basic selection of mv public distribution networks

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Cahier technique no. 203

Basic selection of MV publicdistribution networks

Didier Fulchiron

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no. 203Basic selection of MV publicdistribution networks

ECT 203 first issued May 2001

Didier FULCHIRON

After graduating from the Ecole Supérieure d’Electricité in 1980, hejoined the technical department at Merlin Gerin’s high-power testingstation in 1981.He then took part in various developments of medium-voltageequipment, mainly destined for public distribution.He now works for the Medium Voltage Division of Schneider Electric,participating in the technical specification of Schneider equipment,and is also involved in work on international standards committees.

Cahier Technique Schneider Electric no. 203 / p.2


Basic selection of MV publicdistribution networks

Medium voltage - MV - public distribution networks are constructed on thebasis of two fundamental parameters which influence the majority of theircomponents as well as their operation. These parameters are the neutralmanagement mode and the operating voltage. Selection of theseparameters has a very high impact on the whole network, and it is verydifficult, if not impossible or economically unrealistic, to alter themsubsequently. It is therefore essential to fully understand the influence ofthese decisions on other network parameters such as the protectionsystem, safety, fault management, etc.This document details the limits imposed by these decisions and considersthe various existing solutions, pointing out their advantages anddisadvantages.

Contents

1 Introduction 1.1 The neutral plan p. 41.2 Principal effects of this decision on the network components p. 4

1.3 System for protection against insulation failures (earth fault) p. 5

1.4 Safety of people and animals p. 5

1.5 Electromagnetic compatibility p. 6

1.6 The operating voltage p. 7

2 The existing network situation 2.1 Neutral earthing p. 8

2.2 Operating voltages p. 8

3 Characteristics of “4-wire” systems 3.1 Protection system for networks with distributed neutral (“4-wire”) p. 10

3.2 Operation of networks with distributed neutral (“4-wire”) p. 11

4 Characteristics of “3-wire” systems 4.1 Protection system for networks with non-distributed neutral (“3-wire”) p. 13

4.2 Operation of networks with non-distributed neutral (“3-wire”) p. 16

5 Operating voltage, selection criteria 5.1 Voltage losses and drops p. 18

5.2 Insulation difficulties and associated costs p. 18

5.3 Evolution of MV networks throughout the world p. 19

6 Conclusions p. 20


1 Introduction

1.1 The neutral plan

Impact on electrical characteristics

The neutral earthing mode for the HV/MVtransformer, and the choice of whether or not theneutral conductor is distributed (hence thedistinction between “4-wire” and “3-wire”networks) have a direct influence on a number ofsignificant network parameters.

c The earth fault current: In the event of a faultbetween a phase and earth, the value of the faultcurrent is principally determined by the neutralearthing impedance, and by the capacitance,between earth and phase conductors, present onthe network (lines, cables and capacitors).

c Touch voltage and tread voltage: These twoconcepts concern the safety of people in thevicinity of an electrical fault. These voltages aredirectly linked to the value of the earth faultcurrent and the impedances through which thiscurrent flows.

c The level of overvoltages: Neutral earthing hasan effect on industrial frequency overvoltageswhen earth faults occur, and also on theamplitude and damping of possible oscillating ortransient phenomena.

c The level of disturbance of the surroundingnetworks: In the case of overhead networks, theloop through which an earth fault current flowscauses a strong magnetic field. Voltages arethen induced in the circuits of neighboringnetworks, and typically in telecommunicationcabled networks (copper technology). The levelof these voltages may be unacceptable for

operation or even insulation of neighboringequipment. Electromagnetic compatibility (EMC)studies must therefore be conducted before anMV line is installed.

Impact on the operating characteristics

A number of operating criteria are also affectedby neutral earthing:

c The permitted duration of earth faults (extent ofdamage and safety)

c The behavior of flashovers in the air (whetherself-extinguishing or not)

c The methods used to locate the fault on thenetwork

c Voltage fluctuations at the load terminalsduring earth faults

c The number and duration of faults noted bycustomers

c The possibility and ease of reconfigurationfollowing an incident

Impact on the construction specifications

The neutral earthing mode has a significantinfluence on the construction of networks:

c The values of earth connection impedancesshould be adapted to the fault current.

c The conductors affected by earth faults shouldhave adequate thermal withstand.

c The insulation on the conductors and theequipment should take account of potentialovervoltages, influenced by the neutral plan.

1.2 Principal effects of this decision on the network components

The table in figure 1 lists certain significanteffects generated by the initial choice of aneutral earthing system.The earth fault hypothesis is always a majorconsideration, since multi-phase faults are notaffected by neutral earthing.This table demonstrates that several factors(safety, quality of service and cost) are directly

affected by the value of the earth fault current.The same applies to the safety of people (treadand touch voltages), “voltage dips” on lowvoltage, EMC with neighboring electrical circuits(including telecommunications circuits), anddamage at the location of the fault. It confirmsthere is no “perfect” neutral plan: advantagesand disadvantages are evenly distributed.


Fig. 1: Significant effects generated by the initial choice of a neutral earthing system

1.3 System for protection against insulation failures (earth fault)

The concept of a protection system combinesseveral aspects:

c Determination of the fault situations to behandled

c The selection and location of the switchgearused to eliminate faults

c The selection and configuration of the relaysinstalled to monitor electrical values, diagnosefault situations, then issue commands to open tothe switchgear

c The organization of discrimination between thevarious relays, so that the device closest to thefault is the only one to trip, minimizing the extentof the de-energized zone

Designing a protection system therefore requiresanalysis of the network and of the phenomenapresent in a normal situation and during a fault,familiarity with the possibilities for measurementand analysis (sensors, relays, control systems,etc) and a global approach to the subject inorder to determine the appropriate settings.

1.4 Safety of people and animals

The value of earth fault currents hasconsequences on other aspects than simply theoperation of the network (see fig. 1). In particular,during an earth fault, a current flows through theconductors which are normally de-energized(including the grounds and the earth) whoseconfigurations and impedances are not alwaysfully controlled. Voltage drops due to theseimpedances present risks of electrocution forliving beings standing in the proximity of the fault.

Tread and touch voltagesIn fact, if current is flowing through conductiveparts which are normally de-energized, includingthe floor, the impedance of these conductiveparts may result in two points which areaccessible simultaneously being subjected to adangerous difference in voltage. Depending onthe type of contact made, we talk about touchvoltage (between two parts of the body, mostcommonly between two hands) and tread

Neutral situation Isolated Tuned Impedant Directly earthed(Petersen)

Earth fault Linked to stray Almost zero, dep. Dep. on impedance: High: 2 to 25 kA,current capacitance: on tuning and 100 to 2000 A varies with the

2 to 200 A quality factor (< 40 A) location

Damage Low Almost zero Dep. on impedance High

Voltage None None Low Significantdisturbance

Restrictions Possible Thermal on Thermal on Thermal andovervoltages the coil the impedance electrodynamic

Protection against Difficult Complex Easy (time “3-wire”: easyearth faults discrimination) (current

discrimination)

Moreover, the relative importance of theseadvantages varies according to how much of thenetwork consists of overhead lines andunderground cables, the feeder lengths, etc.

A decision thought to be judicious at a certainpoint in time may therefore be called intoquestion once a network has evolved over anumber of years.


voltage (between two feet or two paws)(see fig. 2).Numerous standards and regulatory texts areconcerned with limiting the risk of electrocution.But if the equipment impedances can be easilydetermined and are stable over time, the samecannot be said for the earth connectionimpedances, whose low values are difficult tomaintain.

Fig. 2: Examples of touch and tread voltages

Safety is therefore enhanced by a greatertolerance vis-a-vis the earth connectionimpedances, and simultaneously limiting thevalue of the earth fault currents. Internationalstandards have introduced a voltage-durationformula to demonstrate what the human bodycan withstand. The longer it takes to eliminate afault, the lower the maximum tread and touchvoltage values to be respected will therefore be.

Touchvoltage

Z

ZTreadvoltage

1.5 Electromagnetic compatibility

Induction with regard to telecommunicationcircuitsIn “3-wire” networks, in a normal operatingsituation, no current is circulating in the earth.Only earth fault situations (see fig. 3) create asignificant magnetic field emission and need tobe considered in the electromagneticcompatibility (EMC) studies.

For “4-wire” networks, in a normal operatingsituation, the permitted load unbalance betweenthe various phases results in a current in theneutral. This current is divided between theneutral conductor and earth, since this neutralconductor is earthed at a number of points. It istherefore necessary to create a permanentsituation of emission of an industrial frequency

Fig. 3: Voltages induced by MV networks in telecommunication cabled network circuits

MV network fault currentInduced current on thetelecommunication network


magnetic field. However, at a comparable faultcurrent, the presence of the fourth conductor,generally placed on the line supports, reducesthe emissive loop.

In all cases, radiation is due to the geometry ofthe lines and is difficult to reduce, particularlywhile adhering to a given system and overheadnetwork technology. The reduction of earth faultcurrents, in 3-wire network systems, maytherefore be a more appropriate solution, since italso participates in the safety of personnel asdescribed earlier.

When the users of these networks (energy andcommunication) can work together, theestablishment of installation rules for theirrespective lines can contribute to a reduction inthe magnetic coupling between their networks,for example by reducing the “receptive” loop ofthe disturbed circuit by using twisted cablesrather than parallel wires, or even by avoiding aparallel line too close to the two networks(see fig. 4).

But it is the increasing strictness of emissionstandards which is leading ever more energydistributors to reconsider their neutralmanagement modes.

Fig. 4: Reduction of the coupling between an MVnetwork and a telecommunication cabled network

HV/MV substation

MV/LV substation

Line B

Line A

The telephone network along line B will be subject to less disturbance than that along line A.

MV overhead lineTelephone line

Hamlet

Town

Note: Networks comprising underground cablesbenefit from low earthing impedance of theground connections and electromagneticmasking, due to the presence andinterconnection of screens. They are thereforeless affected by the factors discussed in sections1.4 and 1.5.

1.6 The operating voltage

A compromise between conflictingparametersThe following information should be consideredbefore choosing an operating voltage for a givennetwork:v the average length of the MV feedersv the delivered powerv the lossesv the cost of insulation and equipmentv the history and the policy of the distributor

c The average length of the MV feeders: In fact,the increased impedance of long feeders risksgenerating unacceptable voltage drops.Therefore, a vast geographical area, suppliedwith a small number of HV/MV substations, willrequire use of a somewhat higher operatingvoltage.

c The delivered power: For zones with a highdensity of use, the feeders, even short ones,have to serve numerous loads; for a givenvoltage, their high currents then fix a limit on thesystem, making a higher operating voltage thepreferred option.

c The losses: Although the voltage drops andcurrents remain within acceptable limits, theenergy losses due to the Joule effect maysometimes have a significant cost. Increasingthe operating voltage allows the energy loss tobe reduced for a given distributed power.

c The cost of insulation and equipment: Thehigher the voltage, the greater the isolation

distances need to be. As a result, the amount ofwork and the size of the switchgear has to beincreased, thus increasing the costs. Moreover,certain technologies which are available for “low”voltages are not available for higher voltages.

c The history and policy of the distributor: Apartfrom the fact that it is very difficult to changeoperating voltage, a distributor often chooses asingle operating voltage in order to rationalize hisequipment. If he has an extensive network, thissingle voltage does not correspond to theoptimum, from a technical and cost point of view,which could be selected for smaller geographicalareas. However, it does offer appreciableopportunities for economy of scale and reuse ofequipment.

The choice is limited to standard valuesIn order to be able to access an importantsupplier market, and therefore benefit fromfavorable competition conditions, a distributorshould limit his choice to standardized values soas to ensure the availability of the variousequipment (isolators, cables, switchgear, etc)from different manufacturers. The use of thesestandardized values also means it is possible tobenefit from experience acquired bymanufacturers and by the standardsorganizations. Finally, the existence of referencestandards simplifies contractual relations whileensuring a good level of performance andquality.


2 The existing network situation

2.1 Neutral earthing

Networks can be classified according to twomajor categories:c Those where the neutral is distributed (4-wirenetworks)c Those where the neutral is not distributed

In theory, each of these categories can useneutral earthing connections with varyingimpedance values (see fig. 5).In fact, all “4-wire” networks use a neutralconnection directly to earth; moreover the neutralconductor can be earthed directly at multiple pointson the network and therefore never presents adangerous voltage. This layout is used in theUnited States, more generally in North America,but also in parts of South America, Australia andother countries influenced by the USA.

“3-wire” networks use four types of earthing:c Directly earthed neutral (Great Britain, Spain,etc)c R//L low-impedance neutral (France, Germany,Spain, etc)c Tuned or “Petersen” neutral (Germany,Hungary, Poland, etc)c Isolated neutral (Spain, Sweden, Norway, Italy,China, etc)

These choices have been made on the basis oflocal features, such as the respective extents ofthe overhead and underground networks.Changes are possible, but often have seriousfinancial implications (major work and expensivechanges of equipment).

Fig. 5: Various earthing layouts for MV distribution networks

Neutral isolatedfrom earth

a tuned circuit

Neutral earthed via

an impedance

Neutral distributed with numerousearthing points

Neutral earthed directly andnon-distributed

Z

2.2 Operating voltages

The operating voltages used by energydistributors are between 5 and 33 kV. The lowestvoltages are only found for networks with alimited coverage, usually in urban areas. Someoperating voltages are currently being phasedout, but this is happening over a long timescaledue to the slow rate at which networks areupgraded.

Two important standards systems coexist andrecommend various preferred voltage values:c The International Electrotechnical Commission(IEC) is the reference for the majority ofcountries.c The American National Standards Institute(ANSI) is the reference for countries which are,or have in the past been, influenced by the USA.


These standards systems have introduced theconcept of “rated voltage”, a value which servesto define all the characteristics of the equipment.

The table in figure 6 gives some of the valueswhich are most commonly found, but the majorityof countries use a number of voltage levels.

Fig. 6: Examples of voltage values used for public distribution

Operating voltage (kV) Country Rated voltage (kV) Standard

6 Japan 7.2 IEC

10 to 12 United Kingdom, Germany, 12 IECChina

13.8 USA, Australia 15 ANSI

15 (being phased out) 17.5 IEC

20 France, Italy, Spain 24 IEC

25 USA 27 ANSI

33 Turkey 36 IEC


When the distributed power on the last segmentis low, the protection furthest away from thesubstation is often in the form of fuses, forreasons of cost.

3 Characteristics of “4-wire” systems

“4-wire” systems are characterized by distributionof the MV neutral right to the loads. This type ofdistribution is used in the USA and in certaincountries influenced by North America, and isalways subject to ANSI regulations. It is only usedin a “directly earthed” neutral plan, and applies aglobal earthing concept consisting of earthing theneutral conductor at multiple points on thenetwork, approximately every 200 meters. Theneutral-earth voltage is therefore fully controlled.

Distribution of the neutral conductor enablespower to be supplied to the loads between theneutral and one phase (to the single voltage): asignificant part of the energy is thereforeconsumed in single-phase. In a normal operatingsituation, this single-phase use, whoseproliferation is not totally controlled by thedistributor, results in the presence of a current inthe neutral conductor or the earth. It is generally

acknowledged that the load unbalance betweenthe various phases can be as much as 40% ofthe rated current for a feeder.

Due to the direct earthing, the current for adirectly earthed fault is mainly limited by theimpedance of the network segment between theHV/MV transformer and the location of the fault.This situation calls for the use of “decentralized”protection, capable of managing increasingly lowthresholds as the distance increases, andnonetheless capable of being coordinated. Theresulting protection system is complex andpoorly suited to network reconfiguration, in theevent of an incident. This system should also beadapted to each significant modification for afeeder, whether in terms of impedance ortopology, which constitutes a major constraint interms of upgradability.

3.1 Protection system for networks with distributed neutral (“4-wire”)

In these networks, the unbalanced current dueto single-phase loads can “mask” an earth faultcurrent. In fact, protection cannot discriminatebetween the current of a phase-neutral loadand the current of a phase-earth fault if theyhave comparable values. The value of thephase-earth fault current is linked on the onehand to the expected impedance of the faultitself, and on the other hand to the networkimpedance between the HV/MV power supplytransformer and the location of the fault. Ittherefore varies according to the distance fromthe fault to the substation. For rather long lines,a phase-earth fault a long way away can causea lower current than the unbalanced currentpermitted at the substation feeder; in this case,a protection device placed in the substation willnot be capable of detecting this fault. Anadditional protection device with lowerthresholds is then necessary to extend the partof the network which is actually monitored,known as the “protection zone”. In a network,the higher the impedance of the faults to beeliminated, the smaller the protection zone foreach device.

Therefore, in order to have adequate detectionof faults on this type of network, where thenormal load currents diminish the greater thedistance from the substation, a number ofprotection devices should be placed in cascade(see fig. 7).

Fig. 7: Example of a North American distributionnetwork comprising a number of protection devicesplaced in cascade: Note how the protection zonesoverlap.

HV/MV transformer

HV/MVsubstation

Recloser: overhead multiplereclosing circuit-breaker

Fuse

Zone protected by the recloser


As far as the underground parts (using cables)of networks are concerned, they generallycover a smaller area than an overhead networkand have lower impedance, therefore the valueof a phase-earth fault current is only slightly

affected by the distance from the fault to thesubstation. Nonetheless, in order to servelimited zones from a trunk cable, thesenetworks also include single-phase junctionsprotected by fuses.

3.2 Operation of networks with distributed neutral (“4-wire”)

Operation of this type of network may becharacterized by two major difficulties:

c Electrical risks due to possible high-impedance faults which are difficult to detecteasily

c When a loop is required for good continuity ofservice, it should be of sufficiently lowimpedance to be in the protection zone.

The previous sections explained the difficulty ofdiagnosing high-impedance earth faults. RecentAmerican publications (1999) note the fact that,in more than half of the operations to re-erectconductors which had fallen to the ground, theconductors on the ground were still energizedwhen the technicians arrived. These situationsrepresent high risks for both people andequipment (electrocution or fire).

When the neutral is distributed, two networkstructures can be distinguished according towhether or not a loopable connection whichdoes not incorporate decentralized protection ispresent.

Presence of a loopable connection whichdoes not incorporate decentralizedprotection

If such a loop exists, it is necessarily at lowimpedance so that it can be entirely in theprotection zone of the HV/MV substationdevices (see fig. 8). This is typically the casefor dense urban geographical areas withunderground distribution.The loop can be used according to the openloop principle in order to benefit from thecapacity to return to service associated with thislayout, if an incident concerning the cableoccurs on the loop itself. From this loop,junctions equipped with protection devices canbe created in single-phase or three-phase(see fig. 9). They may be organized intosub-loops if necessary, to benefit from thesame operating mode, but these sub-loopsshould be entirely in the protection zone of thejunction devices.Due to the limited impedances of the cablesegments, and the existence of only two levelsof protection to be managed, such a systemmay be considered to be satisfactory in terms ofupgradability. Any geographical extension isnonetheless limited by the need to respect theprotection zones.

Fig. 8: For complete protection of a power supply loopagainst earth faults, this loop should be entirelycontained in the protection zone for the HV/MVsubstation devices.

N1

N2

N3 N1

N2

N3

N

Zone protected by the HV/MVsubstation devices

HV/MV substation

Loop

Fig. 9: These are the HV/MV substation deviceswhich protect the main loop, and both ends of eachjunction organized as a sub-loop incorporateprotection devices to ensure the safety of the variouspossible configurations.

N1

Sub-loop

Main loop

Components powered by the sub-loop whoseearth fault protection is provided by A or B depending on the configuration.

N

Zone protected by the HV/MV substation devices

HV/MV substation

A B

Zone protected by A or B


Presence of connections which arephysically capable of being looped, butincorporate decentralized protection devices

When the network is structured around radialfeeders, with cascaded protection devices,“emergency” type layouts are not permitted,although the topology itself would allow it. In fact,a load reconnection, even temporary, whichoccurs at the end of a tree structure wouldnecessitate redefining the protection thresholdsand the discrimination stages of the variousdevices concerned. Since these settings are theresult of fairly complex calculations, taking intoaccount the lengths and the types of the varioussegments, there is no chance that incidentscould be handled by modifying the settings.

Operation is therefore limited to radial mode, andthe incident situations may entail long periodswithout power until they are repaired.For similar reasons, any upgrading of thetopology or the network load level involveschecking the compatibility of the protectiondevices in place. Modifying this type of networkis therefore a very delicate and expensiveoperation. Such a system can only beconsidered mediocre in terms of upgradability.

Whatever the network structure, faults whichcause the current protection devices to work canbe located easily using detectors which react toovercurrents. These fault detectors, placed onthe phase conductors, work with both faultsbetween phases and earth faults.


4 Characteristics of “3-wire” systems

In these systems, the neutral is not distributedand therefore is not available to users. Loads,even single-phase, can only be connected tophases of the network. They do not thereforegenerate any neutral current in the distributionsystem and, excluding any capacitive unbalancebetween the phase conductors, the residualcurrent of such a system is zero.

The network neutral point, which remainsexclusively available to the distributor, can beearthed via an impedance of any value andtype. In practice, four main neutral plans areused: isolated, tuned, impedant or directlyearthed.

If the value of the neutral earthing impedance issignificant compared to the network impedances,

the resulting zero-sequence impedance fixes, defacto, the maximum value of the earth faultcurrent. In this zero-sequence impedance, theneutral impedance should be considered to be inparallel with all the network phase-earthcapacitances. These capacitances may reachhigh values and contribute significantly to theearth fault current. In all cases, due to theabsence of residual current during operation, allearth faults can be detected at the substation.Depending on the neutral impedance, theadopted protection method may vary, but thereis no technical obligation to use decentralizedprotection devices. The protection system cantherefore be kept fairly simple, with theadvantage that it does not require modification ifchanges are made to the network configuration.

4.1 Protection system for networks with non-distributed neutral (“3-wire”)

GeneralIn networks with non-distributed neutral, loadsare necessarily connected between phases andwhen the neutral earthing connection exists, nocontinuous current flows through it. This situationis purely theoretical: capacitive currents whichexist between the phase conductors and earthare never perfectly balanced. This unbalance isdue to the differences in geometry on overheadlines, inside transformers, on the cable runs forthe various phases, etc. However, if when thenetwork is constructed, the distributor takes theprecaution of swapping over the phaseconductors along each feeder, the residualcontinuous current for each feeder can bereduced to less than 1 A, or even much less withan isolated neutral. This type of natural residualcurrent can then be used to diagnose thepresence of low-value earth fault currents fromthe substation.Since the orders of magnitude of these faultcurrents can be noticeably different from thosefor a “4-wire” system, the protection to beprovided is also different.

Network with isolated neutralThe network is said to be with “isolated neutral”when there is no deliberate physical connectionbetween the transformer MV neutral point andearth. The transformer can also have adelta-connected MV winding. The networkaverage voltage in relation to the earth is

therefore fixed by the stray impedance betweenthe phase conductors and earth. Theseimpedances include the capacitance of the linesand cables, which are predominant, but also theleakage impedances of different types ofequipment (lightning arresters, measurementsensors, etc) and those of any faults. Theresidual voltage, which is the vectorial sum of thethree phase-earth voltages, of such a network isnever perfect zero. Monitoring this voltage canprovide a good indication of the networkinsulation quality, since any fault between aphase and earth causes a marked unbalance inimpedance and increase in the residual voltage.However, this information, which is common tothe whole network, does not make it possible tolocate the fault.

In the case of a clear fault to earth (negligibleimpedance direct contact), the phase-earthvoltage is zero for the phase concerned andequals the phase-to-phase network voltage forthe two other phases. The currents present inthe phase-earth capacitance of the three phaseconductors then no longer form a balancedthree-phase set: a residual current which is notzero circulates throughout the network. Itsintensity, at each feeder circuit-breaker, dependson the length and type of the connections and ofthe equipment placed downstream. Use of anovercurrent protection device does not thereforemake it possible to discriminate simply andefficiently which of the healthy feeders is faulty.


Networks with isolated neutral can be used witha sustained earth fault (detected and noteliminated). This operating mode is sometimesused to improve continuity of service, since itenables location of the fault while continuing toserve customers. The risk associated withsustaining an earth fault is that a second fault ofthe same type may appear on one of the otherphases. This second fault creates a short-circuitwhile the healthy phases are, vis-a-vis the earth,maintained at a voltage which equals thephase-to-phase voltage during the whole time ofoperation with a sustained fault.

Network with tuned neutral (Petersen coil)The network is called “tuned” or “earthed via aPetersen coil” when, in connecting the neutral toearth, a high-quality coil whose inductance value

is adjusted in order to obtain tuning (resonanceconditions) is placed between the capacitancesof the network and this coil (see fig. 10). Whenearthing one of the network phases, this tuningresults in a very low current in the fault(Id = IC - IL ). This current is only due to theimperfection in tuning, the capacitive unbalancebetween the phases and the coil resistive losses.

The normal order of magnitude of this type offault current is a few amps (typically 2 to 20 A).The resonance condition is expressed by theformula LCω2 = 1, where:L = neutral inductanceC = zero-sequence capacitance of the network(sum of the phase-earth capacitances of thethree phases)ω = network pulsation (ω = 2πF = 100π for anetwork at 50 Hz).

Fig. 10: Operating principle for an earthing coil whose inductance value is tuned to the capacitive value of thenetwork

HV/MVtransformer

IL

Neutral point current IL

Capacitivecurrents IC

Faultcurrent Id

Tunedinductance

Distributedcapacitances

When a fault occurs: IL ≈ - IC ⇒ Id ≈ 0


If this resonance condition is sustained, duringvariations of the network configuration and evenduring variations in climatic conditions, it impliesthat the coil can be adjusted with good definition.Tuning is generally performed by a controlsystem.

This neutral plan has a major advantage in thatnumerous faults are self-extinguishing, typicallyall air gap flashovers. It therefore offers goodcontinuity of service for networks whichincorporate lots of overhead lines. It is clear thatinsulation faults within equipment and cables(especially underground) do not benefit from thisbehavior. In addition, networks with tuned neutralcan be used with a sustained earth fault, likenetworks with isolated neutral. The limit of thisoperation is most commonly associated with thethermal withstand of the neutral impedancewhich is subjected to single voltage for the wholeduration of the fault.

The main disadvantage of the tuned neutralresides in the difficulty in locating a continuousfault and certain recurrent faults. This difficultycomes from the low value of the current flowingacross a fault compared to the high value of thecapacitive currents which circulatesimultaneously on all the lines. Simple residualcurrent detection cannot therefore tell thedifference between a healthy feeder and thefaulty feeder. It is necessary to introducedirectional residual overcurrent protection, oreven residual overpower protection in order toensure high-performance discrimination. The useof such protection is possible at the sourcesubstations, but is totally unrealistic (complexand expensive) in the plants installed along thenetwork. Finally, fault detectors operating on thesame principle (directional) would be tooexpensive and do not therefore exist. For thisreason, use of the network is seriouslycompromised when continuous fault typeincidents occur: the power can only be switchedback on once the lines have been inspected andsuccessive attempts at reclosing made.

These difficulties make this neutral planunworthy of consideration for networks with ahigh proportion of underground cables. However,recent technological developments have enabledthe creation of new detectors which operate withinexpensive sensors. This development couldremove some of the present operating difficultiesfor networks with compensated neutral.

Network with impedant neutral

For this type of network, a limiting impedance,generally resistive, is inserted in the neutral earth

connection. It can also include an inductive part,in order to compensate partially for thecapacitive contribution from the network. Withpublic distribution, no networks are earthed by asingle non-tuned inductance.The value of the impedance is always highcompared to the line impedances and, therefore,a directly earthed fault current varies accordingto the location of the fault: it is a few hundredamps (100 to 2000 A approx). This high level ofearth fault currents, as well as thepreponderance of the component circulating inthe neutral impedance, make it easy to detectearth faults:

c A “residual overcurrent” type protection device,with sufficiently high threshold values to not beaffected by capacitive or transient phenomena,works correctly on these networks.

c Discrimination between feeders is easy due tothe significant value of the fault current, anddiscrimination between protection devicesarranged in cascade is obtained by time-basedoperation (defined time or IDMT protection)However, the possible existence of impedanceearth faults which are not insignificant comparedto the neutral impedance makes it desirable tosearch for the lowest settings, while guardingagainst unintentional tripping. For faults with veryhigh impedance, residual current protectiondevices can no longer discriminate, andadditional devices, such as automated detectionsystems with successive opening of the variousfeeders, are added in the source substations.

In numerous situations, when the loaddownstream of the protection device is low,protection against directly earthed faults can beprovided by phase overcurrent detection. This iswhy certain distributors do not systematically putresidual current protection on these types ofcircuit (for example: junction supplying an MV/LVtransformer).Locating faults on these networks is made easierby the fact that simple, reasonably-priced faultdetectors can be used, which are capable ofreacting to a directly earthed fault current. Theirnonetheless limited sensitivity means that certainfaults with significant impedance, althoughdiagnosed by the source substation protectiondevices, do not cause them to react (insufficientcurrent). It is, however, possible to choose lowerthresholds with the single inconvenience ofcausing unnecessary signaling, since theunintentional operation of a fault detector doesnot generally have significant consequences.

Network with directly earthed neutral

The directly earthed neutral can be interpretedas a special case (as is insignificant neutralimpedance) of the impedant neutral. These are


therefore exclusively impedances concerning thenetwork (source and line), the fault and return viathe earth, which fix the intensity of the faultcurrent. Therefore, the generally high faultcurrent intensities can present significantvariations depending on the fault location andtype, and as a consequence lead toreconfiguration difficulties; for example whenreconfiguring a network with notably longerfeeders in an emergency situation than in anormal situation. This effect can be achieved bythe adoption of detection thresholds, as low aspossible, designed to diagnose directly earthedfaults and also impedant faults.

The detection of earth faults is simple. Very oftenthe same type of protection can be used inrespect of faults between phases and earthfaults. The “fault detector” function is simple toprovide using phase overcurrent detection, orpossibly residual overcurrent detection.In this layout, where earth faults can have highintensities, they may result in significantdamage. It is therefore desirable to choose shortprotection intervention times. This situation,associated with the ever-present need fordiscrimination in a distribution network, favorsthe use of IDMT protection (often known as“inverse time” protection).

4.2 Operation of networks with non-distributed neutral ("3-wire")

The structure of these networks is mainly radial,or radial with the option of looping, or looped.Numerous other configurations exist whichcombine these basic structures.They operate most commonly on an opencircuit. However, in order to ensure a high levelof availability of energy for their customers,some distributors use closed loops equippedwith circuit-breakers at each MV/LV transformersubstation. But such installations are expensiveand require delicate handling.

In open-circuit operation, the maincharacteristic of "3-wire" networks is that theyallow detection of all earth faults, whatever theirlocation, from the HV/MV substation. Faultsbetween phases are also managed from theHV/MV substation: decentralized protection isonly necessary if there is an exceptionally longline, in fact once it becomes impossible todiscriminate a distant fault between two phaseconductors, in the permitted load current for thefeeder.

Centralized protection offers total freedom tomodify the network. Hence it is possible toprovide a number of emergency layouts withnumerous loop feeders to cope with differenttypes of incident: if the basic layout for normaloperation of a high-density consumption areaencompassing numerous customers is the openloop, loop feeders can also be provided at theend of lines with a radial structure, which maylimit the loads which can be resupplied after theinterruption (see fig. 11).

Freedom of modification also allows extensionsor restructuring of the network to be carried out,without implications for the protection methodsand settings. This ease of upgradability offers avery appreciable level of flexibility.In contrast, if only centralized protection is inplace, all customers of a single feeder areaffected in the event of an incident. This couldinvolve very large numbers… hence the

motivation which is then equally important tocorrect the situation quickly!

The use of remote control in conjunction withfault detectors responds to this imperative forspeed of intervention: depending on the amountof remotely controlled switchgear, it allows asignificant part of the affected customer base tobe reconnected to the supply (usuallyover 60%).A three-stage approach is used by thedistributor’s teams to get the power supplyreconnected:

c 1: Reconfiguration by remote control

c 2: Manual reconfiguration, in the field, usingisolation switchgear which is not remotelycontrolled

c 3: Repair if necessary

Special remote control points can be introducedwith the sole aim of improving quality of service,since no technical imperative makes itnecessary to use a particular location on thenetwork.Some distributors install decentralizedprotection device, simply in order to providequality of service and not because of afundamental need to eliminate faults. It shouldbe noted that the defined discrimination criteriawhich need to be respected between theseprotection devices and those of the HV/MVsubstation are markedly less sensitive than fornetworks with distributed neutral. Moreover,modifications to the topology or load do notgenerally have any adverse effect on thesesettings.


Fig. 11: Example of a distribution network layout where the centralized protection allows different operatingconfigurations to be used (a = configuration for normal operation, b = configuration after an incident reducing thenumber of customers affected, c = configuration for resupply with limitations on the load)Note: The current flows through switch K in opposite directions in configurations a and b; the same applies toswitch X in configurations b and c

a

b

c

HV/MV substation -A- HV/MV substation -B-

Controldevice

Open

Closed

Circuit poweredby Aby B

K

Loads transferredfrom A to B

X


Fault

X

K



5 Operating voltage, selection criteria

5.1 Voltage losses and drops

The losses for an MV distribution networkessentially correspond to the heat dissipationdue to the Joule effect in the conductors. This istherefore a cost criterion, since they downgradethe global efficiency coefficient for the network.They are proportional to the square of thecurrent and, with a given distributed power, areinversely proportional to the square of theoperating voltage. For this reason, it is desirableto use a high operating voltage in order tominimize them, but other factors, mainlycost-related, may counter this advantage.

Voltage drops at the perimeter of the customerdelivery area (see fig. 12) correspond to thedifference (∆U) between the no-load (e) andon-load voltages for the network (ULoad). Thecurrent magnitude and phase (ϕ) are determinedby the power and nature of the load. Since thenetwork behaves in an essentially inductive way,the voltage drop for the line (ULine) is almost 90°out of phase with the current.

Different regulations define the limit voltagevalues that distributors should comply with at thepoint of delivery. For example, a tolerance of± 10% on the low voltage network and atolerance of ± 7.5% on the medium voltagenetwork may be required simultaneously. Itbecomes increasingly difficult to comply withthese tolerances as the distributed powerincreases. For given conductor lengths andcross-sections, a higher power supply voltageauthorizes a higher delivered power with thesame tolerances. Where a maximum voltagedrop is imposed, the table in figure 13 illustratesthe transmission capacity of a line according toits cross-section and operating voltage.

Appreciation of the capacitance of a network is aplanning tool: it can be used to initiate operationsto reinforce lines or create new lines when thepotential growth of the inrush power isapproaching the permitted limits. Although it is

technically possible to change operating voltage,it is rarely done in this situation, since such anoperation presents numerous difficulties:replacement of all the transformers, a large partof the conductors, the switchgear and theisolators, etc. It is therefore very important thatthis voltage is selected correctly at the time ofcreating the network, at a suitable level whichtakes account of the anticipated development ofloads over a period of several decades.

5.2 Insulation difficulties and associated costs

Although the criteria discussed earlier encouragethe use of a high operating voltage, a compromisemust nonetheless be found in order for thenetwork to be cost-effective. It goes withoutsaying that the price of the various componentsincreases significantly with the operating voltage.

Impact of the voltage on the equipmentThe various types of equipment used should bedesigned with insulation adapted to the operating

voltage as well as to the various exceptionalconstraints which apply to this voltage level.

Standardization has resulted in coherent sets ofvalues, known as “insulation levels”, whichassociate a maximum operating voltage withwithstand voltages in specific conditions. Theseconditions are usually described as “lightingimpulse withstand”, and “power frequencywithstand”. Hence a device for a 10 or 11 kV

Fig. 12: Graphic representation of voltage losses anddrops on the line

ULosses

∆U

ϕ

ULinee

I

e

I

ULoad

≈ 90°

Fig. 13: Relationships between power transported by aline and the cross-section and length of this line, foraluminium conductors with a steel core

MW x kmWhere ∆U / U = 7.5% and cos ϕ = 0.9

mm2 kV15 20 33

54.6 22 39 105

75.5 28 49 133

117 37 66 175

148.1 42 76 205


network should be selected in a 12 kV rangewith a power frequency withstand of 28 kV forone minute, and a lightning impulse withstand of75 kV with the standard test wave (example ofIEC values).In terms of equipment design, these variousconstraints result in insulation thicknesses anddistances which differ in the air or in a gas, andtherefore in different overall dimensions.Example: “switch” cells with air insulation and SF6cells, using the same technology have, dependingon the voltage, the following dimensions(Height x Depth x Width):c Un = 24 kV : 1600 x 910 x 375 mmc Un = 36 kV : 2250 x 1500 x 750 mm

The same applies to all the networkcomponents. Only overhead line conductorsescape this criterion: they are insulated by theambient air and the distances are fixed by theconstruction (rails and isolators) of the masts.Another consideration is that technologies areonly available for certain voltage levels, forexample, air break switchgear for indoor use isonly available for the 36 kV level and haspractically disappeared at the 24 kV level.

Finally, use of cables may be limited fortransporting alternating current. In fact, theircapacity relative to earth being a function of theirlength and insulation (and therefore of thevoltage level) seriously reduces the current

available at the end of the cable connections(see fig. 14). As a consequence, in very highvoltage transmission networks, the length of thecable segments is always limited. This is a majorreason why direct current is most commonlychosen for high-voltage insulated connectionswhere overhead lines cannot be used.

Impact on cost

The criteria discussed previously have a directimpact on equipment manufacturing costs, andon market prices. Moreover, equipmentmanufacturers are not all able to offercomprehensive ranges that include the highestoperating voltages (36 to 52 kV). The “supplier”market is therefore more restricted and lesscompetitive than for the lowest operatingvoltages (7.2 to 24 kV). Nonetheless, this effecton the price is sometimes minimal, as is the casefor cables with synthetic insulation which have abroadly similar transmission capacity, forexample:

c 240 mm2 alu / 24 kV: 7.31 ke / km

c 150 mm2 alu / 36 kV: 7.77 ke / km ie. + 6%(1999 list prices)

But the user should also consider the associatedcosts, such as those concerning accessories,the external dimensions of an installation, andsafe distances to be maintained with respect tolines, etc.

Fig. 14: Effect of leakage capacitances on the available current, and therefore on the output power, at the end of acable connection

ID

ISIDIS

IC

IC

LoadDistributedcapacitances

Nominal current of thecable connection

Source

5.3 Evolution of MV networks throughout the world

As we enter the twenty-first century, there is stilla wide disparity in energy requirements betweendifferent countries.

Major growth in MV public distribution can beobserved in numerous countries, with a clearbias in the selection of network structurestowards the intermediate range of operatingvoltages, between 15 and 25 kV, for example:c Japan, where the majority of the distributionnetwork is 6 kV, is preparing to upgrade to 22 kV

c France is gradually replacing its 5.5 kV and15 kV networks with 20 kV networks, whereasthe 33 kV level is stagnating, if not regressingc China, who is looking to use 20 kV in order tosupport its strong economic growth and improvethe overall operation of its networks, presently12 kV


6 Conclusions

The various systems and options discussed inthis document share a fundamental aspectcommon to any MV distribution network, in thatthey all have numerous implications. They aretherefore particularly difficult to consider onexisting networks. However, with the regulargeneral increase of electricity consumption,certain situations mean that barriers todevelopment are reached and have to beovercome, or that opportunities for re-examiningthese basic decisions present themselves.

In this context, it is therefore sensible to initiateor implement far-reaching changes in the contextof large-scale upgrade operations, followinggeneralized obsolescence or exceptional events.

Moreover, the historical situations which justifiedthe initial decisions often no longer apply, and

new factors gain importance. In particular,aspects relating to safety, quality and continuityof service, respect for the living environment,compatibility with other equipment, etc, are nowoverwhelmingly important criteria. A number ofsocial or macro-economic phenomena underlieor encourage these changes.

The major electrical equipment manufacturershave already taken on board the resulting needsof energy distributors, due to these changes. Theequipment offered responds to these needs.But it is the distributors who, in response to rapiddevelopments in their environment, both politicaland geographical, need to think ahead, and viewthe upgradability of a system as a basic selectioncriterion.

Schneider Electric Direction Scientifique et Technique,Service Communication TechniqueF-38050 Grenoble cedex 9Fax: 33 (0)4 76 57 98 60

Transl: Lloyd International - Tarporley - Cheshire - GB.Edition: Schneider Electric.Printing: Imprimerie du Pont de claix - Claix - 1000.- 100 FF- ©

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