Discrimination with LV power circuit-breakers · Discrimination with LV power circuit-breakers The purpose of this “Cahier Technique” is to set out the discrimination techniques

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Cahier technique no. 201

Discrimination with LV powercircuit-breakers

JP. Nereau

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Jean-Pierre NEREAU

An IEG graduate engineer, he joined Merlin Gerin in 1981; during histime in the design office of the Low Voltage Switchgear division, hewas involved in developing a number of products, initially the rangeof moulded case circuit-breakers, then the range of powercircuit-breakers.He is currently the Manager of Schneider Electric’s Advanced DesignOffice for this division.

no. 201Discrimination with LV powercircuit-breakers

ECT 201 first issued April 2001

Cahier Technique Schneider Electric no. 201 / p.2

Lexicon

Breaking capacity: This is the usual name forthe ultimate breaking capacity (Icu). Icu is thehighest short-circuit current intensity which thecircuit-breaker is able to interrupt. It is defined fora given rated operating voltage Ue.

Cascading: Using the limiting capacity of theupstream circuit-breaker to increase the actualbreaking capacity of the unit downstream.Enables use of circuit-breakers with a lowerbreaking capacity than the prospectiveshort-circuit current downstream of acurrent-limiting circuit-breaker.

Current limiting circuit-breaker:Circuit-breaker which, when interrupting ashort-circuit current, limits the current to a valueconsiderably less than the prospective current.

DIN: “Déclencheur INstantané”: Instantaneousself-protection release. By assimilation, thecorresponding threshold.

DINF (or MCR): “Déclencheur INstantané à laFermeture” or “Making Current Release”,instantaneous release intended for self-protectionof the breaker during the closing operation.

Electrodynamic withstand (EDW): Capacity ofa unit to tolerate, by nature of its construction,the electrodynamic effects of a short-circuitcurrent, in particular without repulsion of its mainor plug-in contacts.

Isc: Short-circuit current, given as a peak value,actually crossing the circuit-breaker, takingaccount of any limitation.

Icw: Short-time withstand current. This is themaximum short-circuit current (as an rms value),which the circuit-breaker can withstand for adefined period (0.5 or 1 or 3 s) without alterationof its characteristics.

IDMTL: (Inverse Definite Minimum Time Lag)This refers to long-time delay curves where theslope can take different values (see section onIDMTL trip units).

In: Nominal current of the device.

Ip: Prospective short-circuit current which woulddevelop in the absence of protective devices(rms value).

Ir: Current (as an rms value) corresponding tothe overload protection setting. Generally variesfrom 0.4 to 1 times In.

Instantaneous release : Release withoutintentional time delay (short-circuit protection).

Long-time delay release (LT): Release withintentional time delay lasting several seconds(overload protection). This delay is generallydependent on the current.

Partial discrimination: Discrimination is said tobe partial when it is ensured only up to a currentvalue lower than the prospective short-circuitcurrent.

Rating: Current (= In) corresponding to themaximum trip unit setting.

Selective circuit-breaker: Circuit-breaker withhigh Icw (capable of withstanding a short-circuitcurrent for several hundred milliseconds).

Sellim: Discrimination principle which allowsboth discrimination and current limitation.

Short-time delay release (ST): Release with anintentional time delay from tens to hundreds ofmilliseconds.

tc: Actual breaking time (arc suppression).

Total discrimination: Discrimination is said tobe total when it is ensured up to the prospectiveshort-circuit current.


Discrimination with LV powercircuit-breakers

The purpose of this “Cahier Technique” is to set out the discriminationtechniques which apply specifically to low voltage power circuit-breakers.These devices are characterized by their high rating (800 A to 6300 A),and their location at the head of the LV installation, generally directlydownstream of an MV/LV transformer.This location justifies the strict discrimination requirements which apply tothem.This article begins with a resumé of discrimination techniques, followed byan explanation of the links between discrimination and generalcircuit-breaker characteristics. Finally, some practical examples will beprovided on selection of devices to be installed.

Contents

1 LV discrimination 1.1 Introduction p. 41.2 Discrimination according to the type of fault p. 4

2 Discrimination techniques for 2.1 Current discrimination p. 6

2.2 Time discrimination p. 6

2.3 Pseudo-time discrimination p. 7

2.4 "SELLIM" or energy-based discrimination p. 7

2.5 Zone selective interlocking p. 7

2.6 Combining the different types of discrimination p. 8

3 Discrimination with power circuit-breakers 3.1 Circuit-breaker characteristics p. 9

3.2 Trip unit characteristics p. 12

3.3 Discrimination on closing p. 16

4 Examples of circuit-breaker selections for 4.1 Presentation of the installation concerned p. 18

4.2 Dimensioning the protective equipment p. 19

4.3 Selecting breakers to ensure discrimination p. 19

4.4 Variant with zone selective interlocking p. 22

4.5 Variant with two more powerful incoming lines p. 23

Bibliography p. 26

an LV installation

short-circuits


1 LV discrimination

1.1 Introduction

In a radial feeder layout (see fig. 1) the purposeof discrimination is to disconnect only the faultyload or feeder from the network and no others,while keeping as much as possible of theinstallation powered up.

Safety can thus be combined with continuity ofservice, and the fault easily located. It is anespecially important concept for high-powerequipment, since this is generally located at thehead of the installation and therefore has evengreater consequences in the event of falsetripping.

Discrimination is said to be total if it is assuredirrespective of the value of the fault current, up tothe maximum value available in the installation.If this is not the case, it is said to be partial.

The faults encountered in an installation are ofdifferent types:c overloadc short-circuitas well as:c earth fault

c voltage dip or momentary loss of supply

Fig. 1: Discrimination

1.2 Discrimination according to the type of fault

The techniques for using discrimination have tobe adapted to the phenomena involved, andtherefore differ according to the type of fault.

OverloadsThese are currents between 1 and 10 times theduty current. They should be eliminated within aperiod which is compatible with the thermalwithstand of the conductors concerned. The triptime is generally inversely proportional to thesquare of the current (this is known as “inversetime” tripping).

Circuit-breaker discrimination works bycomparing the time/current curves for thelong-time delay releases affected by the fault(see fig. 2)

It is effective if, for any overload current value,the time during which the upstreamcircuit-breaker D1 does not trip is greater thanthe maximum breaking time for the downstreamcircuit-breaker D2 (including the arc suppressiontime). In practice, this condition is achieved if theratio Ir1/Ir2 is greater than 1.6.

For each type of fault there is a specificcorresponding protective device (protectionagainst overload, short-circuit or earth faultcurrents, or against loss of voltage, etc).

Each of these faults can cause a loss ofdiscrimination if coordination of the protectivedevices has not been taken into account.

D1

D2D3

Fig. 2: Discrimination in the overload zone

Overloaddiscrimination zone

I ins2 Ip

tc

Overloads Short-circuits

D2 D1

I r2 I r1


Short-circuits

Because of the magnitude of short-circuitcurrents, and especially the presence ofelectrical arcs which generally accompany them,the circuits concerned should be interruptedalmost instantly, in less than a few hundredmilliseconds.

Discrimination can work, to some extent, bycomparing the time/current curves, provided thattime tc is at least thirty or forty milliseconds.Below that time, these curves are not sufficientlyprecise to reach a verdict with certainty.

Moreover, the time and current are not then theonly discriminating criteria. Depending on thesituation, it may be necessary to take account ofthe peak current, limiting, or a combination oftime and current (for example, i dt2∫ ). It is thennecessary to refer to the discrimination tablespublished by the relevant circuit-breakermanufacturer.

Various techniques can be used to achievediscrimination in the event of a short-circuitbetween 2 circuit-breakers, and these areoutlined in the following section.

Earth leakage currents

Here too, discrimination has to be taken intoaccount so as to prevent an insulation fault atsome point in the installation leading to trippingof the main devices.

There are 2 major protection “families” withregard to leakage currents. For low or very lowcurrent values (typically between 30 mA and30 A), a sensor is used which surrounds all thelive conductors. This sensor naturally adds upthe total current, and provides a signal which isproportional to the fault current. The presence ofan earth (or ground) fault current causes the sumof I1+I2+I3+In to be other than zero.This system is generally known as “residualprotection” or “vigi”.For higher leakage current values, above 20% ofthe nominal current, one sensor is used per liveconductor.

Is2 Is2 Ip

tc

D2 D1

t1

t2

Fig. 3: D1 is selective with regard to D2.

The system, which is called simply “groundfault protection”, adds up all the signals providedby each of these sensors.

In both cases, discrimination works bydifferentiating between thresholds and timedelays. It can be controlled by time/currentcurves (see fig. 3).

Voltage dips or loss of supply

These phenomena can be generated by ashort-circuit in the installation, or by a faultupstream of it, and lead to tripping of the maindevices if they are equipped with anundervoltage trip unit.The solution consists of using time-delayedundervoltage trip units, with a reaction timewhich is longer than the short-circuit trip time ofthe equipment located downstream.

Even without a time delay, undervoltage tripunits should offer immunity againstundervoltages lasting approximately tenmilliseconds, in order that they are not affectedduring short-circuits eliminated by equipmentlocated near the loads.


D2

D1 : Circuit-breaker with short-time bands 0-1-2-3, selective with bands 1, 2 and 3 with D2

D2 : Instantaneous circuit-

breaker with Iins2 threshold

: Time delay obtained with band 1

tc

D1

ICR1Iins2 Ip

0

23

1

2 Discrimination techniques for short-circuits

2.2 Time discrimination

To ensure discrimination above the short-timethreshold (ICR1) of the upstream device, it ispossible to use a time delay, which may or maynot be adjustable, on the trip unit for theupstream device D1 (see fig. 5).

This solution can only be used if the device canwithstand the short-circuit current during thistime delay. It therefore only applies to deviceswith high electrodynamic withstand, which arealso called “selective”.

On two circuit-breakers in series, the differenttime bands, when they exist, are arranged sothat they discriminate between one another.The maximum operating time of one band,including the breaking time, should be less thanthe minimum detection time of the followingband.

Fig. 4: Current discrimination

Discrimination limitfor short-circuits

Iins2 Ip

tc

Iins1

Discrimination zonefor short-circuits

D2 D1

Improving discrimination generally comes downto “restraining” tripping by the circuit-breakerconcerned as opposed to the circuit-breakerslocated downstream in the installation.

This goal can be achieved by:c Creating a difference between the tripthresholds, which is current discrimination

c Delaying - by a few tens or hundreds ofmilliseconds - tripping of the upstreamcircuit-breaker, which is time discrimination

c Using a more sophisticated discriminationcriterion, for example detection of the number ofcurrent waves, or the form of these waves ( idt∫ ,

i dt2∫ , etc), which is “Sellim” or “energy-based” discrimination

c Communicating threshold overshootinformation from one circuit-breaker to the other,which is zone selective interlocking

2.1 Current discrimination

This results from the difference between thethresholds of the instantaneous or short-time delayreleases of circuit-breakers in series in a circuit.

It is applied in the event of short-circuit faults andgenerally leads, unless associated with anothertype of discrimination (time, Sellim orenergy-based), to partial discrimination limited tothe intervention threshold of the upstream device(see fig. 4).

Discrimination is ensured if the maximumthreshold of the trip unit for the downstreamdevice is less than the minimum threshold of thatfor the upstream device, including all tolerances.

Fig. 5: Time discrimination


2.3 Pseudo-time discrimination

If a limiting circuit-breaker is being useddownstream, the magnitude and duration of theactual short-circuit current is significantlyreduced, especially if the prospective current ishigh. The trip unit on the upstream devicetherefore detects a much weaker current than ifthere is no downstream circuit-breaker. This canbe shown on the time/current trip curve for thedownstream device by an “equivalent” time,which diminishes considerably when theprospective short-circuit current increases.

The comparison with the detection curve for thedevice highlights the discrimination between thetwo devices. It is called pseudo-time, since itdoes not use an intentional time delay(see fig. 6).

This solution, with its limiting effect and thespeed with which the fault is eliminated, can alsobe used to limit the thermal and electrodynamicstresses in the installation.

2.4 “SELLIM” or energy-based discrimination

These principles, developed bySchneider Electric, are particularly useful formedium power equipment (100 to 630 A),where current limiting is a necessity. This typeof equipment, with very active electrodynamicrepulsion, cannot even withstand a delay of afew hundred milliseconds. Time discriminationrelating to downstream equipment is thereforeunsuitable, or limited to a very low currentvalue.

The solution consists of using moresophisticated trip criteria than just the value ofthe current or time, generally a combination ofboth these values, for example i dt2∫ . The type

Fig. 6: Pseudo-time discrimination

2.5 Zone selective interlocking

Fig. 7: Zone selective interlocking

of criterion, and the threshold value, are adaptedvery precisely to the upstream/downstreamcombination of equipment under consideration.They can be used to ensure discrimination overseveral stages, while limiting considerably thethermal and electrodynamic stresses on theinstallation.

This discrimination is used in Merlin Gerin’sCompact NS circuit-breakers.

For a more detailed explanation, the reader maylike to refer to “Cahier Technique” no. 167entitled “Energy-based discrimination forlow voltage protective devices”.

This technique requires data transmissionbetween the trip units of the circuit-breakers atthe various levels in the feeder network.

The operating principle is simple (see fig. 7):

c A trip unit that detects a current greater thanits trip threshold sends a logic wait instruction tothe trip unit for the circuit-breaker which is nextupstream. The time delay will be that displayedon the trip unit.

c The trip unit of the circuit-breaker locatedimmediately upstream of the short-circuit doesnot receive a wait instruction and reactsimmediately, regardless of the time delaydisplayed.

Tripunit

Logic waitinstruction

D1

D2Tripunit

Ip

D2

tcD1

D2 : Current limiting

Note: If a short-time dependent release (dotted line) is used on D1, discrimination will be much improved.


Fig. 8: Example of uses for different types of discrimination

Zone selective interlocking is a technique used inaddition to time discrimination. It is used toreduce fault clearing times, which reduces thestress on the installation. It is applied tohigh-amp selective LV circuit-breakers, but it is

also used on HV industrial networks. It requiresthe trip units to be compatible with one another.

For further details, see “Cahier Technique” no. 2entitled “Protection of electrical distributionnetworks by the logic selectivity system”.

2.6 Combining the different types of discrimination

The choice of a type of discrimination in anelectrical feeder network depends on the type ofdevice and their location in the installation.Different techniques can be combined betweentwo devices in order to obtain the best

availability of electrical energy; see example infigure 8.

Current discrimination is, without exception, thefirst link in the discrimination chain.

Circuit Type of discrimination Type ofconcerned Current Time + zone Time Pseudo Sellim and circuit-

selective int. time energy-based breaker

Head of Selectiveinstallation

Powerdistribution Limiting

Final Limitingdistribution


LV power circuit-breakers, due to their positionat the head of the installation, are especiallyconcerned by discrimination requirements.

Their natural robust nature means that timediscrimination is mainly used in the event of ashort-circuit. This does not exclude the additionaluse of “pseudo-time” discrimination (currentlimiting circuit-breaker downstream of a selectivecircuit-breaker), and zone selective interlocking

3.1 Circuit-breaker characteristics

Short-time withstand current (Icw)The short-time withstand current (Icw)characterizes the capacity of devices to withstandshort-circuit currents, which may be very high, fora sufficient period for them to be eliminated bycircuit-breakers or protective devices locateddownstream. It is therefore an essentialcharacteristic for power circuit-breakers which arealways found at the head of the installation.

The higher the Icw, the higher the usage limit fortime discrimination. This is why devices with highIcw are often known as “selective” devices. It is,of course, essential that the switchboard wherethe device is installed, and all the conductorslocated upstream, are capable of withstandingsuch currents.

c ConstraintsShort-circuit currents generate 2 types ofphenomenon:

v Electrodynamic forces between the variousparts of the circuit conducting the current:These forces may be either repulsion orattraction depending on the respective directionof the currents; they appear instantly, and theresistance of the device to these forces, called“electrodynamic withstand” (marked EDW) willtherefore be characterized by the maximuminstantaneous value of the current it canwithstand, measured in “peak” kA.Above this value, parts may be irreversiblydeformed, or electrical arcs may be producedwhich could damage the parts concerned.

v Temperature rise in the parts conducting thecurrent:This temperature rise is not a function of theinstantaneous value of the current, but of its rmsvalue and its duration; the device withstand cantherefore be expressed in kArms and in seconds.

(logical wiring between the various levels in thefeeder network).

In this section we will examine the characteristicswhich have an influence on this discrimination,first considering those of the circuit-breaker itself,and then those of the trip unit fitted on it.

The special case of discrimination oncircuit-breaker closing is then analyzed, alongwith the characteristics which determine it.

The “short-time withstand current” is defined by anumber of standards, including IEC 60947-2which has allocated it the symbol “Icw”. Theassociated test can be used to test the behaviorof the device both from the electrodynamic pointof view, when the short-circuit occurs, and fromthe thermal point of view, since the current ismaintained for a predefined period (usually0.5 s, 1 s or 3 s). Since the maximum peakcurrent is fixed by the standard as a function ofthe rms current, if this is known, the Icw can bedefined.It is clear that the Icw is limited by the mostsevere phenomena, whether electrodynamic orthermal, and its value therefore often diminisheswhen the associated time increases: an Icwlasting 3 s is thermally 9 times more restrictivethan an Icw lasting 1 s.

The Icw value to be taken into account fordiscrimination is that which corresponds to themaximum time setting for the short-time delayrelease, generally 0.5 s. As this value is usuallydetermined directly by the electrodynamicwithstand, the thermal stress is easily controlled.Values at 1 s, or even 3 s, are only an indicationof extra robustness in this case.

c Construction recommendations to obtain agood Icw

All these requirements specify:

v Robust and rigid device construction, whichholds the current-carrying parts firmly in place;compared to the old construction techniquesbased on metal parts which were cut, bent andthen assembled, the use today of thermosettingpolyester moulded cases offers a notableimprovement in the structural rigidity ofcircuit-breakers.

3 Discrimination with power circuit-breakers


v Excellent rigidity of the mechanism to keep thecontacts in the closed position

v Special arrangement of the moving contactsand disconnecting contact fingers (see fig. 9) toensure automatic compensation of the repulsionforces generated between the contact points:- The disconnecting contact fingers are locatedon either side of the conductors to be linked; theparallel currents circulating in these fingerscreate an attraction force Fm whichcompensates the repulsion forces Fr generatedat the contacts (fig. 9a).- The moving contacts incorporate a hinge pinlocated approximately one-third of the distancebetween the incoming conductors. Therefore,the result of the repulsion forces Fm producedby the current loop creates a torque on thecontacts which compensates that generated bythe repulsion Fr at the contact points (fig. 9b).Compensation of these forces does howeverhave the effect of increasing the forcestransmitted to the mechanism, which constitutesa restriction for the manufacturer.

v Generous dimensioning of the power circuitcross-section, so as to avoid reaching anexcessive temperature when the time delay onthe trip unit is set to maximum

v Use of thermosetting moulded materials (withno melting point), or special thermoplastics witha high melting point, near the power circuit

Breaking capacity

In order to use a circuit-breaker on a givencircuit, its ultimate breaking capacity (Icu) mustbe higher than the prospective short-circuitcapacity of this circuit at the point underconsideration.

Usually, on low voltage power circuit-breakers,this breaking capacity equals the Icw at 0.5 s. Inthis case, time discrimination can be used up tothe breaking capacity, since the device iscapable of withstanding these currents for thecorresponding time. Total discrimination istherefore achieved.

However, the values of Icw obtained, even withthe best types of construction, are at presenttypically limited to around 85 kArms, whichtherefore limits the breaking capacities. Yet anincreasing number of installations can generateshort-circuit currents above this value, reaching150 kA in some cases, or even more. This isparticularly the case for installationsincorporating several high-power transformers inparallel, or networks looped with multiplegenerators.

There is now a response to this need, withcircuit-breakers which have a breaking capacityhigher than the Icw.

Fig. 9: Construction recommendations to ensurecompensation of repulsion forces in a circuit-breaker

Isym

Iasym

Irms

-150

-100

-50

0

50

100

150

200

250

0 10 20 30 40 50

Isym

-200

-150

-100

-50

0

50

100

150

200

0 5010 20 30 40

Irms

a: Asymmetrical energization

b: Symmetrical energization

Fig. 10: Timing diagram of the current in the event ofasymmetrical or symmetrical energization

b

aFr Fr

Fr Fr

Fm

(i/2)

(i/2)

i

i

i

1/3

2/3

Fr

Fm

i

A


c Devices with breaking capacity higher than theIcw, construction requirements

For their own protection, circuit-breakers with abreaking capacity higher than the Icw/0.5 srequire instantaneous tripping (DIN) as soon asthe current exceeds their electrodynamicwithstand, as they cannot withstand arcs of suchintensity produced on contact repulsion forseveral hundred milliseconds.

However, this condition is not sufficient of itself,and controlling the breaking capacity of thesedevices requires the expertise of specialists ininterruption of electrical arcs. In particular, asthese devices are not limiting, the current onbreaking powerful short-circuits can reachapproximately 2.3 times the rms value of theprospective current in asymmetrical operation,ie. 230 kApeak in the case of a prospectiveshort-circuit of 100 kArms (see fig. 10). Theelectrodynamic constraints are thereforesignificant on the device itself, with theconsequences being amplified by effectiveopening of the device at the very moment whenthese stresses are strongest.

These considerations limit the maximumbreaking capacity that can be obtained withdevices with high Icw, and only an extremelyrobust construction combined with anexceptional ability to control phenomenaassociated with breaking high currents enablesvalues higher than 100 kA rms to be obtained.Merlin Gerin’s type H3 Masterpact NW devices,which offer a breaking capacity of 150 kA at440 V, for an Icw of 65 kA/3 s are an excellentillustration of this expertise.

Note that in this case, the withstand of theswitchboard and the installation also requiresvery robust construction of the busbar sets andtheir supports. The use of factory-produced LVswitchboards, tested to standard IEC 439,ensures the reliability of this construction(see “Cahier Technique” no. 162).

c Current limiting devices

When, on devices with high Icw, the maximumbreaking capacity indicated by the manufactureris insufficient, the only remaining option is to usecurrent limiting circuit-breakers, which usuallyhave breaking capacities of as much as 150 kAat 400 V.

By their very nature, these devices limit themaximum value reached by the current, andprovide a high breaking capacity, while reducingthe effects of the short-circuit on the installationand the device itself.

High-rated current limiting circuit-breakers dohowever suffer from a handicap, with regard todiscrimination with devices located downstream,as their EDW is always relatively low. In fact,

current limiting is usually obtained by using anelectrodynamic contact repulsion effect, whichconflicts directly with the EDW. The threshold forthe instantaneous self-protection release (DIN)should therefore be set very low, which restrictsdiscrimination with downstream equipment to lowvalues, unless more sophisticated trip criteriaare used (see “Cahier Technique” no. 167,“Energy-based discrimination for low voltageprotective devices”).

Here too, it is the clever design of current limitingpower circuit-breakers that enablesmanufacturers to offer high breaking capacityand effective current limiting, while still ensuringgood EDW. This is particularly the case withMerlin Gerin’s Masterpact NW current limitingdevices, which have an EDW of as high as37 kArms!This EDW would never be as high, however, asa non-limiting device.

Thus, the maximum breaking capacity of deviceswith high Icw, by avoiding the need to use currentlimiting devices at the head of the installation, isa fundamental element of discrimination.

Current limiting

The instantaneous value of a sinusoidalalternating current, in steady state, oscillatesbetween + r and - r times its rms value.During energization, this instantaneous valuecan reach approximately 2.3 times the rms valueon the first wave, due to the asymmetry of thecurrent.The actual value depends on the circuitinductance; in practice, it is also related to thelevel of short-circuit in question, and increaseswith it. If the incoming circuit-breaker is fitted withan instantaneous self-protection release (DIN),because its breaking capacity is higher than itsIcw, discrimination with the downstream device islimited by the presence of this instantaneous

Fig. 11: Asymmetry coefficients as a function of theprospective rms current acc. to standard IEC 60947-1

1.7

2.042.16

2.29

0

0.5

1

1.5

2

2.5

6 to 10 10 to 20 20 to 50 > 50

Prospective Isc in kA


Long-timethreshold

Short-timethreshold

DINthreshold

0.5 s

20 ms

Long time

Short time

Instantaneous

Instant.threshold

I

t

Fig. 13: Trip curve for a circuit-breaker, illustrating thesetting parameters

release. If the value of its threshold is known (inkApeak), this value simply has to be divided bythe asymmetry coefficient (see fig. 11) to find outthe discrimination limit (in kArms). However, if thedevice located downstream of thecircuit-breaker in question is a current limitingtype, and if the short-circuit occurs downstreamof this current limiting device, the maximuminstantaneous value mentioned earlier will notbe achieved. In this case, the discriminationlimit obtained is increased, even more so if thecircuit-breaker downstream has a high currentlimiting capacity (pseudo-time discrimination).

In extreme circumstances, if the maximumcurrent limited by the downstream circuit-breakeris less than the instantaneous threshold of theupstream device, there is total discriminationbetween the two devices (see fig. 12).

3.2 Trip unit characteristics

Fig. 12: Partial or total discrimination obtained betweentwo devices, depending on the current limiting capacityof the downstream device

The discrimination potential of a device can onlybe fully exploited by using an appropriate tripunit.

Types of trip unit

On high-rated equipment, trip units areexclusively electronic nowadays. There are anumber of different types, with different settingoptions (see fig. 13).

c Simple trip units

These usually offer an inverse time curve with anadjustable threshold, for overload protection, andan instantaneous trip threshold (< 10 ms), alsoadjustable, for short-circuit protection.This instantaneous threshold generally has amaximum value of 10 to 12 In.It is this maximum value which limits the actualdiscrimination which can be obtained using thistrip unit.

c “Selective” trip units

These offer, in addition to the overload andshort-circuit protection described above, a tripthreshold with time delay, where both thethreshold and the time delay can be adjusted(from 0 to 500 ms), and an instantaneous

release which can be adjusted up to themaximum permissible value for the breaker.

Limitedcurrent

Upstream device threshold

Discrimination limit withnon-limiting device

Discrimination limit withlimiting device

TOTAL discrimination with very limiting device

Prospective Isc


If the circuit-breaker Icw equals its breakingcapacity, this maximum value can be “infinite”,equivalent to the “Off” position: instantaneoustripping will never occur - see section onBreaking capacity). In this case, discriminationis then total, otherwise the actual discriminationis limited by the value of the instantaneousthreshold set as for a simple trip unit.

If the Icw is less than the breaking capacity, thisinstantaneous threshold can nonetheless be veryhigh (much greater than 12 In) when the EDW ishigh (see section on Breaking capacity).Discrimination is then partial, up to the rmscurrent corresponding to this instantaneousthreshold, or even total if the downstreamprotective device is sufficiently limiting for thisvalue never to be reached (see section onCurrent limiting).

Below this threshold, time discrimination has tobe used, for example: a 3rd level device istime-delayed by 100 ms, a level 2 device by200 ms, and a level 1 device by 300 ms.

c Trip units with “zone selective interlocking”.

A hard-wired link connects the circuit-breaker tripunits on a single circuit.A trip unit which detects a short-circuit sends atime delay command to the upstream trip unit.This trips instantaneously above its “short-time”threshold (whatever its time delay setting), if ithas not received a wait command fromdownstream.

This function does not modify the rules fordiscrimination, but it reduces the stresses on theinstallation since the circuit-breaker immediatelyupstream of the fault will always tripinstantaneously.

c Trip units with “IDMTL” curves

In a very different area from the previousconsiderations, which concern circuit-breakerdiscrimination in short-circuit situations, some“top-of-the-range” trip units offer trip curvesknown as “IDMTL”, as defined by standardIEC 60255-3. These curves can be used toimprove circuit-breaker discrimination in the areaof overloads, where discrimination can easily bestudied by comparing the trip curves for theupstream and downstream protective devices(see fig. 14).

With these trip units it is possible to set not onlythe threshold and time delay for the “long-time”delay release, but also the slope of the triptime as a function of the current. As standard,this slope is at I2 t = constant (the trip time isinversely proportional to the square of thecurrent) and it offers constant thermal stressprotection.

IDMTL curves permit different trip times, asrequired by the user:

v Constant(t = constant; DT = “Definite Time”)

v Inversely proportional to the current(I t = constant; VIT = “Very Inverse Time”)

v Inversely proportional to the square of thecurrent(I2 t = constant; EIT = “Extremely Inverse Time”)

v Inversely proportional to power 4 of the current(I4 t = constant; HVF = “High Voltage Fuse”)

This offers improved discrimination, especiallywith medium-voltage circuit-breakers locatedupstream, which often have constant trip times,or with medium-voltage fuses, which have aslope higher than I2t (see section 4.3).

The self-protection release function

As we saw earlier (see section on Currentlimiting devices), a circuit-breaker whosebreaking capacity is higher than the Icw needs tohave an instantaneous release (DIN) for its ownprotection.

c Standard DIN

The DIN threshold should be selected so that,even in the least favorable withstand conditions,it is still less than the ultimate circuit-breakerwithstand. In particular, the tolerance of thecurrent measurement system should be takeninto account.If there is a wide degree of tolerance, thenominal threshold must be reduced by the sameamount. However, if this system is very precise,

Fig. 14: Circuit-breaker “IDMTL” trip curve

Ir 6Ir

Short time

I

I4t constant

I2t constant

It constant

t constant

tr

t


Fig. 16: Detection principle for a short-circuit, based onthe current instantaneous differential coefficient andlimiting obtained

the nominal threshold can be set nearer the limitwithstand value for the device (see fig. 15).

With a trip unit which is sensitive to the currentdifferential coefficient, the trip command can begiven instantaneously, although still with a veryshort time delay in order to avoid false trippingdue to interference.

v Effect on discriminationThis type of self-protection release does,however, behave in a particular way in terms ofdiscrimination. In fact, even a very limiting deviceplaced downstream of this device has noimmediate effect on the differential coefficient forthe current of a fault that it detects: some time,however minimal, is necessary for its contacts toopen and for the arc voltage generated to slowdown the current rise, before stopping italtogether. In this case, discrimination willtherefore be limited by the threshold value ofthe current differential coefficient,irrespective of the downstream protectivedevice.It is therefore vital for discrimination that themanufacturer sets this threshold at the highestpossible value, compatible with the desiredcurrent limiting and the device electrodynamicwithstand.In the previous example, if the threshold is set ata value of 44.3 kA/ms, corresponding to aprospective current of 100 kArms at 50 Hz,current limiting only comes into effect above thisprospective current value and then discriminationwill occur with downstream devices up to thissame value.

v For 60 Hz:di/dt max = 0 531. Irms so a threshold set at44.3 kA/ms corresponds to a limit of 83 kArms(instead of 100 kArms at 50 Hz).

c Contact separation detector

One way of completely eliminating currentsensor inaccuracy is not to use them. Usingphotoelectric light sensors to detect, between the

c DIN with di/dt

To improve breaking performance, and obtain acertain degree of short-circuit current limiting onnon-limiting devices, a self-protection releasecan be used which is not based on theinstantaneous current value, but on itsdifferential coefficient (di/dt).

v Principle

At known frequency, the maximum differentialcoefficient of the current is in fact directly linkedto its rms value by the equation:

di/dt max = Irms f2 2π ,where f is the network

frequency, which gives

di/dt max = 0 443. Irms at 50 Hz (differential

coefficient in kA/ms if Irms is in kA)

di/dt max = 0 531. Irms at 60 Hz

The least favorable case in terms of speed ofestablishing the short-circuit current consists of asymmetrical wave, which develops in the form ofa sinusoidal equation:

Irms f t2 2 sin π( ) (see fig. 16)

To limit the maximum current produced by thistype of wave, it is essential to act extremelyquickly. The current differential coefficient offersthis opportunity, since its maximum value, in thiscase, is reached as soon as the short-circuit isinitiated, while the value of the currentinstantaneous threshold may only be reached afew milliseconds later. Thus, for a short-circuit of100 kArms at 50 Hz, the symmetrical wave willgenerate a maximum current of 140 kApeak at theend of 5 ms (see fig. 16).With a self-protection release based on aninstantaneous threshold value of 100 kApeak, it isnecessary to wait approximately 2.5 ms beforereaching the threshold. Too little time thenremains to limit the current in any significantfashion.

0

100

140

2.5 5 7.5 10 Time

Current

Limited wave

di/dt

Variation of EDW

Imprecise DINPrecise DIN

Discrimination zone Discrimination zonePro

spec

tive

curr

ent

Variation of EDW

Fig. 15: Effect of the accuracy of a circuit-breaker’scurrent measurement system on discrimination


us = k dip / dt

ip

plasticsupport

contacts of each of the poles, the appearance ofan arc when the electrodynamic withstand isexceeded, is one technique for commanding thecircuit-breaker to open. In this case,discrimination is no longer affected by thetolerance of the current sensor, but only by thatof the electrodynamic withstand itself.In practice, these complex devices can only bejustified to remedy the inaccuracy ofconventional magnetic sensors on high currents.

Current sensors

The accuracy of the measurement systemdepends on the sensor accuracy. Two majorfamilies of sensors are used on circuit-breakers:v Magnetic circuit current transformersv Current transformers with non-magnetictoroids

c Magnetic circuit current transformers

v Their technologyThis is the oldest type and offers satisfactoryaccuracy for equipment with a low EDW. Thesetransformers are fitted with a secondary windingwith n turns around a core of magnetic material,with the passage of the main conductor acrossthe magnetic circuit constituting the primary(see fig. 17).

Moreover, when the circuit-breaker closes due toa short-circuit, the response of the sensor on thefirst current wave depends to a large extent onthe magnetic state (remanent induction) in whichit was left by the previous current. If the primarycurrent on closing is in the same direction as theprevious current, the secondary current Is isnoticeably attenuated on the first wave; if it is inthe opposite direction, Is is increased.Consequently, the measurement system may bemarred by a significant error, the DIN thresholdtherefore needs to be set well below the EDWvalue by the manufacturer.

c Current transformers with non-magnetictoroids

v A new technology (see fig. 19)

Is = Ip / n

Ip

n turns

Magneticcircuit

Fig. 18: Variation in the secondary current of atransformer as a function of the primary current (effectof saturation of the magnetic circuit)

Primary current Ip

Secondarycurrent Is

Tolerance on Is

Saturation

Toleranceon Ip

Fig. 17: Schematic diagram of a current transformerwith magnetic circuit

This transformer draws a current (Is) from thesecondary equal to the primary current (Ip)divided by the number of turns (n) in thesecondary.v Their accuracyAccuracy is satisfactory as long as the magneticcircuit is not saturated, ie. up to 5 to 10 times thenominal current. Thereafter, the secondarycurrent is significantly weaker than Ip/n(see fig. 18).

Fig. 19: Schematic diagram of a current transformerwith non-magnetic toroid


These current transformers, or Rogowski toroids,consist of a secondary winding wound ontonon-magnetic material surrounding the primarycircuit. They provide the secondary with avoltage proportional to the variation of theprimary current. The integration of this voltage bythe electronic circuits gives an image of theprimary current.

v Their accuracyThe absence of a magnetic circuit gives thesesensors perfect linearity for all current values.They enable optimum usage of circuit-breakers.The accuracy of these sensors means that thetrip threshold can be set as close as possible tothe limit value of the device electrodynamicwithstand (EDW), and the discrimination limit iscorrespondingly increased.

3.3 Discrimination on closing

Risks associated with switch-on-to-fault

When a device closes, the mechanism shouldsupply the necessary power for contactoperation, and in particular for compression ofthe springs which provide the bearing force forthe moving contacts on the fixed contacts. It isthis force which ensures that the current flowscorrectly through the mechanism, withoutcausing an excessive temperature rise.

When the device closes on a normal or overloadcurrent, the above conditions are not significantlyaltered.

However, when the device closes on ashort-circuit current, considerableelectrodynamic forces are generated betweenthe contacts even before the mechanism closescompletely, and may lead to closing beingprevented, and then unwarranted reopening.This situation should be avoided, otherwise thedevice may be rapidly destroyed by anuninterrupted series of make and breakattempts, without intervention by the trip unit.

Necessity of distinguishing betweeninstances of closing on a normal current, or ashort-circuit current

There is therefore a clear difference between thecurrent which the device can withstand when it isclosed (electrodynamic withstand), and thecurrent which the device can fully make (makecapacity), also known as “close & latch”.

It is possible to control the value of the currentwhich the device is capable of fully making, bycontrolling the power of the control mechanism.By increasing this power, the limit current is alsoincreased.However, as this additional power is notconsumed to overcome electrodynamic forcesduring operations where there is no current or

with “normal” currents, it is dissipated by jolts inthe mechanism. This power cannot therefore beincreased with impunity without compromisingthe endurance of the mechanism, a value whichis essential to the user as it determines theservice life of the device.

The 2-step release solutionThere is a solution which enables a device to beused on circuits where the current can reachvalues higher than its make capacity. It consistsof tripping the device if the current exceeds thiscapacity on closing the circuit. Opening thenoccurs in controlled conditions which do not leadto any particular difficulties.

Of course, since this make capacity is less thanits electrodynamic withstand, it is not desirable tosimply have an instantaneous release with athreshold lower than this capacity: the wholepoint of high electrodynamic withstand wouldthen be lost. It is therefore necessary to have atwo-step instantaneous release; one “low” step,which is only active on closing (called DINF), theother “high”, which is active when the device iscompletely closed (DIN).

This system can be used in two ways:

c The first solution, which is widely used,consists of activating the low threshold for thirtyor forty milliseconds after the trip unit detects acurrent. This solution is easy to use, as it onlyconcerns the trip unit, and can therefore beimplemented entirely electronically. It does,however, have a major disadvantage: it is notpossible to distinguish between an open devicewhich is closing, and a device which, havingbeen closed with no current or with a very weakcurrent, is suddenly affected by a short-circuitcurrent. This is what happens with a closedincoming circuit-breaker, with no current, whenone of the downstream feeder circuit-breakers is

The downside of this characteristic is:- the low value of the voltage delivered- the fact that this sensor delivers no power- the sensitivity of the secondary signal to thetoroid dimensions.These various points can be solved respectivelyby:- careful signal processing- installation of a second, magnetic, sensorwhich delivers the power required to supply thetrip unit electronics- controlling the toroid dimensions by the use ofsuitable materials, ie. which are stable, notsensitive to temperature and reproducible


closed due to a short-circuit. In this case, theDINF of the first circuit-breaker is activatedunnecessarily and adversely affectsdiscrimination, whereas the device would havebeen sufficiently protected by the DIN threshold.

c A second more satisfactory solution consists ofdetecting a closure movement by the device,delaying this information for as long asnecessary to ensure that the device has closedcompletely, and to use this information in theform of an electrical contact to switch the trip unitfrom the DINF state to the DIN state. Thissolution ensures that the low threshold is onlyactivated at an appropriate moment, and doesnot reduce discrimination unnecessarily for adevice which is already closed.

Advantage of discrimination in the event ofclosing on short-circuit

Finally, remember that when a circuit-breakercloses, the loss of discrimination caused byDINF protection is of limited consequence, sincethe device tripping is not likely to switch off partof the installation which would have beensupplied with power previously. Nonetheless,discrimination is still useful since it makes itpossible, at least up to the DINF threshold, toclose the upstream device, and to allow thedownstream device affected by the fault to trip,thus making it easier to locate the short-circuit.


The LV installation which forms the subject ofthis study is shown in figure 20. This studyincludes coordination of protection between theLV equipment and the protection locatedupstream of each MV/LV power supplytransformer. The selections refer to Merlin Gerinproducts.

The installation includes 2 medium voltage 20 kVincoming lines protected by a fuse, each

Fig. 20: 1st installation example (with 1600 kVA MV/LV transformers) with indication of the order in whichdiscrimination is examined

equipped with an MV/LV transformer withcharacteristics 20 kV/410 V, 1600 kVA,and an incoming LV circuit-breaker (A) or (B).A section switch (C) can be used to operateboth parts of the installation together orseparately, in order to optimize availability ofpower in the event of failure of one of the twoincoming lines.

4 Examples of circuit-breaker selections for an LV installation

4.1 Presentation of the installation

20 kV

NW25H1

80 A MV fuse 80 A MV fuse

20 kV / 410 V1600 kVAIn 2253 AIsc 36 kA

Isc 50 kA

Isc 72 kA Isc 72 kA

A

4

3

1

NT08L1

NW25H1

NS250H

700 A

185 A

D

C

F

Cable

NW25H1

20 kV / 410 V1600 kVAIn 2253 AIsc 36 kA

Isc 50 kA

B

2

4'

3'

1'

2' NW10H2

NS400H

750 A

330 A

E

G

Cable

Main LVswitchboard


4.2 Dimensioning the protective equipment

Rating of devices (A) and (B) installed on LVincoming linesDetermining the nominal current for the LVincoming lines:1600 kVA at 410 V corresponds to a nominalcurrent of 1,600,000 / 410 x 2253 A.3( ) =Incoming devices with a rating of 2500 A aretherefore chosen.

Rating of fuses installed on MV incominglinesThe nominal current for the MV incoming lines is:

In =1,600,000 / 20 000 x 4 A3 6( ) =

Based on the manufacturers’ selection tables,fuses with a rating of 80 A are therefore chosen(in order to take account of inrush and overloadcurrents, while providing thermal protection forthe transformer).

Breaking capacity for the various devicesc Determining the short-circuit currents atdifferent points in the installationEach transformer has a short-circuit current Iscequal to 36 kA (current linked to the transformerpower and short-circuit voltage).

4.3 Selecting devices to ensure discrimination

When the section switch is closed, downstreamof devices (A) and (B) and ignoring the busbarimpedances, the short-circuit current is2 x 36 = 72 kArms .Given the cable impedances, the short-circuitcurrent crossing the circuit-breakers located at(F) and (G) is no more than approximately 50 kA.

c Selecting breaking capacityThe breaking capacity required for each devicemust be determined according to the short-circuitcurrent values at different points in theinstallation.Circuit-breakers (D) and (E) must have abreaking capacity higher than 72 kA, while forcircuit-breakers (A), (B) and (C), a breakingcapacity higher than 36 kA is adequate.Circuit-breakers (F) and (G) must have abreaking capacity of 50 kA minimum.

PrincipleDiscrimination is determined by comparing thecharacteristics of each circuit-breaker with thoseof the protective device (circuit-breaker or fuse)located immediately upstream.

The circuit-breakers located furthest downstreamin the installation should be selected and set inorder to trip “as quickly as possible”, so as tolimit the stresses on the installation in the eventof an overload.

Once the characteristics of these circuit-breakershave been established, one can work back upthe installation, ensuring discrimination betweencircuit-breakers 2 by 2 (downstreamcircuit-breaker/upstream circuit-breaker).

Discrimination between circuit-breakers (F)and (D) 1

c At F: In = 185 A; Isc = 50 kAA circuit-breaker with a rating of 250 A issuitable, for example a Compact NS 250 H(breaking capacity 70 kA at 415 V).

c At D: In = 700 A; Isc = 72 kAA circuit-breaker with a rating of 800 A issuitable, for example a Compact NS 800 L or aMasterpact NT 08 L1 (breaking capacity 150 kAat 415 V).

c Discrimination mechanismDevice (F) is very limiting (the maximum currentwhich can cross it is 22 kApeak for a prospective

short-circuit of 50 kArms), and this circuit-breakertherefore allows “pseudo-time” discriminationwith circuit-breaker (D).This discrimination is improved by applying the“SELLIM” principle to circuit-breaker (D). Thisdevice, which is also limiting (with low EDW toensure very good current limiting), enables totaldiscrimination between (F) and (D) since,according to the SELLIM principle ofdiscrimination, the device on (D) does not trip onthe 1st current wave.Note: The “SELLIM” function is systematicallyincluded in Micrologic - Merlin Gerin trip units,and automatically activated on the devicesconcerned.

Discrimination between circuit-breakers (G)and (E) 1’

c At G: In = 330 A; Isc = 50 kAA circuit-breaker with a rating of 400 A issuitable, for example a Compact NS 400 H(breaking capacity 70 kA at 415 V).

c At E: In = 750 A; Isc = 72 kAThe same circuit-breaker (current limiting) canbe used as for (D), but since the NS 400 Hcurrent limiting is weaker than on the NS 250 H,this combination will not be totally discriminating.To achieve this discrimination, a selectivecircuit-breaker must be selected, for exampleMasterpact NW 10 H2 (In 1000 A, breakingcapacity 100 kA at 415 V, Icw = 85 kArms /1 s).


In addition, the current limiting power of device(G) enables, if necessary, pseudo-timediscrimination.

c Discrimination mechanismSince the Icw (85 kA) is less than the breakingcapacity (100 kA), this device has aninstantaneous self-protection release (DIN) witha threshold of 170 kApeak.With an Isc = 72 kArms, the maximum current at(E) is 72 x 2.3 = 165 kApeak. Since the DINthreshold is therefore never reached, no trip willbe generated which would hinderdiscrimination.

Moreover, in the event of a short-circuit at (G),the maximum current, which corresponds to anIsc of 50 kA, will be limited for (G) to 30 kApeak!Discrimination will therefore be total, as longas device (E) is fitted with a trip unit with aninstantaneous threshold higher than 30 kApeak,say 30/r = 21 kArms = 21 In, and that theshort-time release delay is set on the 0.1 s band.

c VariantA current limiting device can also be used at (E),with a better EDW than (D), for example anNW 10 L1 (In 1000 A, breaking capacity 150 kAat 415 V, Icw 30 kA/1s).Because of its current limiting (125 kApeak to72 kArms, against 165 kApeak with no currentlimiting), the choice of this type of circuit-breakerconsiderably reduces the electrodynamic stresseson the cables between (E) and (G). Thiscircuit-breaker is fitted with an instantaneousself-protection release 80 kApeak, which is nevertherefore called on if there is a fault downstreamof (G) (Isc limited to 30 kApeak). This also providestotal discrimination, of the pseudo-time type, dueto the current limiting of the device downstream.Note: A non-limiting device at (G) would allow apeak current of 50 kA x 2.3 = 115 kApeak to passin the event of a short-circuit, which would causecircuit-breaker (E) to trip.

Discrimination between circuit-breakers (E)and (C) 2

This discrimination is not essential if bothincoming lines are operational (since opening ofthe section switch does not interrupt the powersupply via (A) and (B)). Conversely, it isessential if incoming line (B) is non-operational.

c Value of the nominal current In at (C):To offer the maximum flexibility, the sectionswitch devices have identical dimensions to theincoming devices, ie. In = 2500 A.As Isc = 36 kA, a selective circuit-breaker placedat (C) allows time discrimination with (E) andeven more with (D) which is current limiting, forexample a Masterpact NW 25 H1 (In 2500 A,breaking capacity 65 kA at 415 V, Icw 65 kA/1 s).

c Reason for this selection

Since the Icw for the device equals the breakingcapacity, it does not incorporate an instantaneousself-protection release; time discrimination cantherefore be applied without restriction up to thebreaking capacity. The circuit-breaker (C) musttherefore be fitted with a selective trip unit, withits instantaneous release set to the “Off” position,and the short-time delay on the 0.2 s band (sincethe short-time release delay of circuit-breaker (E)is set on the 0.1 s band).

Discrimination between circuit-breakers (D)and (C) 2’

The solution chosen for discrimination between(E) and (C) is also suitable between (D) and (C)since (C) is totally discriminating up to itsbreaking capacity.

Discrimination between circuit-breakers (C)and (B) or circuit-breakers (C) and (A) 3 3’

(A) and (B) are selective devices, without aself-protection instantaneous release. Here too,time discrimination applies up to the breakingcapacity, with for (A) and (B): their instantaneousrelease set to the “Off” position and theirshort-time delay set on the 0.3 s band (since theshort-time release delay of circuit-breaker (C) isset on the 0.2 s band).

Discrimination between circuit-breakers (A)or (B) and MV fuses 4 4’

To analyze this discrimination, we need tocompare the trip curves for LV circuit-breakersand MV fuses.To do this, transpose the MV fuse curve to LV,by multiplying the current scale by thetransformer ratio, or here 20,000/410 = 48.8(see fig. 21).

Discrimination is considered with 2 types of tripunit: a standard selective trip unit, and a trip unitwith IDMTL curves.

c Settings for standard selective trip units

v Long-time thresholdNo problem, the non-tripping limit current for thefuse is well above the limit current forcircuit-breaker tripping; the long-time thresholdcan therefore be set to maximum (ie. Ir = In).

v Long-time delay and short-time thresholdThe blowing characteristic for MV fuses has amuch steeper slope than that of the long-timedelay release (LT) tripping, with a slope of I2t(see fig. 21). To avoid the curves intersecting,the long-time delay (tr), or short-time threshold(Isd), must be set to sufficiently low values.A good compromise in this example consists ofsetting tr = 12 s (at 6 Ir, in a range generallygoing from 1 to 24 s), and Isd = 4 Ir (in a rangefrom 1.5 to 10 Ir).


Fig. 21: Analysis of discrimination between an LV circuit-breaker and MV fuses - applied to the example of theinstallation concerned

These values allow the passage of peak currentsat switch-on or starting currents for loads locateddownstream without false tripping; a detailedstudy needs to be undertaken on these loads.With a higher short-time delay threshold, 5 Ir forexample, tr should be reduced to 4 s.

v Short-time delayAs the short-time delay is set on the 0.3 s band,to ensure discrimination with the devicesdownstream as indicated above, the fuse andcircuit-breaker curves cross at around 10 In(see fig. 21). Discrimination between thecircuit-breaker and the MV fuse is thereforelimited to approx. 25 kArms, for a maximumshort-circuit current Isc of 36 kArms.

c Settings for trip units with IDMTL curves (seesection Trip units with “IDMTL” curves)With these trip units, it is possible to select theslope of the long-time curve. In this case, we canopt for the “HVF” (High Voltage Fuse) slope,which is the closest to that for the fuse (slope ofI4t). With a delay at 6 Ir of 2s, better immunity tohigh transient currents (peak currents atswitch-on or starting) is possible, in the zone forcurrents between 5 and 10 Ir, since theshort-time threshold can be set at any desiredvalue up to 10 Ir (see fig. 21).

0.1 1 10 100 I/In

MV 80 A fuse corrected to LV

HVF curve tr = 2 s

I2t curvetr = 12 s

Short-time threshold = 4 Ir

Max. Isc 36 kA

Short-time band 0.3 s

10000

1000

100

10

1

0.1

0.01

Td (s)


Direct wiring between (E) and (B) on the onehand, (D) and (A) on the other hand, is a meansof ensuring discrimination between thesedevices when the section switch (C) is open. Inthis case the diodes ensure the independence ofboth halves of the installation: they avoid (D)acting on (B) and (E) acting on (A).

Operationc In the event of a fault downstream of (G) (seefig. 20):v (G) trips instantly.v (E) is delayed for 100 ms and does nottherefore trip, but sends a signal to (C).v (C) is then delayed for 200 ms and does nottherefore trip, but retransmits a signal to (A) and(B) which are then delayed like (C).v Hence only (G) will trip.

c In the event of a fault between (G) and (E):v (E) trips after 100 ms and sends a signal to(B) and (C) which are then delayed for 200 msand therefore do not trip.v (C) retransmits the signal to (A) which is thendelayed like (C).If (C) is open, it does not send a signal to (A),which is of no importance since the short-circuitsupplied by (B) does not affect (A).

c In the event of a fault between (E) and (C):v If (C) is closed, power is supplied to the faultby both incoming lines in parallel.

Fig. 22: Use of zone selective interlocking - illustration of time delay settings and trip unit wiring

4.4 Variant with zone selective interlocking

This variant requires, for the relevant circuit-breakers, trip units with this function (typeMicrologic 5.0 A - Merlin Gerin).

Description

The principle and operation of zone selectiveinterlocking are explained in a “CahierTechnique” dedicated to this type ofdiscrimination.Remember that each trip unit has four terminals:c 2 input terminals, for connection todownstream equipmentc 2 output terminals, for connection to upstreamequipment

When a trip unit detects a fault above itsshort-time threshold, it short-circuits both itsoutput terminals.When a trip unit has a short-circuit on both itsinput terminals, it activates the short-time delay.Otherwise, it trips instantly.

Implementation in this example

c The first devices (D) and (E) have a permanentshort-circuit on their inputs, so that theirshort-time delay is activated. This ensuresdiscrimination with the stage below (Compact NScircuit-breakers).

c Next the wiring is effected and the short-timedelays set according to figure 22.

OUT

AST band 2

IN

OUT

CST band 2

IN

OUT

BST band 2

IN

OUT

DST band 1

IN

OUT

EST band 1

IN

Otherfeeders

Otherfeeders


20 kV

NW40H1

Main LVswitchboard

MV circuit-breaker MV circuit-breaker

20 kV / 410 V2500 kVAIn 3520 AIsc 54 kA

Isc 78 kA

Isc 108 kA Isc 108 kA

A

4

3

4'

3'

2 2'

1

NT08L1

NW40H1

NS250H

700 A

185 A

D

C

F

Cable

NW40H1

20 kV / 410 V2500 kVAIn 3520 AIsc 54 kA

Isc 95 kA

B

1'

NW25H3

NS400H

2200 A

330 A

E

G

Cable

- (C) trips instantly and send a signal to (A) and(B) which are then delayed by 200 ms andtherefore do not trip.- (C) then interrupts the current supplied by theincoming line (A).- (A) stays closed and maintains the powersupply to the part of the installation downstreamof its busbar.- (B) interrupts the fault current after 200 ms.v If (C) is open, it does not send a signal and(B) trips instantly.

c In the event of a fault between (C) and (B):(B) trips instantly.NB: A fault between (E) and (C) or between (C)and (B) is extremely unlikely, as these devices aregenerally located in the same switchboard.

Zone selective interlocking, by reducing or eveneliminating the always considerable time delaysat the head of the installation, can thus be usedto limit stresses on the installation even moreeffectively when the feeders are close to thenetwork. Therefore, using this technique in thisinstallation, a fault immediately downstream of(A) or (B) is eliminated within tens ofmilliseconds instead of taking longer than300 ms.

In addition, since device tripping is instantaneousin nearly all instances, discrimination with theMV fuses is total, whereas using timediscrimination it was limited to 25 kArms due tothe 300 ms short-time delay of devices (A) and(B).

4.5 Variant with two more powerful incoming lines

This is the same diagram as before, except forthe following:

c The transformer power has been increased to2500 kVA, and the current on the outgoingcircuit at (E) increased to 2200 A.

c MV protection is provided by MV circuit-breakers (see fig. 23).

The nominal current at (A) and (B) becomes3520 A and the Isc 54 kA. On the main busbar,Isc becomes 108 kA.

Fig. 23: 2nd installation example (MV/LV transformer power 2500 kVA)


I2t curvetr = 12 s

Short-time threshold = 5 Ir

10000

1000

100

10

1

0.1

0.010.1 1 10 100

Td (s)

I/In

Max. LV Isc 54 kA

Max. MV Isc

MV circuit-breakerhigh threshold 1400 A corrected to the LV = 68 kA

MV circuit-breakerlow threshold 600 A corrected to the LV = 29 kA

Selecting the MV circuit-breakerFor an MV voltage of 20 kV, it is possible to usea Merlin Gerin “MC-Set” circuit-breaker, with a“transformer” type Sepam protective relay.This relay has two trip thresholds (see fig. 24):c The first provides protection in the event of ashort-circuit between the transformer and theLV circuit-breaker, or in the event of failure of theLV protection.c The second provides protection in the event ofa short-circuit upstream of the transformer.

Selecting the LV circuit-breakersc Circuit-breaker (E)Since the Isc is higher than 100 kA, it is notpossible to use an NW 25H2 (breakingcapacity 100 kA). Nor is it possible to use acurrent limiting circuit-breaker, since thenominal current does not exceed 2000 A(NW 20L1).The solution lies in selecting a selectivecircuit-breaker with high breaking capacity,such as the NW 25H3, which offers a breakingcapacity of 150 kA, with an Icw of 65 kA/3 s.

c Circuit-breakers (A), (B) and (C)For In = 3520 A, NW 40H1 type circuit-breakers(In 4000 A, breaking capacity 65 kA,Icw 65 kA/1 s) is chosen.

Setting devices to ensure discrimination

On the basis of the rules defined in section 4.3, theonly modifications to be made are for:

1’ the delay for device (E) to be set on the 0.1 s band

2’ device (C) on the 0.2 s band

3 3’ devices (A) and (B) on the 0.3 s bandIf zone selective interlocking is being used, thediagram in figure 22 is still valid.

Setting the protective relay for the MVcircuit-breaker 4 4’The first threshold should be lower than theshort-circuit current downstream of thetransformer, ie. 54 kA on the LV side, equivalentto 1100 A on the MV side. It should discriminateagainst the short-time threshold of circuit-breakers (A) or (B). If this threshold is set at 5 Ir,

Fig. 24: Analysis of discrimination between LV circuit-breakers and MV protection of the transformer


the corresponding maximum value will equalIr x threshold x tolerance, or 4000 x 5 x 1.1 = 22 kA,corresponding to 450 A for the MV incoming line.The first MV threshold can therefore be set at600 A.

To avoid interference with the short-time delay of0.3 s, the delay associated with this firstthreshold is set for example at 0.6 s.

The second threshold should be higher than theabove short-circuit current (1100 A), and lowerthan the short-circuit current upstream of thetransformer. Assuming that the network hasshort-circuit power of 150 MVA, thecorresponding current is 4 kA on the MV side.The second threshold can therefore be set at1400 A.


Bibliography

Standards

c IEC 60255-3Electrical relays - Part 3: Single input energizingquantity measuring relays with dependent orindependent time

Schneider Electric Cahiers Techniques

c Protection of electrical distribution networks bythe logic selectivity systemCahier Technique no. 2 -R. CALVAS - F. SAUTRIAU

c Behaviour of the SF6 MV circuit-breakersFluarc for switching motor starting currentsCahier Technique no. 143 -D. GIBBS - J. HENNEBERT

c Development of LV circuit-breakers tostandard IEC 947-2Cahier Technique no. 150 -E. BLANC

c Electrodynamic forces on busbars in LVsystemsCahier Technique no. 162 -JP. THIERRY - C. KILINDJIAN

c Energy-based discrimination for LV protectivedevicesCahier Technique no. 167 -R. MOREL - M. SERPINET

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Documents

Discrimination with LV power circuit-breakers · Discrimination with LV power circuit-breakers The purpose of this “Cahier Technique” is to set out the discrimination techniques