Ect158 Calculation of Short-circuit Currents

8/12/2019 Ect158 Calculation of Short-circuit Currents

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Cahier technique no. 158

Calculation of short-circuitcurrents

B. De Metz-NoblatF. DumasG. Thomasset

Collection Technique


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"Cahiers Techniques" is a collection of documents intended for engineersand technicians, people in the industry who are looking for more in-depthinformation in order to complement that given in product catalogues.

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Benot de METZ-NOBLAT

Graduate engineer from ESE (Ecole Suprieure dElectricit), heworked for Saint-Gobain first as a research engineer, then inmaintenance and new projects on a production site.He joined Schneider Electric in 1986, and is currently responsible forthe Electrical System Analysis Group at the Research andDevelopment Department.

Frdric DUMAS

After graduating as doctor engineer from UTC (Universit deTechnologie de Compigne) in 1993, he joined Schneider Electric.Works for the the Electrical System Analysis Group at the Researchand Development Department, and is in charge of research projects inthe field of industrial and distribution networks, including thedevelopment of software for electrotechnical calculations.

Georges THOMASSETA graduate engineer from IEG (Institut dElectrotechnique deGrenoble) in 1971, has since conducted numerous research, designand implementation of complex industrial network projects inMerlin Gerins Technical Management Department.He has long and valuable experience as responsible of the technicalsection for the industrial unit in the Engineering and ContractingDepartment from 1984 to 1996. He is now responsible of the generaltechnical development of offers at the Schneider Electric Applicationsand Services Activity.

no. 158

Calculation of short-circuitcurrents

ECT 158 updated June 2000


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Cahier Technique Schneider Electric no. 158 / p.2

Lexicon

AbbreviationsBC Breaking capacity.MLVS Main low voltage switchboard.

SymbolsA Cross sectional area of conductors. Angle between the initiation of the fault

and zero voltage.c Voltage factor.cos Power factor (in the absence of

harmonics).e Electromotive force.E Electromotive force (maximum value).

Phase angle (current with respect tovoltage).i Instantaneous current.ia Alternating sinusoidal component of

the instantaneous current.idc Aperiodic component of the

instantaneous current.ip Maximum current value (first peak of

the fault current).I Maximum r.m.s. current.Ib Short-circuit breaking current

(IEC 60909).

Ik Steady-state short-circuit current(IEC 60909).Ik Initial short-circuit current

(IEC 60909 ).Ir Rated current of a generator.Is Service current.Isc Steady-state short-circuit current (Isc 3

= three-phase, Isc 2 = two-phase, ).

Factor depending on the saturationinductance of a generator .

k et K Constants (tables and graphs).Ra Equivalent resistance of the upstream

network.RL Line resistance per unit length.S Cross-sectional area of conductors.Sn Transformer kVA rating.Ssc Short-circuit power.tmin Minimum dead time for short-circuit

development time, often equal to thetime delay of a circuit breaker.

u Instantaneous voltage.

usc Transformer short-circuit voltage in %.U Network phase-to-phase voltage with

no load.Un Network rated voltage with load.x Reactance, in %, of the rotating

machinesXa Equivalent reactance of the upstream

network.XL Line reactance per unit length.Xsubt Reactance in % of rotating machines.ZL Link impedance.Zsc Network upstream impedance for a

three-phase fault.Zup Equivalent impedance of the

upstream networkZ(1), Z (2) , Z (0)

Positive-sequence, negative- sequence and zero-sequenceimpedances of a network or anelement.


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Calculation of short-circuit currents

Summary

1 Introduction p. 41.1 The main types of short-circuits p. 51.2 Etablishing the short-circuit current p. 71.3 Standardised Isc calculations p. 101.4 Methods presented in this document p. 111.5 Basic assumptions p. 11

2 Calculation of I sc by the impedance method 2.1 Isc depending on the different types of short-circuit p. 122.2 Determining the various short-circuit impedances p. 132.3 Relationships between impedances at the differentvoltage levels in an installation p. 182.4 Calculation example p. 19

3 Calculation of I sc values in a radial 3.1 Advantages of this method p. 23

3.2 Symmetrical components p. 23 3.3 Calculation as defined by IEC 60909 p. 24

3.4 Equations for the various currents p. 263.5 Calculation example p. 27

4 Computerised calculations and conclusion p. 31

Bibliography p. 32

In view of sizing electrical installation and the required equipment, as wellas determining the means required for the protectiuon of life and property,short-circuit currents must be calculated for every point in the network.

This "Cahier Technique" reviews the calculation methods for short-circuitcurrents as laid down by standards such as IEC 60909. It is intended forradial low-volktage (LV) and high-voltage (HV) circuits.

The aim is to provide a further understanding of the calculation methods,essential when determining short-circuit currents, even when computerisedmethods are employed.

network using symmetrical components


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1 Introduction

Electrical installations almost always requireprotection against short-circuits wherever thereis an electrical discontinuity. This most oftencorresponds to points where there is a changein conductor cross-section. The short-circuitcurrent must be calculated at each level in theinstallation in view of determining thecharacteristics of the equipment required towithstand or break the fault current.The flow chart in figure 1 indicates theprocedure for determining the various short-circuit currents and the resulting parameters forthe different protection devices.

In order to correctly select and adjust theprotection devices, graphs of figures 2, 3 & 4

are used. Two values of the short-circuit currentmust be evaluated:c the maximum short-circuit current, used todetermine:v the breaking capacity of the circuit breakers;v the making capacity of the circuit breakers;v the electrodynamic withstand capacity of thewiring system and switchgear.

The maximum short-circuit current correspondsto a short-circuit in the immediate vicinity of thedownstream terminals of the protection device. It

must be calculated accurately and used with asafety margin.

Fig. 1 : short-circuit ( I sc) caculation procedure when designing an electrical installation.

Upstream Ssc

u (%)

Iscat transformer

terminals

Iscof main LV switchboard

outgoers

Isc

at head of secondaryswitchboards

Iscat head of finalswitchboards

Iscat end of final

outgoers

Conductor characteristicsc Busbars:- length,- width,- thickness.c Cables:- type of insulation,- single-core or multicore,- length,- cross-section,c Environment :- ambient temperature,- installation method,- number of contiguous circuits.

Breaking capacity

Inst. trip settingMaincircuit breaker

Breaking capacity

Inst. trip setting

Main LVswitchboarddistributioncircuit breakers

Breaking capacity

Inst. trip setting

Secondarydistributioncircuit breakers

Breaking capacity

Inst. trip setting

Finaldistributioncircuit breakers

Feeder currentratingsvoltage drops

Power factor,coincidence factor,duty factor,foreseeable expansionfactor

HV / LVtransformer rating

Loadrating

sc


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a 5 s

Iz1 < Iz2

t 1 2

1 > 2

I2t = k 2A2

I

Fig. 2 : the I 2 t characteristics of a conductor depending on the ambient temperature (temperature of case 1: t1 being higher that temperature of case 2 : t2, the admissible current limit I z1 is lower than the admissible current limit I z2 ).

Transientoverload

I

t

Designcurrent

Cable or I2tcharacteristic

Circuit breakertime-currentcurve

IB Ir Iz Isc BC

Fig. 3 : circuit protection using a circuit breaker.

Transientoverload

Cable or I2tcharacteristic

Fuse time-currentcurve

I

t

IB Ir Iz

Fig. 4 : circuit protection using an aM fuse.

1.1 The main types of short-circuits

v protection of life depends on circuit breaker orfuse operation, essentially the case for TN andIT electrical systems.

Note that the minimum short-circuit currentcorresponds to a short-circuit at the end of theprotected line, generally phase-to-earth for LV andphase-to-phase for HV (neutral not distributed),under the least severe operating conditions (faultat the end of a feeder and not just downstreamfrom a protection device, one transformer inservice when two can be connected, etc.).

Note also that whatever the case, for whatevertype of short-circuit current (minimum ormaximum), the protection device must clear theshort-circuit within a time tc that is compatiblewith the thermal stresses that can be withstoodby the protected cable:

i2 dt k A2 2 (see fig. 2, 3 and 4 )where A is the cross-sectional area of theconductors and k is a constant calculated on thebasis of different correction factors for the cableinstallation method, contiguous circuits, etc.

Further practical information may be found in the"Electrical Installation Guide" published bySchneider Electric (see the bibliography).

Various types of short-circuits can occur inelectrical installations.

Characteristics of short-circuitsThe primary characteristics are:c duration (self-extinguishing, transient andsteady-state);c origin:v mechanical (break in a conductor, accidentalelectrical contact between two conductors via a

foreign conducting body such as a tool or ananimal);v internal or atmospheric overvoltages;v insulation breakdown due to heat, humidity ora corrosive environment;c location (inside or outside a machine or anelectrical switchboard).

Short-circuits can be:c phase-to-earth (80 % of faults);

c the minimum short-circuit current, essentialwhen selecting the time-current curve for circuitbreakers and fuses, in particular when:v cables are long and/or the source impedanceis relatively high (generators, UPSs);


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c phase-to-phase (15 % of faults). This type offault often degenerates into a three-phase fault;c three-phase (only 5 % of initial faults).These different short-circuit currents arepresented in figure 5 .

Consequences of short-circuitsThe consequences are variable depending onthe type and the duration of the fault, the point inthe installation where the fault occurs and theshort-circuit power. Consequences include:c at the fault location, the presence of electricalarcs, resulting in:v damage to insulation;v welding of conductors;v fire and danger to life;c on the faulty circuit:v electrodynamic forces, resulting in:- deformation of the busbars;- disconnection of cables;

Fig. 5 : different types of short-circuits and their currents. The direction of current is chosen arbitrarily.(See IEC 60909).

v excessive temperature rise due to an increasein Joule losses, with the risk of damage toinsulation;c on other circuits in the network or in near-bynetworks:v voltage dips during the time required to clearthe fault, ranging from a few milliseconds to afew hundred milliseconds;v shutdown of a part of the network, the extent ofthat part depending on the design of the networkand the discrimination levels offered by theprotection devices;v dynamic instability and/or the loss of machinesynchronisation;v disturbances in control / monitoring circuits,v etc...

Short-circuit current, Partial short-circuit currents in conductors and earth.

In calculations, the various currents ( Ik") are identified by an index.

Ik"

L3

L2L1

Ik"

L3

L2L1

Ik"

Ik" Ik

"

L3

L2L1

Ik"

L3

L2L1

a) Symmetrical three-phase short-circuit.

b) Phase-to-phase short-circuit clear ofearth.

c) Phase-to-phase-to-earth short-circuit.

d) Phase-earth short-circuit.


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1.2 Establishing the short-circuit current

A simplified network comprises a source ofconstant AC power, a switch, an impedance Zscthat represents all the impedances upstream of

the switch, and a load impedance Zs (see fig. 6 ).In a real network, the source impedance is madeup of everything upstream of the short-circuitincluding the various networks with differentvoltages (HV, LV) and the series-connectedwiring systems with different cross-sectionalareas (A) and lengths.In figure 6 , when the switch is closed, thedesign current Is flows through the network.

When a fault occurs between A and B, thenegligible impedance between these pointsresults in a very high short-circuit current Isc thatis limited only be impedance Zsc.

The current Isc develops under transientconditions depending on the reactances X andthe resistances R that make up impedance Zsc:

Zsc = R X2 2+In power distribution networks, reactanceX = L is normally much greater than resistance

R and the R / X ratio is between 0.1 and 0.3. Theratio is virtually equals cos sc for low values:

cosR

R Xsc 2 2 =

+However, the transient conditions prevailing whilethe short-circuit current develops differ dependingon the distance between the fault location and thegenerator. This distance is not necessarilyphysical, but means that the generatorimpedances are less than the link impedancebetween the generator and the fault location.

Fault away from the generatorThis is the most frequent situation. The transientconditions are those resulting from the applicationof a voltage to a reactor-resistance circuit. This

voltage is:e = E sin t + ( )Current i is then the sum of the two components:i = i ia dc+ .

c The first (i a) is alternating and sinusoidal:

i = sin t +a ( ) whereI = maximum current = E

Zsc,

= angle characterising the difference betweenthe initiation of the fault and zero voltage.

c The second (i dc ) is an aperiodic component,

i - sin edc -RL t = . Its initial value depends on

a and its decay rate is proportional to R / L.

At the initiation of the short-circuit, i is equal tozero by definition (the design current Is isnegligible), hence:i = i ia dc+ = 0Figure 7 shows the graphical composition of i asthe algebraic sum of its two components i a et i dc .

Fault initiation

It

i = ia + idc

ia = I sin ( t + )

idc = - I sin eR

Lt-

Fig. 6 : simplified network diagram.

R

A

Zsc

B

X

Zse

Fig. 7 : graphical presentation and decomposition of a short-circuit current occuring away from the generator.


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Fig. 8 : graphical presentation of the two extreme cases (symmetrical and asymmetrical) for a short-circuit current.

The moment the fault occurs or the moment of closing,with respect to the network voltage, is characterised byits closing angle a (occurrence of the fault). The voltagecan therefore be expressed as: u = E sin ( t + ) .The current therefore develops as follows:

i =EZ

sin t + - - sin - e-

RL

t ( ) ( )

with its two components, one being alternating with ashift equal to with respect to the voltage and thesecond aperiodic and decaying to zero as t tends toinfinity. Hence the two extreme cases defined by:c = /2 , said to be symmetrical (or balanced),(see figure a ).

The fault current can be defined by: i =EZ

sin t which, from the initiation, has the same shape as forsteady state conditions with a peak value E / Z.c = 0, said to be asymmetrical (or unbalanced),(see figure b ).

The fault current can be defined by:

i =EZ

sin t - - sin e-

RL

t ( )

Its initial peak value i p therefore depends on , i.e. onthe R / X = cos ratio of the circuit.

i

u

I = 2 Ia

ip

u

idci

b) Asymmetrical

a) Symmetrical

Figure 8 illustrates the two extreme cases forthe development of a short-circuit current,

presented, for the sake of simplicity, with asingle-phase, alternating voltage.

The factor e- R

L t

is inversely proportional to theaperiodic component damping, determined bythe R / L or R / X ratios.

The value of i p must therefore be calculated todetermine the making capacity of the requiredcircuit breakers and to define the electrodynamicforces that the installation as a whole must becapable of withstanding.

Its value may be deduced from the rms value ofthe symmetrical short-circuit current Ia using theequation:

i K 2 ap = where the coefficient K isindicated by the curve in figure 9 , as a functionof the ratio R / X or R / L.

Fault near the generatorWhen the fault occurs in the immediate vicinity ofthe generator supplying the circuit, the variationin the impedance of the generator, in this casethe dominant impedance, damps the short-circuitcurrent.

The transient current-development conditionsare complicated by the variation in theelectromotive force resulting from the short-circuit. For simplicity, the electromotive force is

assumed to be constant and the internalreactance of the machine variable. The

reactance develops in three stages:c subtransient (the first 10 to 20 milliseconds ofthe fault);c transient (up to 500 milliseconds);c steady-state (or synchronous reactance).

Note that in the indicated order, the reactanceacquires a higher value at each stage, i.e. thesubtransient reactance is less than the transientreactance, itself less than the steady-statereactance. The successive effect of the threereactances leads to a gradual reduction in the

2.0

1.8

1.6

1.4

1.2

1.00 0.2 0.4 0.6 0.8 1.0 1.2 R / X

K

Fig. 9 : variation of coefficient K depending on R / X or R / L (see IEC 60909).


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short-circuit current which is the sum of fourcomponents (see fig. 10 ):c the three alternating components(subtransient, transient and steady-state);c the aperiodic component resulting from thedevelopment of the current in the circuit(inductive).

Practically speaking, information on thedevelopment of the short-circuit current is notessential:c in a LV installation, due to the speed of thebreaking devices, the value of the subtransientshort-circuit current, denoted Ik", and of themaximum asymmetrical peak amplitude i p is

Fig. 10 : total short-circuit current I sc ( e ), and contribution of its components: a) subtransient of each reactance; b) transient reactance; c) steady-state reactance; d) aperiodic component.Note that the decrease in the generator reactance is faster than that of the aperiodic component. This is a rare situation that can cause saturation of the magnetic circuits and interruption problems because several periods occur before the current passes through zero.

0 t (s)

0 t (s)

0 t (s)

0 t (s)

0 t (s)

0.3

Subtransient Transient Steady-state

0.50.1

a)

b)

c)

d)

e)


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sufficient when determining the breakingcapacities of the protection devices and theelectrodynamic forces;c in LV power distribution and in HV applications,however, the transient short-circuit current is

often used if breaking occurs before the steady-state stage, in which case it becomes useful touse the short-circuit breaking current, denoted Ib,which determines the breaking capacity of thetime-delayed circuit breakers. Ib is the value ofthe short-circuit current at the moment

interruption is effective, i.e. following a time t afterthe development of the short-circuit, wheret = tmin Time t min (minimum time delay) is the sumof the minimum operating time of a protectionrelay and the shortest opening time of theassociated circuit breaker, i.e. the shortest timebetween the appearance of the short-circuitcurrent and the initial separation of the polecontacts on the switching device.Figure 11 presents the various currents of theshort-circuits defined above.

1.3 Standardised I sc calculations

The standards propose a number of method.c Application guide C 15-105, whichsupplements NF C 15-100 (Normes Fran aises)(low-voltage AC installations), details fourmethods:v the "impedance" method, used to calculatefault currents at any point in an installation with a

high degree of accuracy.This method involves adding the variousresistances and reactances of the fault loopseparately, from (and including) the source to thegiven point, and then calculating thecorresponding impedance. The Isc value isfinally obtained by applying Ohm's law:

Is c = U n / (Z)All the characteristics of the various elements inthe fault loop must be known (sources and wiringsystems).Note that in the application guide, a number offactors are not taken into account, notably:- the reactances of the circuit breakers and thebusbars;- the resistances of rotating machines.

The results obtained may be different from thosepresented in the next chapter, because thesefactors are taken into account;v the "composition" method, which may be usedwhen the characteristics of the power supply arenot known. The upstream impedance of thegiven circuit is calculated on the basis of anestimate of the short-circuit current at its origin.

Power factor cos sc = R / X is assumed to beidentical at the origin of the circuit and the faultlocation. In other words, it is assumed that theelementary impedances of two successivesections in the installation are sufficiently similarin their characteristics to justify the replacementof vectorial addition of the impedances byalgebraic addition. This approximation may beused to calculate the value of the short-circuitcurrent modulus with sufficient accuracy for theaddition of a circuit. This very approximatemethod should be used only for installationsrated up to 800 kVA;v the "conventional" method, which can be used,

when the impedances or the Isc in theinstallation upstream of the given circuit are not

Fig. 11 : short-circuit currents near a generator (schematic diagram).

i

Asymmetrical

Symmetrical

Subtrans. Transient Steady-state


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known, to calculate the minimum short-circuitcurrents and the fault currents at the end of aline. It is based on the assumption that thevoltage at the circuit origin is equal to 80 % ofthe rated voltage of the installation during theshort-circuit or the fault.This method considers only the resistance of theconductors and applies a coefficient greater than1 to conductors with large cross-sectional areasto take into account their inductance (1.15 for150 mm 2, 1.20 for 185 mm 2, etc.). It is mainlyused for final circuits with their origin at adistance that is sufficiently far from the powersource (network or power-station unit);v the "simplified" method (presented in detail inthis application guide), which, via tables basedon numerous simplifying assumptions, indicatesfor each conductor cross-sectional area:- the current rating of the overload protectiondevice;- maximum lengths of wiring systems to maintainprotection against indirect contact;- permissible lengths in view of line voltagedrops.The data in the tables is in fact the result ofcalculations run using essentially the

composition and conventional methods. Thismethod may be used to determine thecharacteristics of a circuit to be added to anexisting installation for which sufficientinformation is not available. It is directlyapplicable to LV installations, and can be usedwith correction coefficients if the voltage is not230 / 400 V.c Standard IEC 909 (VDE 0102) applies to allnetworks, radial or meshed, up to 230 kV. Thismethod, based on the Thevenin theorem,calculates an equivalent voltage source at theshort-circuit location and then determines thecorresponding short-circuit current. All networkfeeders as well as the synchronous andasynchronous machines are replaced in thecalculation by their impedances (positivesequence, negative-sequence and zero-sequence). All line capacitances and the paralleladmittances of non-rotating loads, except thoseof the zero-sequence system, are neglected.c Other methods use the superposition principleand require that the load current first becalculated. Note also the method proposed bystandard IEC 865 (VDE 0103) which calculatesthe thermally equivalent short-circuit current.

1.4 Methods presented in this document

In this "Cahier Technique" publication, twomethods are presented for the calculation ofshort-circuit currents in radial networks:c the impedance method, reserved primarily forLV networks, was selected for its high degree ofaccuracy and its instructive value, given thatvirtually all characteristics of the circuit are takeninto account.

c the IEC 60909 method, used primarily for HVnetworks, was selected for its accuracy and itsanalytical character. More technical in nature,

it implements the symmetrical-componentprinciple.

1.5 Basic assumptions

To simplify the short-circuit calculations, anumber of assumptions are required. Theseimpose limits for which the calculations are validbut usually provide good approximations,facilitating comprehension of the physicalphenomena and consequently the short-circuitcurrent calculations. They nevertheless maintaina fully acceptable level of accuracy, "erring"systematically on the conservative side.The assumptions used in this document are asfollows:c the given network is radial with rated voltagesranging from LV to HV, but not exceeding230 kV, the limit set by standard IEC 60909;c the short-circuit current, during a three-phaseshort-circuit, is assumed to occur simultaneouslyon all three phases;c during the short-circuit, the number of phasesinvolved does not change, i.e. a three-phase

fault remains three-phase and a phase-to-earthfault remains phase-to-earth;c for the entire duration of the short-circuit, thevoltages responsible for the flow of the currentand the short-circuit impedance do not changesignificantly;c transformer regulators or tap-changers areassumed to be set to a medium position (if theshort-circuit occurs away from the generator, theactual position of the transformer regulator ortap-changers does not need to be taken intoaccount;c arc resistances are not taken into account;c all line capacitances are neglected;c load currents are neglected;c all zero-sequence impedances are taken intoaccount.


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2 Calculation of I sc by the impedance method

2.1 I sc depending on the different types of short-circuit

Three-phase short-circuitThis fault involves all three phases. Short-circuitcurrent Isc 3 is equal to:

sc U / 3Zsc3

=

where U (phase-to-phase voltage) correspondsto the transformer no-load voltage which is 3 to5 % greater than the on-load voltage across theterminals. For example, in 390 V networks, thephase-to-phase voltage adopted is U = 410, andthe phase-to-neutral voltage is U / 3 = 237 V .Calculation of the short-circuit current thereforerequires only calculation of Zsc, the impedanceequal to all the impedances through which Isc

flows from the generator to the location of thefault, i.e. the impedances of the power sourcesand the lines (see fig. 12 ). This is, in fact, the"positive-sequence" impedance per phase:

Zsc = R X + 2 2

where

R = the sum of series resistances,X = the sum of series reactances.It is generally considered that three-phase faultsprovoke the highest fault currents. The faultcurrent in an equivalent diagram of a polyphasesystem is limited by only the impedance of onephase at the phase-to-neutral voltage of the

Fig. 12 : the various short-circuit currents.

ZLZsc

VZL

ZL

ZL

ZLU

Zsc

Zsc

ZL

ZLn V

ZLn

Zsc

ZL

Z(0)

V

Z(0)

Zsc

Three-phase fault

Phase-to-phase fault

Phase-to-neutral fault

Phase-to-earth fault

sc U / 3Zsc3

=

sc U / 3Zsc + Z1 Ln

=

sc U2 Zsc2

=

sc U / 3Zsc + Z0 0

( )( )

=


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network. Calculation of Isc 3 is therefore essentialfor selection of equipment (maximum current andelectrodynamic withstand capability).

Phase-to-phase short-circuit clear of earthThis is a fault between two phases, supplied witha phase-to-phase voltage U.In this case, the short-circuit current Isc 2 is lessthan that of a three-phase fault:

sc U2 Zsc

=3

2 sc 0.86 sc2 3 3 =

Phase-to-neutral short-circuit clear of earthThis is a fault between one phase and theneutral, supplied with a phase-to-neutral voltageV = U / 3 .The short-circuit current Isc 1 is:

sc = U / 3Zsc + Z1 Ln

In certain special cases of phase-to-neutralfaults, the zero-sequence impedance of the

source is less than Zsc (for example, at theterminals of a star-zigzag connected transformeror of a generator under subtransient conditions).In this case, the phase-to-neutral fault currentmay be greater than that of a three-phase fault.

Phase-to-earth fault (one or two phases)This type of fault brings the zero-sequenceimpedance Z (0) into play.Except when rotating machines are involved(reduced zero-sequence impedance), the short-circuit current Isc (0) is less than that of a three-phase fault.Calculation of Isc (0) may be necessary,depending on the neutral system (systemearthing arrangement), in view of defining thesetting thresholds for the zero-sequence (HV) orearth-fault (LV) protection devices.

Figure 12 shows the various short-circuitcurrents

2.2 Determining the various short-circuit impedances

This method involves determining the short-circuit currents on the basis of the impedancerepresented by the "circuit" through which theshort-circuit current flows. This impedance maybe calculated after separately summing thevarious resistances and reactances in the faultloop, from (and including) the power source tothe fault location.

(The circled numbers X may be used to comeback to important information while reading theexample at the end of this section.)

Network impedancesc Upstream network impedanceGenerally speaking, points upstream of the powersource are not taken into account. Available dataon the upstream network is therefore limited tothat supplied by the power distributor, i.e. only theshort-circuit power Ssc in MVA.The equivalent impedance of the upstreamnetwork is:

1 Zup = USsc

2

where U is the no-load phase-to-phase voltageof the network.The upstream resistance and reactance may bededuced from Rup / Zup (for HV) by:Rup / Zup 0.3 at 6 kV;Rup / Zup 0.2 at 20 kV;Rup / Zup 0.1 at 150 kV.

As, Xup Zup - Rup = 2 2 ,

XupZup

= 1 -RupZ pu

2

2 Therefore, for 20 kV,

Xup

Zup 1 - 0,2 ,

= ( ) =2 0 980

Xup 0.980 Zup= at 20 kV,hence the approximation Xup Zup .c Internal transformer impedanceThe impedance may be calculated on the basisof the short-circuit voltage u sc expressed as apercentage:

3 Z uUSnT sc

2= where

U = no-load phase-to-phase voltage of thetransformer;Sn = transformer kVA rating;

U u sc = voltage that must be applied to theprimary winding of the transformer for the ratedcurrent to flow through the secondary winding,when the LV secondary terminals are short-circuited.

For public distribution MV / LV transformers, thevalues of u sc have been set by the EuropeanHarmonisation document HD 428-1S1 issued inOctober 1992 (see fig. 13 ).

Fig. 13 : standardised short-circuit voltage for public distribution transformers.

Rating (kVA) of the HV / LV transformer 630 800 1,000 1,250 1,600 2,000Short-circuit voltage u sc (%) 4 4.5 5 5.5 6 7


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Note that the accuracy of values has a directinfluence on the calculation of Isc in that an errorof x % for u sc produces an equivalent error (x %)for ZT.

4 In general, R T 150 mm 2).v the reactance per unit length of overhead lines,cables and busbars may be calculated as:

X L 15.7 144.44 LogdrL

= = +

Fig. 14 : resultant error in the calculation of the short-circuit current when the upstream network impedance Zup is neglected.

500 1,000 1,500 2,0000

5

10

12

Pn(kVA)

Psc = 250 MVA

Psc = 500 MVA

Isc/ Isc(%)


17/36


expressed as m / km for a single-phase orthree-phase delta cable system, where (in mm):r = radius of the conducting cores;d = average distance between conductors.

N.B. Above, Log = decimal logarithm.

For overhead lines, the reactance increasesslightly in proportion to the distance between

conductors (Log dt

), and therefore inproportion to the operating voltage .

7 the following average values are to be used:

X = 0.3 / km (LV lines);X = 0.4 / km (MV or HV lines).The table in figure 16 shows the variousreactance values for conductors in LVapplications, depending on the wiring system.The following average values are to be used:

- 0.08 m / m for a three-phase cable ( ),and, for HV applications, between 0.1 and0.15 m / m.

8 - 0.09 m / m for touching, single-conductor

cables (flat or triangular ) ;

9 - 0.15 m / m as a typical value for busbars

( ) and spaced, single-conductor cables

( ) ; For "sandwiched-phase" busbars(e.g. Canalis - Telemecanique), the reactance isconsiderably lower.Notes:v the impedance of the short links between thedistribution point and the HV / LV transformermay be neglected. This assumption gives aconservative error concerning the short-circuitcurrent. The error increases in proportion to thetransformer rating.v the cable capacitance with respect to the earth(common mode), which is 10 to 20 times greaterthan that between the lines, must be taken intoaccount for earth faults. Generally speaking, the

Fig. 15 : conductor resistivity values to be taken into account depending on the calculated short-circuit current (minimum or maximum). See UTE C 15-105.

Current Resistivity Resistivity value Concerned(*) ( mm 2 / m) conductors

Copper Aluminium

Maximum short-circuit current 1 = 1.25 20 0.0225 0.036 PH-NMinimum short-circuit current 1 = 1.5 20 0.027 0.043 PH-NFault current in TN and IT 1 = 1.25 20 0.0225 0.036 PH-N (**)systems PE-PENVoltage drop 1 = 1.25 20 0.0225 0.036 PH-N (**)Overcurrent for conductor 1 = 1.5 20 0,027 0.043 Phase-Neutralthermal-stress checks PEN-PE if incorporated in

same multiconductor cable

1 = 1.25 20 0.0225 0.036 Separate PE(*) 20 is the resistivity of the conductors at 20 C. 0.018 mm 2 / m for copper and 0.029 mm 2 / m for aluminium.(**) N, the cross-sectional area of the neutral conductor, is less than that of the phase conductor

Fig. 16 : cables reactance values depending on the wiring system.

Wiring system Busbars Three-phase Spaced single-core Touching single- 3 touching 3 "d" spaced cables (flat)cable espac s core cables (triangle) cables (flat) d = 2r d = 4r

Diagramd d r

Average reactance 0.15 0.08 0.15 0.085 0.095 0.145 0.19per unit lengtvalues (m / m)Extreme reactance 0.12-0.18 0.06-0.1 0.1-0.2 0.08-0.09 0.09-0.1 0.14-0.15 0.18-0.20per unit lengthvalues (m / m)


18/36


capacitance of a HV three-phase cable with across-sectional area of 120 mm 2 is in the orderof 1 F / km, however the capacitive currentremains low, in the order of 5 A / km at 20 kV.c The reactance or resistance of the links may

be neglected.If one of the values, R L or X L, is low with respectto the other, it may be neglected because theresulting error for impedance Z L is consequentlyvery low. For example, if the ratio between R Land X L is 3, the error in Z L is 5.1 %.The curves for R L and X L (see fig. 17 ) may beused to deduce the cable cross-sectional areasfor which the impedance may be consideredcomparable to the resistance or to the reactance.

Examples:v first case. Consider a three-phase cable, at20 C, with copper conductors. Their reactanceis 0.08 m / m. The R L and X L curves(see fig. 17 ) indicate that impedance Z Lapproaches two asymptotes, R L for low cablecross-sectional areas and X L = 0.08 m / m forhigh cable cross-sectional areas. For the low andhigh cable cross-sectional areas, the impedanceZL curve may be considered identical to theasymptotes.The given cable impedance is thereforeconsidered, with a margin of error less than5.1 %, comparable to:

- a resistance for cable cross-sectional areasless than 74 mm 2;- a reactance for cable cross-sectional areasgreater than 660 mm 2.v second case. Consider a three-phase cable, at

20 C, with aluminium conductors. As above, theimpedance Z L curve may be considered identicalto the asymptotes, but for cable cross-sectionalareas less than 120 mm 2 and greater than1,000 mm 2 (curves not shown).

Impedance of rotating machinesc Synchronous generatorsThe impedances of machines are generallyexpressed as a percentage, for example:Isc / In = 100 / x where x is the equivalent of thetransformer u sc .Consider:

10 Z =x

100USn

2

where

U = no-load phase-to-phase voltage of thegenerator,Sn = generator VA rating.

11 What is more, given that the value of R / X islow, in the order of 0.05 to 0.1 for MV and 0.1to 0.2 for LV, impedance Z may be consideredcomparable to reactance X. Values for x aregiven in the table in figure 18 for turbo-generators with smooth rotors and for "hydraulic"generators with salient poles (low speeds).

On reading the table, one may be surprised tonote that the steady-state reactance for a short-circuit exceeds 100 % (at that point in time,Isc < In) . However, the short-circuit current isessentially inductive and calls on all the reactivepower that the field system, even over-excited,can supply, whereas the rated current essentiallycarries the active power supplied by the turbine(cos from 0.8 to 1).c Synchronous compensators and motorsThe reaction of these machines during a short-circuit is similar to that of generators.

12 They produce a current in the

network that depends on their reactance in %(see fig. 19 ).c Asynchronous motorsWhen an asynchronous motor is cut from thenetwork, it maintains a voltage across itsterminals that disappears within a few

Fig. 18 : generator reactance values, in x %.

Subtransient Transient Steady-statereactance reactance reactance

Turbo-generator 10-20 15-25 150-230

Salient-pole generators 15-25 25-35 70-120

Fig. 17 : impedance Z L of a three-phase cable,at 20 C, with copper conductors.

m / m1

0.2

0.1

0.02

0.01

Cross-sectional area A (in mm 2)10

0.05

0.08

0.8

20 20050 100 500 1,000

RL

ZL

XL


19/36


hundredths of a second. When a short-circuitoccurs across the terminals, the motor supplies acurrent that disappears even more rapidly,according to time constants in the order of:v 0.02 seconds for single-cage motors up to100 kW;v 0.03 seconds for double-cage motors andmotors above 100 kW;v 0.03 to 0.1 seconds for very large HV slipring

motors (1,000 kW).In the event of a short-circuit, an asynchronousmotor is therefore a generator to which animpedance (subtransient only) of 20 to 25 % isattributed.

Consequently, the large number of LV motors,with low individual outputs, present on industrialsites may be a source of difficulties in that it is noteasy to foresee the average number of in-servicemotors that will contribute to the fault when ashort-circuit occurs. Individual calculation of thereverse current for each motor, taking intoaccount the impedance of its link, is therefore atedious and futile task. Common practice, notablyin the United States, is to take into account thecombined contribution to the fault current of all theasynchronous LV motors in an installation .

13 They are therefore thought of as a unique

source, capable of supplying to the busbars acurrent equal to ( Istart / In) times the sum of therated currents of all installed motors.

Other impedancesc CapacitorsA shunt capacitor bank located near the faultlocation discharges, thus increasing the short-circuit current. This damped oscillatory discharge

is characterised by a high initial peak value thatis superposed on the initial peak of the short-circuit current, even though its frequency is fargreater than that of the network.Depending on the coincidence in time betweenthe initiation of the fault and the voltage wave,two extreme cases must be considered:v if the initiation of the fault coincides with zerovoltage, the discharge current is equal to zero,whereas the short-circuit current is asymmetrical,with a maximum initial amplitude peak;v conversely, if the initiation of the fault coincideswith maximum voltage, the discharge currentsuperposes itself on the initial peak of the fault

current, which, because it is symmetrical, has alow value.

It is therefore unlikely, except for very powerfulcapacitor banks, that superposition will result inan initial peak higher than the peak current of anasymmetrical fault.

It follows that when calculating the maximumshort-circuit current, capacitor banks do not needto be taken into account.

However, they must nonetheless be considered

when selecting the type of circuit breaker. Duringopening, capacitor banks significantly reduce thecircuit frequency and thus produce an effect oncurrent interruption.c Switchgear

14 Certain devices (circuit breakers, contactors

with blow-out coils, direct thermal relays, etc.)have an impedance that must be taken intoaccount, for the calculation of Isc, when such adevice is located upstream of the deviceintended to break the given short-circuit andremain closed (selective circuit breakers).

15 For LV circuit breakers, for example, areactance value of 0.15 m is typical, with theresistance negligible.

For breaking devices, a distinction must be madedepending on the speed of opening:v certain devices open very quickly and thussignificantly reduce short-circuit currents. This isthe case for fast-acting, limiting circuit breakersand the resultant level of electrodynamic forcesand thermal stresses, for the part of theinstallation concerned, remains far below thetheoretical maximum;v other devices, such as time-delayed circuitbreakers, do not offer this advantage.c Fault arcThe short-circuit current often flows through anarc at the fault location. The resistance of the arcis considerable and highly variable. The voltagedrop over a fault arc can range from 100 to 300 V.For HV applications, this drop is negligible withrespect to the network voltage and the arc hasno effect on reducing the short-circuit current.For LV applications, however, the actual faultcurrent when an arc occurs is limited to a muchlower level than that calculated (bolted, solidfault), because the voltage is much lower.

16 For example, the arc resulting from a short-

circuit between conductors or busbars mayreduce the prospective short-circuit current by

Fig. 19 : synchronous compensator and motor reactance values, in x %.

Subtransient Transient Steady-statereactance reactance reactance

High-speed motors 15 25 80

Low-speed motors 35 50 100

Compensators 25 40 160


20/36


20 to 50 % and sometimes by even more than50 % for rated voltages under 440 V.However, this phenomenon, highly favourablein the LV field and which occurs for 90 % of faults,may not be taken into account when determiningthe breaking capacity because 10 % of faults takeplace during closing of a device, producing a solidfault without an arc. This phenomenon should,however, be taken into account for the calculationof the minimum short-circuit current.

c Various impedancesOther elements may add non-negligibleimpedances. This is the case for harmonics filtersand inductors used to limit the short-circuit current.They must, of course, be included in calculations,as well as wound-primary type currenttransformers for which the impedance values varydepending on the rating and the type ofconstruction.

c For the system as a whole, after havingcalculated all the relative impedances, the short-circuit power may be expressed as:

S1

scZR

= from which it is possible to deducethe fault current Isc at a point with a voltage U: sc =

Ssc3 U

13 U Z R

=

ZR is the composed vector sum of all therelative upstream imedances. It is therefore therelative impedance of the upstream network asseen from a point at U voltage.Hence, Ssc is the short-circuit power, in VA, at apoint where voltage is U.For example, if we consider the simplifieddiagram of figure 20 :

At point A, Ssc =U

ZUU Z

LV2

T LVHV

L

+

2

Hence, Ssc = ZU

ZU

T

HV2

L

LV2

1

+

UHV

ZT

ULV

ZL

A

;

Fig. 20 : calculating Ssc at point A.

2.3 Relationships between impedances at the different voltage levels in an installation

Impedances as a function of the voltageThe short-circuit power Ssc at a given point inthe network is defined by:

Ssc = U 3UZsc

2 =

This means of expressing the short-circuit powerimplies that Ssc is invariable at a given point inthe network, whatever the voltage. And the

equation sc U3 Zsc3

= implies that allimpedances must be calculated with respect tothe voltage at the fault location, which leads tocertain complications that often produce errors incalculations for networks with two or more voltagelevels. For example, the impedance of a HV linemust be multiplied by the square of the reciprocalof the transformation ratio, when calculating afault on the LV side of the transformer:

17 Z ZUULV HV

LV

HV =

2

A simple means of avoiding these difficulties is therelative impedance method proposed by H. Rich.

Calculation of the relative impedancesThis is a calculation method used to establish arelationship between the impedances at thedifferent voltage levels in an electrical installation.This method proposes dividing the impedances(in ohms) by the square of the network line-to-line voltage (in volts) at the point where theimpedances exist. The impedances thereforebecome relative.

c For lines and cables, the relative resistancesand reactances are defined as:

RR

U and X

XUR 2 R 2

= = where R is in ohmsand U in volts.c For transformers, the impedance is expressedon the basis of their short-circuit voltages u sc andtheir kVA rating Sn:

ZUSn

2 = usc

100c For rotating machines, the equation isidentical, with x representing the impedanceexpressed in %.


21/36


2.4 Calculation example

(with the impedances of the power sources,the upstream network and the power supplytransformers as well as those of the electrical

links)ProblemConsider a 20 kV network that supplies aHV / LV substation via a 2 km overhead line, anda 1 MVA generator that supplies in parallel thebusbars of the same substation. Two 1,000 kVAparallel-connected transformers supply the LVbusbars which in turn supply 20 outgoers to20 motors, including the one supplying motor M.All motors are rated 50 kW, all connection cables

Fig. 21 : diagram for calculation of I sc values at points A, B, C and D.

Upstream networkU1 = 20 kVPsc = 500 MVA

Overhead line3 cables, 50 mm 2, copper,length = 2 km

Generator1 MVAXsubt = 15 %

2 transformers1,000 kVAsecondary winding 237 / 410 Vusc = 5 %

Main LV switchboardbusbars3 bars, 400 mm 2 / ph, copper,length = 10 m

Link 13 single-core cables, 400 mm 2,aluminium spaced, flat,length = 80 m

LV sub-distribution board

Link 23 three-phase cables,35 mm 2, copper,length = 30 m

Motor50 kW(efficiency: 0.9, cos : 0.8)usc = 25 %

are identical and all motors are running when thefault occurs.The Isc value must be calculated at the various

fault locations indicated in the network diagram(see fig. 21 ), that is:c point A on the HV busbars, with a negligibleimpedance;c point B on the LV busbars, at a distance of10 meters from the transformers;c point C on the busbars of an LV sub-distribution board;c point D at the terminals of motor M.Then the reverse current of the motors must becalculated at C and B, then at D and A.

3L

3L

B

C

G

M

D

10 m

A

3L


22/36


In this example, reactances X and resistances Rare calculated with their respective voltages in

the installation (see figure 22 ). The relativeimpedance method is not used.

(3 x 400 mm 2)

Solution

Section Calculations Results

(the circled numbers X indicate where explanations may be found in the preceding text)

20 kV X () R ()

1. upstream network Zup 10 3 /= ( )20 500 102 6x x 1

Xup Zup .= 0 98 2 0.78

Rup 0.2 Zup 0.2 Xup = 0.15

2. overhead line Xco .= 0 4 2x 7 0.8

Rc o .,= 0 018 2 00050

x 6 0.72

3. generator XG = ( )15

100

20 10

10

3 2

6xx

10 60

R XG G .= 0 1 11 6

20 kV X (m) R (m )Fault A

4. transformers ZT =12

5100

41010

2

6x x 3 5

X ZT T 4.2

R 0.2 XT T

= 4

0.84

410 V

5. circuit-breaker Xcb .= 0 15 15 0.15

6. busbars X 0.15 x 10B-3 = x 10 9 1.5

RB .= 0 022510

3 400x

x 6 0

Fault B

7. circuit-breaker Xcb .= 0 15 0.15

8. cable link 1 Xc1 .= 0 15 10 803x x 12

Rc 1 0 03680

3 400 .= x x 6 2.4Fault C

9.circuit-breaker Xcb .= 0 15 0.15

10 . cable link 2 Xc2 .= 0 09 10 303x x 8 2.7

Rc 2 0 02253035

.= x 19.2

Fault D

11 . motor 50 kW Xm =25

100 50 / 0.9 x 0.8x

41010

2

3( )12

605

Rm = 0.2 Xm 121

(3 x 400 mm 2)

(50 mm 2)

(35 mm 2)

Fig. 22 : impedance calculation.


23/36


I- Fault at A (HV busbars)Elements concerned: 1, 2, 3.The "network + line" impedance is parallel to thatof the generator, however the latter is muchgreater and may be neglected:

XA . . .= + 0 78 0 8 1 58 R 0.87A . .= + 0 15 0 72

Z R XA A2

A2 .= + 1 80 hence

A 6,415 A .

= 20 103 1 80

3xx

IA is the "steady-state Isc" and for the purposesof calculating the peak asymmetrical Isc:RX

A

A .= 0 55

hence k = 1.2 on the curve in figure 9 andtherefore Isc is equal to:

1.2 x 2 x 6,415 = 10,887 A.

II - Fault at B (main LV switchboard busbars)Elements concerned: (1, 2, 3) + (4, 5, 6).The reactances X and resistances R calculatedfor the HV section must be recalculated for theLV network via multiplication by the square ofthe voltage ratio 17 , i.e.:

410 / 20,000( ) = 2 30 42 10 . x henceX X 0.42B A

-3 . . .= ( ) + + +[ ]4 2 0 15 1 5 10X mB .= 6 51 andR R 0.42B A

-3 .= ( ) +[ ]0 84 10R m

B .=

1 2 These calculations make clear, firstly, the low

importance of the HV upstream reactance, withrespect to the reactances of the two paralleltransformers, and secondly, the non-negligibleimpedance of the 10 meter long, LV busbars.

Z R X 6.62 mB B2

B2 = + = hence

B -3 x 6.62 x 10 35,758 A = 410

3

RX

B

B .= 0 18 hence k = 1.58 on the curve in

figure 9 and therefore the peak Isc is equal to:

1.58 x 2 ,758x 35 = 79,900 AWhat is more, if the fault arc is taken into

account (see c fault arc section 16 ), IB is

reduced to a maximum value of 28,606 A and aminimum value of 17,880 A.

III - Fault at C (busbars of LV sub-distributionboard)Elements concerned: (1, 2, 3) + (4, 5, 6) + (7, 8).The reactances and the resistances of the circuitbreaker and the cables must be added to X B andRB.X X 10 mC B

-3 . .= + +( ) =0 15 12 18 67 andR R 10 mC B

-3 . .= +( ) =2 4 3 6

These values make clear the importance of Isclimitation due to the cables.

Z R X mC C2

C2 = + 19

C 12,459 A =

410

3 19 10 3x xRX

C

C .= 0 19 hence k = 1.55 on the curve in


1.55 x 2 ,459x 12 27,310 A

IV - Fault at D (LV motor)Elements concerned:(1, 2, 3) + (4, 5, 6) + (7, 8) + (9, 10).The reactances and the resistances of the circuitbreaker and the cables must be added to X Cand R C.

X X 10 mD C

-3 , , ,= + +( )

=0 15 2 7 21 52 and

R R 10 mD C-3 . .= +( ) =19 2 22 9

Z R X mD D2

D2 .= + 31 42

D -3 534 A .,= 410

3 31 42 107

x x

RX

D

D .= 1 06 hence k 1.05 on the curve in


1.05 2 7,534x x 11,187 A

As each level in the calculations makes clear,the impact of the circuit breakers is negligiblecompared to that of the other elements in thenetwork.

V - Reverse currents of the motorsIt is often faster to simply consider the motors asindependent generators, injecting into the fault a"reverse current" that is superimposed on thenetwork fault current.c Fault at CThe current produced by the motor may becalculated on the basis of the "motor + cable"impedance:

X 10 mM -3 .= +( ) 605 2 7 608 R 10 mM

-3 .= +( ) 121 19 2 140 Z mM = 624 , hence

M -3 A = 410

3 624 10379

x x

For the 20 motorsMC 7,580 A = .Instead of making the above calculations, it is

possible (see 13 ) to estimate the currentinjected by all the motors as being equal to

(I start / In) times their rated current (95 A), i.e.(4.8 x 95) x 20 = 9,120 A.


24/36


This estimate therefore allows protection byexcess value with respect to IMC (7,580 A).On the basis of

R / X = 0.3 => k = 1.4 and the peak

sc 2 7. ,= 1 4 580x x 15,005 A.Consequently, the short-circuit current(subtransient) on the LV busbars increases from12,459 A to 20,039 A and Isc from 27,310 A to42,315 A.c Fault at DThe impedance to be taken into account is1 / 19th of Z M, plus that of the cable.

X 10 3 mMD-3 . ,= +

60519

2 7 4 5

R 10 2 .5 mMD-3 .= +

12119

19 2 5

Z 43 mMD = henceMD -3 5,505 A = =

4103 43 10x x

giving a total at D of:7,534 + 5,505 = 13,039 A rms, andIsc 20,650 A .c Fault en BAs for the fault at C, the current produced by themotor may be calculated on the basis of the"motor + cable" impedance:

X 10 mM-3 .= + +( ) 605 2 7 12 620

R 10 mM-3 . . .= + +( ) 121 19 2 2 4 142 6

Z mM = 636 hence

M -3 A = 410

3 636 10372

x x

For the 20 motors IMB = 7,440 A.Again, it is possible to estimate the currentinjected by all the motors as being equal to 4.8times their rated current (95 A), i.e. 9,120 A. Theapproximation again overestimates the realvalue of IMB. Using the fact thatR / X = 0.3 => k = 1.4 and the peak

sc 2 x 7 0. .= =1 4 44 14,728 AConsequently, the short-circuit current(subtransient) on the main LV switchboard

increases from 35,758 A to 43,198 A and thepeak Isc from 79,900 A to 94,628 A .However, as mentioned above, if the fault arc istaken into account, Isc is reduced between45.6 to 75 kA.

c Fault at A (HV side)Rather than calculating the equivalentimpedances, it is easier to estimate(conservatively) the reverse current of themotors at A by multiplying the value at B by the

LV / HV transformation value 17 , i.e.:

7,440 x410

20 x 10 A-3 .= 152 5

This figure, compared to the 6,415 A calculatedpreviously, is negligible.

Rough calculation of the fault at D

This calculation makes use of all theapproximations mentioned above (notably 15and 16).

X = 4.2 + 1.5 + 12 + 0.15

X = 17.85 m = X'

R = 2.4 + 19.2 = 21.6 m R'

D

D =

Z' R' X' mD D2

D2 .= + 28 02

'.

D -3 8,448 A= 410

3 28 02 10x xhence the peak Isc:

2 x 8,448 11,945 ATo find the peak asymmetrical Isc, the abovevalue must be increased by the contribution of

the energised motors at the time of the fault 13

i.e. 4.8 times their rated current of 95 A:

sc = 11,945 + 4.8 x 95 x 2 x 20( )= 8 A24 42, .

Compared to the figure obtained by the fullcalculation (20,039), the approximate methodallows a quick evaluation with an error remainingon the side of safety.


25/36


Calculation using symmetrical components isparticularly useful when a three-phase network isunbalanced, because, due to magneticphenomena, for example, the traditional"cyclical" impedances R and X are, normallyspeaking, no longer useable. This calculationmethod is also required when:c a voltage and current system is notsymmetrical (Fresnel vectors with different

moduli and imbalances exceeding 120 ).This isthe case for phase-to-earth or phase-to-phaseshort-circuits with or without earth connection;c the network includes rotating machines and/orspecial transformers (Yyn connection, forexample).

This method may be used for all types of radialdistribution networks at all voltage levels.

3.2 Symmetrical components

Similar to the Leblanc theorem which states thata rectilinear alternating field with a sinusoidalamplitude is equivalent to two rotating fieldsturning in the opposite direction, the definition ofsymmetrical components is based on theequivalence between an unbalanced three-phase system and the sum of three balancedthree-phase systems, namely the positive-sequence, negative-sequence and zero-sequence (see fig. 23 ).

The superposition principle may then be used tocalculate the fault currents.In the description below, the system is definedusing current 1 as the rotation reference, where:c 1 1( ) is the positive-sequence component;c 1 2( ) is the negative-sequence component;c 1 0( ) is the zero-sequence component;

and by using the following operator

a = e = -12

j3

2

j23

+ between 1, 2 ,and 3 .This principle, applied to a current system, isconfirmed by a graphical representation(see fig. 23 ). For example, the graphicaladdition of the vectors produces, for, thefollowing result:

2 1 1 2 3 = + +( ) ( ) ( )a a 1 12Currents 1 and 3 may be expressed in thesame manner, hence the system:

1 1 1 1 0 = ( ) ( ) ( )1 2+ + 2 1 1 1 0 = ( ) ( )a + a +2 1 2 3 1 1 1 0 = ( ) ( ) ( )a + a +1 2 2

Fig. 23 : graphical construction of the sum of three balanced three-phase systems (positive-sequence, negative-sequence and zero-sequence).

+ + =I3(1)

I1(1)

I2(1) t

Positive-sequence

I1 (2)

I2 (2)

I3(2) t

Negative-sequenceI1 (0)

I2 (0)

I3 (0)

t

zero-sequence

t

I 3

I 2

I 1

Geometric construction of I1

I 1

I1 (1) I1 (2) I1 (0)

I1 (0)I1 (1)

I 2

I1(2)

a 2 I1(1)a I1(2)

Geometric construction of I2

3 Calculation of I sc values in a radial network usingsymmetrical components

3.1 Advantages of this method


26/36


These symmetrical current components arerelated to the symmetrical voltage componentsby the corresponding impedances:

Z =V

Z =V

and Z =V

11

12

2

20

0

0( )

( )

( )( )

( )

( )( )

( )

( ) ,

These impedances may be defined from thecharacteristics (supplied by the manufacturers)of the various elements in the given electricalnetwork. Among these characteristics, we cannote that Z (2) Z(1) , except for rotating machines,whereas Z (0) varies depending on each element(see fig. 24 ).

For further information on this subject, a detailedpresentation of this method for calculating solidand impedance fault currents is contained in the"Cahier Technique" n 18 (see the appendedbibliography).

3.3 Calculation as defined by IEC 60909

Elements Z (0)

Transformer(seen from secondary winding)

No neutral Yyn or Zyn free flux

forced flux 10 to 15 X (1)Dyn or YNyn X (1)primary D or Y + zn 0.1 to 0.2 X (1)Machine

Synchronous 0.5 Z (1)Asynchronous 0Line 3 Z (1)

Standard IEC 60909 defines and presents amethod implementing symmetrical components,that may be used by engineers not specialised inthe field.

The method is applicable to electrical networkswith a rated voltage of less than 230 kV and thestandard explains the calculation of minimumand maximum short-circuit currents. The formeris required in view of calibrating overcurrentprotection devices and the latter is used todetermine the rated characteristics for theelectrical equipment.

In view of its application to LV networks, thestandard is accompanied by application guideIEC 60781.

Procedure1- Calculate the equivalent voltage at the fault

location, equal to c Un / 3 where c is a voltagefactor required in the calculation to account for:c voltage variations in space and in time;c possible changes in transformer tappings;c subtransient behaviour of generators andmotors.

Depending on the required calculations and thegiven voltage levels, the standardised voltagelevels are indicated in figure 25 .2- Determine and add up the equivalent positive-sequence, negative-sequence and zero-sequence impedances upstream of the faultlocation.3- Calculate the initial short-circuit current usingthe symmetrical components. Practicallyspeaking and depending on the type of fault, the

equations required for the calculation of the Iscare indicated in the table in figure 26 .4- Once the Isc ( Ik") value is known, calculate theother values such as the peak Isc value, thesteady-state Isc value and the maximum, steady-state Isc value.

Effect of the distance separating the faultfrom the generatorWhen using this method, two differentpossibilities must always be considered:c the short-circuit is away from the generator,the situation in networks where the short-circuitcurrents do not have a damped, alternating

component.This is generally the case in LV networks, exceptwhen high-power loads are supplied by specialHV substations;c the short-circuit is near the generator(see fig. 11), the situation in networks where theshort-circuit currents do have a damped,alternating component. This generally occurs inHV systems, but may occur in LV systems when,for example, an emergency generator suppliespriority outgoers.The main differences between these two casesare:c for short-circuits away from the generator:v the initial ( Ik" ), steady-state ( Ik) and breaking(Ib) short-circuit currents are equal ( Ik" = Ik = Ib);

Fig. 24 : zero-sequence characteristic of the various elements in an electrical network.

Rated Voltage factor cvoltage for calculation ofUn I sc max. I sc min.

LV

230 - 400 V 1 0.95

Others 1.05 1

HV

1 to 230 kV 1.1 1

Fig. 25 : values for voltage factor c (see IEC 60909).


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Phase-to-phase-to-earth

(Zsc between phases = 0)

Type I k Fault occurringof short-circuit General situation far from the generators

Three-phase (any Ze) =( )

c Un

3 Z 1

=( )

c Un

3 Z 1In both cases, the short-circuit current depends only on Z (1), which is generally replaced by Zk,

the short-circuit impedance at the fault location, defined by Zk = Rk Xk2 2 + whereRk is the sum of the resistances of one phase, connected in series;Xk is the sum of the reactances of one phase, connected in series.

Phase-to-phase clear of earth (Ze = ) =( ) ( )

c Un

Z + Z1 2=

( )

c Un

2 Z 1

Phase-to-earth =( ) ( ) ( )

c Un 3

Z + Z + Z1 2 0=

( ) ( )

c Un 3

2 Z +Z1 0

= ( )

( ) ( ) ( ) ( ) ( ) ( )

c Un 3 Z

Z Z + Z Z + Z Z

2

1 2 2 0 1 0=

( ) ( )

c Un 3

Z + 2 Z1 0

Symbols used in this tablec phase-to-phase rms voltage of the three-phase network = U c short-circuit impedance = Zscc modulus of the short-circuit current = I k" c earth impedance = Ze.c symmetrical impedances = Z (1) , Z (2), Z (0)Fig. 26 : Short-circuit values depending on the positive-sequence, negative-sequence & zero-sequence impedances of the given network (see IEC 60909).

v the positive-sequence (Z (1)) and negative-sequence (Z (2)) impedances are equal(Z(1) = Z(2));c for short-circuits near the generator:v the short-circuit currents are not equal, in factthe relationship is Ik < Ib < Ik";v the positive-sequence impedance (Z (1)) is notnecessarily equal to the negative-sequenceimpedance (Z (2)).

Note however that asynchronous motors mayalso add to a short-circuit, accounting for up to30 % of the network Isc for the first30 milliseconds, in which case Ik" = Ik = Ib nolonger holds true.

Conditions to consider when calculating the

maximum and minimum short-circuitcurrentsc Calculation of the maximum short-circuitcurrents must take into account the followingpoints:v application of the correct voltage factor ccorresponding to calculation of the maximumshort-circuit currents;v among the assumptions and approximationsmentioned in this document, only those leadingto a conservative error should be used;

v the resistances per unit length R L of lines(overhead lines, cables, phase and neutralconductors) should be calculated for atemperature of 20 C;c Calculation of the minimum short-circuitcurrents requires:v applying the voltage factor c corresponding tothe minimum permissible voltage on the network;v selecting the network configuration, and insome cases the minimum contribution fromsources and network feeders, which result in thelowest short-circuit current at the fault location:v taking into account the impedance of thebusbars, the current transformers, etc.;v neglecting the motors;v considering resistances R L at the highestforeseeable temperature:

RC

- 20 C RL e L20 .

= + ( ) 1 0 004

where R L20 is the resistance at 20 C;e is the permissible temperature ( C) for theconductor at the end of the short-circuit.

The factor 0.004 / C is valid for copper,aluminium and aluminium alloys.


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3.4 Equations for the various currents

Initial short-circuit current I k"The different initial short-circuit currentsIk" are calculated using the equations in the table

in figure 26.

Peak value i p of the short-circuit currentIn no meshed systems, the peak value i p of theshort-circuit current may be calculated for alltypes of faults using the equation:

i K 2p k "=

Ik = is the initial short-circuit current;

K is a factor depending on the R / X ratio anddefined in the graph in figure 9, or using thefollowing approximate calculation:

K = 1.02 + 0.98 e-3 RX

Short-circuit breaking current I bCalculation of the short-circuit breaking currentIb is required only when the fault is near thegenerator and protection is ensured by time-delayed circuit breakers. Note that this current isused to determine the breaking capacity of thesecircuit breakers.

This current may be calculated with a fair degreeof accuracy using the following equation:

Ib = Ik where where is a factor defined bythe minimum time delay tmin and the

Fig. 27 : factor used to calculate the short-circuit breaking current I b (see IEC 60909).

Minimum time delay t min

0

Three-phase short-circuit current Ik" / Ir

2 3 4 5 6 7 8 9

0.02 s

0.05 s

0.1 s

0.5

0.6

0.7

0.8

0.9

1.0

> 0.25 s

1

Ik" / Ir ratio (see fig. 27 ) which expresses theinfluence of the subtransient and transientreactances with Ir as the rated current of the

generator.Steady-state short-circuit current I kThe amplitude of the steady-state short-circuitcurrent Ik depends on generator saturationinfluences and calculation is therefore lessaccurate than for the initial symmetricalcurrent Ik". The proposed calculation methodsproduce a sufficiently accurate estimate of theupper and lower limits, depending on whetherthe short-circuit is supplied by a generator or asynchronous machine.

c The maximum steady-state short-circuitcurrent, with the synchronous generator at itshighest excitation, may be calculated by:

Ikmax = max Irc The minimum steady-state short-circuit currentis calculated under no-load, constant (minimum)excitation conditions for the synchronousgenerator and using the equation:Ikmin = min Ir where Ir is the rated current at thegenerator terminals; is a factor defined by the saturation inductanceXd sat.

The max and min values are indicated infigure 28 for turbo-generators and in figure 29for machines with salient poles.


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Fig. 28 : factors max and min for turbo-generators (see IEC 60909).

Fig. 29 : factors max and min for generators with salient poles (see IEC 60909).

01 2 3 4 5 6 7 8


min

max

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0.6

0.8

1.01.2

1.72.0

Xd sat

3.5 Calculation example

ProblemConsider four networks, three 5 kV networks andone 15 kV network, supplied via a 30 kV networkby transformers in substation E (see fig. 30 ).During construction of line GH, calculation ofthe breaking capacity of circuit breaker M isrequested.

The following information is available:c only the secondary windings of thetransformers in substation E are earthed;c for a 30 kV line, the reactance value is0.35 / km (positive-sequence and negative-sequence conditions) and 3 0.35 / km (zero-sequence conditions);c the short-circuit reactance is 6 % for thetransformers in substation E and 8 % for theother transformers;c the factor c for U is set to 1;c all loads connected to points F and G areessentially passive;c all resistances are negligible with respect tothe reactances.

Fig. 30

1.2

1.41.61.82.02.2

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

00 1 2 3 4 5 6 7 8


min

max

Xd sat

4 MVA

40 km15 km

30 km20 km

F G

10 MVA

E

H

60 kV network290 MVA

2 MVAcos : 0.8

4 MVA4 MVA

6 MVAcos : 0.8

2 MVAcos : 0.8

2 MVAcos : 0.8

30 kV

8 MVA

10 MVA

15 kV 5 kV5 kV

5 kV

M


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Solution

c On the basis of the positive-sequence andnegative-sequence diagrams (see fig. 31 ), thefollowing may be calculated:

a = USsc

j 3.12 = 30290

2

b = uUSn

j 5.4sc2

= 6100

3010

2x

c1 = 0.35 x 40 j 14 c2 = 0.35 x 30 j 10.5 c3 = 0.35 x 20 j 7 c4 = 0.35 x 15 j 5.25

d = uUSn

j 9sc2

= 8100

308

2x

e =US j 90

2

. .x x0 630

6 0 6

2

=

f = uUSn

j 18sc2

= 8100

304

2x

g =US

j 2702

. .x x0 630

20 6

2=

c Note on the zero-sequence diagram(see fig. 32 ):v the delta windings of the transformers insubstation E block zero-sequence currents, i.e.the network is not affected by them;

v similarly, the transformers in the substations F,H and G, due to their delta windings, are notaffected by the zero-sequence currents and,therefore, have an infinite impedance for thefault.b = b1 = j 5.4 c1 = 3 c1 = j 42 c2 = 3 c2 = j 31.5 c3 = 3 c3 = j 21 c4 = 3 c4 = j 15.75 d = f = c Calculations may therefore be made using twosimplified diagrams:v with line GH open (see fig. 33 ):

Z(1) = Z(2) = j 17.25 This result is obtained by successive calculationsas shown in figure 34 .

A similar calculation for the zero-sequencediagram gives the result:

Z(0) = j 39.45 sc c Un

Z 31.104 kA3

1

= ( )

sc c Un 3Z + Z + Z

0 kA11 2 0

.= ( ) ( ) ( )

773

Note the network is HV, hence coefficientc = 1.1.

Fig. 31.

f g

a

b

f

g

g fF G

E

c4

c3 c2

c1d

e

b

H

f'

b'

f'

f'F G

E

c' 4

c' 3 c' 2

c' 1d'

b'

H

Fig. 32.


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j 18

j 270

j 3.1

j 5.4

j 270

j 18F

G

E

j 5.25

j 7

j 9

j 90

j 5.4

H

H

Positive-sequence andnegative-sequence diagram

j 17.25

j 14

Z(1) , Z(2)

j 5.4

F G

E

j 15.75

j 21

j 5.4

H

H

Zero-sequence diagram

j 39.45

j 42

Z(0)

Z(1) , Z (2) Z(0)

j 18

j 270

j 3.1

j 5,4

j 270

j 18G

E

j 5.25

j 7

j 9

j 90

j 5,4

H

H

j 14

Z(1) , Z (0)

Za

Zb

Zc

Za = j 3,1 +

= j 3.1 + j 2.7 = j 5.8

j 5.25 Z(1) =

j 9 + j 90= j 99

Zc = j 14 + j 18

+ j 270 = j 302

Z'

j 5.4 j 5.4 j 5.4 + j 5.4

j 288

j 7

H

j 5.25

j 288

j 7

Z' = = j 5.381Za x Zb x Zc

Za Zb+Za Zc+Zb Zc

H

Z = + j 7

= j 17.253

j 10.631 j 288 j 10.631 + j 288

F

Fig. 34.

Fig. 33.


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v with line GH closed (see fig. 35 ):Z(1) = Z(2) = j 13.05 Z(0) = j 27.2 Isc 3 = 1.460 kAIsc 1 = 1.072 kA

Given the highest short-circuit current(I sc 3 = 1.460 kA ), the line circuit breaker atpoint M must be sized for:P = U 3 = 30 x 1.460 x 3P 76 MVA .

j 18

j 270

j 3.1

j 5.4

j 270

j 18F G

E

j 5.25

j 7 j 10,5

j 9

j 90

j 5.4

H

H

Positive-sequence diagram

j 13,05

j 14

Z(1), Z(2)

Z(1) = Z (2) = j 13.05

j 5.4

F G

E

j 15.75

j 21 j 31.5

j 5.4

H

H

Zero-sequence diagram

j 27.2

j 42

Z(0)

Z(0) = j 27.2 Z(1), Z (2) Z(0)

Fig. 35.


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Various methods for the calculation of short-circuit currents have been developed. Somehave been included in a number of standardsand are consequently included in this"Cahier Technique" publication as well.

Several standardised methods were designed insuch a way that short-circuit currents could becalculated by hand or using a small calculator.When computerised scientific calculationsbecame a possibility in the 1970's, electrical-installation designers devised software for theirparticular needs. This software was initially runon mainframe computer systems, then onminicomputers, but was difficult to use, andtherefore limited to a small number of experts.

This software was finally transferred to the PCmicrocomputing environment, proving mucheasier to use. Today, a wide range of softwarepackages are available which comply with theapplicable standards defining the calculation ofIsc currents in LV applications, for example

Ecodial, a program designed and marketed bySchneider Electric.

All computer programs designed to calculateshort-circuit currents are predominantlyconcerned with determining the requiredbreaking and making capacities of switchgearand the electro-mechanical withstand capabilitiesof equipment.

Other software is used by experts specialising innetwork design, for example, research on thedynamic behaviour of electrical networks. Suchcomputer programs can be used for precisesimulations of electrical phenomena over timeand their use is now spreading to include theentire electro-mechanical behaviour of networksand installations.

Remember, however, that all software, whateverits degree of sophistication, is only a tool. Toensure correct results, it should be used byqualified professionals who have acquired therelevant knowledge and expertise.

4 Computerised calculations and conclusion


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Bibliography

Standardsc IEC 60909: Short-circuit current calculation inthree-phase AC systems.c IEC 60781: Application guide for calculation ofshort-circuit currents in low voltage radialsystems.c NF C 15-100: Installations lectriques bassetension.c C 15-105: Guide pratique,D termination des sections de conducteurs etchoix des dispositifs de protection.

Schneider Electric Cahiers Techniquesc Analyse des r seaux triphas s en r gimeperturb l'aide des composantes sym triques,Cahier Technique n 18 - B. DE METZ-NOBLATc Neutral earthing in an industrial HV network.

Cahier Technique no. 62 - F. SAUTRIAU.c LV circuit-breaker breaking capacity.Cahier Technique no. 154 - R. MOREL.

Other publication by Institut SchneiderFormation (ISF)c Electrical Installation Guide,Ref.: MD1ELG2E(information on this 400 page pbulication isobtainable on www.schneiderformation.com)


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