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..........................................................................Collection
Technique
Cahier technique n° 167
Energy-based discrimination forlow-voltage protective
devices
M. SerpinetR. Morel
■ Merlin Gerin ■ Modicon ■ Square D ■ Telemecanique
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Robert MOREL
Graduated with an engineering degree from ENSMM in Besançon
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 «anticipating»future developments.
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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 ≈∫ I
ibLimited 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).
I rCorresponds 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.
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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.
Contents
1. Discrimination in low-voltage 1.1 Definition p. 4
1.2 Enhanced safety and availability p. 5
1.3 Discrimination zones p. 5
2. Discrimination techniques for short-circuits 2.1 Current
discrimination p. 7
2.2 Time discrimination p. 7
2.3 “SELLIM” discrimination p. 8
2.4 Zone selective interlocking p. 9
2.5 Combining the different types of discrimination p. 9
3. Energy-based discrimination 3.1 Choice of operating curves p.
10
3.2 Characterisation of a Compact NS circuit-breaker p. 11
3.3 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. 16
4.2 Discrimination with Compact NS circuit-breakers p. 18
4.3 Combination with traditional protective devices p. 19
5. Conclusion p. 21
6. Appendix - indications concerning breaking with current
limiting p. 22
energy-based discrimination
protective devices
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Cahier Technique Schneider n° 167 / p.4
1 Discrimination in low-voltage protective devices
1.1 Definition
In 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.
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Cahier Technique Schneider n° 167 / p.5
Fig. 2 : circuit-breaker behaviour during a fault.
1.2 Enhanced safety and availability
A 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.
Examples
c 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 zones
Two 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.
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Cahier Technique Schneider n° 167 / p.6
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 zone
Discrimination 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.
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Cahier Technique Schneider n° 167 / p.7
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 discrimination
This 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 < I ins
of CB1).The minimum ratio between I ins1 and I ins2 mustbe 1.5 to
take into account threshold accuracies.
2.2 Time discrimination
To 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 I ins1 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-breakers
The 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).
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Cahier Technique Schneider n° 167 / p.8
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 and“pseudo-time”
discriminationThese circuit-breakers have two
maincharacteristics:
Fig. 5 : time discrimination.
IDIN1
CB2
CB2 : rapidCB1 : selective with1-2-3 ST settings
tb
CB1
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 ensure“pseudo-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 kÂ
Fault at B
i3
u3
i2
u2
i1
u1
3.5 ms
12 ms
Fault at A
34 kÂ
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Cahier Technique Schneider n° 167 / p.9
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,
entitled“Protection 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 discrimination
The 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.
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Cahier Technique Schneider n° 167 / p.10
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 curves
The 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
ST1
ST2
t(s)
Ip(A)
LT
(A2 s)
Ip(A)
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Cahier Technique Schneider n° 167 / p.11
(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) ⇒
îbf 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 (îb).
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
î = 40 kA
î = 20 kA
î = 10 kA
Ip (kA)
î = 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 =
îbf 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-breaker
Display of the breaking I 2 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
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Cahier Technique Schneider n° 167 / p.12
irrespective of the moment at which the faultoccurs (upper
limits) (see fig. 12 ).
Curve analysis
A 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 (îb) 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
î = 40 kA
î = 20 kA
î = 10 kA
Ip (kA)
î = 5 kA
Ι2 t
2.5 ms
(A2 s)
5 30(10 In)
DC
(B)
(E)A F
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Cahier Technique Schneider n° 167 / p.13
3.3 Characterisation of the trip units
Trip 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 units
c 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 a“clock-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
î = 40 kA
î = 20 kA
î = 10 kA
Ip (kA)
î = 5 kA
Ι2 t
2.5 ms
(A2 s)
5 30(10 In)
20 m
s fix
edTi
me
dela
y (S
T)
Depend
ent
time d
elay (D
ST)
Instantaneous (INS)
Hig
h se
t
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Cahier Technique Schneider n° 167 / p.14
current value. For high fault currents, their I2 tcharacteristic
is theoretically a straight line(îb = 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
î = 40 kA
î = 20 kA
î = 10 kA
Ip (kA)
î = 5 kA
I2 t
2.5 ms
(A2 s)
5 30
(INS)
40 m
s tim
e de
lay
(ST)
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 detection
Generally 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
-
Cahier Technique Schneider n° 167 / p.15
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.
-
Cahier Technique Schneider n° 167 / p.16
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 îand 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
system
The 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
î = 40 kA
î = 20 kA
î = 10 kA
Ip (kA)
î = 5 kA
I2 t
2.5 ms
(A2 s)
5 30
Pressure
trip system
ST
40 m
sS
T 10
ms
-
Cahier Technique Schneider n° 167 / p.17
Stresses in the installation
Stresses 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 dependability
The 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 dips
Voltage 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.
-
Cahier Technique Schneider n° 167 / p.18
4.2 Discrimination with Compact NS circuit-breakers
Using 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
î = 40 kA
î = 20 kA
î = 10 kA
Ip (kA)
î = 5 kA
I2 t
2,5 ms
(A2 s)
5 30
Breaking
Non-tripping
Breaking
Non-tripping
Breaking
ST
160
ST
200
ST
250
100 A
100 A
Mag
netic
100
A
ST
400
ST
500
630 AST
630
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 discrimination
Figure 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.
-
Cahier Technique Schneider n° 167 / p.19
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
î = 40 kA
î = 20 kA
î = 10 kA
Ip (kA)
î = 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(1
0 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 devices
Standard 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
-
Cahier Technique Schneider n° 167 / p.20
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
î = 40 kA
î = 20 kA
î = 10 kA
Ip (kA)
î = 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.
-
Cahier Technique Schneider n° 167 / p.21
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.
-
Cahier Technique Schneider n° 167 / p.22
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 (îb) 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 = ∫ 20
where ib is a sinusoidal function:
Eb tb vb ( ).=
12
12î
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 ;= =î 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 = = ′ω ωI 2 î
where ′ =ω π tvb
t pvb b ω πI 2 = îhence:
îb vbt f p = 2 2Ior
t
f pvbb
=
î
2 2I
If we express equation (1) as:
îb
vb
Ebt
2 2 =
we obtain:
22 2
2
Ebt
t f pvb
vb= ( )Ihence:
Eb f p tvb ( )= 4 32 2 3I
Again on the basis of (1), but with îb in mind:
t
Eb
f pvb b
b
= =22 22î
î
I
we obtain:
Eb
f pb
( )=
î3
4 24
I
Formulas (3) and (4) can be used to plot the timeand peak
current curves.
-
Cahier Technique Schneider n° 167 / p.23
Fig. 21 : breaking with current limitation.
Ua
Ur
ip
t
îb
ir
0 tr ta tb
ib
^t tvb T/2
di/dto
Ua: arcing votlageUr: network voltageip: prospective currentib:
break current (limited)îb: maximum break currentir: contact
repulsion current
: time corresponding to îbta: time at which the arc appearstb:
breaking timetr: time at which contact repulsion occurstvb: virtual
breking timeω: angular frequency of the interrupted wave
t̂
-
Cahier Technique Schneider n° 167 / p.24
-
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Real.: Sodipe - ValenceEdition: SEST Grenoble03.98 - 1500 -
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© 1
998
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neid
er
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