the hottestequipment on a transformer because it indicates the hottest--spotspottemperature of the transformer. While the thermal relay works witemperature of the transformer. While the thermal relay works withthfluid expansion and a bourdon gauge, the hotfluid expansion and a bourdon gauge, the hot--spot temperaturespot temperatureequipment works electrically using a Wheatstone bridge method. Iequipment works electrically using a Wheatstone bridge method. Innother words, it measures the resistance of a resistance temperatother words, it measures the resistance of a resistance temperatureuredetector (RTD) that is responsive to transformer temperaturedetector (RTD) that is responsive to transformer temperaturechanges and increases with higher temperature.changes and increases with higher temperature.


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ForcedForced--air coolingair cooling

Another means of protecting against overloads is toAnother means of protecting against overloads is toincrease the transformerincrease the transformers capacity by auxiliary cooling.s capacity by auxiliary cooling.ForcedForced--airair--cooling equipment is used to increase thecooling equipment is used to increase thecapacity of a transformer by 15% to 33% of base rating,capacity of a transformer by 15% to 33% of base rating,depending upon transformer size and design.depending upon transformer size and design.


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Fuses or over current relaysFuses or over current relays

Other forms of transformer protection,Other forms of transformer protection,such as fuses or over current relays,such as fuses or over current relays,

provide some degree of thermal protectionprovide some degree of thermal protection

to the transformer.to the transformer.


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Over excitation protectionOver excitation protection

OverOver--excitation may be of concern on directexcitation may be of concern on direct--connectedconnected

generator unit transformers. Excessive excitation currentgenerator unit transformers. Excessive excitation currentleads directly to overheating of core and unleads directly to overheating of core and un--laminatedlaminatedmetal parts of a transformer.metal parts of a transformer.

Such overheating in turn causes damage to adjacentSuch overheating in turn causes damage to adjacentinsulation and leads to ultimate failure.insulation and leads to ultimate failure.


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Nonlinear loadsNonlinear loads

Nonlinear electrical loads may cause severeNonlinear electrical loads may cause severeoverheating even when the transformer isoverheating even when the transformer isoperating below rated capacity. This overheatingoperating below rated capacity. This overheatingmay cause failure of both the winding and themay cause failure of both the winding and the

neutral conductor. Electronic equipment such asneutral conductor. Electronic equipment such ascomputers, printers, uninterruptible powercomputers, printers, uninterruptible powersupply (UPS) systems, variablesupply (UPS) systems, variable--speed motorspeed motordrives, and other rectified systems are nonlineardrives, and other rectified systems are nonlinearloads. Arc furnace and rectifier transformers alsoloads. Arc furnace and rectifier transformers alsoprovide power to nonlinear loads.provide power to nonlinear loads.


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The nonlinear load causes transformerThe nonlinear load causes transformeroverheating in three ways:overheating in three ways:

HysteresisHysteresis

Eddy currentsEddy currentsSkin effectSkin effect



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Overheating of neutral conductors fromOverheating of neutral conductors from

nonlinear loads is due to the following:nonlinear loads is due to the following:ZeroZero--sequence and oddsequence and odd--order harmonicsorder harmonics

Skin effectSkin effect



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Failures of transformers due to nonlinearFailures of transformers due to nonlinearloads can be prevented by deloads can be prevented by de--rating therating the

transformer.transformer.



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Protecting the transformer from electricalProtecting the transformer from electrical

disturbancesdisturbances

Transformer failures arising from abusiveTransformer failures arising from abusiveoperating conditions are caused byoperating conditions are caused by

Continuous overloadingContinuous overloading

Short circuitsShort circuits Ground faultsGround faults

Transient over voltagesTransient over voltages


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Overload protectionOverload protection

An overload causes a rise in the temperature of theAn overload causes a rise in the temperature of the

various transformer components. If the final temperaturevarious transformer components. If the final temperature

is above the design temperature limit, deterioration ofis above the design temperature limit, deterioration of

the insulation system occurs and causes a reduction inthe insulation system occurs and causes a reduction in

the useful life of the transformerthe useful life of the transformer Protection against overloads consists of both loadProtection against overloads consists of both load

limitation and overload detection. Loading on thelimitation and overload detection. Loading on the

transformers may be limited by designing a systemtransformers may be limited by designing a system

where the transformer capacity is greater than the totalwhere the transformer capacity is greater than the totalconnected load when a diversity in load usage isconnected load when a diversity in load usage is

assumedassumed


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Over current relaysOver current relays

Transformer overload protection may be provided byTransformer overload protection may be provided by

relays. These relays are applied in conjunction withrelays. These relays are applied in conjunction with CTsCTs

and a circuit breaker or circuit switcher, sized for theand a circuit breaker or circuit switcher, sized for themaximum continuous and interrupting duty requirementsmaximum continuous and interrupting duty requirements

of the applicationof the application


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ShortShort

--circuit current protectioncircuit current protection

In addition to thermal damage from prolongedIn addition to thermal damage from prolonged

overloads, transformers are also adversely affected byoverloads, transformers are also adversely affected byinternal or external shortinternal or external short--circuit conditions, which cancircuit conditions, which canresult in internal electromagnetic forces, temperatureresult in internal electromagnetic forces, temperaturerise, and arcrise, and arc--energy release.energy release.

Protection of the transformer for both internal andProtection of the transformer for both internal andexternal faults should be as rapid as possible to keepexternal faults should be as rapid as possible to keepdamage to a minimum. This protection, however, maydamage to a minimum. This protection, however, maybe reduced by selectivebe reduced by selective--coordination system design andcoordination system design and

operating procedure limitations.operating procedure limitations.


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TimeTime

--current Characteristics (current Characteristics (

TccsTccs

))


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Different types of protectionDifferent types of protection

Over current relay protectionOver current relay protection Over current relays may be used to clear theOver current relays may be used to clear the

transformer from the faulted bus or linetransformer from the faulted bus or line

before the transformer is damaged. On somebefore the transformer is damaged. On somesmall transformers, over current relays maysmall transformers, over current relays may

also protect for internal transformer faults. Onalso protect for internal transformer faults. On

larger transformers, over current relays maylarger transformers, over current relays may

be used to provide backup for differential orbe used to provide backup for differential or

pressure relays.pressure relays.


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Time over current relaysTime over current relays Over current relays applied on the primaryOver current relays applied on the primary

side of a transformer provide protection forside of a transformer provide protection for

transformer faults in the winding, and providetransformer faults in the winding, and providebackup protection for the transformer forbackup protection for the transformer for

secondarysecondary--side faults.side faults.


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Instantaneous over current relaysInstantaneous over current relays Phase instantaneous over current relaysPhase instantaneous over current relays

provide shortprovide short--circuit protection to thecircuit protection to the

transformers in addition to overloadtransformers in addition to overloadprotection. When used on the primary side,protection. When used on the primary side,

they usually coordinate with secondarythey usually coordinate with secondary

protective devices. Fast clearing of severeprotective devices. Fast clearing of severe

internal faults can be obtainedinternal faults can be obtained


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Phase differential relaysPhase differential relays Differential relaying compares the sum ofDifferential relaying compares the sum of

currents entering the protected zone to thecurrents entering the protected zone to the

sum of currents leaving the protected zone;sum of currents leaving the protected zone;these sums should be equal. If more than athese sums should be equal. If more than a

certain amount or percentage of currentcertain amount or percentage of current

enters than leaves the protected zone, a faultenters than leaves the protected zone, a fault

is indicated in the protected zone; and theis indicated in the protected zone; and the

relay operates to isolate the faulted zone.relay operates to isolate the faulted zone.

Phase differential relaysPhase differential relays


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y


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Ground differential relaysGround differential relays

Protection of the transformer by percentage differentialProtection of the transformer by percentage differential

relays improves the overall effectiveness in detectingrelays improves the overall effectiveness in detectingphasephase--toto--phase internal faults. However, linephase internal faults. However, line--toto--groundgroundfaults in afaults in a wyewye winding may not be detected if thewinding may not be detected if thetransformer is lowtransformer is low--resistanceresistance--grounded where groundgrounded where ground

fault current is limited to a low value below thefault current is limited to a low value below thedifferential relay pickup level.differential relay pickup level.



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Protection against over voltagesProtection against over voltages

Transient over voltages produced by lightning,Transient over voltages produced by lightning,

switching surges, switching of power factorswitching surges, switching of power factor

correction capacitors, and other systemcorrection capacitors, and other system

disturbances can cause transformer failures.disturbances can cause transformer failures.

High voltage disturbances can be generated byHigh voltage disturbances can be generated bycertain types of loads and from the incomingcertain types of loads and from the incoming

line. A common misconception is thatline. A common misconception is that

underground services are isolated from theseunderground services are isolated from thesedisturbances.disturbances.


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Surge arrestersSurge arresters

Ordinarily, if the liquidOrdinarily, if the liquid--insulated transformer isinsulated transformer is

supplied by enclosed conductors from thesupplied by enclosed conductors from the secondariessecondaries

of transformers with adequate primary surgeof transformers with adequate primary surge

protection, additional protection may not be required,protection, additional protection may not be required,depending on the system design. However, if thedepending on the system design. However, if the

transformer primary or secondary is connected totransformer primary or secondary is connected to

conductors that are exposed to lightning, theconductors that are exposed to lightning, the

installation of surge arresters is necessaryinstallation of surge arresters is necessary


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Surge capacitorsSurge capacitors

Additional protection in the form of surgeAdditional protection in the form of surge

capacitors located as closely as possible to thecapacitors located as closely as possible to the

transformer terminals may also betransformer terminals may also be

appropriate for all types of transformersappropriate for all types of transformers

FerroresonanceFerroresonance


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is a phenomenon resultingis a phenomenon resulting

in the development of a higher thanin the development of a higher than

normal voltage in the windings of anormal voltage in the windings of a

transformer. These overtransformer. These over--voltages mayvoltages mayresult in surge arrester operation, damageresult in surge arrester operation, damage

to the transformer, and electrical shockto the transformer, and electrical shock

hazard.hazard.



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The following conditions combine to produceThe following conditions combine to produce

ferroresonanceferroresonance:: a) No load on the transformera) No load on the transformer

b) An open circuit on one of the primary terminals ofb) An open circuit on one of the primary terminals ofthe transformer and, at the same time, an energizedthe transformer and, at the same time, an energized

terminal. In the case of threeterminal. In the case of three--phase transformers,phase transformers,either one or two of the three primary terminals mayeither one or two of the three primary terminals maybe disconnected.be disconnected.

c) The location of the point of disconnection if it is notc) The location of the point of disconnection if it is notclose to the transformerclose to the transformer

d) A voltage potential between the disconnectedd) A voltage potential between the disconnectedterminal conductor and groundterminal conductor and ground



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Protection from the environmentProtection from the environment


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Undesirable conditions include:Undesirable conditions include:

a) Average ambient temperatures above 30a) Average ambient temperatures above 30 C when theC when thetransformer is loaded at rated kilotransformer is loaded at rated kilo voltamperevoltampere or moreor more

b) Corrosive agents, abrasive particulate matter, andb) Corrosive agents, abrasive particulate matter, andsurface contaminants derived from the surroundingsurface contaminants derived from the surroundingatmosphereatmosphere

c) Conditions that can lead to moisture penetration or toc) Conditions that can lead to moisture penetration or tocondensation on windings and other internal electricalcondensation on windings and other internal electrical

componentscomponents d) Submersion in water or mudd) Submersion in water or mud

e) Obstruction to proper ventilation of liquid transformere) Obstruction to proper ventilation of liquid transformerradiators or, in the case of dry transformers, ventilatingradiators or, in the case of dry transformers, ventilating

openingsopenings f) Exposure to damage from collision by vehiclesf) Exposure to damage from collision by vehicles

g) Excessive vibrationg) Excessive vibration

h) Exposure to vandalismh) Exposure to vandalism

ConclusionConclusion


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Protection of todayProtection of todays larger and mores larger and moreexpensive transformers can be achievedexpensive transformers can be achievedby the proper selection and application ofby the proper selection and application ofprotective devices. Published applicationprotective devices. Published application

guides covering transformers are readilyguides covering transformers are readilyavailable, for example, ANSI C37.91available, for example, ANSI C37.91--2000.2000.The system design engineer should relyThe system design engineer should rely

heavily on sound engineering judgment toheavily on sound engineering judgment toachieve an adequate protection system.achieve an adequate protection system.

power transformer 3

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