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n°62

Neutral earthing inan industrial HV

network

photographie

François Sautriau

Having graduated from the EcoleSupérieure d'Electricité in 1968, hejoined Merlin Gerin in 1970. Afterworking on the design of networksand protective devices, he was headof the design office for industrialprojects and then for naval

equipment projects.He is now, consultant in theMarketing Division of the Protectionand Control Department

E/CT 62 up dated April 1996

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Merlin Gerin Technical Specification n° 62 / p.2

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neutral earthing in an industrial HV network

contents

1. Introduction p. 4

2. Earthing Direct earthing p. 4

Earthing through a reactor p. 4

Earthing through a resistor p. 4

3. Requirements imposed by Earthing through a current p. 5overvoltages limiting reactor

Earthing through a resistor p. 4

4. Requirements imposed by networks p. 5

5. Requirements imposed by receivers p. 6

6. Calculating fault currents p. 6

7. Earth protection mode Earth protection adjustment p. 7

Earthing with accessible neutral p. 8

Earthing with an artificial neutral p. 8

Appendix 1: Comments on determining p. 10network capacitance values

Appendix: Bibliography p. 11

HV electrical networks can be earthedin different ways. This documentanalyzes the constraints imposed bythe different parameters of the

installation (overvoltages, network,receivers) and calculates the faultcurrents.Different protection modes aredescribed along with the settings andadjustments suggested according tothe requirements.

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1. introduction

[1] See bibliography

When designing an industrial HVnetwork, a suitable neutral eartharrangement must be selected: theneutral can either be insulated, or it canbe connected to earth. The use of aninsulated neutral in an HV network hasthe advantage of ensuring operationalcontinuity since it does not trip on thefirst fault, however the networkcapacitance must be such that an earthfault current is not likely to endangerpersonnel or damage equipment.

On the other hand, an insulated neutral

2. earthing

neutral generally implies mandatorytripping on the first fault, however:s it reduces overvoltages,s it provides a simple, reliable,selective means of protection,s it allows the use of equipment, and inparticular cables, with lower insulationlevels than for an insulated neutral.

implies the following:s the risk of high overvoltages likely tofavourize multiple faults,s the use of superinsulated equipment,s compulsory monitoring of theinsulation,s protection against overvoltages,which will become compulsory in thenear future,s the need for complex, selectiveprotection against earth faults whichcannot usually be ensured by simple

current-measuring relays. An earthed

earthing through a resistor

This is often the most satisfactorysolution.

A study is necessary to choosebetween these two earthing, (through areactor or through a resistor) :accurate determining of these earthingmodes depends on the voltage level,the size of the network and the type ofreceivers.Depending on the earthing mode, acriterion then determines a maximumimpedance value corresponding to theovervoltage problem.Next, it is necessary to check its

compatibility with the requirements ofthe network and the receivers.

However, in the event of an earth fault,the current is not limited, damage and

interference occur and there isconsiderable danger for the personnelduring the time the fault persists.This solution is not used for HVdistribution.

earthing through a reactor

Tuned reactor (Petersen coil)This solution is sometimes used forpublic HV networks. It is rarely used forindustrial distribution.Protective relays sensitive to the activecomponent of the residual current must

be used to obtain selectivity.

Current limiting reactorThis solution can result in seriousovervoltages, as demonstrated by LeVerre (the Research and Developmentdivision of the E.D.F.) [1]. It can beused only where there are low limitingimpedances.

The purpose of this study is not tocompare the different neutral earth

arrangements, but rather, once theneutral earth solution has beenadopted, to determine the earthingmode by finding a compromisebetween three often contradictoryrequirements:s to sufficiently damp overvoltages,s to limit damage and disturbancescaused by an earth fault,s to provide simple, selective protectivedevices.Earthing can be of different types:s direct (without impedance-dependentcurrent limiting),

s through a reactor,s through a resistor.

direct earthingThis type of earthing is the mostefficient in limiting overvoltages;protection selectivity presents nodifficulties.

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3. requirements imposed by overvoltages

4. requirements imposed by networks

its path and in particular to the cableshields. The maximum currentwithstood by the cable shields may bespecified by the constructors. As ageneral rule, the value used is between500 and 3 000 A for 1 second.

The above criterion is used to definethe lower limit of the phase to earthfault current.To determine the upper limit, it isnecessary to check that the faultcurrent does not cause damage along

fig. 1 : a zigzag or neutral point coil provides an earth fault current limiting reactor.

current IC in the event of an earth fault.Hence the relation IL ≥ 2 IC.

Determination of the cable capacitancevalues depends on their design (seeappendix for this calculation).

earthing through a currentlimiting reactor(see fig. 1)

The study of overvoltages that occurwhen short-circuits are eliminated fromnetworks with the neutral earthedthrough a reactor gives the followingresults:

s let, I0ω be the earth fault currentlimiting reactance,

s and Lω the network three-phase

short-circuit reactance.The neutral-to-earth overvoltageoccurring when short-circuits areeliminated is:

∆V

V=

1

2

I0

Lfor a radial field cable

network,

∆V

V=

1

2

I0

Lfor all other cases.

In practise, the earth fault current islimited to at most 10 % of the three-

phase short-circuit current, as appliedby the EDF to its HV power distributionnetwork.

s

U

3 ris the value of the earth fault

current IL in the earthing connection,

s 3 C ωU

3is the network capacitive

earthing through a resistorAs recommended by EDF forhydroelectric power networks. Theresistance value r is détermined inorder to obtain a total active power

loss :U

2

3 requal to or greater than the

capacitive power 2 C ωU2

in the event

of a phase-earth fault, i.e.:

U2

3 r ≥ 2 C ω U

2

.

When dividing byU

3, this become

U

3 r

≥ 2 . 3 C ωU

3

where:

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5. requirements imposed by receivers

between 3 kV and 15 kV, mostfrequently 5.5 kV in France; the earthfault current should not exceed 20 A inorder to avoid damage to the steelplating of the machines, for if reworkinga winding is a regular repair, repairing a

machine when the metal plating isdamaged is much more timeconsuming and more costly.

6. calculating fault currents

The currents in the different circuits areeasily calculated using a simpleapproximative method.This consists in ignoring the short-circuit impedance of the source and thecoupled impedances with respect to theneutral earth impedance and the

network capacitances. In other words,we consider that earth fault currents aremuch lower than three-phase short-circuit currents (see fig. 2).To calculate the neutral-to-earthpotential, the sum of the currentsflowing to earth is considered to bezero (see diagram).

IN + IrD + Σ IrS = 0

0 = g VN + [G + jω C] (VN + E)+ jωC (VN + a

2E) + jωC (VN + aE)

0 = VN [g + G + 3jωC] + GE+ jω CE (1 + a

2+ a)

Since 1 + a2

+ a = 0

This gives:

VN =−GE

g +G + 3 jωC

whereVN =− zE

z + Z + 3 jω C z Z

fig. 2: Earth fault current calculation parameters.

In HV networks, receivers aretransformers which have no particularrequirements as concerns the neutralearthing in a power supply network.However, industrial HV networks cansupply rotating machines with voltages

z =1

g

Z =1

G

Neutral earth impedance

Phase to earth fault impedance

CD Phase to earth capacitanc e of the faulty outgoing

feeder

CS Phase to earth capacitance of a sound outgoing

feeder

C = Σ CS Total phase to earth capacitance of the network

E Network phase voltage

VN Neutral point to earth potential

IN Neutral to earth current

ID Fault current

IrD Residual current of the faulty outgoing feeder

IrS Residual current of a sound outgoing feeder

EVN

3

2

1

g =1z

IN

G =1

Z CD

I r D I r S

CS

ID

aE

a E2

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In the event of a short-circuit Z = 0, theabove formulae become:

s

s

s

s

s

s

Since we know VN, the differentcurrents (IN neutral to earth current, ID

fault current, IrD and IrS residual currentsin outgoing feeders) are calculated as

shown below:s

s

I N =−E

z

7. earth protection mode

The neutral earthing impedance affectsthe required method of protectionagainst phase to earth faults.As a general rule, the higher the faultcurrents, the easier they are to detect;the lower they are, the harder they areto detect .

Moreover, it is advisable, evenessential, to ensure protection not atone single point, but in all branches ofthe network, since the relays operateselectively.

Phase to earth protection is provided byovercurrent relays supplied by the earthcurrent.This current can be measured asfollows:

s either by a core balance transformeraround the three phase conductors and

which directly detects the sum of theircurrents (zero if no earth fault) ;

s or by three current transformers thesecondary windings of which areconnected to form a neutral conductorthrough which the sum of the threecurrents flows.The core balance transformer solutionis the most accurate, however it canonly be installed on cables, not onbusbars or overhead lines. The3 current transformers solution is often

used, in particular when these3 transformers are already required foranother application. But themeasurement obtained is degraded bythe inaccuracies of all threetransformers, in particular in the eventof transient overcurrents when thetransformers become saturated.

earth protection setting

This must be adjusted according to themeasurement accuracy. It must ensuremaximum protection and authorizeselectivity.

If the measurement is carried out usingthe sum of the secondary currents ofthe three transformers, it will bedegraded by the dispersion of thetransformers. In particular, a residual

current is measured if there is no earthfault when the transformers becomesaturated.

Saturation is caused by excessiveamplitude of the phase current, butmore specifically by the DC componentinduced in a short-circuit or unbalancedinrush current.

Note that, in transient conditions, theDC component can induce saturation ofthe transformers even though the peakvalue of the transient current is around

10 times less than the saturation valuefor a steady-state balanced current. Anearth protection device supplied by3 transformers must therefore include atime delay in order to avoid spurioustriggering resulting from transients. Thecurrent setting must not be lower than6 % of the transformer rating at best, or15 or 20 % of the transformer ratings inthe most unfavourable cases.Moreover, if an earth fault occurs in astar winding near a neutral point, themaximum fault current is only a smallpart of the maximum fault current whichis limited by the neutral earthimpedance. For this reason, the currentsetting is usually 20% of the maximumcurrent limited by the neutral earth inorder to protect 80% of the windings.

But, as the calculation shows, in theevent of a fault, a residual capacitivecurrent flows through the sound parts ofthe network. So, to prevent theprotective device of a sound line fromtripping spuriously, the threshold mustbe set to 30% higher than thecapacitive current flowing through thissound line when a phase to earth short-circuit affects the network.Moreover, we must take into accountthe possible presence of voltageharmonics likely to produce currents

I rD = ID+ 3jωC D VN

=g + 3 jω C − CD( )

g+G+3jωC GE

=1+ 3jω C − CD( ) z

Z + z + 3jωCzZ E

I rD =1

z+ 3jω C − CD( )

E

ID =1

z+3jωC

E

I rS = − 3jω CS E

IN= gVN

−gGE

g +G + 3jωC

=−E

Z +z + 3jωCzZ

ID = G VN +E( ) =g + 3jωC

g + G + 3jωC GE

1+ 3jωCz

Z + z + 3jωCzZ

I rS = 3jωC S VN

=−3jωC S

g + G+ 3jωC GE

=1−3jωC Sz

Z +z + 3jωCzZ E

VN = − E

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which increase as the order of theharmonics increases. Note that 3rdharmonics and multiples of 3 exist evenin steady-state conditions. Finally, the

neutral earth impedance characteristicsmust be coordinated with the protectivedevices so that this impedance itself isnot degraded by the fault current beforeit is eliminated.

Note: that this concerns circuitprotection and not personnel protection.

Conclusion: when the earthed neutralsolution is selected for a mediumvoltage network, it is advisable to useearthing through a resistor rather thanother systems.

Calculating r and IL

This resistance r and the maximum

current IL =U

3 rare determined

taking the following requirements intoaccount:

s the current IL must be greater (orequal) to twice the network capacitivecurrent in the event of an earth faultIL ≥ 2 LC in order to limit overvoltages,

s the current IL must be less than themaximum overcurrent that can bewithstood by the cable shields, normallybetween 500 and 3 000 A, dependingon the cable cross section,

s in a network containing HV motors, itis preferable to respect the relation:5 A ≤ IL ≤ 20 A but, if this isincompatible with the first condition, IL

can be up to 50 A,

s in order to ensure correct protectionof receivers, the Ir threshold settingsshould not exceed 0,2 IL, i.e. Ir ≤ 0,2 IL,

s in order to ensure selectivity withregard to sound connections, therelation Ir ≥ 1,3 IC, must be respected,

where IC is the capacitive current of theline when a phase to earth fault occurs,

s if the earth current is measured by3 transformers of rating In , then Ir mustbe ≥ 0,06 In,

s the thermal withstand of resistor r

must allow for current IL to flow duringthe maximum fault clearing timerequired (1 to 1.5 s) or, conversely, theearth fault must be cleared sufficientlyrapidly to avoid damaging the resistor.

earthing with accessibleneutral

The resistor is connected to the neutraloutput terminal and to the earthingsystem, either directly, or through asingle-phase transformer the secondarywinding of which is loaded by anequivalent resistance, as is used innetworks supplied through atransformer which has a star-connectedsecondary winding with accesibleneutral and for AC generators withaccessible neutral (see fig. 3).

If the network is supplied throughseveral transformers or AC generators,it is advisable to have one single

neutral earth connection to prevent themaximum earth fault current fromvarying with the number of sources inservice.

a) b)

R r

fig. 3: neutral earthing at the secondary

winding of a star-coupled transformer,

through a resistor either connected directly

(a) or connected through a single-phase

transformer (b).

earthing with an artificialneutral

If the neutral of the source isinaccessible (as for delta-connectedwindings), or if there are severalparallel-connected sources, earthingcan be ensured by an artificial neutral(see figs. 4 and 5) (or earthingtransformer)

fig. 4: earthing a network neutral by means

of a star-delta transformer associated to:

a) a resistor connected to the HV side; in

this case the transformer secondary winding

can supply the auxiliaries;

b) a resistor series-connected to the

secondary winding.

r

b)

I ∆

R

a)

I ∆

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Several solutions are possible:

s a star/delta transformer with aresistor;s

a zigzag coil (see figs. 1 and 6): thisis used in cases where the maximumearth fault current is limited to valuesover 100 A;s or, a special transformer, since to setup an artificial neutral it may be moreeconomical to use the transformerwhich supplies the substation LVauxiliaries (see figs. 7 and 8).

The resulting impedance :ro + jIoω appears as a resistanceif ro ≥ 2Ioω,with ro and Io referenced to the samevoltage.

r

I ∆

fig. 7: neutral earthing by means of a

double-star transformer with compensating

delta windings around a resistor.

fig. 8: neutral earthing by means of a zigzag

transformer.

fig. 5: special transformer for neutral earting

RI ∆

fig. 6: neutral earthing by means of a zigzag

coil.

R

I ∆

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appendix 1:how to determine the capacitance values of a network

The capacitance of the cables dependson their design:

s single-pole cableThe conductor is surrounded by ashield and the capacitance C is thatmeasured between the conductor andthe earthed shield.s three-pole radial field cableEach conductor is surrounded by ashield and the capacitance C is thatmeasured between each conductor andits earthed shield.s three-pole belted type cableA single shield surrounds the threeconductors. There is a capacitance Kbetween conductors and a capacitanceC between each conductor and theearthed shield.

be known is therefore:

Ic = 3 Cω V

which only takes into account thecapacitance C regardless of the cabletype.

In practise, cable manufacturers usethe term star connection capacitance,for which they indicate the following:s the value of C for radial field cables,s the value of 3 K + C for belted typecables.

They do not normally give this Ccapacitance value for belted typecables. On request, they give threemeasurement results for these cables:s the capacitance C1 measuredbetween a conducting core and theother cores connected to the metalsheathing; this gives the relationC1 = 2 K + C,

s the capacitance C2 measuredbetween the three conducting coresbound together and the metalsheathing; this gives the relation:

C2

= 3 C,

s the capacitance C3 measuredbetween two conducting cores, with thethird connected to the metal sheathing;this gives the relation:

C3 =3 K + C

2

Thus, capacitance C2 must be known inorder to directly obtain the value of:

C =C2

3

C C

C

K K

K

For the single-pole cable and thethree-pole radial field cable, there isno ambiguity since there is only onecapacitance, C which is a phase-to-earth capacitance. If there is no faultand in three-phase steady-stateoperating conditions, a capacitivecurrent ic flows through each phaseand is absorbed by the cablecapacitance C under the phasevoltage V, flowing from phase to earth,at the network frequency:

i c = CωV = CωU

3

Since this capacitive load is three-phased and balanced, it does not

disturb the network and does not affectthe protection operation.

On the other hand, if an earth faultoccurs in the network, in other wordswhen a phase is earthed, the cablecapacitance is equivalent to anunbalanced load composed of thecapacitance C between the two soundphases and the earth under phase tophase voltage U.The two currents CωU which are out ofphase by 60° flow through the two

sound phases; the sum of these twocurrents is known as the capacitivecurrent IC of the network in the event ofan earth fault.

i.e. IC

= 3 CωV.

For the three-pole belted type cable,if there is no fault, current i

cflowing in

balanced conditions is:

i c = 3 KωU + CωV = 3 KωV + CωV

i.e. ic = (3 K + C) ω V per phase, and

the sum of the currents of the three

phases is zeroCable manufacturers generally givethis 3 K + C capacitance value forbelted-type cables.On the other hand, if there is an earthfault in the network, in other words if aphase is earthed, the capacitive loadincludes:

s the three K capacitance values underthe phase to phase voltage which forma balanced load;s the three C capacitance values, twoof which are under phase to phasevoltages and are out of phase by 60°;

the third under a zero voltage.The sum of these currents (ic perphase), known as the capacitive currentIc of the network in the event of anearth fault is:

i c = 2 CωV c osπ

6= 3 CωU

i.e. Ic = 3 Cω V.

Conclusion: when determining theresistance of an earth connection or theadjustment of an earth protectiondevice, the capacitive current that must

I c = 2 Cω V cosπ6 3 CωU

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[1] Le Verre: «Overvoltages occurringwhen elimining short-circuits innetworks with the neutral earthedthrough a resistor». Société Françaisedes Electriciens (French Electricians'Guild) Bulletin, 8th series, Volume 1,No. 4 (April, 1960).

[2] E.D.F.: Memo on protection ofhydraulic power generators. NP 69 03.

appendix 2:bibliography

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M li G i T h i l S ifi ti ° 62 / 12Réal. DTE - Photo IPV

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