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

Cahier technique n 167

Energy-based discrimination forlow-voltage protective devices

M. SerpinetR. Morel

n Merlin Gerin n Modicon n Square D n Telemecanique

Cahiers Techniques are a collection of documents intended for engineersand technicians people in the industry who are looking for information ingreater depth in order to complement that given in display productcatalogues.

These Cahiers Techniques go beyond this stage and constitute praticaltraining tools.They contain data allowing to design and implement electrical equipement,industrial electronics and electrical transmission and distribution.Each Cahier Technique provides an in-depth study of a precise subject inthe fields of electrical networks, protection devices, monitoring and controland industrial automation systems.

The latest publications can be downloaded on Internet from theSchneider server.code: http://www.schneiderelectric.comsection: mastering electrical power

Please contact your Schneider representative if you want either a CahierTechnique or the list of available titles.

The Cahiers Techniques collection is part of the Groupe SchneidersCollection Technique.

ForewordThe author disclaims all responsibility further to incorrect use of informationor diagrams reproduced in this document, and cannot be held responsiblefor any errors or oversights, or for the consequences of using informationand diagrams contained in this document.

Reproduction of all or part of a Cahier Technique is authorised with theprior consent of the Scientific and Technical Division. The statementExtracted from Schneider Cahier Technique no..... (please specify) iscompulsory.

Robert MOREL

Graduated with an engineering degree from ENSMM in Besanon andjoined Merlin Gerin in 1971. Specialised in designing low voltageswitchgear and participated in designing the Sellim system.In 1980, took over development of Compact circuit-breakers andInterpact switches.In 1985, became manager of the Low-Voltage Current Interruptiondesign office in the Low-voltage Power Components division.

n 167Energy-based discriminationfor low-voltage protectivedevices

ECT167 first issued, march 1998

Marc SERPINET

Joined Merlin Gerin in 1972 and worked until 1975 in the low-voltageequipment design offices, in charge of designing electrical cabinetsfor various installation layouts. Since 1975, he has managed researchand development testing for low-voltage circuit-breakers. Graduatedin 1981 from the ENSIEG engineering school in Grenoble.In 1991, after managing a Compact circuit-breaker project from thepreliminary studies on through to production, he was appointed headof the electromechanical design office in charge of anticipatingfuture developments.

Cahier Technique Schneider n 167/ p.2

Lexicon

EbEnergy let through by the protective deviceduring breaking. This energy is characterised by

i dt tb b2 2

IibLimited short-circuit current actually flowingthrough the circuit-breaker (the break current isless than Ip).

IpProspective short-circuit current that woulddevelop in the absence of protective devices(rms value).

IrCorresponds to the overload protection setting.

tbThe actual breaking time (arc extinction).

UTElectronic processing unit.

ActuatorDevice capable of producing a mechanicalaction.

Circuit-breaker ratingCorresponds to the models of the range(ex. 160 A, 250 A, 630 A, 800 A, etc.).

Current-limiting circuit-breakerCircuit-breaker which, when interrupting a short-circuit current, limits the current to a valueconsiderably less than the prospective current (Ip).

High-set instantaneous release (HIN)Instantaneous release used to limit thermalstress during a short-circuit.

Instantaneous release (INS)Release without an intentional time delaysystem. It trips at a low multiple of In to ensureshort-circuit protection.

Long-time release (LT)Release with an intentional time delay system(several seconds) for overload protection.

Partial discriminationDiscrimination is said to be partial when it isensured only up to a certain level of theprospective current (Ip).

Selective circuit-breakerCircuit-breaker with an intentional time delaysystem (time discrimination).

Short-time release (ST)Release with an intentional time delay systemranging from ten to several hundredmilliseconds. If the time delay is reduced as Ipincreases, the system is referred to asdependent short-time (DST).

Total discriminationDiscrimination is said to be total when it is ensuredfor all values of the prospective fault current.

Trip-unit ratingCorresponds to the maximum current setting ofthe trip unit.

Cahier Technique Schneider n 167 / p.3

Energy-based discrimination forlow-voltage protective devices

The purpose of this Cahier Technique publication is to present the newenergy-based discrimination technique that ensures tripping discriminationbetween protective devices during a short-circuit. Both simpler and moreeffective than standard discrimination techniques, it has been implementedon the Compact NS range of circuit-breakers used in low-voltage powerdistribution networks. Discrimination is ensured for all prospective faultcurrents on the condition that upstream and downstream circuit-breakershave different current ratings (ratio u 2.5) with a trip-unit rating ratio u 1.6.Following a brief review of standard discrimination techniques, the authorsexamine the behaviour of circuit-breakers and various trip units from theenergy standpoint.They then demonstrate that total discrimination is possible up to thecircuit-breaker breaking capacity, over several levels, without using timediscrimination techniques.

Contents1. Discrimination in low-voltage 1.1 Definition p. 4

1.2 Enhanced safety and availability p. 51.3 Discrimination zones p. 5

2. Discrimination techniques for short-circuits 2.1 Current discrimination p. 72.2 Time discrimination p. 72.3 SELLIM discrimination p. 82.4 Zone selective interlocking p. 92.5 Combining the different types of discrimination p. 9

3. Energy-based discrimination 3.1 Choice of operating curves p. 103.2 Characterisation of a Compact NS circuit-breaker p. 113.3 Characterisation of the trip units p. 13

4. Advantages and implementation of 4.1 Current-limiting circuit-breaker fitted with a pressure trip system p. 164.2 Discrimination with Compact NS circuit-breakers p. 184.3 Combination with traditional protective devices p. 19

5. Conclusion p. 216. Appendix - indications concerning breaking with current limiting p. 22

energy-based discrimination

protective devices


1 Discrimination in low-voltage protective devices

1.1 DefinitionIn an electrical installation, loads are connectedto sources via a succession of protection,isolation and control devices. This CahierTechnique publication deals essentially with theprotection function using circuit-breakers.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,thus ensuring maximum continuity of service.If discrimination studies are not or are incorrectlycarried out, an electrical fault may cause severalprotective devices to trip, thus provoking aninterruption in the supply of power to a large partof the network. That constitutes an abnormal lossin the availability of electrical power for thoseparts of the network where no fault occurred.

Fig. 1: several circuit-breakers are concerned by the fault If.

Several types of overcurrents may beencountered in an installation:c overloads,c short-circuits,c inrush currents,as well as:c earth faults,c transient currents due to voltage dips ormomentary loss of supply.To ensure maximum continuity of service, theremust be coordination between protectivedevices.Note that voltage dips may provoke unnecessaryopening of circuit-breakers by actuatingundervoltage releases.

ACB1

CB3

CB4

BCB2

If

If passes through CB1, CB2, CB3, CB4.


Fig. 2: circuit-breaker behaviour during a fault.

1.2 Enhanced safety and availabilityA specific type of protective device exists foreach type of fault (overloads, short-circuits, earthfaults, undervoltages, etc.). However, a fault maysimultaneously bring several types of protectivedevices into play, either directly or indirectly.

Examplesc A high short-circuit current creates anundervoltage and may trip the undervoltageprotective device.c An insulation fault may be interpreted as azero-phase sequence fault by an earth-leakageprotective device and as an overcurrent by theshort-circuit protective device (applicable for TNand IT earthing systems).c A high short-circuit current may trip the earth-leakage protective device (in TT earthingsystems) due to local saturation of thesummation toroid which creates a false zero-phase sequence current.For a given network, discrimination studies andthe evaluation of the protection system in generalare based on the protective devicecharacteristics published by the manufacturers.

Studies begin with an analysis of requirementsconcerning protective devices needed foreach type of fault. The next step is an evaluationof coordination possibilities between theprotective devices concerned by a given fault.The result is improved continuity of service whilestill guaranteeing protection of life and property.The following chapter will deal exclusively withdiscrimination in the event of overcurrents(overloads and short-circuits).In this context, the existence of discriminationbetween circuit-breakers is determined quitesimply by whether several circuit-breakers openor not (see fig. 2 ).Total discriminationDiscrimination is said to be total if and only if,among the circuit-breakers potentially concernedby a fault, only the most downstream circuit-breaker trips and remains open, for all faultcurrent values.

Partial discriminationDiscrimination is said to be partial if the abovecondition is no longer valid for fault currentsexceeding a certain level.

CB2

CB1

CB2

CB1

a) CB1 and CB2 open. discrimination is not ensured, i.e. power is notavailable for the feeders where no fault occurred.

b) CB1 opens CB2, remains closed. discrimination is ensured, i.e. power is available for thefeeders where no fault occurred (continuity of service).

1.3 Discrimination zonesTwo types of overcurrent faults may beencountered in an electrical distribution network:c overloads,c short-circuits.Overcurrents ranging from 1.1 to 10 times the ser-vice current are generally considered as overloads.

Overcurrents with higher values are short-circuitsthat must be cleared as rapidly as possible byinstantaneous (INS) or short-time (ST) releaseson the circuit-breaker.Discrimination studies are different for each typeof fault.


Overload zoneThis zone starts at the ILT operating threshold ofthe long-time (LT) release. The tripping (or time-current) curve tb = f (Ip) is generally of theinverse-time type to remain below thepermissible thermal stress curve of the cables.Using the most common method, the curves ofthe LT releases concerned by the fault are plottedin a system of log-log coordinates (see fig. 3 ).For a given overcurrent value, discrimination isensured during an overload if the non-trippingtime of the upstream circuit-breaker CB1 isgreater than the maximum breaking time(including the arcing time) of circuit-breaker CB2Practically speaking, this condition is met if theratio ILT1/ILT2 is greater than 1.6.

Short-circuit zoneDiscrimination is analysed by comparing thecurves of the upstream and downstream circuit-breakers.The techniques that make discriminationpossible between two circuit-breakers during ashort-circuit are based on combinations ofcircuit-breakers and/or releases of different typesor with different settings designed to ensure thatthe tripping curves never cross.A number of such techniques exist and arepresented in the next chapter.

Overload discri-mination zone

ILT2 ILT1 I ins2

Ip

tb

Overloads Short-circuits

CB2 CB1

Fig. 3: overload discrimination.


2 Discrimination techniques for short-circuits

Several techniques can be used to ensurediscrimination between two circuit-breakersduring a short-circuit:c current discrimination,c time discrimination,

2.1 Current discriminationThis type of discrimination is the result of thedifference between the thresholds of theinstantaneous or ST releases of the successivecircuit-breakers.Applied primarily in final distribution systems, it isimplemented using rapid circuit-breakers notincluding an intentional tripping time-delay system.It protects against short-circuits and generallyresults in only partial discrimination.This form of discrimination is all the moreeffective when the fault currents are different,depending on where they occur in the network,due to the non-negligible resistance ofconductors with small cross-sectional areas(see fig. 4 ).The discrimination zone increases with thedifference between the thresholds of theinstantaneous releases on circuit-breakers CB1and CB2 and with the distance of the fault fromCB2 (low Isc < Iins of CB1).The minimum ratio between Iins1 and Iins2 mustbe 1.5 to take into account threshold accuracies.

2.2 Time discriminationTo ensure total discrimination, the time-currentcurves of the two circuit-breakers must nevercross, whatever the value of the prospectiveshort-circuit current. For high fault currents, totaldiscrimination is guaranteed if the horizontalsections of the curves to the right of Iins1 are notone on top of another.Several solutions may be implemented toachieve total discrimination:c the most common involves installing selectivecircuit-breakers including an intentional time-delay system,c the second applies only to the last distributionstage and involves using current-limiting circuit-breakers.

Fig. 4: current discrimination.

Short-circuitdiscrimination limit

Iins2

Ip

tb

Iins1

Short-circuitdiscrimination zone

CB2 CB1

Use of selective circuit-breakersThe term selective means that:c the circuit-breaker trip unit has a fixed oradjustable time-delay system;c the installation and the circuit-breaker canwithstand the fault current for the duration of theintentional time delay (sufficient thermal andelectrodynamic withstand capacities).A selective circuit-breaker is generallypreceded in the network by another selectivecircuit-breaker that has a longer intentional timedelay.Use of this type of circuit-breaker, correspondingto time discrimination solutions, results in totalbreaking times greater than 20 ms (one period)

c SELLIM discrimination,c zone selective interlocking,c energy-based discrimination (see chapters 3and 4).


in the event of a fault. This figure may run up to afew hundred milliseconds (see fig. 5 ).When the installation (and perhaps even thecircuit-breaker) cannot withstand a high short-circuit current (Isc) for the entire time delay,circuit-breaker CB1 must be equipped with ahigh-set instantaneous release (HIN).In this case, the discrimination zone is limited tothe high-set threshold of the upstream circuit-breaker (see fig. 5 ).Use of current-limiting circuit-breakers andpseudo-time discriminationThese circuit-breakers have two maincharacteristics:

Fig. 5: time discrimination.

IDIN1

CB2

CB2 : rapidCB1 : selective with1-2-3 ST settings

tbCB1

Iins1

Installation and/or circuit-breaker thermal withstandcapacity limit

Ip1

32

Note: use of a high-sed instantaneous releasedetermines the discrimination limit.

Ip

CB2tb CB1 CB2 : rapid current limitingCB1 : rapid

Fig. 6: pseudo-time discrimination.

Note: use of dependent ST releases (dotted line) onCB1 improves discrimination.

c they severely limit short-circuit currents due tofast opening times and high arcing voltages,c the higher the prospective short-circuit current,the faster they act.Use of a current-limiting circuit-breakerdownstream thus makes it possible to ensurepseudo-time discrimination between twoprotection levels. This solution, due to thecurrent limiting effect and rapid clearing of thefault, limits thermal and electrodynamic stressesin the installation (see fig. 6 ).

2.3 SELLIM discrimination

Fig. 7: SELLIM discrimination.(CB1 - Compact C250 L SBCB2 - Compact C125 N).

CB1

CB2

A

B

2.5 ms

i3

u3

i2

u2

i1

u1

26 k

Fault at B

i3

u3

i2

u2

i1

u1

3.5 ms12 ms

Fault at A

34 k


2.5 Zone selective interlocking

The SELLIM system offers a number ofadvantages: discrimination, cascading, reducedstresses in the installation.Upstream from a rapid circuit-breaker CB2, thesystem requires an ultra current-limiting circuit-breaker CB1 fitted with a special release thatdoes not trip during the first half-wave of the faultcurrent (see fig. 7 ).A major fault at B is detected by both circuit-breakers.CB2, equipped with an instantaneous release,opens as soon as the fault current exceeds its

This technique requires data transmissionbetween the trip units of the circuit-breakers atthe various levels in a radial feeder network.The operating principle is simple (see fig. 8 ):c each trip unit that detects a current greaterthan its tripping threshold sends a logic waitorder to the next trip unit upstream,c the trip unit of the circuit-breaker located justupstream of the short-circuit does not receive await order and reacts immediately.With this system, fault clearing times remain lowat all levels in the network.Zone selective interlocking is a technique usedwith high-amp selective LV circuit-breakers,though its main application remains HV industrialnetworks. For further information, refer to CahierTechnique Publication Number 2, entitledProtection of electrical distribution networks bythe logic selectivity system.

Fig. 8: zone selective interlocking.

Logicrelay

Logicrelay

Logic waitorder

CB1

CB2

2.6 Combining the different types of discriminationThe different types of discrimination presentedabove are generally combined to ensure thehighest degree of availability of electrical power.See figure 9 for an example.Discrimination studies are still carried out using thetables supplied by manufacturers. The tables indi-cate the discrimination limits for each combinationof circuit-breakers and for the various trip units.

The costs of non-discrimination and of thevarious devices selected are taken into account.The energy-based discrimination techniquepresented in the next chapter constitutes a trueinnovation that will considerably simplify LVdiscrimination studies and make possible totaldiscrimination over several levels at minimumcost.

Fig. 9: example of uses for different types of discrimination.

Circuitsconcerned Zone selective

interlocking

Powerdistribution

Finaldistribution

Type of discrimination Type ofcircuit-breakerTime Pseudo-time SELLIM

SelectivelogicSelectiveRapid/currentlimiting SELLIMRapid

Head ofLV network

Rapid/currentlimiting

trip threshold and clears the fault in less than ahalf-period. CB1 detects only a single currentwave and does not trip. The fault currentnonetheless causes contact repulsion, thuslimiting the current and the resulting stresses.This limiting of the fault current means thatdownstream circuit-breakers may have breakingcapacities less than the prospective fault current.A fault at A causes repulsion of the contacts of thecurrent-limiting circuit-breaker, thus limiting thestresses produced by the fault current. CB1 opensafter the second half-wave of limited current.


3 Energy-based discrimination

Energy-based discrimination is an improved andgeneralised version of the pseudo-timetechnique described in the preceding chapter.Discrimination is total if, for all values of Ip, theenergy that the downstream circuit-breaker letsthrough is less than that required to actuate thetrip unit of the upstream circuit-breaker.The actual implementation of the energy-based dis-crimination principle is covered by a Merlin Gerinpatent and has been incorporated in the design ofthe new Compact NS range of circuit-breakers.These rapid and highly current-limiting circuit-breakers meet the rapidly evolving criteria of themarket concerning:

c increases in installed power, which lead tohigher short-circuit currents and correspondinglyhigher breaking capacities;c the need to limit stresses in the installation aswell as the level and duration of fault currents.When reasoning in terms of energy and in orderto understand energy-based discrimination, thechoice of the means of presenting the operatingcurves is essential and the subject of the nextsection.Following that discussion is an analysis of thebehaviour in terms of energy for current-limitingcircuit-breakers and the various trip units.

3.1 Choice of operating curvesThe tb = f (Ip) curves commonly used fordiscrimination studies are of no use with current-limiting circuit-breakers when currents exceed25 In (breaking times are less than 10 ms at afrequency of 50 Hz).Discrimination studies may no longer be carriedout on the basis of periodic phenomena, butrather require analysis of transient phenomena.An understanding of energy-baseddiscrimination requires that the followingelements be characterised:c the current wave that the circuit-breaker letsthrough during breaking, which is characterisedby its Joule integral i dt2 (often expressed asI2 t ), and corresponds to the breaking energy Eb,c the sensitivity of the releases to the energycorresponding to the current pulse.Thus, quite logically, the above characteristicsare represented using I2 t = f (Ip) curvesinstead of tb = f (Ip) curves (see fig. 10 ).It should be noted that standard IEC 947-2specifies characterisation of circuit-breakersusing such curves.For practical reasons the I2 t = f (Ip) curve ispresented in a system of log-log coordinates.For discrimination studies, the limits of thebreaking I2 t value (Eb for circuit-breakers) arebetween 104 and 107 A2 s for prospectivecurrents ranging from 1 to 100 kA. Three powersof ten are therefore used for Eb and two for thecurrent.Assuming that the half-wave of the interruptedcurrent is equivalent to half of a sine-wave withthe same initial slope as the prospective current,the breaking energy Eb may be expressed as afunction of Ip using the following expressions

Fig. 10: tb = f (Ip) and I2 t = f (Ip) curves for a circuit-breaker equipped with an electronic trip unit.

10 In 15 In 30 In

10 In

I2 t

INS

ST1ST2

t(s)

Ip(A)

LT

(A2 s)

Ip(A)


(see the appendix on breaking with currentlimiting):v for t u 10 ms(2) Eb = Ip2 tv for t < 10 ms(3) Eb = 4 f2 Ip2 tvb3or

(4)

bf p

3

4 2 IOn the basis of these equations, the Ip2 t / Ipsystem can be improved, thus providing furtherinformation on the virtual breaking time (tvb) andthe limited peak current value (b).Time lines (see fig. 11 )A series of lines representing constant breakingtimes can be included in the log-logrepresentation for a given frequency.For example, when f = 50 Hz, the line for:c t = 20 ms corresponds to the most commonbreaking time when Ip is greater than the

Fig. 11: graph representing energies.1

40 m

s20

ms

10 m

s

7 m

s

5 m

s

3 10 50 100

107

106

105

104

= 40 kA

= 20 kA

= 10 kA

Ip (kA) = 5 kA

I2 t

2.5 ms

(A2 s)

5 30

instantaneous thresholds and less than thecontact repulsion threshold:(2) Eb = Ip2 x 2 x 10-2.c t = 10 ms is the breaking time at the current-limiting threshold:(2) Eb = Ip2 x 10-2.c t = 9 to 4 ms which indicate circuit-breakerbehaviour when current limiting:(3) Eb = Ip2 tvb3 x 104.Peak-current linesSimilarly, on the basis of equation (4)Eb =

bf p

3

4 2 Ia series of lines corresponding to constant,limited peak currents can be included in therepresentation (see fig. 11 ).It should be noted that this method ofrepresentation makes it possible to characterisecircuit-breakers and trip units at 50 Hz for three-pole, two-pole and single-pole faults.

3.2 Characterisation of a Compact NS circuit-breakerDisplay of the breaking I2 t valueThe I2 t values that a circuit-breaker letsthrough are determined by standardised typetests or by computer models run for a givenvoltage and frequency.

The curves presented here correspond to three-phase faults at 400V/50Hz.The same curves may be generated for othervoltages and other frequencies. The indicatedvalues are the maximum values obtained


irrespective of the moment at which the faultoccurs (upper limits) (see fig. 12 ).Curve analysisA great deal of information is available from thegraph in figure 12 which corresponds to a250 A Compact NS circuit-breaker, equippedwith a dependent ST (DST) electro-mechanicalrelease with a 10 In threshold.The information characterises the differentphases in the breaking behaviour of the current-limiting circuit-breaker depending on the value ofthe prospective short-circuit current Ip.c Point A: when the fault current reaches the tripthreshold of the release, the breaking time istypically 50 ms for an INS or DST release.c Point B: when the fault current is greater thanthe trip threshold of the release, the breakingtime drops and stablises at 20 ms beginningat 16 In.c Point C: when the fault current reaches thecontact repulsion level, current limiting starts dueto the insertion of an arc voltage in the circuit.Current limiting results in the return to in-phaseconditions for the voltage and the current andconsequently a drop in fault clearing timesfrom 20 ms to 10 ms as Ip increases.

Fig. 12: breaking curve for a current-limiting circuit-breaker.

c Point D: when the fault current reachesapproximately 1.7 times the contact repulsionlevel, the energy is sufficient to totally open thecontacts. At that point, the breaking time istypically 10 ms.This reflex-type breaking is autonomous and atrip unit is required only to confirm the trippedstatus of the circuit-breaker and avoid untimelyreclosing of the contacts.c Zone E: when the fault currents runs beyond2 times the contact-repulsion level, currentlimiting is increasingly effective and results inincreasingly short breaking times.c Point F: the end of the curve represents thebreaking capacity limit of the circuit-breaker.The curve provides a great deal of information:c tripping threshold (I threshold, point A);c breaking I2 t value as a function of theprospective current;c contact-repulsion level (Ir, point C);c breaking capacity (point F);c breaking time (tvb) as a function of the prospec-tive current;c limited peak current (b) as a function of theprospective current;c current value above which tvb < 10 ms(beginning of current limiting).

1

40 m

s20

ms

10 m

s

7 m

s

5 m

s

3 10 50 100

107

106

105

104

= 40 kA

= 20 kA

= 10 kA

Ip (kA) = 5 kA

2 t

2.5 ms

(A2 s)

5 30(10 In)

DC

(B)

(E)A F


3.3 Characterisation of the trip unitsTrip units are characterised by their responsetime to a given current (full-wave, half-wave,etc.).By modifying the duration and the peak value ofthe current, which corresponds to the variouscurrents limited by a circuit-breaker, a number oftests can be run to obtain a series of pointswhich may be plotted on the previouslydescribed graph, thus producing the curvecharacterising a trip unit.

Magnetic trip unitsc Instantaneous releaseGenerally made up of a magnetic U and a blade,it ensures short-circuit protection. The responsetime is under 50 ms at its operating threshold(between 5 and 10 times the rated current), thendrops rapidly to below 10 ms when the currentincreases (see fig. 13 ).c High-set releaseAs indicated in the time discrimination section,the role of high-set releases in timediscrimination systems is to limit thermalstresses (see fig. 5 ) in the installation and thecircuit-breaker.

The high-set release is an instantaneous unitwith a threshold of 15 to 50 In.The release may be either electro-mechanical orelectronic.c Constant time-delay releaseThis is an instantaneous release fitted with aclock-type time delay system intended to maketripping selective with respect to the downstreamcircuit-breaker.The time delay may range from 10 to 500 msand is generally set using notched dials.Figure 13 shows the curve (20 ms setting) for ashort-time delay.If the thermal stess (I2 t) resulting from a longtime delay must be limited, the high-set releaseenters into play (see fig. 13 ).c Dependent time delay release (function of Ip,dependent short-time - DST).The time delay results from the inertia of a massand is therefore inversely proportional to Ip(see fig. 13 ).Electronic trip unitsThe instantaneous thresholds in electronic tripunits are sensitive to the rms value or the peak

Fig. 13: curves for various magnetic releases.

1

40 m

s20

ms

10 m

s

7 m

s

5 m

s

3 10 50 100

107

106

105

104

= 40 kA

= 20 kA

= 10 kA

Ip (kA) = 5 kA

2 t

2.5 ms

(A2 s)

5 30(10 In)

20 m

s fixe

dTi

me

delay

(ST)

Depende

nt

time dela

y (DST)

Instantaneous (INS)

Hig

h se

t


current value. For high fault currents, their I2 tcharacteristic is theoretically a straight line(b = constant).In fact, the above is true for current pulsedurations greater than the response time of theactuating elements of the trip unit (generally4 ms). Below this value, the inertia of themechanical elements of the trip unit produces,for high Ip values, a characteristic similar tothat of an instantaneous electro-mechanicalrelease.The trip unit must therefore be characterised byits Eb = f (Ip) curve by carrying out testsidentical to those for magnetic trip units.These trip units may be of either theinstantaneous or time delay type.It is possible to combine several types ofelectronic trip units, for example:c 10 to 15 In - ST (40 ms),c 15 to 30 In - ST (10 ms),c > 30 In - INS.Figure 14 is an illustration of this example. Thecurves for this combination should be compared

Fig. 14: examples of combinations of electronic trip-unit curves.

1

40 m

s20

ms

10 m

s

7 m

s

5 m

s

3 10 50 100

107

106

105

104

= 40 kA

= 20 kA

= 10 kA

Ip (kA) = 5 kA

I2 t

2.5 ms

(A2 s)

5 30

(INS)

40 m

s tim

e de

lay (S

T)

10 m

s tim

e de

lay

(10 In)

with those in figure 10 for the breakingenergies of the circuit-breaker.

Trip units with arc detectionGenerally combined with electronic trip units,arc detectors may be used to provide protectionfor:c a cubicle: if an arc occurs in a cubicle, thedetector orders opening of the incoming circuit-breaker,c a selective circuit-breaker: positioned in thebreaking unit, the detector provokes via theelectronic trip unit the instantaneous tripping ofthe circuit-breaker.The circuit-breaker is thus self-protected and cantherefore be used up to the limit of itselectrodynamic withstand capacity.Pressure trip systemThe pressure that develops in the breaking unitof a circuit-breaker is a result of the energyproduced by the arc.Above a certain fault current level, this pressuremay be used for detection and tripping.This is possible by directing the expanding gases


in the unit toward a piston that trips the circuit-breaker (see fig. 15 ).Pressure trip systems may be used to:c ensure self-protection of a selective circuit-breaker (similar to the arc detector),c improve breaking and operating reliability of arapid current-limiting circuit-breaker.If each circuit-breaker is fitted with a correctlydesigned pressure trip system, discrimination is

P1 P2 P3 Breaking units

Flap valves

PistonFault on phase 1pressure P1 pressure P2 and P3

Fig. 15: operation of the pressure trip system.

ensured between circuit-breakers with differentratings for all overcurrents greater than 20 In.It is this energy-based trip system (constant I2 tvalue) that makes possible the energy-baseddiscrimination technique employed in theCompact NS current-limiting circuit-breakers.


4 Advantages and implementationof energy-based discrimination

Note that the circuit-breaker trip-unit system,whether electromechanical, electronic or acombination of the two, must offer the followingfeatures:c minimum stresses in the installation (limited and I2 t values),

c tripping dependability (safety),c minimum disturbance for correctly functioningcircuits (voltage dips),c ease of discrimination studies.

4.1 Current-limiting circuit-breaker fitted with a pressure trip systemThe above requirements may best be met with apressure trip system, combined with either anelectromechanical or electronic trip unit.Figure 16 indicates the energy sensitivity of thiscombination. The higher the prospective short-

Fig. 16: trip-unit combination curves (electromagnetic and pressure or electronic and pressure).

circuit current, the shorter the response time, whichleads to a virtually constant tripping time at I2 t.The energy let through by the current-limitingcircuit-breaker during a break follows the samecurve, but with a slight shift.

1

40 m

s20

ms

10 m

s

7 m

s

5 m

s

3 10 50 100

107

106

105

104

= 40 kA

= 20 kA

= 10 kA

Ip (kA) = 5 kA

I2 t

2.5 ms

(A2 s)

5 30

Pressure trip system

ST 40

ms

ST 10

ms


Stresses in the installationStresses are limited compared to those observedin current-limiting circuit-breakers of the previousgeneration.On the basis of the example in figure 16 , thefigures for a Compact NS 250 A and an Ip of40 kA are:c 4 ms for the breaking time;c 20 kA for the peak current;c 8 x 105 A2 s for the I2 t.

Tripping dependabilityThe pressure trip system is a part of the openingmechanism for short-circuits and thereforedepends on the current rating of the circuit-breaker.The adjustable DST release, whetherelectromechanical (see fig. 13 ) or electronic(see fig. 14 ), is physically independent of thepressure trip system. Physical independenceenhances operating dependability.

Voltage dipsVoltage dips in an installation can tripundervoltage releases in circuit-breakers andcontactors.Unnecessary opening, following a voltage dipcaused by a short-circuit, results in reducedcontinuity of service.Consequently, discrimination studies must alsotake into account the reactions of undervoltagereleases and contactors during voltage dips.A voltage dip in a network lasts until the arcvoltage that opposes the source voltage enablesinterruption of the current. It follows that thevoltage dip depends on the type of circuit-breaker and/or trip unit used:c with non-limiting circuit-breakers, the voltagedip is more pronounced and can last from 10 to15 ms (see fig.17 ),c with current-limiting circuit-breakers, the rapiddevelopment of a high arc voltage reduces thevoltage dip both in duration and in amplitude(see fig.17 ).The voltage dip lasts approximately 5 ms andamounts to 50 % of the rated voltage for currentsclose to the level required for contact repulsion.The voltage dip amounts to 30 % of the ratedvoltage for higher currents, but the duration isreduced to 3 to 4 ms. The higher the Isc, theshorter the voltage dip.Any undervoltage releases equipping the circuit-breakers are not affected by such voltage dips.

DiscriminationThe severely limited energy let through by thecircuit-breaker is insufficient to trip the trip unit on

Fig. 17: the voltage dip on the network depends onthe type of circuit-breaker.

a) non-limiting circuit-breaker

b) highly limiting circuit-breaker

Ur

t (ms)10 20

Ua

Ur

i

Ua

i

i

Uaic

Ur 5 10 20

t (ms)

the upstream circuit-breaker which remainsclosed.


4.2 Discrimination with Compact NS circuit-breakersUsing Compact NS circuit-breakers,discrimination is total up to 150 kA.To ensure total discrimination, the energy that acircuit-breaker lets through must be less thanthat required to trip the upstream circuit-breaker.

General ruleDiscrimination is total and without anyrestrictions if:ccccc the ratio between the ratings of thesuccessive circuit-breakers is equal to orgreater than 2.5,ccccc the ratio between the trip unit ratings isgreater than 1.6.

Fig. 18: total discrimination between 100 A, 160 A and 250 A Compact NS circuit-breakers.

1

40 m

s20

ms

10 m

s

7 m

s

5 m

s

3 10 50 100

107

106

105

104

= 40 kA

= 20 kA

= 10 kA

Ip (kA) = 5 kA

I2 t

2,5 ms

(A2 s)

5 30

Breaking

Non-tripping

Breaking

Non-tripping

Breaking

ST 1

60ST

200

ST 2

50

100 A

100 A

Mag

netic

100

A

ST 4

00ST

500

630 AST 6

30

630 A

250 A

250 A

Non-tripping2.5 ms

NoteST 160, ST 200 and ST 250: electronic trip unitsfor 250 A circuit-breakers.ST 400, ST 500 and ST 630: electronic trip unitsfor 630 A circuit-breakers.

Using the energy-based discrimination techniqueand depending on the ratios between theupstream and downstream circuit-breaker ratingsand the trip unit ratings, the Compact NS range(100, 160, 250, 400 and 630 A) offers eitherpartial or total discrimination up to the breakingcapacity.

Total discriminationFigure 18 provides an example of totaldiscrimination up to 100 kA over three levels with100 A, 250 A and 630 A circuit-breakers fittedwith various trip units.


Fig. 19: partial discrimination between two Compact NS circuit-breakers, 160 and 250 A.

1

40 m

s20

ms

10 m

s

7 m

s

5 m

s3 10 50 100

107

106

105

104

= 40 kA

= 20 kA

= 10 kA

Ip (kA) = 5 kA

I2 t

2.5 ms

(A2 s)

5 30

Breaking 160 ANon-tripping 250 A

Discrimination limit

160

A( 8

In)

250

A(10

In)

Partial discriminationIf the general rule presented above is notrespected, discrimination is only partial.Figure 19 indicates that between a 160 A circuit-breaker and a 250 A circuit-breaker fitted with a250 A trip unit, discrimination is ensured up to aprospective short-circuit current of 4 800 A. Thislevel is higher than that observed, under the sameconditions, with standard Compact circuit-breakers.Cascading with the Compact NSCascading, covered by standard NF C 15-100,enables the upstream circuit-breaker to help the

downstream device to break high short-circuitcurrents.Note that this is detrimental to discrimination(except with the SELLIM system).For the Compact NS, cascading in no way modifiesthe total and partial discrimination characteristicsmentioned above.A Compact NS circuit-breaker can however alwaysassist a downstream circuit-breaker of a differenttype and with insufficient breaking capacity.

4.3 Combination with traditional protective devicesStandard circuit-breakersIn an existing installation, the highly limitingCompact NS circuit-breakers may be used forextensions or to replace existing circuit-breakers

without reducing the previous discrimination limit. Onthe contrary, if the new circuit-breaker is installed:c downstream, its current-limiting capacity canonly improve the discrimination level, possibly to


Fig. 20: replacement of a Compact C250 N, H or L by a Compact NS 250 provides improved discrimination. In thisexample, discrimination becomes total.

1

40 m

s20

ms

10 m

s

7 m

s

5 m

s

3 10 50 100

107

106

105

104

= 40 kA

= 20 kA

= 10 kA

Ip (kA) = 5 kA

I2 t

2.5 ms

(A2 s)

5 30

H

N

C 250 L

NS 250

C 250

Non-tripping

Mag

netic

630

A

the point of making discrimination total(see fig. 20 ),c upstream, the discrimination level is at leastequal to the previous level and the high current-limiting capacity of the Compact NS can be usedto reinforce cascading.

FusesThe I2 t = f (Ip) curves (supplied bymanufacturers) concern:c the energy required to blow the fuse(prearcing),

c the energy that flows through the fuse duringthe break.To ensure discrimination between an upstreamcircuit-breaker and a fuse, the circuit-breaker tripunit must not react to the sum of these twoenergies.


5 Conclusion

Using a few simple rules, highly limiting circuit-breakers that operate faster for higherprospective short-circuit currents can beimplemented to provide total discrimination overseveral network levels. They may alsoimplement time-discrimination techniques.This is a major technical innovation that can beused to:c considerably simplify discrimination studies,c minimize electrodynamic forces, thermalstresses and voltage dips resulting from short-circuits.This new discrimination technique, referred to asenergy discrimination and based on total controlover the energy let through by the circuit-breakers during breaking and on the sensitivityof the trip units to the same energy, is animportant contribution to improving theavailability of electrical power.


6 Appendix - indications concerning breakingwith current limiting

Figure 21 shows the currents and voltages for ahalf-period current-limiting phenomenon.The short-circuit current (ib) obeys the followingrelationship:

Ur Ua = r i + L didt

L didt

c at the beginning of the short-circuit, Ua is zero,ib and ip are equal and have identical slopes,c when Ua is equal to the network voltage Ur, ibattains its maximum value (b) because itsderivative is equal to zero,c when Ua is greater than Ur, ib declines to zeroat tb.The interrupted current wave is equivalent to asinusoidal half wave with a period equal to twicethe virtual breaking time (tvb).With the above information, it is easy todetermine the energy dissipated in theimpedances of the concerned circuit.Expressed in other terms, the formula for thisenergy, called the breaking energy, is:

Eb i dtbtvb

= 20where ib is a sinusoidal function:

Eb tb vb ( ).=

12

12

It is useful to express Eb as a function of Ip andthe duration (tvb) of the break:c tvb u 10 msFor such a duration, the fault current is low, thecircuit-breaker contacts do not repel each otherand there is therefore no arcing voltage:

i i andb p b p ;= = 2 I

and formula 1 may be expressed as:

Eb p t ( )= I 2 2

c tvb 10 msThe circuit-breaker limits the fault current.ib and ip have the same initial slope, therefore:

didt

p b = = I 2

where = pi tvb

t pvb b piI 2 = hence:

b vbt f p = 2 2Ior

t

f pvbb

=

2 2I

If we express equation (1) as:

b

vb

Ebt

2 2

=

we obtain:

2 2 22

Ebt

t f pvb

vb= ( )Ihence:

Eb f p tvb ( )= 4 32 2 3IAgain on the basis of (1), but with b in mind:

t Eb

f pvb bb

= =

22 22

I

we obtain:

Eb

f pb

( )= 3

4 24

I

Formulas (3) and (4) can be used to plot the timeand peak current curves.


Fig. 21: breaking with current limitation.

Ua

Ur

ip

t

b

ir

0 tr ta tb

ib

^t tvb T/2

di/dto

Ua: arcing votlageUr: network voltageip: prospective currentib: break current (limited)b: maximum break currentir: contact repulsion current

: time corresponding to bta: time at which the arc appearstb: breaking timetr: time at which contact repulsion occurstvb: virtual breking time: angular frequency of the interrupted wave

t

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

Real.: Sodipe - ValenceEdition: SEST Grenoble03.98 - 1500 - Printing: ClercPrinted in France

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ider

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