Cahier technique n 192 - studiecd.dk

..........................................................................Collection Technique

Cahier technique no192

Protection of MV/ LV substationtrans forme rs

D. Fulchi ron

Merlin Gerin Modicon Square D Telemecanique

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Didier FULCHIRON

Graduating as an engineer from the “Ecole Supérieure d'Electricité”in 1980, he joined the technical department in Merlin Gerin in 1981 atthe high power testing station. Now in the Medium Voltage Division,he uses his knowledge of public distribution applications to work onexpert assessments, specifications and standards.

n° 192Protection of MV/LV substationtransformers

ECT192 first issued july 1998

Cahier Technique Schneider n° 192 / p.2

Lexicon

Chopped wave: part of an overvoltage wave,generally lightning generated, which continuespropagating beyond arcing in an air gap (sparkgap or insulator breakdown). The high gradientof the downward slope generated by arcing isvery severe for certain equipment.

GRPT: device useable on hermetically sealedimmersed type transformers with integral fillingcombining monitoring features for gas release,pressure and temperature.

Overlaying: technical and/or time-baseddifferences in the users of a network that

enables a maximum power rating to be used thatis much less than the sum of the individualmaximum powers.

Take-over current: value of currentcorresponding to the intersection of the time-current characteristics of two overcurrentprotection devices (VEI 441-17-16).

Transfer current: value of the symmetricalthree-phase current at which the fuses and theswitch exchange the breaking function (in acombined fuse-switch) (IEC 420).


Protection of MV/LV substationtransformers

The choices involved in the protection of MV/LV transformers can appearto be simple since they are often the result of usual practices of electricalnetwork designers, or even of policy dictated by technical and economicconsiderations. In fact, the choices must be made as a function of thetransformer technology, the type of loads that they are supplying, andabove all the external environment that they are subjected to.

This “Cahier Technique” discusses the stresses to which the transformersare subjected during operation and the consequences of these stressesand goes on to present the various protection devices that can be used. Itis necessarily simple, due to the large number of criteria and solutions thatexist. Electrical engineers should however find this document provides themain information needed to make the right choices.

Contents

1. Introduction 1.1 MV/LV transformers and protection policy p. 4

1.2 A review of transformer technology and uses p. 5

2. Operating stress and failure modes 2.1 Energizing and de-energizing p. 7

2.2 External overvoltages p. 7

2.3 Overloads p. 9

2.4 Short-circuits on the LV network p. 10

2.5 Progression of internal faults p. 10

2.6 Faults related to technology types p. 13

3. Overvoltage protection 3.1 General p. 14

3.2 Lightning arrestors and spark gap protection p. 14

4. Overload protection 4.1 Current measurement protection p. 16

4.2 Temperature measurement protection p. 17

5. Protection by MV fuses and 5.1 Characteristics of MV fuses p. 18

5.2 Limits of fuses p. 19

5.3 Using a fuse switch combination p. 21

6. MV circuit-breaker protection, associated 6.1 Trip curve selection criteria p. 22

6.2 Advantages of earthing protection p. 24

6.3 Independent protection devices: TFL and relays p. 24

6.4 Protection devices with auxiliary power supply:GRPT, temperature sensors and relays p. 25

7. Conclusion p. 27

Appendix 1: Rules governing selection of a fuse to protect a transformer p. 29

Appendix 2: Calculating transfer and take-over currents of a fuse switch combination p. 30

Bibliography p. 32

Logic diagram of situations, criteria and solutions

fuse switches combinations

tripping devices


1 Introduction

1.1 MV/LV transformers and protection policy

Why transformers existTransformers are included in distributionnetworks in order to:c minimize energy losses due to the Joule effect;increasing voltage by a factor of 10 reducesthese losses by a factor of 100(Losses = R (Pconsumed / U)2),c minimize voltage drops, both resistive (R) andreactive (X) at the given transmitted power(U I cosφ) (∆ U ≈ R I cosφ + X I sinφ),c and possibly ensure electrical separationbetween networks of the same voltage(boundary limits, changes in the neutralarrangement, etc.).

Even though it is rare to voluntarily interruptpower distribution, transformers neverthelesshave to be “switched” under normal operatingconditions, e.g.:c for network reconfiguration,c for reasons of maintenance and security,c to meet a consumption peak,c to start or stop a process.

These operations are carried out on the transfor-mer either under load or with no load which hasa notable influence on the operating conditionsand the resulting transitory electrical phenomena.Distribution transformers are very reliable passivedevices with a life expectancy of several dozensof years. A Norwegian utility has cited an annualfailure rate of 0.09 % (9 for 10,000), all reasonsincluded, for an equipment base of 5,000 transfor-mers monitored over four years. For undergroundnetworks, the observed failure rate still remainsless than 0.2 %: It can increase to 0.5 % on cer-tain overhead networks. It is often obsolescence- the evolution of the power or voltage levels -which leads to their replacement. Faults in serviceare very rare, but the need to provide safety ofproperty and people as well as continuity of servicenevertheless leads to the use of protection devices.

Stresses suffered by transformersTransformers are subjected to many externalelectrical stresses from both upstream anddownstream. The consequences of any failurecan be very great in terms of damage as well asin terms of operating losses. Transformers musttherefore be protected against attacks of externalorigin on one hand, and isolated from thenetwork in case of internal failure on the otherhand. The term “transformer protection” is veryoften associated with the action of disconnectingfrom the network, even though the transformer isalready failing, and the amalgam is made betweenpreventative measures (overvoltages, downstream

faults, overloads, temperature) and correctivemeasures to isolate the failed transformer.

Protection policyIt is the electrical network designer'sresponsibility to define the measures to beimplemented for each transformer as a functionof criteria such as continuity and quality ofservice, cost of investment and operation andsafety of property and people as well as theacceptable level of risk. The solutions chosenare always a compromise between the variouscriteria and it is important that the strengths andweaknesses of the chosen compromise areclearly identified. E.g., an operator and a utilitycan choose very different solutions for urban andrural network sections since the criteria of unitpower, of cost and the consequences of anincident, are not the same.

The high reliability level of transformers is adecisive factor in the choices made by utilities,faced with the unit cost of the protection deviceswhich can be associated with them. For example,it means that rather than looking to protect thetransformer, in order to save the equipment, weseek to limit the consequences of a failure.

This situation can be illustrated using certaincommonly encountered, although by no meanssystematic, choices such as:c “protection” exclusively targeted to “prevent therisk of explosion and safeguard the MV network”for transformers connected to the publicdistribution network,c temperature monitoring for industrial or tertiarysector installation transformers in which loadshedding arrangements can be implemented,c non-monitoring of overloads for publicdistribution transformers; customer overlayingmaking overloading relatively unlikely and,moreover, load shedding only being possible toconsider in the case of an incident. If thetransformer supplies a uniform group ofcustomers, a need arises for overload protectionsince there is no longer any overlaying.

Since these various choices are always theresult of a technical-economic compromisetogether with policy considerations, it isimpossible to offer a solution providingsatisfaction in every case. Therefore, after brieflyreviewing transformers and their characteristics,we will go on to examine the stresses to whichtransformers can be subjected and the variousmeans of protection. The chosen solutionremains the network designer's responsibility, ona case by case basis.


1.2 A review of transformer technology and uses

Liquid filled or dry-type transformer technologyhas an influence on certain characteristics, onthe protections to be implemented and on thepossible installation locations.It is necessary to know transformers' electricaland thermal characteristics in order tounderstand their behaviour and their resistanceto stresses in operation or fault situations.

Technologies

c In general, liquid filled transformers arehermetically sealed with integral filling.

These transformers are particularly suited to:v unsupervised substations (zero maintenance),v severe environments if the tank is suitablyprotected (active parts protected),v cyclic consumption applications (with goodthermal inertia).

On the other hand, the liquid dielectric has someinherent risks:v ground water pollution (in case of leaks of thedielectric), from which results the obligation, incertain cases, to provide for a back-up retentiontank,v fire (see fig. 1 ) which is why they areprohibited in certain buildings.

These risks are taken into account in the variousregulatory texts and standards concerning theconditions of installation and limits of use.

c “Dry”-type transformers are more appropriatefor:v locations with controlled environments: dust- humidity - temperature, etc. and must beperiodically cleaned and dusted,v buildings, in particular high-rise buildings; sincethey can have good fire behaviour (e.g. class F1according to NF C 52-726) and meet non-toxicityof fumes criteria.

Characteristics

The various rated values are defined by IEC 76(power transformers). Certain electricalcharacteristics are required in order to be able toknow how the transformer withstands stresses inoperation and in fault situations; they are alsodecisive factors in the choice and setting ofprotection devices:

c Rated primary voltage (Ur )Applying IEC standard 71 (insulationco-ordination) enables the insulating voltageand the lightning impulse withstand to beselected (see fig. 2 ).

c Short-circuit voltage (Usc )This enables calculation of the current absorbedby the primary in case of short-circuit across thesecondary's terminals:

II

scn

scU

= 100 %

,

c Retention tanksc Distances or screens to prevent propagationof firec Device to achieve spontaneous extinctionc Automatic de-energizing device on gasreleasec Automatic de-energizing device ontemperature risec Automatic de-energizing and extinctiondevice on fire detectionc Automatic closing of fire doors

Fig. 1 : fire protection devices when using liquiddielectric transformers.

Insulation level 17.5 24 36according to IEC 71Highest voltage for 17.5 24 36the equipmentIndustrial frequency 38 50 70withstand, 1 min.Lightning impulse 75 95, 125 145withstand or 95 or 145 or 170Network operating 12 to 17.5 to 24 tovoltage 17.5 24 36

N.B.: switching impulse withstand is not specifiedbelow 245 kV

Fig. 2 : standard insulation levels (kV).

Rated power Usc according toSn in kVA CEI 76 H426.S1 (Europe)Sn < 50 4 % unspecified50 < Sn < 630 4 % 4 %630 < Sn < 1250 5 % 6 %1250 < Sn < 2500 6.25 % 6 %

Fig. 3 : standard short-circuit voltages for distributiontransformers.

if the source impedance is disregarded. It alsogives the transformer impedance, required tocalculate the short-circuit current when thisoccurs in the LV distribution system:

ZU Usc r

r

= %

100 I.

Short-circuit voltages are standardized and are afunction of transformer power: 4 to 6% for MV/LVtransformers (see fig. 3 ).

c Switching currentIn particularly unfavorable conditions(transformer under no load, large residual fluxand zero voltage tripping with an initial half-waveflux of the same polarity as the residual flux),the magnetic core becomes very saturated, withthe winding taking in up to three times its ratedflux.Due to this saturation, the apparent inductanceof the coil significantly drops, approaching the


behaviour of an air coil (increasing the leakageflux): the resulting current in the winding maytherefore reach very high peak values, up to adozen times the peak rated current, with anextremely distorted current wave form due tosaturation phenomena (see fig. 4 ).

These switching phenomena damp down with atime constant that is dependent on the trans-former, related to its magnetic characteristicsand leakage flux. The time constant is of theorder of a few hundreds of milliseconds fordistribution transformers (a table of numericalvalues is given further on in the document).

Knowing the switching current is necessary todetermine the choice and/or the settings of short-circuit protection devices located on thetransformer's primary.

c Thermal inertia of the transformerThis varies according to transformer type (dry orliquid filled) and power. Knowing it is useful indetermining the overload protection to be used.Readers wishing to gain a more in-depthknowledge of transformers (technology,characteristics, and use) are invited to read thecorresponding “Cahier technique”.

Fig. 4 : profile of making currents with asymmetrical saturation.

t

I


2 Operating stresses and failure modes

2.1 Energizing and de-energizing

Distribution transformer “operation” is limited toenergizing and de-energizing. In publicdistribution, these operations are exceptionaland do not really correspond to the operationaluse. Nevertheless, the transformers areenergized and de-energized during networkcircuit-breaker operation, including duringreclosing cycles. Rapid reclosing can causeenergizing with a strong residual flux, which inturn generates particularly high switchingcurrents.

In industrial or tertiary sector processes, thesame switching operations can be performedsystematically e.g. for process start-up/shut-down or site opening/closing, etc. When the loadconnected to the transformer is controlled,energizing can take place under load or underno-load conditions.

Since the damping of switching currents isrelated to the transformer's magneticcharacteristics (mainly its hysteresis losses), thepresence of a load has little effect on behaviour.

Energizing generally occurs with the loadsconnected. If these themselves have transitoryphenomena, it is the overall behaviour whichmust be taken into consideration. E.g., in thecase of motors units, the transformer's transitorycurrent is superposed on the motor's start-upcurrent, but the duration is significantly differentand the transformer's impedance is dimensionedto limit current demand during the start-upphase. Such well-identified cases must be thesubject of special study. They do not correspondto “distribution” type applications.

Switching currents require monitoring devices(associated current relays and sensors, fuses,etc.) to integrate the idea of time delay in ordernot to generate spurious actions. This aspect isdealt with further in the correspondingparagraphs.

2.2 External overvoltages

Origin and severity

Distribution transformers are subjected totransient overvoltages resulting from thenetworks to which they are connected. Theseovervoltages are either the result of direct orinduced lightning strikes on the MV orLV networks (see “Cahier Technique” n°168:Lightning and MV electrical installations), or oftransmission at MV level of switching over-voltages generated on the upstream network.

During de-energizing by switchgear situatedimmediately upstream, overvoltages can begenerated by the combined transformer- supplycircuit switchgear set leading to a dielectricstress on the transformer. This stress causespremature ageing, or even an insulation fault

between turns or to earth. The most criticalconditions are obtained during the de-energizingof transformers under no load, or by switchingmechanisms capable of breaking high frequencycurrents such as vacuum circuit-breakers. Theuse of such switchgear as a means ofoperational switching should therefore beconsidered with caution.

The criteria determining the severity of theovervoltage for transformers are the peak value,naturally, as well as the voltage rising rate(increase gradient, or decrease gradient in caseof near - by flash-over - “chopped wave”) whichleads to the uneven distribution of stresseswithin the windings and therefore results inexceeding the inter-turns withstand limits even ifthe peak value across the primary winding


terminals does not exceed the accepted values(see fig. 5 ).

Risks of exposure

The risks of exposure to overvoltages for a giventransformer are related to its environment, withcriteria such as:c MV supplied by an overhead or undergroundnetwork,c the eventual presence, sizing and installationconditions of overvoltage limiting devices(lightning arrestors or spark gap protectors),c the length and the type of connectionsbetween the network and the transformer,c the switchgear type and the conditions ofoperation,c the quality of earthing connections and of thedesign of the earthing network at the substationlevel,c overhead or underground LV network,c the earthing of the LV network and its possiblecoupling with the substation's earthing system.

The standard definitions relating to ideas ofinsulation level do not fully cover all stresses thattransformers can be subjected to since certainnetwork phenomena are poorly taken intoconsideration i.e. very steep-gradient transientvoltages.

In practice, assessing the risks of overvoltageremains very global, since the stakesrepresented by an MV/LV transformer do notjustify an in-depth insulation coordination study.Furthermore, it is wise for the network designerto avoid specifying characteristics which mayrequire custom manufacturing. We therefore limitourselves to a choice between standardizedinsulation levels (see fig. 2 ).

Insulation failuresc Internal failures caused by overvoltages canbe observed in the following forms:v insulation faults between the turns in the samewinding (the most frequent case),v insulation faults between windings,v insulation faults between the involved windingand a neighboring conductor (other winding, coreor tank).

The behaviour associated with these two failurecategories is further detailed in the following pages.

c External insulation of immersed transformersis over-dimensioned and cases of dielectricfailure on these transformers are rarelyobserved, except for in certain cases ofoverhead network transformers in particularlypolluted regions. As previously mentioned, dry-type transformers can be subject to externaldielectric failures where there is pollution of theinsulating surfaces.

Fig. 5 : distributed capacitance and stresses along a winding.

Capacitance to earth

Capacitance betweenwindings or layers

V %

40

1 τ (µs)

a) Representative diagram b) Percentage of an impulse wave seen by the firstturns as a function of the rise gradient


2.3 Overloads

GeneralThe acceptable temperature rises in the variousparts of the transformer, taking account of thetemperature rise threshold values provided bythe standards, based on a life-expectancyrelated to the aging of the insulation material, aredefined for continuous operation. A highercurrent value than the rated value correspondsto operation under overload conditions.Maintaining an overload situation leads toexceeding the temperature rise calculated atcertain points on the transformer (according tohow it is built) and, in the instance of a highambient temperature, to the exceeding ofacceptable temperatures.The distinction between temperature rises andtemperatures is important because it enables thecriticality of certain overload conditions to beassessed differently. E.g. an overload related toelectrical heating during the winter in a coldclimate does not have the same consequencesthat an overload of the same level due to airconditioners in a hot climate during the summer.Nevertheless, under abnormal or exceptionaloperating conditions it is acceptable to exceedthe thresholds, possibly to the detriment of thelife-expectancy. This may be preferable tointerrupting service due to a momentary powerpeak.The acceptable overload criteria, such asambient temperature, operating with cyclic loads,etc. are discussed in the “Cahier Technique” ondistribution transformers.

Overloads are often transitory and thermalequilibrium is not affected; the transformersthermal inertia, essential for “oil filled” technologytransformers, enables these high values to besustained, according to a law that is “inverselyproportional to time” (see fig. 6 ).

Acceptable overload currents vary according towhether or not we are interested in steady-stateoperation; simply monitoring a current thresholdin each phase can be unnecessarily penalizing.

Public distributionIn public distribution, overloads do not generallylead to transformer disconnection, continuity ofservice having been given short-term priority.Moreover, low voltage circuits are always over-dimensioned and a transformer overload nevercorresponds with an LV conductor overload. Ifoverload situations are repeated too frequently,the utilities company is led to replace thetransformer by a more powerful model. Certainutilities use current maxi-meters in order to beable to monitor the progression of the peakpower demand on each transformer.

Fig. 6 : order of magnitude of the overload capacity ofan oil filled transformer.

Industrial distributionIn an industrial installation, an overload situationcan be of a short duration, e.g. related to amachine start-up phase, or likely to be prolongedin the case of poor load overlaying. In theseinstallations, the general low voltage switchboardimmediately downstream of the transformer isequipped with circuit-breakers which protectagainst a prolonged overload situation.Management is therefore performed on theLV side, either by load shedding procedures forcomplex installations, or by a general tripping ifno other downstream tripping occursbeforehand.

Tertiary sector distributionIn “large tertiary” sector installations, such asoffice buildings, shopping malls, etc. thecontinuity of service criteria is important. Thereare no periodic loads with start-up arrangementsor similar behaviour. Load shedding is essentialin the case of transformer overload and can beexecuted at the expense of non-priorityapplications, e.g. air conditioning or heatingsystems.The “load shedding” function is increasingly inte-grated in Technical Building Management systems.

5 In

10 In

In

2 In

In

5 s10 s

20 s1 mn

2 mn5 mn

10 mn20 mn

1 h2 h

5 h t


2.4 Short-circuits on the LV network

In case of a fault downstream of the transformer,the impedance of low voltage circuits quicklybecomes preponderant in short-circuit currentcalculations (see “Cahier Technique” n°158:Calculating short-circuit currents), and the onlyfaults representing a significant stress for thetransformer are those located within itsimmediate proximity. These faults are eithermanaged by the LV protection concerned (fusesor circuit-breakers), or by the MV protectionupstream from the transformer in the case of afault upstream of the LV protections.Remember that a transformer having a short-circuit voltage of 5 % has a short-circuit currentof 20 In, with an infinite power source and a lowvoltage short-circuit impedance of zero. Thehypothesis of an infinite power source is oftenrealistic in public distribution, where the unitpower of distribution transformers is low incomparison with the short-circuit power of theMV network. This is not generally the case in

industrial and large tertiary sectors, anddisregarding source impedance imposesunnecessarily elevated stresses for the design ofthe low voltage part of the network and itsassociated protection devices.For transformers, a low voltage fault near to theterminals is translated into thermal stresses,according to the value and the duration of thefault, and mechanical stresses, due to theelectrodynamic effect especially when the faultfirst appears. Transformers are generallydesigned to be able to withstand a short-circuitacross their terminals (infinite source and boltedshort-circuit), corresponding to a situation moresevere than any foreseeable situations duringoperation. Nevertheless, repeated faults canhave a cumulative effect, e.g. coil displacement,and contribute to premature ageing. In any case,the duration of the fault must be limited by aprotection device otherwise it risks leading todestruction by thermal effects.

2.5 Progression of internal faults

Faults between turnsFaults between medium voltage winding turnsare the most frequent failure mode as well asbeing the most difficult to detect.They result from the localized deterioration ofconductor insulation, due to thermal or dielectricstresses. The initial effect is limited to a slightincrease in the primary current, due to the

modification of the transformation ratio on theone hand and the appearance of a short-circuited turn phenomena on the windingconcerned.This faulty turn behaves as a secondary windingand is the seat of a current limited solely by itsown impedance and the resistance at the pointof fault (see fig. 7 ).

Fig. 7 : functioning of a transformer with a short-circuited turn in the primary.

MVn1 n1 n1-1

n2LV

Rfault


According to the current that passes through thisturn, the progression of the fault will be more orless rapid. In the case of high currents, the localtemperature rise will lead to the deterioration ofthe neighboring turns and the fault will quicklyspread. The order of magnitude corresponds toapproximately one hundred times the ratedcurrent or around 1 kA for the primary winding ofa transformer of 400 kVA under 20 kV(CIRED 1991/1.14). In any case, the presence oflocal arcing will lead to a gaseous release,whether or not the transformer is of oil filled ordry type. This release can lead to a largeincrease in pressure, until part of the structureruptures (tank or solid insulation).

If the fault causes a low primary current, thephenomena can be slow and difficult to detectthrough monitoring of the supply current.Laboratory tests on oil filled transformers haveshown current of between 1 and 6 times therated current, accompanied by large gaseousrelease, for faults involving up to 8 % of theprimary turns (CIRED 1991/1.14). This is whymonitoring of gaseous emissions or pressurecan be used in a complementary manner toprotection devices based on currentmeasurement.

Faults between windings

c MV windingsFaults between MV windings are rare but cancause high fault currents, up to the networkshort-circuit current in the case of a fault at theterminals, with significant effects. Certain

locations in particular, such as a fault betweenwindings neighboring neutral point connectionsof a star coupling, are similar to a fault betweenturns since the points coming into contact are notat greatly differing voltages.

c LV windingsFaults between LV windings are exceptionalsince these windings are placed closest to themagnetic core and are surrounded by theMV windings. In the case of multiple LV windingson the same magnetic core column (e.g. zig-zagcoupling), the possibility of a fault exists. In anycase, the fault current remains less than that of ashort-circuit across the secondary terminals, butprogression can be quick due to the presence ofan arc of significant intensity.

c MV/LVA fault between windings can also lead to acontact between the primary and secondary, withthe appearance of a dangerous potential on thelow voltage network (see “Cahier Technique”n°172: Earthing systems in LV). The risk toequipment and people depends on the neutralarrangement of the two networks (see fig. 8 ). Incertain applications, for enhanced safety of thelowest voltage winding, the use of a shieldconnected to earth, positioned between theprimary and secondary windings enables thisfault hypothesis to be eliminated by favoringphase-earth faults. In this case, earthingconnections of the transformer frame and of theLV neutral are different, thus avoiding increasedLV network potential relative to earth.

Fig. 8 : example of a fault between primary and secondary windings.

Non zeroimpedanceearth

Current to earthbetween primary andsecondary windings

Ifault

MV/LVinsulationfault


Faults to earth and the influence of theneutral earthing arrangement

Faults between MV windings and earth mostfrequently originate from a break in insulationfollowing an overvoltage. Nevertheless they canalso be the result of mechanical type faults or theprogression of an electrical fault as previouslyseen. The characteristics of an earthing fault, aswell as the capacity to detect it, depend on thesupply network earthing arrangement and on thelocation of the fault in the transformer(see fig. 9 ).

c In the case of a non-distributed mediumvoltage neutral, connected to earth by animpedance of some sort, the fault will cause acurrent to earth to appear varying as a functionof the neutral impedance and the position of thefault on the winding. In the case of a very lowfault current, there is a risk of a slow increase inpressure similar to that for faults between theturns. Arbitrarily fine detection of the current toearth would be an effective means of protection;nevertheless, such protection is not alwaystechnically and/or economically achievable.

c In the case of a tuned neutral network (earthedby a Petersen coil), an insulation fault in an oilfilled transformer will be of recurring self-extinguishing type. The low value of the faultcurrent enables its spontaneous extinction in theoil and progressive reappearance of the voltage,characteristic of a tuned neutral network, leadingto another breakdown several hundreds of

milliseconds later. The frequency of thephenomena will increase if there is progressivedeterioration by successive breakdowns leadingto a lowering of the dielectric withstand.

c In the case of a neutral network directlyconnected to the earth and distributed (4 wiresnetwork, of North American type), the presenceof neutral current is normal, due to the existenceof single-phase loads, and the appearance of afault will increase this current (as a function ofthe impedance of the winding section not inshort-circuit). The situation is thereforeanalogous with the short-circuitedautotransformer. The fault current will always besignificant and require quick response orotherwise risk resulting in an explosion. It risks,however, not being seen by the network'sprotection devices which are set to allow a largeneutral current (up to 40 % of the line's ratedcurrent). It is therefore the transformer'sprotection which must be able to act.

A significant proportion of faults concern thetransformer's frame, then the ground Protectionagainst earth faults is therefore useful. Thecurrent to earth being zero under normalconditions (except in networks with an earthedand distributed neutral arrangement), suchprotection can be set with a low threshold, e.g.10 % of the rated current with a time delay of100 ms, in cases with current transformers and afew amperes in cases using a residual currentsensor.

Fig. 9 : fault current to earth as a function of MV coupling and the fault position.

100 %

Mid-point Phase

100 %

50 %

Phase Phase


2.6 Faults related to technology types

Fig. 10 : summary of operating stresses and their consequences.

Stress Possible cause Most probable failure Initial signsOvervoltages Nearby lightning strike Breakdown between Gas or smoke release

Network switching MV turns Slight increase in phase currentBreakdown between Current to earthwinding and earth

Slight overcurrent Overload Destruction of windings Gas or smoke releaseImpedent fault on the at hot spots with short Slight increase in phase currentLV network circuiting of turns

Violent overcurrent Nearby LV fault Destruction of windings Quick and random progressionat hot spots with short- towards a fault between windingscircuiting of turns andshifting of windings

Ageing Cumulative effect of Breakdown between Gas or smoke releasepast faults MV turns Slight increase in phase current

Possible progress Current to earthtowards the earth

N.B.: all failure modes, if not remedied in their initial stages, will develop to become generalized in the variouswindings and violent consequences such as rupturing of the tank and/or explosion of the windings possiblyfollowed by a fire.

Internal transformer faults are primarily theconsequence of external stresses (overvoltages,overintensities). We have previously seen thevarious failure modes and the manner in whichthe situation can progress.Nevertheless, other failure modes areforeseeable according to the type of transformertechnology.c Oil filled-type transformersv A dielectric leak not detected in time results inan electrical fault through the loss of insulationabove the coils. Such a leak can be caused forexample by corrosion of the tank or by animpact.v Pollution of the dielectric though the presenceof particles from the tank itself, the core or theinsulation, or water penetration, can also cause asituation of dielectric breakdown. Such pollution isnot usually monitored in distribution transformers.

c Solid insulation transformersv Abnormal mechanical stresses (impacts,efforts to tighten connections, etc.) can crack the

insulation, causing arcing between turns or toneighboring earthing.v Insulation cracking can also be the result ofabnormal thermal ageing related to wrongtransformer use.v Molding imperfections in solid insulation cancreate partial discharge phenomena, if bubblesare present in the insulation in areas with a highelectrical field. This phenomena causes internalbreakdown of insulation material and can lead toa major failure.v The presence of external pollutants (dust) onsuch transformers disturbs the distribution ofsurface dielectric stresses and can causeinsulation faults.v The presence of metallic earthing at a distanceof less than that recommended by themanufacturer can cause excessive local stresson the insulation.

A summary of stresses in operation and theirconsequences is presented in figure 10 .


3 Overvoltage protection

3.1 General

A single feeder supplied transformer, or onepositioned at the opening point of a ring,represents a very high impedance at highfrequency compared with the cable or supplyline's wave impedance. Because of this, duringwave propagation phenomena, the transformerrepresents a point of almost total reflection andthe stress that it is subjected to can reachapproximately twice the maximum voltage of theincident wave. It is essential that limiting devicesare positioned in the immediate vicinity of the

transformer in order to be effective. Thecorresponding order of magnitude is of around adozen meters. Installation conditions, inparticular the length of the connections and theearthing impedance values, have a largeinfluence on protection device performancelevels(see “Cahier Technique” n°151: Overvoltagesand insulation co-ordination HV and MV, and“Cahier Technique” n°168: Lightning and HVelectrical installations).

3.2 Lightning arrestors and spark gap protection

Two means of overvoltage protection are widelyused: spark gap protection and lightningarrestors.Spark gap protection devices are simplest andleast expensive. They are exclusively used onoverhead networks.Lightning arrestors provide protection withgreater performance, but at a noticeably highercost.

Spark gap protection devices

Spark gap protection devices are simplemechanisms comprising two electrodes in air.Voltage limiting across its terminals is achievedby arcing in the air gap.This has a certain number of disadvantagessuch as:c High variations in flash-over level as a functionof environmental conditions (humidity, dust,foreign bodies, etc.).c Dependence of the level of protection inrelation to the steepness of the overvoltagegradient.In fact, air behaves with an “arcing delay” whichmeans that a high overvoltage with very steepgradient does not lead to arcing until reaching apeak value noticeably greater than the desiredprotection level (see fig. 11 ).c The appearance of an earth fault current afterspark gap protection operation.This “follow-up” current, whose intensity dependson the network's neutral earthing arrangement,cannot in general extinguish itself spontaneouslyand requires the intervention of an upstream

Fig. 11 : behaviour of spark gap protection relative to asteep gradient; the more dV/dt increases, the higherthe overvoltage.

protection device. Reclosing performed a fewhundreds of milliseconds later enables service tobe restored.Devices such as the shunt circuit-breakers, forimpedance earthed networks, extinguish the arcand suppress the fault without leading to aninterruption in supply.

t

U

Arcing levelat “sustained”voltage

Overvoltagereached

Incident waveArcing point

Plot of arcingpoints


Lightning arrestors

Lightning arrestors enable this detrimentalbehaviour to be eliminated by having reversiblebehaviour. They are extremely non-linearresistors with a large decrease in internalresistance above a certain terminal voltage value(see figure 12 ). Operational reproducibility ismuch better than with spark gap protections anddelay phenomena are non-existent.

The old silicon carbide (SiC) models are not ableto withstand the operating voltage on a continuousbasis since their residual current is too great andgenerates an unacceptable dissipated power. Theyare therefore associated with serial spark gap pro-tection devices capable of interrupting the residualcurrent and of maintaining the operating voltage.

The more recent zinc-oxide (ZnO) models havemore accentuated non-linearity which enablesthem to have leakage currents less than 10m Aat the operating voltage. Because of this, it ispossible to permanently keep the active partsenergised. Their extreme non-linearity alsoimproves the efficiency of their protection againsthigh currents (see fig. 12 ).

Zinc-oxide lightning arrestors, whose use isbecoming widespread, are available in thereferences suited to use on overhead networks,in cubicles or in extension to the plug-inconection accessories. All installationpossibilities can therefore be covered.

Fig. 12 : example of the characteristic curve of a zincoxide (ZnO) lightning arrestor intended for 20 kVnetworks, insulated to 125 kV “impulse”.

5 kA

10 mA

I

75 kV U15 kV


4 Overload protection

4.1 Current measurement protection

Protection against overloads must act at athreshold of between 110 and 150 % of the ratedcurrent and preferably operate in a timedependant manner. It can be placed on eitherthe MV or LV side.The lower the transformer power, the more thepositioning of the protection on the low voltageside is suitable. As opposed to this, the higherthe power, the more the choice of a protection onthe MV side is wise.

MV side protectionProtection against overloads on the MV side isof interest for high power transformers with anMV circuit-breaker associated with auxiliarysource protection devices. These protections canbe constant time or time dependant. They alsoguarantee protection against high fault currents(i.e. MV fault). In any case, selectivityrequirements with low voltage protection devicesmust be complied with.

LV side protectionLV side protection is easy to achieve with a mainLV circuit-breaker. This type of device employsan inverse time curve (so-called thermal or longdelay) which generally overprotects thetransformer. In fact, the time constant and theinertia taken into consideration to define thiscurve are those of the low voltage ducting, whichis lower than that of the transformer.

In order to protect the transformer, the circuit-breaker is not set as a function of the thermalwithstand of the downstream conductors, as isoften the case in low voltage networks, but as afunction of the rated current of the transformerplaced upstream which is generally lower thanthe rated current of the conductors. If the generalcircuit-breaker is time delayed, in order to ensurethe time-based selectivity with the low voltagefeeders, then selectivity (possibly with mediumvoltage protection) can become difficult. Thissubject is further developed in paragraphsdiscussing medium voltage protection.

Remember that in this type of low voltageprotection scheme, we choose to protect thetransformer against overloads and short-circuitson the low voltage network, without takingaccount of internal failure modes.

In public distribution networks, it is commonpractice to use fuses on low voltage feederswhen the fault current throughout the network issufficiently high. These fuses are rated to onlyoperate during short-circuits between the publiclow voltage network conductors and are notintended to protect the overloaded transformer.The use of fuses, therefore, with quick responserates at high fault currents, makes coordinationeasy with any protection devices on the mediumvoltage side.

One case in particular in overhead publicdistribution is seen when the low voltage networkhas high impedance due to long distances anduse of unshielded conductors. Faults can occur along way from the transformer between phasesor from phase to earth for which the currentremains low, e.g. of the order of 2 or 3 In transfo.

Such a fault situation presents a public hazard atthe fault location, as well as a risk for thetransformer if it persists. These faults are notdetected by the usual short-circuit protectiondevices such as fuses and can justify theadoption of a circuit-breaker “overload”protection capable of responding in this situation.

The release switches associated with such low-voltage circuit-breakers can be equipped with a“thermal imaging” function which tolerates single-phase overloads, if the other phases are hardlyloaded and the resulting temperature within thetransformer remains at an acceptable level. Thisoperating mode is only valid for “oil filled”technology transformers in which the liquiddielectric favors heat exchange between itsvarious components.

This solution is of particular interest in publicdistribution where the increase in loadsconnected to a low power transformer is difficultto optimize. It is used in circuit-breakers intendedto for pole-mounted transformers and therebyhelps eliminate unjustified customer poweroutages. The technology chosen involvesrecreating an interaction by heat exchange in therelease between the three current measuringcomponents - generally positive temperaturecoefficient resistors - as well as an overallthermal inertia which is representative of a


Phase 1 Phase 2 Phase 3Limit case 0 0 2.15Frequent case 0.8 0.8 1.6Without thermal imaging 1.2 1.2 1.2

Fig. 13 : thermal imaging protection - various cases ofpossible unbalanced operation.

4.2 Temperature measurement protection

Temperature control of windings is the mostrelevant action since it is the temperature whichages the insulation.Nevertheless, for temperature rises occurring inenergized sections, the measurement cannotgenerally be taken directly on these points. Theslow rate of temperature variation for thecurrents during overloading, due to the thermalinertia of the transformer, enables themeasurement to be considered representative.A quick rise in winding temperature is normallymanaged by overcurrent detection.

For oil filled type transformers, it is generally thetemperature of the oil that is taken as anindication. In fact, the liquid dielectric functionsas a cooling fluid for the winding and tends toeven out the internal temperature of thetransformer. Temperature measurement can beachieved by a thermostat capable ofindependently supplying information to an output

contact. Two thresholds may be used to definean alarm thresholds, e.g. leading to loadshedding or assisted cooling, and a tripthreshold. This function is included in devicessuch as the “GRPT” described below.

For cast resin transformers, it is not possible toonly take one measurement since thetemperatures can be very different from onewinding to another in the case of imbalance.Moreover, their technology does not lend itself tothe use of thermostats in which the active partsare fairly bulky. Manufacturers offer transformersequipped with platinum sensors, as on certainmedium voltage motors. It is common practice toequip each winding with two sensors, in order tobe able to closely monitor the spots known asbeing the hottest. These sensors are connectedto electronic processing which can manageseveral thresholds used to cause either loadshedding or general circuit-breaking.

protected transformer. For the same maximumhot spot temperature in the transformer, thetripping current in a permanently unsteady statecan thereby be increased to values noticeablygreater than those achieved by independentphase protection. Moreover, taking into accountof thermal inertia enables a more efficient use tobe made of the transformer during temporaryoverloads (see fig.13 ).


5 Protection by MV fuses andfuse switches combinations

For operating requirements - switching,changing fuses, isolating - fuses are installeddownstream of a switching device. Suchswitchgear often takes the form of fuse switches.In this case, the fuses are installed in theswitchgear unit without there necessarily being alink between the melting of the fuses and theoperation of the switch. When the fuse has a

striker capable of opening the switch on melting,the device is then designated by the term “fuseswitch combination”.

5.1 Characteristics of MV fuses

General

Fuses are a very widely used means ofprotecting distribution transformers, mainly dueto their simplicity and the correspondinglyreduced cost of the equipment.Nevertheless, their technological limits leadto a certain number of disadvantages orimperfections which mean that fuseprotection can be considered to be ratherbasic.

Fuses are characterized by their rated current,the highest current value that the fuse canaccept on a continuous basis in an open-airinstallation, and by their current/time fusingcharacteristic.The rated current depends on temperature risecriteria in steady state on the contact surfacesand on the insulating enclosures. It does notcorrespond to melting.There is still a zone of current values betweenthe rated current and the start of melting.A current in this zone generates unacceptabletemperature rises, deteriorating both the fuseand its environment to a greater or lesserextent.Certain fuses integrate temperature sensitivemechanisms intended to trigger the switch in thecase of a fuse switch combination.

Classification of MV fuses

There are two main families of fuses: expulsionfuses and limiting fuses.Expulsion fuses are widely used in North-American type overhead distribution, in unitswhich often provide an automatic disconnectingfunction.

Nevertheless, the fact that they are non-limiting,their limited breaking capacity and especiallytheir external use means they tend to be lessfrequently used.Because of this we will look in more detail atlimiting fuses, such as those defined in theIEC 282.

c Of these fuses, the most common belong tothe “back-up” (or “associated”) category offuses. They provide a minimal breaking current(I3 in the standards) greater than their minimummelting current.

c Fuses in the “general purpose” category aredefined as having a minimal breaking currentsuch that the corresponding melting time isgreater than one hour.

c The fuses in the “full range” categoryguarantee clearing of all melting currents, up tothe short-circuit breaking capacity.These fuses are generally more expensive thanthose in the “back-up” category, which limitstheir use.Moreover, they still enable overheating and donot provide a solution in all installations.

Looking at characteristic fuse curves we canobserve that:

c the minimal fusing current is between 2 and 5times the rated current, according to the types offuses,

c the response time is extremely dependant onthe current, and very variable (current toleranceof ± 10 %). The exact shape of the curvedepends on the type of fuse, and its technology.This time is very low for high currents (greaterthan 20 times the rated current) (see fig. 14 ).


Selection criteria

The ability of limiting fuses to respond withinaround a few milliseconds at high currents istheir main advantage, excepting the cost.This characteristic enables the fuses to provide alimiting effect on the current that is very useful onhigh short-circuit current installations.In fact, the designer can dimension downstreamcircuit conductors and components takingaccount of this limiting effect and, therefore usefault current withstand values less than thenetwork's short-circuit current.This limiting also helps to reduce the destructiveeffects of a major fault.

The rules regarding the selection of fuses, givenby the manufacturers and dependent upon thecharacteristics of each fuse type, cover thefollowing criteria:

c the transformer's operating voltage,

c the switching currents,

c the generally accepted possibility oftemporarily overloading a transformer,

c the need for a near-by low voltage fault(upstream of the LV protection devices) to beeliminated within a sufficiently short period oftime,

c compliance with LV protection selectivity(see fig. 15 ).These criteria are further covered in appendix 1.

Taking into account all of these criteria, as wellas the MV short-circuit current, the installationconditions and the possible need for selectivity,makes the choice of fuses fairly complex.Because of this, a number of installationsoperate with fuses that do not correctly ensurethe protection for which they have beeninstalled.This can result, either in spurious melting duringenergizing or in non-protection due to theunsuitability of characteristics.

Fig. 14 : characteristic curve typical of an “combined” fuse.

Fig. 15 : selectivity between MV fuses and LVprotection devices.

5.2 Limits of fuses

Handling precautions

Fuse technology - metal wires or ribbonsparallel connected in sand - makes themmechanically fragile during handling ortransport.Deterioration is frequently observed due to therupture of one or more conductors, in theabsence of all electrical stresses.The use of a damaged fuse is equivalent tousing an abnormal low fuse rating and quickly

leads to temperature rise phenomena. Suchphenomena can have a disastrous effecton the switchgear and thereby on the wholeinstallation.In order to avoid this type of incident, operatorscan measure resistance just before installation,in order to ensure that the fuse's resistance is inconformity with its specifications and thereforethe fuse does not have a broken conductorelement.

30 s

1 ms

1 h

t

Imin.break

Non

-bre

akin

g zo

ne

Ove

rhea

ting

zone

Nor

mal

ope

ratin

g zo

ne

Imin.melting

In I

Pro

tect

ion

zone

t

MV or LVfault zone

MV fault zone

MV fuseLV fuse

In IMV

LV circuit-breaker


Prohibited operating points

c The “prohibited” operating zone for “back-up”fuses extends from the rated current to theminimal breaking current. In this zone, twosuccessive behaviours can be observed:

v between the rated current and the minimalmelting current, the excessive temperature risescan damage the fuse envelope and itsenvironment within the switchgear;

v between the minimal melting current and theminimal breaking current an arc appears thatdoes not self-extinguish and which quickly leadsto a major medium voltage fault if no otherdevice intervenes.

Because of this, these fuses must be used withcare, only in applications in which the occurrenceof a current of value located in this critical zoneis impossible. If these fault situations arepossible, it is necessary to use the fuse switchcombination. This solution is discussed below.The selection guide, IEC 787, regarding fuseprotection of transformers reviews the variouscriteria.

c “Full range” fuses do not have a minimalbreaking current. Their “prohibited” zone istherefore limited to the current values betweenthe rated current and the minimal meltingcurrent, in order to comply with temperature riselimits. This is not a problematic zone except forsemi-permanent phenomena which can lead todetrimental thermal effects. The order ofmagnitude of the time is one hour.

c In transformer protection applications, faultsare often progressive, based on low currents.This type of fault can subject the protectiondevice to a current that very gradually increasesbeyond the rated current. Such progression, in acircuit protected by fuses of whatever type canbe considered as dangerous due to the fact thatit will systematically take the fuse into the criticalzone. A slowly progressing fault in thetransformer, can result in failure of the device,through overheating or non-breaking of the fuse.E.g: a 400 kVA transformer at 11 kV is protectedby back-up fuses with a 40 A rated current,according to the fuse manufacturer's selectionguide, while the rated current of the transformeris 21 A. The melting curve for such a fuseshows a minimal melting current ofapproximately 100 A with a minimal breakingcurrent of approximately 130 A. In the case of afault between the primary turns, there is a highprobability that this fuse will be required tohandle a dangerous level of current, the minimalbreaking current being to the order of 6 times thetransformer's rated current.

Single phase operation

Assuming only one fuse melts, the transformer isthen supplied by the two remaining phases.

Depending on the transformer coupling, lowvoltage loads will observe a different situation.In the case of a delta-star coupling, two low-voltage phases out of three will find themselvesin a reduced voltage situation and the phasedisplacements no longer complied with.This situation is mainly harmful to three-phasemotors, as well as single phase motorsconnected to the phases with reduced voltage.Other applications can also be affected byreduced voltage, e.g. relay beats or dischargelamps.Separation on a single phase is therefore mostoften a situation to be avoided and can beconsidered as being worse than a completeoutage bv.

Parallel connected transformers

In the case of using parallel connectedtransformers, it is essential to protect them usinga common device.This avoids the re-supply a transformer faultacross the low voltage coupling (see fig. 16 ).

If we want to achieve such protection usingfuses, the above mentioned dimensioning criteriaare applied to select fuses using the currentresulting from both transformers.Because of this, the minimal melting andbreaking currents are seen to increase by afactor of nearly 2, compared with fuses dedicatedto a single transformer.The protection given against internal faults in oneof these two transformers is therefore notablyreduced; There is therefore an increased risk ofthese fuses being subjected to critical over-heating situations or melting below I3.The use of fuse protection is therefore notrecommended in such installations.

Fig. 16 : current circulation after opening of an MVprotection device during a primary fault.


5.3 Using a fuse switch combination

Advantages

Spurious melting, caused by ageing or transitoryphenomena are the main cause of situations inwhich one MV phase is missing.Single phase separation is avoided by the use ofa fuse switch combination, in which the fuses areequipped with a striker. In this type of device thefirst tripped fuse's striker activates the switchmechanism and causes it to open. Theinterruption of supply is therefore across allthree phases whatever be the reason for themelting of the fuse.This operating mode also enables the switch toclear low value fault currents situated in thefuse's prohibited zone (between the minimalmelting current and I3). The risk of the non-breaking of the fuse is thereby eliminated.As opposed to this, since the combination'sswitch does not have a fault breaking capacityup to the short-circuit current, the selection of theswitchgear-fuse pairing must comply withcoordination rules. The objective of these rules isto guarantee that the switch will never be placedin a situation in which it will be incapable ofbreaking. The IEC publication 420 discussesthese criteria.

In the fuse switch combination, we are thereforeseeking to achieve separation of fault situations:c high currents are eliminated by the fuses,using their breaking capacity and limiting effect,c and lower currents are eliminated by theswitch, by the striker or another external order.

Complexity

Among the parameters taken into considerationin deciding on a switchgear-fuse pairing is theability of the switch to interrupt “transfer”currents. Transfer currents are defined as thevalue of the three phase current at which thefuses and the switch exchange the breakingfunction: immediately below this value, thecurrent in the first pole that trips is cleared bythe fuse, and the current in the two other polesby the switch; above this value, the current in allthree phases is cleared by the fuses. Thecalculation of the transfer current is shown inappendix 2.The calculations and the tests performed tocover this situation are all based on assumingconstant fault impedance. This is not necessarilythe case since the fault current is progressiveand may have increased.

The positioning of the transfer current must alsoguarantee that fuses act in fault situationsgenerating severe transient recovery voltages.E.g. for a fault across the transformer's lowvoltage terminals. Certain cases of low voltagefaults between only two phases can, accordingto the transformer coupling, generate criticalsituations not covered by IEC 420.

Limits

The choice of fuse in a fuse-switch combinationfor a transformer protection application mustsatisfy a large number of criteria. Switchgearmanufacturers supply the list of fuses that can beused in their combination (brand, types andratings) for each type of application. In the casewhere these recommendations are not compliedwith, protection may be deteriorated, or safetycompromised according to the faults occurring.The basic rules are further discussed inappendix 1 but alone they cannot guaranteecoverage of all possible fault cases.The overheating zone still exists, for virtually allfuses, and the use of a combined fuse-switchdoes not provide any means of protectionagainst thermal damage if the current ismaintained in this zone. This is why certainmanufacturers offer fuses with an integratedtemperature limiter which, in the case ofabnormal temperature rises, trips the striker andthereby the combination.

Additional protection possibilities

The use of fuse-switch combination can bebeneficial when adding an additional protectiondevice such as earth fault protection or whentaking account of pressure or temperature. Thetime delay must in all cases guaranteecompliance with the combination's take-overcurrent. The take-over current is defined as thecurrent value at the intersection of the time-current characteristic curves of two maximumcurrent protection devices (VEI 441-17-16),therefore being the current value at theintersection of the fuses' curves on one handand the protection device on the other (IEC 420)(see appendix 2).

In conclusion, combined protection is relativelycomplex and involve risks. For this reason, theelectrical installation designer may prefer circuit-breaker protection which he can more easilyassociate with high performance functions.


6 MV circuit breaker protection,associated tripping devices

The use of a circuit-breaker has the mainadvantages of not creating critical currents- the circuit-breaker is capable of breaking allcurrents lower that its breaking capacity - and ofoffering great flexibility in the choice of operatingcriteria. The technical solutions offered are

frequently more costly with circuit-breakers thanwith fuses, fuse switches or fuse-switchecombinations. Nevertheless, certainarrangements, particularly compact Ring MainUnit type devices, offer circuit-breaker solutionsat a unit cost similar to fuse based solutions.

6.1 Trip-curve selection criteria

General

Maximum current protection devices operatewhen the current exceeds a set value for a setperiod of time.So called “time dependant” protection devices,for which the trip time depends upon the value ofthe circulating current, are those most commonlyused.In fact, they make it possible to reconcilelarge time delays in low current zones (overloador “early” internal failure) with fast operation incase of major faults. The current-time curve ofthe relay also guarantees non-tripping duringtransitory phenomena such as inrushcurrents.Several curves are provided in internationalstandards (IEC 255) which have the advantageof offering selectivity between medium voltagecircuit-breakers.Other curves are provided by manufacturers,better suited to the protection of distributiontransformers.

Selectivity

Selectivity involves only tripping the protectiondevice closest to the fault, in order to minimizethe portion of the installation or network takenout of service. In the specific application ofprotecting an MV/LV transformer, selectivitymust be sought relative to the upstreamMV circuit-breaker and, possibly, relative todownstream low voltage protection devices. Inpublic distribution, the circuit-breakerimmediately upstream of the transformerprotection device is a feeder or branching circuit-breaker; its protection parameters are generallygoverned by much higher values and selectivityis achieved without any additional constraint.Selectivity relative to downstream devices is onlyuseful in cases where several low voltageprotection devices are parallel connected. Even

if there is only one LV protection device, a loss ofselectivity does not change the fact that all theLV consumers are disconnected. One cantherefore consider that the MV circuit-breakerand general LV protection device make up asingle selectivity level. In fact, regulatory orcontractual aspects between the utilities and theLV customers mean that operators rarely haveaccess to both circuit-breakers. In privateMV installations, and when the equipment usedpermits, the incorporating of logical selectivitybetween the MV circuit-breaker and generalLV protection device provides considerablesimplification (see “Cahier Technique” n°2:Protection of electrical distribution networks bythe logic selectivity system).

Example

Figure 17 shows the fault current levels in aninstallation. One can observe that the low

Fig. 17 : impedances and fault currents – an exampleof an installation.

15 kV

400 kVA

15.4 A

580 A

In Icc MV Icc LV

400 V

D2

20 m away

28 mΩ

0.2 mΩ

Z

20 kA

11.6 kA

10 kA

309 A

266 A


voltage short-circuit level varies quickly solelydue to the impedance of the conductors.If one considers that the D2 circuit-breaker is notlimiting and that it is set at 5 mohms from thetransformer (15 to 30 meters of LV conductor),the fault current which can immediately beestablished across the downstream terminals isof around 16 times the transformer's ratedcurrent. It is therefore necessary to check thatselectivity is achieved at this current value.The quickest standardized curve (extremelyinverse), set to obtain 20 ms at 20 In, thereforegives a tripping time equal to 31 ms.Selectivity is obtained if the D2 circuit-breakereliminates the fault within 15 ms, to take intoaccount the memory time of MV relays.In the case of complex installations, inindustrial distribution, it is possible that the D2circuit-breaker is itself time delayed at high faultvalues.It is therefore necessary to use an operatingmode on the MV relays which enables timebased selectivity to be guaranteed up to this faultvalue of 16 In (see fig.18 ) or to use logicalselectivity.In the case of public distribution, one never findscascading circuit-breakers without theimpedance between them being fairly significantthus enabling current based selectivity.

Practical solutions

The thresholds available on relays only rarelycorrespond exactly with the rated current of themonitored transformer, which leads to a shift inthe protection curve towards higher currents.This leads to increasing the selectivity margin.Manufacturers can therefore offer curvesdifferent from standardized curves, enablingoperation to be better targeted towards theoperational requirements of transformers.

A dedicated MV transformer protection devicemust meet the following criteria:

c always be quicker than the MV protectiondevice immediately upstream,

c be as quick as possible for current valuesgreater than the low voltage short-circuit current(20 to 25 In transfo depending on Zsc),

c let inrush currents pass (see fig. 19 ),

c be able to guarantee monitoring of theoverload zone, or the zone immediately abovethe overload threshold acceptable to theoperator.

This is what justifies using a curve such as thatillustrated in figure 20 , used in certainSchneider group integrated protection devices.It can be noted that such a curve guaranteesselectivity with any low voltage fuses, the latter

Fig. 18 : co-ordination with a low voltage circuit-breakerby staggering time settings. Overloading is managedon the LV circuit. Internal faults are less well protectedagainst.

P (kVA) Ipeak/In Time cnst (ms)50 15 100100 14 150160 12 200250 12 220400 12 250630 11 300800 10 3001000 10 3501250 9 3501600 9 4002000 8 450

Fig. 19 : making currents relative to the rated current(peak value) in oil filled transformers.

Fig. 20 : trip curve for a relay dedicated to transformerprotection.

t

2 10 20 I/In1Fault across theLV circuit-breaker

LV circuit-breaker

MV circuit-breaker

1 2 5 10 20 50

20 ms50 ms

100 ms0.2 s0.5 s

1 s2 s5 s

10 s20 s

t

Threshold: 1.2 Isetting

I/Isetting


still achieving very rapid fault elimination (of theorder of a few milliseconds) for fault currents

Fig. 21 : operation relative to overload protection and internal faults by an MV circuit-breaker.

6.2 Advantages of earthing protection

The behaviour observed during an internalground fault depends on the MV network neutralearthing arrangement.Residual current detection can cover all or partof these earth faults.Furthermore, detection of residual current is alsosensitive to the faults between primary andsecondary windings, corresponding to earthfaults detected by the upstream network.Such protection is useful for a distributiontransformer, apart from in earthed anddistributed neutral networks.

Its operating threshold must be as low aspossible; in fact there are certain limitationssince:

c it must allow the “normal” residual currents toflow. In fact, in certain network operationsituations, the imbalances of simple voltages - inrelation to earth - can generate a residual currentthat is not zero across the transformer's straycapacitances and any connecting wires. Evenoutside of a fault situation, all parts of the

network have a “natural” capacitive imbalancegenerating a residual current.

c it can be limited by the errors of instrumenttransformers in the case of a summation of threephase current measurements.

The technological limitations of the currenttransformers and protection devices require theuse of threshold detection generally greater than10 % of the rated current to avoid spurioustripping on the occurrence of transitoryphenomena or in short-circuit.

In certain cases, “earthed tank” type detection,which implies being able to install thetransformer insulated from the earth, can beconsidered.Nevertheless, this type of protection posesdifficulties in implementation related to thephysical installation of the transformers and tothe possible distance between these and theprotection device. It is never used for distributiontransformers.

6.3 Independent protection devices: Time Fuse Links (TFL) and relays

In many situations, particularly in publicdistribution and of course in small installations, itis not always conceivable to use an auxiliarypower sypply to achieve protection. In fact, directuse of low voltage from the transformer does notenable a simple response to all fault hypothesesand the presence of an auxiliary source leads toa more expensive installation and unacceptable

maintenance. Several types of protectiondevices without an auxiliary source exist, andfuses belong to this category.

Regarding the opening of a circuit-breaker, onefinds three categories of mechanisms:

c Direct relays, in which the monitored currentactivates the release mechanisms by a thermal

1

t

2 10 20 I/In

LV fuse

MV circuit-breaker

near to the low voltage network's short-circuitcapacity (see fig.21 ).


or magnetic effect, without currenttransformation.

This is the case of many low voltage circuit-breakers, but the direct relays are also suitableto medium voltage devices. They are tendingnevertheless to disappear, basically due to theirsimple nature, their mediocre accuracy and theirlimited adjustment capacity.

c “Time Fuse Links” are mainly used by theBritish (see fig.22 ).Under normal operating conditions, the coil isshort-circuited by a low voltage fuse which isused to determine the protection parameters. Incase of a fault, there is fusing and the currenttransformer's secondary current activates thecoil. This basic principle is simple and efficient.Nevertheless, it implies having replacementfuses and it only offers a choice of limitedcharacteristics related to the fuses' fusingcurves.Earthing protection can be achieved using a coilplaced in the common conductor of currenttransformers. The current normally being zero inthis section, there is no parallel connected fuseon this coil.

c Self powered electronic relays, in which theenergy required for electronic and circuit-breakertripping operation is taken from the sensors'

secondary. These relays are combined with lowenergy release devices, generally with magneticlatching, which requires rearming by the circuit-breaking mechanism itself.These relays are often combined with sensors,specially designed for this type of application,less voluminous and less costly thanstandardized current transformers.The protection chain so formed can beintegrated in a given switchgear, which enablesa global solution to be offered with greaterpossibilities than the direct relay or TFLsolutions.

The performance levels offered cover virtually allinstallation cases, using standardized curves ormanufacturer's curves, and with very widesetting ranges.The principle is nevertheless limited on lowthreshold values, due to the lack of availableenergy in the case of low MV current unlessvoluminous current sensors are used whose costwould be prohibitive.Current limits (1998) for autonomous operationare approximately 10 amperes. Lower “groundfault” threshold values can be used, but will notbe activated unless a load current - phasecurrent - exists above the autonomous operatinglimit.

Fig. 22 : wiring principle for a Time Fuse Links type protection device with two “phase” coils and one“earth” coil.

6.4 Protection devices with auxiliary power supply:GRPT, temperature sensors and relays

When an auxiliary source is decided upon tosupply all or a part of the protection functions, itis possible to use other information than merelymeasurements of electrical values. Themonitored transformer's low voltage can supplythese functions if protection against the short-

circuits is guaranteed by an independentmechanism.Two widely used applications are dedicated tofaults not yet causing a noticeable overcurrentand to overload situations: the GRPT andtemperature sensors.


c The GRPT, standing for “Gaseous Release,Pressure, Temperature” is used for liquid filledand hermetically sealed transformers and iscombined in one single auxiliary devicemonitoring these parameters. It thereforeincludes a pressostat function, a thermostatfunction, possibly with two thresholds, and a floatmechanism which reacts to the abnormalpresence of gas. It can be used for hermeticallysealed immersed transformers. Severalindication contacts are available for the variousevents which can take place (see fig. 23 ).

The function of monitoring gaseous release alsoacts in the case of accidental loss of liquiddielectric, in a preventative manner.

These functions are limited to slow phenomena.Quickly progressing faults requiring quickresponse, still require relaying in terms ofanalyzing electrical values.

c Temperature sensors, generally associatedwith dry transformers, supply accurateinformation on internal thermal stresses. They

are combined with electronic processing whichcan manage various thresholds (overload alarm,load shedding, tripping). This information is usedby the control system to manage the surroundingswitchgear.

Moreover an auxiliary power supply providesaccess to low threshold protection values, tophase or to earth.When a relay supplied by an auxiliary sourceachieves the basic protection functions (includingprotection against short-circuits), it is essential tohave a back-up supply available. Thisguarantees the ability to manage all faultsituations, whatever the LV voltage during thefault. The existence of a back-up source, as wellas the monitoring and maintenance which mustbe associated, are a heavy constraint whichlimits the use of such devices to installationsalready with a back-up supply at their disposalfor another reason. Such relays are therefore notfound apart from in industrial or tertiary sectorsubstations.

Fig. 23 : operation of a GRPT device.

Normal

T° T° T°

T° T° T°

Low drop in level Large drop in level

Internal overpressure Temperature rise(threshold 1)

Temperature rise(threshold 2)

121110

987

654

321

121110

987

654

321

121110

987

654

321

121110

987

654

321

121110

987

654

321

121110

987

654

321


Choice of transformerprotection

Sensors

Installation

Functional Networktechnology

LVoperation

Switchingrequirements,hierarchy ...

Selectivitywith MV

Type of curvesthresholds,

thermal imaging

Bare overhead/braided,underground

Power, insulation ...

Overloads,hazards/safety

Regulations, fire,indoor/outdoor ...

Operation

MV networkneutral voltage ...

Specificationand choiceof transfo

technology ...

Choice ofLV protection

fuses, circuit-breakers ...

7 Conclusion

The choice of distribution transformer protection(MV/LV) is a relatively complex matter since itmust take account of a large number ofparameters and several technical choices maybe suitable and provide the same type ofprotection.The transformer is generally specified initially.However, beyond criteria related to transformerfunctional requirements such as power oroperating voltages or those related to installationconditions (presence of harmonics, risk ofoverload), the user should define his choice interms of the operation and protection policy:c the safety of people and installations orexternal effects in the case of a fault,c continuity of service or life expectancy of theequipment,

c investment cost relative to the probability offault.

Since the protection devices downstream of thetransformer are directly dependent on the type ofLV network and on the load characteristics, theyare normally defined before the upstreamprotection devices.

The choice of protection devices used with thetransformer is made at this moment; an iterativeprocess is then required to ensure theconsistency of the whole system: transformer,LV protection device and MV protection device(see fig. 24 ).

The various protection options are summarisedin the logic diagram shown on the back cover. Itshows the many different interrelations betweenthe technical choices and also illustrates the

Fig. 24 : processus itératif de choix d’une protection transformateur.


Result ⇒ Protection of a healthy transformer Separation of damaged transformer

Situation ⇒ MV Overloads Nearby Internal Internal Majorovervoltages Far LV fault LV fault fault 1 fault 2 MV fault

Device Risk ⇒ Internal Temperature Thermal Progresses Progresses Explosion,⇓ type 1 or 2 fault rise, reduction destruction towards towards fire

in life span (several explosion explosionseconds)

LV fuse

LV circuit-breaker (thermal imagingfor immersed type)

Spark gap

Lightning (znO)

arrestors

MV fuse

Combined IEC 420 (2) (3)

MV circuit-breaker Time dependent

relay

Temperature With combination

or circuit-breaker

Pressure With combination (1) or circuit-breaker(only oil filled)

Type 1 fault: fault to earth of a value less than the rated currentType 2 fault: fault generating a current of a value between one and five times the rated currentMajor MV fault: fault generating a current greater than 5 In(1): overpressure detection can be used for faults generating a gas release, whatever the value of the current(2): by combining with a earth fault relay(3): as long as there is appropriate co-ordination

: risk of fuse failure in these situations

multi-criteria approach required to determinewhich protection device to use. The table infigure 25 provides an overview of the possibletechnical criteria. It highlights the complexity ofthe interactions and the absence of an ideal andabsolute solution.In fact, MV protection devices are an integral partof the switchboards and the choice can be affectedby others criteria. E.g. in choosing whether to usemodular switchboards or compact switchboards,the choice is often made based on criteria not

involved with protecting the transformer, such asthe environment or the upgradability leading to avery different economic positioning for possiblesolutions. Indeed, the use of fuses in compactswitchgear technology implies placing them insealed enclosures which represents significantextra cost. With such technology a circuit-breakersolution becomes particularly competitive. Incontrast, modular switchgear ranges offer fusesolutions which are more economical than thecircuit-breaker solutions.

Fig. 25 : table summarizing the various cases and possibilities of transformer protection.


Appendix 1: Rules governing selectionof a fuse to protect a transformer

The selection guides offered by fuse andswitchgear manufacturers take account of thefollowing rules, for the part that concerns them,as well as any particularities of the switchgear inquestion (confining of fuses modifying theirconditions of cooling for example) (see IEC 787).

Int: rated current of the transformer.

Isc LV : primary current in case of an LV short-circuit.

Inf: rated current of the fuse.

If t( ): current leading to melting in time t (thefuse's characteristic time-current curve).

I3: minimal breaking current of the fuse.

Rules to avoid spurious melting

c Withstand operating current (and possibleoverloads)1.4 < I In nt f .c Withstand inrush currents1 < (0.1s)2 I In ft .

Rule to eliminate a major LV fault

c Act before the destruction of the transformerI If sc LV (2s) < .

Rule for correct functioning of the fuse in the absence of a combination

c Do not operate the fuse in its critical zone.Manage situations with

I I Inf < < 3 using a

complementary means.

Rules for coordination to ensure the correct functioning of a fuse-switch combination

(see IEC 420)ts: minimal opening time of the combined switchcaused by the striker.td: opening time of the combined device underthe action of the tripping device.I4: rated transfer current of the combination.I5: rated take-over current of the combination.

c Do not operate the switch beyond itsperformance levels: transfer current less than therated valueI Itransfer 4 < .See appendix 2 for details of the calculation.

c Do not operate the switch beyond itsperformance levels: transfer current less thancurrent in the instance of a fault across the lowvoltage terminals,I Itransfer sc LV < (this rule does not cover all cases of faults onlyinvolving two phases on the low voltage side).

c Do not operate the switch above itsperformance levels: take-over current less thanthe rated value

1.065 t < f d 5I I . +( )0 02 s .See appendix 2 for the details of this calculation.


Appendix 2: Calculating transfer and take-overcurrents of a fuse switch combination

Transfer current

To characterize the operating limits of acombined device, the search for the most severeconditions leads to considering the followingoperation (see fig. 26 ):c when subjected to a fault current Id, the firstfuse to melt is on the minimal limit of the time-current curve,c the two other fuses are on the maximal limitand are subjected, starting from the moment of

Fig. 26 : determining the transfer current.

Fig. 27 : determining the coefficient at the slope of thefuse melting curve.

Fig. 28 : principle for determining the transfercurrent.

clearing the first phase, to a current with areduced value of 0.87 Id .

IEC standard 420, which discusses thesecombined devices, provides a detailedcalculation which leads to the followingconclusion: the transfer current is the currentcorresponding to a melting time at the minimalcharacteristic equal to

ttI = 0.87 t 1.13 -1s

α α/ ( )[ ]where ts is the opening time of the combineddevice under the action of a striker, and a theslope of the fuse's characteristic time-currentcurve near the point under consideration(see fig. 27 ).An iterative calculation, of a few steps, isgenerally necessary due to the variation of theslope along the characteristic curve. One canuse the ts value as an initial value of t

tI for suchan iteration (see fig. 28 ).

Manufacturer's settings for fuses can vary fromone rating to another within the same range. E.g.within Merlin Gerin's FUSARC range, the

t2

t

t1

Maximumline

Minimumline

Id I0.87 Id

t

ts

Slope

Fuse melting curve

I

Fuse meltingcurve

Combinationopening time ts

Slope ofcurve at ts

Melting time atthe transfer current

Value of thetransfer current

Tolerance ofthe fuse curve


coefficient α varies from 2.2 to 5.2; the transfercurrent for each fuse used in the combinationmust be less than the rated transfer current ofthe combination.

Numerical examples:

Considering a fuse-switch combination equippedwith 80 A/24 kV fuses, where the time of openingafter striker action is 60 ms (ts).If one chooses SIBA fuses, the slope obtainedfrom the characteristic curve is α = 3.32; thisgives a time from melting to transfer equal to:

ttI = 0.87 60 1.13 -1 = 75.5 ms3.32 3.32 / × ( )

or, according to the fusing curves, It = 850 A.Choosing Merlin Gerin fuses, gives α = 3.34which is similar. The transfer current is obtained

from the melting curves, It = 800 A. Both fusesused in a combined RM6 device thereforeprovide equivalent operation.

Now let us consider the same combinationequipped with 125 A/12 kV fuses. In the case ofSIBA fuses, the curves provide us with acoefficient a equal to 3.1 giving a melting time of85 ms. The transfer current is then 300 A. In thecase of Merlin Gerin fuses, the curves give aequal to 2.65, or a melting time of 108 ms. Thetransfer current is therefore only 870 A. In thiscase the choice of fuse strongly influences thedemands which can be placed on the switch ofthe combination, even if both these values canbe acceptable.

Take-over current

The rated take-over current of a combination(designate by I5) is the maximal take-overcurrent acceptable. The switchgear manufacturerprovides the opening time td of the switch underthe action of the release device. All fuses used inthe combination must guarantee compliance withthe rated take-over current (see fig. 29 ).

In the most severe case for a given fuse, it ischaracterized as follows:c “instantaneous” operation of the external relay;the standard proposes using a reaction time of20 ms for such instantaneous operation. Theresulting opening time is therefore the openingtime of the combination under the action of therelease device (td) increased by 20 ms;c a fuse in a cold state and at the maximum ofits tolerances (the standard considers that thetolerance for the melting curves is of ± 10 % ofthe current, enabling the use of a value of twostandard deviations, or ± 6.5 %).The take-over current is taken from thecharacteristic time-current curve under the abovestated conditions, for the melting time oftd + 20 ms.

Fig. 29 : determining the take-over.

t

Take-overcurrrent

Min.relaytime+ 0.02 s

Relaycurve

Fuse curve

I


Bibliography

Others publications

c Trends in distribution transformer protection,Blower / Klaus / Adams, IEE conference, 90/04.

c Tenue des transformateurs en cas de défautsinternes, Raux / Leconte / Gibert, CIRED 89.

c Protection contre les défauts dans lestransformateurs de distribution MT/BT,Bruggemann / Daalder / Heinemeyer / Blower,CIRED 91.

Standards

c IEC 71-1: Insulation co-ordination.

c IEC 71-2: Insulation co-ordination, applicationguide .

c IEC 76: Power transformers.

c IEC 255: Electrical relays.

c IEC 787: Application guide for the selection offuse-links of high-voltage fuses for transformercircuit application.

c IEC 420: High-voltage alternating currentswitch-fuse combinations.

c NF C 52-726: Dry-type power transformers.

Cahiers techniques Schneider

c Overvoltages and insulation co-ordination inMV and HV,Cahier Technique n°151, D. Fulchiron.

c Calculation of short-circuit currents,Cahier Technique n°158, B. De Metz Noblat.

c Lightning and H.V. electrical installations,Cahier Technique n°168, B. De Metz Noblat.

c Earthing systems in LV,Cahier Technique n°172, B. Lacroix andR.Calvas.

Internal faultprotection

Overvoltageprotection

“Single phase”operation protection

LV faultprotection

Overloadprotection

Mixed oroverheadnetwork?

Should allrisk be

eliminated?

Do weaccept external

effects?

Do wewant to favour

upstreamcontinuity?

Is the riskof fuses not

breakingacceptable?

Do we want todisconnect the

transformer as soonas possible?

Cast resintransformer?

Do we wantto save the

transformer?

Is therelow voltageprotection?

Possibilityof faults< 4In?

Is therelow voltageprotection?

Are thereseveral

feeders?

Does therisk exist?

Do we acceptusers to be

cut-off?

Are thereany priority

loads?

Lightningarrestors

Circuit-breaker+ relay

Fuse switchcombination

Full rangefuses

Associatedfuses

Appraisal ofthe cost/riskcompromise

Residual current,temperatureprobes

Dangeroussingle phase

operation

We acceptthe risk ofdestroyingthe transformer

Thermostat,thermalimaging

Thermostat,thermal imaging,max I > on LVprotection

Selectivitybetweenfeeders andupstream level

Load sheddingand selectivitybetweenfeeders andupstream level

Residual current,pressure, gasrelease,thermostat

Suitable LVprotection:max I >>

NOTHING

NOTHING

NOTHING NOTHING

Fig. 30 : logic diagram of situations, criteria and solutions.

Schneider Direction Scientifique et Technique,Service Communication TechniqueF-38050 Grenoble cedex 9Télécopie : (33) 04 76 57 98 60

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