mv public distribution networks throughout the world public distribution networks throughout ......

E/CT 155 first published March 1992

Christian Puret

A graduate Engineer from theE.N.S.E.R.G. (Institut NationalPolytechnique de Grenoble) and theI.A.E. in Paris, he joined Merlin Gerinin 1977.His first assignment gave himexpertise in the field ofprogrammable logic controllers.He was then put in charge ofcustomer training for theMerlin Gerin Group.In 1986, he rejoined the MediumVoltage Division in which he nowholds a position involving strategicmarketing. He is in charge ofintegration of developments inprotection systems and telecontrolin MV equipment, and morespecifically in those destined forpublic distribution networks.

n°155

MV public distributionnetworks throughoutthe world

Cahier Technique Merlin Gerin n° 155 / p. 2

Configuration : an operation, in bothdigital protection and telecontrolsystems, that involves assigning, viaeither a built-in or a loaded softwarepackage, a standard piece ofequipment to a specific application.The latter operation, loading a softwarepackage, is carried out using a tool :the configurator, normally a personalcomputer PC. This allows for example:■ definition of functions that theequipment will perform,■ devising of connections with theenvironment,■ creation of mimic diagrams andlabelling of alarms for users.

Draw-out (part of an assembly)(IEC 50 - chapter 441, NF C 01- 441) :withdrawable part of an assembly that,whilst remaining mechanically attachedto the assembly, can be moved to theposition or one of the positions thatgives an insulating distance or a metalseparation between open contacts.This insulating distance or metalseparation always intervenes in themain circuit. It may or may not alsointervene in auxiliary circuits.

Fixed (fixed device),(IEC dictionary of electricity) :a device that is designed to bemounted on a fixed support andintended to be connected to externalcircuits using fixed electricalconductors.

Cut-out type fuse : MV striker-typefuse, that performs the two functions ofprotection and isolation. Isolationoccurs when the fuse melts by ejectionof the striker which in turn automaticallytriggers switch-over of the fusecartridge.

Fault passage indicator : a devicefitted to MV networks that indicates,fault passage either locally or at adistance. In order to improve quality ofservice, the operating company makesevery effort to reduce the interruption insupply to that part of the network inwhich the fault exists. To achieve thisthey must know which portion of thenetwork is affected by the fault. Withthis aim, the operating company installsfault passage indicators. Analysis of theinformation provided enables the faultyarea to be closed off, followed byreconfiguration of the network (typicalremote control application)

Prediction : a new science with theobjective of event prediction, it is basedon reasoning and scientific deduction.

Recloser : MV circuit breakerincorporating a multishot autoclosingrelay, installed on an overhead MVfeeder and coordinated with protectiondevices (fuses) placed on the feeder(upstream and downstream). It is usedin North-American type distributionnetworks.

Sectionaliser : MV switch equippedwith a fault passage counter, it isinstalled downstream of a MVoverhead feeder protected by arecloser. Its automatic control countsthe number of fault current passages(corresponding to the number of timesthe recloser is activated), and when apreset number has been reached itopens the circuit breaker. Selectivitycan thus be obtained by installingseveral sectionalisers in series on aMV feeder, the last sectionaliser(which is furthest from the recloser)opening on the second instance ofcurrent fault, the precedingsectionaliser opening on the thirdoccasion and so on. This device isused in North-American typedistribution networks.

glossary


In a country, the Transmission andPublic Distribution networks ensure thetransfer of electrical energy from pointsof production to consumer units.

The points of production are powerstations that generate electrical energyfrom various primary energy sources(nuclear, hydro-electric, coal....)

The points of consumption inMV - Medium Voltage -, aresubstations, from which the energy isdelivered to customers (subscribers).This takes place via the “MVdistribution system “wich is the object ofthis “Cahier Technique“ report.

In this “Cahier Technique“ report, afterhaving described the various types ofdistribution networks and thedistributor’s role, the reader who is notfamiliar with MV will find details on:■ topologies of MV networks,■ substations,■ protection and telecontrol devices.

Comment: In this “Cahier Technique“report, the term MV applies to anyvoltage from a few kV to 40 kV.

MV public distribution networksthroughout the world

table of contents

1. Different types of electrical The transmission and p. 4networks interconnection network

The subtransmission network p. 5The MV network p. 5The LV network p. 6Type of electric current p. 6Network planning p. 6

2. The distributor Reason of existence p. 7His role p. 7His development p. 8

3. MV network topologies Criteria in choosing a topology p. 10Items that depend on p. 10the chosen topologyVarious MV network layouts p. 10Neutral earthing layouts p. 12Protection system p. 13Telecontrol system p. 14

4. MV Public distribution Substations on MV networks p. 15Other MV installations p. 16MV switchgear p. 16French and p. 18North-American layouts

5. Protection and control MV protection device technology p. 20of MV networks Electromagnetic compatibility p. 21

MV control applications p. 21MV telecontrol architectures p. 23Communications networks p. 24

6. Conclusion p. 25Appendix 1: some MV product standards p. 26Appendix 2: various selectivity techniques p. 26Appendix 3: EDF architecture and Merlin Gerin equipment p. 27Appendix 4: references p. 28


corresponding voltage levels can bedifferent from country to country.However, in general the number ofvoltage levels is limited to three; indeedin 1983 the IEC publication 38formulated recommendations forvoltage levels for 50 and 60 Hznetworks.

However, in order to gain betterunderstanding of this split, the followingparagraphs present each grid with:■ its object,■ its voltage level,■ its structure.

the transmission andinterconnection networkThe geographical distance betweenproduction sites and consumer centres,the irregularity of consumption and theimpossibility of storing electrical energycreate the need for an electricalnetwork that is capable of directing andtransmitting it across large distances.

These lines can stretch acrossthousands of kilometers , forexample in the French network thereexists 20 000 km.

Producing electrical current in powerstations is not enough in itself, it mustalso be brought to the end-user.

In order to link production andconsumption, which in turn can betranslated into financial benefit, acountry’s electrical structure isgenerally broken down into severallevels that correspond to different typesof electrical networks. (see fig. 1).

It should be noted that there is nostandard structure that existsworldwide, and that the split intoseveral networks with their

1. different types of electrical networks

fig. 1: illustrated layout of an electrical network showing that electricity is produced, carried and distributed at various voltage levels.


The object of this network is threefold:■ a "transmission" function with the aimof carrying electricity from producingpower stations to the main consumerzones;■ a "national interconnection" functionthat manages the product distributionby relating the production to thegeographical and time-dependantnature of demand;■ an "international interconnection"function - that manages energy fluxbetween countries dependant onprogrammed exchanges or as back-up.

In general only very few customers witha high consumption are connected tothese networks.

These networks are essentially ofoverhead type structure.

The voltages are normally between 225and 400 kV, and sometimes 800 kV(e.g.: 765 kV in South Africa). Use ofsuch high voltages is tied in with costsaving objectives. Indeed for a givenpower, the line losses by Joule effectare inversely proportional to the squareof the voltage:p = k/U2, withU= network voltage,k = constant - dependant on the line.

In addition the transmitted powervalues are such that using low voltageswould require totally unrealistic cablecross-sections. Use of high voltages isthus imposed in spite of the drawbacksinvolved in higher equipment insulationcosts, the easiest solution being theuse of overhead lines.

In any case, the choice of transmissionvoltage is above all a technico-economicone, dependant on the power to betransmitted and the distances to becovered.

The safety aspect is fundamental forthese networks. Indeed any fault at thislevel leads to important supply failuresfor all consumer units. In 1965 in theUnited States, 30 million people werewithout electricity for 12 hours due tosuch a fault.

These networks’ protection systemsmust be very high-performance. As fortheir operation on a national level, thisis the task of a control centre fromwhich the electrical energy ispermanently monitored and managed.

the subtransmissionnetworkThe object of this network is essentiallyto carry electricity from the transmissionnetwork to the main consumer centres.

These consumer centres are:■ either from the public sector withaccess to the MV network,■ or from the private sector with accessto high-consumption customer (greaterthan = 10 MVA) supplied directlywith HV. In any country the number ofsuch consumers is very small (e.g. 600in France). They are essentiallyindustries such as iron and steel,cement, chemicals, rail transport,....

The structure of these networks isgenerally of overhead type (sometimesunderground near urban areas). In thisrespect environmental concerns (carefor the environment and protection ofcertain natural sites) are often raised inopposition to the construction of lines.As a result it is more and more difficultand expensive for subtransmissionnetworks to reach high populationdensity areas.

Voltage in these networks is between25 kV and 275 kV.

The protection systems are of the samekind as those for transmissionnetworks, the control centres beingregional.

the MV networkThis level of a country’s electricalstructure will be covered in more detailin the chapters that follow. Thus, herewe only give a few distinguishingfeatures.

The object of this network is to carryelectricity from the subtransmissionnetwork to points of mediumconsumption (greater than 250 KVAin France).

These consumer points are:■ either in the public sector, withaccess to MV/LV public distributionsubstations,■ or in the private sector, with access todelivery substations for mediumconsumption users. The number ofthese customers (e.g. 160 000 inFrance) is only a small proportion of thetotal number of customers supplieddirectly with LV. They are essentiallyfrom the tertiary sector, such ashospitals, administrative buildings,small industries,...

The structure is of overhead orunderground type.

Voltage in these networks ranges froma few to 40 kV (see fig. 2).

Operation of these networks, can becarried out manually or, morefrequently, by remote control from fixed/mobile control centres. However, inorder to account of specific needs for

fig. 2: proportion of various voltage levels within national MV networks as a function of thelengths of lines of each country.

12 < U < 17 kV 17 < U < 24 kV

% 100

90

80

70

60

50

40

30

20

10

6070

95

80

95

10

103

Belgium France Great Britain Italy Japan

< 12 kV > 24 kV

5 5

25

5

35

2


control of MV networks, these controlcentres are different to those used ontransmission and subtransmission grid.The number and the geographicaldispersion of remote control units, themanagement of several simultaneouscontrol centres, the number and level ofqualifications of the operators allrequire appropriate solutions:ergonomics and user-friendly type workstations, control assistance tools,control centre configuration tools, andmanagement of the varioustransmission devices that are used.

the LV networkThe object of this network is to carryelectricity from the MV network topoints of low consumption (less than250 KVA in France) in the public sectorwith access to LV customers.It represents the final level in anelectrical structure.

This network enables supply to a verylarge number of consumers (26 millionin France) corresponding to thedomestic sector.

Its structure, whether overhead orunderground is often influenced by theenvironment.

Voltages in these networks arebetween 100 and 440 V.

Such networks are often operatedmanually.

type of electric currentEnergy transmission on these variousnetworks is accomplished using electriccurrent.

Direct current or HVDC (high voltagedirect current) links are used forexchanges between countriesexclusively on a transmission networklevel. The choice of this techniqueenables optimization of the use oftransmission cables, particularly bycancelling the "skin" effect. Suchintercontinental and even continentallinks exist, for example a link betweenItaly and Sardinia via Corsica (300 MW/200 kV).

In other cases, particularly public MVnetworks, links are through alternatingcurrent. Indeed, in these networks, theuse of direct current would not beprofitable:■ losses reduced on short networks(less than 100 km),■ equipment made more expensive(requirement of numerous direct/alternating converters).

In addition, alternating current is verywell adapted to voltage changes(transformers) in the course of itstransmission as electrical energy.

With very few exceptions (SaudiArabia), and outside of the Americancontinent where 60 hertz is usedthroughout, the current frequencyis 50 hertz.

Of particular note is the case of Japanwhere half of the country is on 60 hertz,and the other half on 50 hertz.

network planningThe installation and evolution of thestructure of an electrical supply networkfor a country corresponds to planningoperations.

For transmission and subtransmission,these operations are generallycentralised, since:■ the decisions leading to amodification in the structure of suchnetworks, for example the introductionof a new MV/LV substation, requires totake into account numerous technicaland economical parameters;■ the number of these parametersinvolved and their possible interactionsmeans that help from computerizedtools is required, such as use of a data-base and of expert systems.

For MV and LV networks, the planningis, however, often decentralized.


2. the distributor

reason of existance:to supply electricityThe electrical energy distributors existto supply electrical energy toconsumers taking into account severalobjectives such as:■ continuity and quality of service,■ safety of people and goods,■ flexibility and ease of operation,■ commercial competitiveness.

his roleIf electricity supply is satisfactory inindustrialized countries, the degree ofelectrification still remains variable incertain other countries.

Varying objectives depending on thedegree of electrification...For countries that are not 100%electrified, the priority objective remainsthe improvement of this degree ofelectrification. To this end, investmentis mostly in construction of networksand installations (see fig. 3).

However, the investment capabilities,sometimes reduced, can lead tosolutions based on simplification of thenetworks’ structure to the detriment ofthe system performance. In the sameway, sometimes a lack of availabilityand competence of the operators canlead to over-simplified operation.

Varying situations in industrializedcountriesIn countries that are 100% electrified,there are considerable differences inuses of electrical energy:■ national electrical energyconsumptions show great differences(see fig. 4). These differences are dueto the size of country, to its economicgrowth (GNP) and to the weight of theindustrial sector (example 40% of theFrench consumption).■ per capita consumption can vary by afactor of 10 between certain countries(see fig. 4). These differences aremostly due to the pricing policy ofdistributors, but also to climaticconditions.

The role of the MV distributor is notclear-cut: it often covers LV - LowVoltage - distribution and in somecases he is also in charge oftransmission, for example:

■ in Japan, nine regional privatecompanies are each responsible for theproduction, transmission anddistribution for their area,

■ in Germany around a thousandcompanies are involved in thedistribution of electricity. Around 1/3have their own production facilities.

■ in Great Britain production is theresponsibility of two companies (NP -National Power - and PG - Power Gen).The NGC (National Grid Company) arein charge of transmission, and theregional distribution is looked after byaround twelve Regional ElectricityCompanies. This structure is a result ofa privatisation bill for British distributorsvoted in 1990.

■ in Italy a law founded the E.N.E.L. in1962. This is a public serviceresponsible for the production,

fig. 4: net consumption per country and per inhabitant.

19

1860

27

5200

108

2790183

3190294

5320595

4900

2355

11760

Net consumption / country TWh

Portugal Denmark Spain Italy France Japan United states

30

100

300

1000

3000

300

1000

3000

10000

30000

Net consumption / inhabitantKWh

fig. 3: break-down of costs of MV overheadlines.

transformers= 5% MV switchgear

= 5%

lines + poles + total installation = 90%

transmission and distribution; itmanages around 80% of the electricitydistributed in Italy.

■ in France the situation is similar withthe E.D.F.


These examples would thus appear toshow that the number of partiesinvolved, particularly in MV distribution,can vary considerably from country tocountry (see fig. 5).

In the case of MV distribution thedistributor generally has completeresponsibility for the network, from theHV/MV substation to the MV/LVtransformer substation. In addition thedistributor’s job now includes acommercial aspect with respect to thesale of "electricity" as a product in theform of a kWh. He must thuscontinually improve the quality of theproduct to satisfy the demands of hisvarious customers, and remaincompetitive in relation to other sourcesof energy. This objective leadsdistributors to consider several pricelevels dependant on the quality of kWhbeing sold.

Equally, the electrical networkrepresents an important investment forthe distributor. He has to make thisinvestment as profitable as possibleand it is for this reason that it isincreasingly frequent that distributors’requirements include the idea of energymanagement.

Lastly, distributors play an importantsocial and political role, a role whichcan have a bearing on his choices, orat least on their priorities, as in thefollowing two examples:■ supplying new customers mayrequire extension of the network,■ the price per kWh may be limited inline with government economic policy.

his development: supply ofhigh quality energyIncreasingly, the energy distributor isrequired to supply a high qualityelectricity product. In order to do thishe must:■ reduce the number and the length ofinterruptions in supply to his customers,■ minimise the consequences of them,■ avoid disruptions such as voltage andfrequency fluctations (see fig. 6).

A permanent fault implies aninterruption lasting between severalminutes to several hours ; it requireshuman intervention.

Overhead networks, that are naturallyconsiderably more exposed thanunderground grid, require specificsolutions to problems encounteredsuch as:■ tree branches falling on overheadlines;■ birds landing on the line or itssupports;■ faults due to lightning, wind, frost,snow;■ vandalism.

As a consequence, the type of failuresencountered differ between overheadand underground networks:■ on overhead networks, the faults aremostly momentary (80 to 90%) andone-phased (75%) since they are oftendue for example to storms, to a linefallen to the ground or to shortingacross an insulator.■ on underground networks, the faultsare mostly permanent (100%) andmulti-phased (90%) since they are veryoften the result of severing a cable.

A need for informationThe importance of understanding theincidents occuring on the networkincreasingly justifies a need forinformation that distributors satisfy byusing statistical studies.

This analytical work aims to:■ classify and code the incidents,■ determine their origins and causes,■ statistically calculate their frequencies,■ search for correlations,■ study the comparative performancesof the various topologies,■ analyse the results according tooperating methods and installedequipment involved.

These statistics provide a tool forhelping distributors in the design,operation and maintenance of publicnetworks.

In addition, in order to be able to decideon the best solutions, the quality ofservice must be able to be quantifiedand measured, and no longer just beapproached in a subjective manner.In order to achieve this, new tools havebeen designed (based on mathematicalmodels), particularly including the ideaof "undistributed energy". Notably, tomeasure the cost of low quality in

country number of distributors the most important

total distributing 80%of nationalconsumption

Germany 600 20 R . W . E.

Saudi arabia 5 5 S . C . E . C . O .

Spain 200 6 Hydro Electrica

France 200 1 E . D . F .

Great Britain 15 10 Regional Electricities Cie

Italy 150 1 E . N . E . L .

Japan 9 9 Tokyo Electric Power Co

The type of faults depend on thetype of networkFor customers, the consequences ofsuch phenomena depend above all onthe type of fault.

A fault can be:■ momentary or permanent in duration;■ one-phase or three-phase dependanton the type of its cause.

A momentary fault often means a briefinterruption of the order of several100 ms, essentially related to theoperating time of a recloser.

parameters nominal tolerancesvalues

Frequency (50 Hz) ± 1Hz

MV Voltage (12 to 24 kV) ± 7%

LV Voltage (230 or 400V)overhead ± 10%underground ± 5%

fig. 6: examples of a distributors "quality"constraints (EDF France).

fig. 5: electrical energy distributors in several countries.


MV networks, the E.D.F. use theformula:A*N*N*P + B*N*P*T, withN = number of permanent breaks perfeeder,P = average power per feeder in kW,T = average length of interruption perfailure,A and B = economic value coefficients(in 1990, for the EDF in FranceA = 6 FF/ kW and B = 13.5 FF/ kWh).

However, measurement of the qualityof service can require inclusion of evenmore parameters. The complexity ofthe calculations and simulations to becarried out, justifies the development ofsoftware tools that are increasinglypowerful to help with the decision.

To measure the reliability of energysupply to a residential LV customer,distributors prefer to use the criteria of"degree of unavailability": this is thetotal annual time during which anaverage customer has his supplyinterrupted due to a fault on theelectrical network (HV, MV or LV).

Lastly, it is important to note that for aLV customer, numerous incidents aredue to the MV network (60% accordingto an EDF study) (see fig. 7).

Networks, equipment and operators,are all evolvingIt should not be forgotten that anetwork’s performance depends aboveall on its topology. However, throughoutthe world, present networks are simplythe result of years of laying ofstructures one on top of the other asneeds have increased. In addition anetwork ages and is constantly in needof maintenance and renovation work toretain its performance level and toavoid incidents, the sources of"undistributed energy".

To answer these needs, themanufacturers thus propose"maintenance-free" or reducedmaintenance equipment; equipment forwhich maintenance, modification andaddition type operations do notadversely effect the continuity ofservice.In addition, energy distributors are notwilling to undertake preventivemaintenance, particularly devicemonitoring by recording and analysingincidents that occur on the network(use of "disturbance recorders" andtime-stampers).

Advances in protection and telecontrolequipment with digital technology(microprocessor) and expansion ofcommunication networks, offer theperspective of innovative solutions interms of event prediction (seeglossary).

Lastly, the practice of live-line repairwork and remote control of thenetworks are also points that favourincreased quality of service, byreducing the number of interruptionsand their duration.

Of course, all of these developmentsrequire that the workforces shouldadapt quickly, following the example ofthe current changes in work practices in

control centres:■ there still exists control centres inwhich:■■ the status of various networks isindicated by manually displacedsymbols on large charts several meterssquare in area,■■ and the instructions relating tooperations are hand written in log-books;■ in new centres all these operationsare performed on computer terminals,with:■■ all information available on screens inreal time (network layouts,geographical description),■■ events logged automatically (datalogging).

0 1 2 3 4 5 >5

10

20

30

40

50

0

60

1986 1995

duration of interruptionhours

% number of LV customers

fig. 7: degree of unavailability of electrical energy on one LV network (EDF - France).


■ the choice of an overhead orunderground distribution system has abig influence on the installation costsand the quality of service (e.g.: costs ofa trench / vulnerability to momentaryfaults...). For MV, in industrializedcountries, this choice can be brokendown into three cases:■■ highly populated urban area with anunderground distribution system,■■ highly populated suburban area withunderground or part-underground part-overhead distribution system;■■ scarcely populated rural area withoverhead distribution.

However it should be noted thathistorically, due to high initial costs,numerous urban areas have overheaddistribution systems, as in Japan andthe United States.

various MV network layoutsThe choice of layout is important to acountry: particularly for MV networkssince they are of great length. Thus, forexample, the total MV structure inFrance is around 570 000 km long, thatof Italy being 300 000 km long and thatof Belgium being 55 000 km long.

Several topologies exist:■ lattice type, closed loop topology,■ simplified lattice type, open looptopology,■ open loop topology,■ radial topology.

items that depend on thechosen topologyThe choice of a topology fixes the maindesign elements of a distributionsystem, such as:■ the rated power and the maximumvalue of earthing currents, e.g. for MV,the EDF limits the value of thesecurrents to 300 A at 20 kV overheadand to 1000 A underground;■ the rated voltages, e.g.:for MV Japansupplies at 6.6 kV, Great Britain at 11and 33 kV and France mostly at 20 kV;■ voltage surge ratings andcoordination of isolation as well asprotection systems against atmosphericvoltages surges;■ the earthing connection layouts, aswell as the number of distributed wires,■ the maximum feeder lengths (tens ofkilometers at MV);■ the type of distribution: overhead orunderground (see fig. 8);■ the type of operation: manual,automatic, remote controlled.

It is important to note that:■ the choice of short circuit current hasrepercussions on the rating of theequipment used in the network;■ the choice of voltage rating(s) isalways a compromise between theinstallation and operating costs of thenetwork;■ the choice of insulation rating ofequipment is generally in line withinternational and/ or national standards;

The topology of an electrical network isdefined here as all of the principlesinvolved in carrying electrical energy inpublic distribution (layout, protection,operation).

In practice for a distributor, defining atopology means fixing a certain numberof physical factors, whilst takingaccount of criteria dependant onobjectives aimed for and technicalconstraints. Since these factors areclosely interrelated, choice of a certaintopology is always the result oftechnico-economic compromises.

Here, the graphical representation of atopology will be by simplified single-linelayouts.

criteria in choosing atopologyThe choice of a topology depends onmeeting objectives:■ to ensure the safety of people andgoods,■ to attain a pre-defined level of qualityof service,■ to produce the desired profitability.

However, it must also meet certainrequirements:■ to correspond to the housing densityand/or to consumption, known as theload density, it plays an ever increasingrole. Calculated in units of MVA/km2,this density enables the expression ofvarious geographical zones in terms ofload concentration. Certain distributorsdistinguish two types of consumptionzones by defining:■■ low load density zone:< 1 MVA/km2,■■ high load density zone:> 5 MVA/km2.■ to account for the geographicalspread, terrain and constructionproblems,■ to satisfy environmental constraints,particularly climatic (maximum andminimum temperatures, frequency ofstorms, snow, wind, etc.) and respectfor surroundings.

fig. 8: proportion of lengths of overhead (lines) and underground (cables) on MV networks, forseveral countries.

3. MV network topologies

0 25 50 75 100%

overhead

Germany

Canada

United States

Denmark

Great Britain

Netherlands

underground


Other topologies are also used, forexample the double shunt found onFrench MV networks.

Although none of them are "standard"in MV, distributors lean towards twobase topologies: radial and open loop.Each of these two topologies will thusbe covered in more detail and definedin terms of:■ its operating principle,■ its typical line layout,■ its typical application,■ its strengths and weaknesses.

Radial layoutThis layout can also be calledantenna-type.

Its operating principle is based on usinga single supply line. This means that allconsumer units in such a structure onlyhave one possible electrical feed path.It is of arborescent type (cf. fig. 9).

This arborescence originates fromsupply points, that are MV/MV orHV/MV distribution substations.

This layout is particularly used for MVdistribution in rural areas. Indeed, itenables easy and low-cost supply tolow load density (≈ 10 kVA) consumerunits with a wide geographicaldispersion (≈ 100 km2).

Very often a radial layout is used withan overhead type distribution system.

Its strengths and weaknesses aresummarized in the table in figure 10.

Open loop layoutIts operating principle uses two lines ofsupply. This means that any consumer

fig. 9: the two basic layouts of a MV distribution network, radial (or antenna) and open loop (or artery break).

fig. 10: comparative table of the two base MV network layouts.

unit on this structure can be suppliedvia two possible electrical paths, whilstonly one of those paths is activated atany one time, back-up is provided bythe possibility of using the other loop. Insuch a layout, there is always an openpoint in the loop, which leads to asimilar operation to two antennalayouts.

The typical line layout is, of course, aloop on which are connected consumerunits (see fig. 9) which can be publicdistribution substations MV/LV, and/ ordelivery substations for a MV customer.

technology strengths weaknesses

radial ■ simplicity ■ quality of service■ operation■ installation costs.

open loop ■ simplicity ■ operation with more■ quality of service frequent switching

■ installation costs

HV/ MV substations

HV/ MV transformers

radial layout open loop layout

supply path for all substations except n°1

open point

substation n°1MV/ LV transformer

supply path for substation n°1

MV/ LV transformer

MV/ LV transformer

MV/ LV transformer


Each point (between 15 and 25 pointsper loop) is connected to the loop bytwo MV switches. All these switchesare closed , except for one of them thatforms the opening in the loop anddefines the feed path for eachconsumer unit. This opening can bedisplaced within the loop, particularlyduring reconfiguration operationsfollowing a fault.

Very often this layout is used inassociation with an underground typedistribution system.

It is typically used in highly populatedurban areas, and its strengths andweaknesses are detailled in the table,figure 10.

Double shunt layoutThis rather uncommon layout is mostlyused by the EDF in the Paris region, itis shown in figure 11.

The operating principle is the following:■ the MV network is doubled up,containing two circuits A and B, bothnormally live,■ every MV/LV substation■■ is connected to both MV cables ("A"and "B"), but it is only actually live toone of the cables (MV switch closed oncable "A"),■■ is equipped with a simple localautomatic control device,■ on failure of cable "A", the automaticcontrol detects the lack of voltage in thecable, checks the presence of voltagein cable "B" and commands the closingof one MV switch and the opening ofthe other MV switch.

neutral earthing layoutsThe choice of neutral earthing layouts(or neutral MV systems) definesamongst other things the voltage surgeratings and earth fault currents thatcould be found on a network. It must benoted that these two parameters arecontradictory, in view of the fact thatobtaining a low fault current level leadsto a high voltage surge and vice versa.These values thus pose electricalconstraints that the electrotechnicalequipment must be capable ofwithstanding. However, in choosing theconnecting layout, we simultaneouslyselect the possible solutions forprotection of the electrical network andinfluence the operating methods.

The five neutral earthing layoutsused in MV throughout the worldHere again, no one standard neutralearthing layout exists. However, it ispossible to bring together all thevarious cases met around the worldinto five categories (see fig. 12):■ direct distributed neutral earthing,■ direct non-distributed neutral earthing,■ neutral earthing via an impedance,■ neutral earthing via a designatedcircuit,■ neutral insulated from earth.

As has already been said, none of thecategories is dominant throughout theworld: some solutions are specific tosome countries, and several categoriescan be found within one single country,or even within the network of one singleelectricity distributor.

But in the end, the choice of a MVneutral earthing layout is always theresult of a compromise betweeninstallation and operating costs.

The differences between the fivecategoriesIt has been previously stated that thechoice of neutral earthing layoutinfluences the performance of thenetwork and the design of its protectionsystem. Indeed, the main differencesbetween the five categories lie in thebehaviour of the network in an earthfault situation.

These differences translate in realterms to the degree:■ of ease of detection of these faults,■ of security achieved for people,■ of impact on the requirements ofelectrotechnical equipment.

The distributed neutral earthing layoutthat allows single phase distributionshould, however, be consideredseperately. This possibility can beconsidered in certain countries on thebasis of its reduced installation costs.However, the more complex protectiondevices require stricter maintenance.

fig. 11: double shunt distribution layout, used by EDF - France. In the inset, automatic controlsequence of Merlin Gerin permutator, conforming to EDF - France specifications.

HV/ MVsubstation

HV/ MV substation

1

0

t f

A B

t f = 5 or 25 s

network supplied by...

inlet A

inlet B

t f

1

0

1

0

A

U

t

circuit "A"circuit "B"


Independantly of this particular case,the table in figure 13, a summary of thestrengths and weaknesses of thesecategories, shows why no one of thesecategories is predominantly usedthroughout the world.

protection systemThe electrical structure of a countrycorresponds to a group of electricalnetworks.

An electrical network can itself bebroken down into areas.

Each of these areas is generallyprotected by a circuit breaker inconjunction with detector devices(measurement sensor: current orvoltage transformer...), protection andcontrol devices (protection relays), andtrip devices (actuators).

All these elements together make up achain of protection (see fig. 14) thatensures isolation of the faulty part ofthe network in the event of a fault.Its role is to ensure safety by protectingagainst insulation faults betweenphases or between phase and earth,and against prolonged overloading. Thechain of protection must especiallyreduce the consequences of a shortcircuit, such as the risks of fire,explosion of mechanical damage,...

The network’s protection system ismade up of all of these chains ofprotection, integrating their installationwith the organization of the operationbetween them. This protection systemorganization, including the trip times ofthe associated circuit breakers, definesthe maximum duration of a current faultin the different parts of the electricalnetwork.

The effectiveness of a protectionsystem depends on several criteria:reliability, selectivity, rapidity,sensitivity, adaptability.

ReliabilityThis criterion describes the level ofquality concerning safety of people andproperty, particularly in terms ofdangers of electrocution by increasedearth potential. Although a protectiondevice is rarely activated, when there isa fault it must act effectively, throughoutits many years of service. This criteriondirectly influences the networkperformance, thus, for example, any

country

neutral insulated from earth

neutral earthed via a designated circuit

neutral earthed via an impedance

neutral distributed with numerous earthing points

neutral earthed directly and undistributed

method

GermanyJapanFrance

Spain

Australia

Canada■

■

■

■

■

■

■

■

■

fig. 12: various MV neutral earthing layouts, and their applications throughoutthe world.

neutral earthing method strengths weaknesses

direct earthing authorises one-phase ■ requires numerous high qualityand distributed and three-phase earthing points (safety)

distribution ■ requires a complex protectionsystem■ leads to high values of earthfault currents

direct to earth eases detection of leads to high values of earthand undistributed earthing faults fault currents

insulated limits earth fault leads to surge overvoltagecurrents

designated favours auto-extinction requires complexof earth protection systemsfault currents

impedant(compared with neutral limits earth fault requires more complexdirect to earth) currents protection systems

(compared with neutral reduces surge leads to higher earth faultinsulated from earth) overvoltage currents

fig. 13: a summary of strengths and weaknesses of the five MV neutral earthing methods.

measurementsensor

protectionunit actuator

breakerdevice

electrical network

fig. 14: MV chain of protection, and photograph of a SF set (Merlin Gerin), an example ofcomplete integration.


interruption in supply must be "justified"since it entrains a loss of operation forthe customer ... and the distributor.

SensitivityThis criterion also has a bearing interms of security and costs: it describesthe ability to detect weak fault currentswithout being sensitive to transitoryphenomena due to the network(operations) or to surroundingelectromagnetic effects, thus beforethere is any risk to people or property,and that without tripping prematurely.

SelectivityThis criterion has a particular bearingon costs of operation, since it indicatesto what degree it is possible to maintainthe network operation whilst one part ofit is not working as it should do. Inpractice, this leads to shutting off thefaulty element, and only the faultyelement (see appendix 2: differentselectivity techniques).

RapidityThis criterion, as in the preceding case,has an effect on costs: it enablesreduction of the damage due toelectrical arcs and short circuit currents,it particularly reduces the risk of fireand the costs of repair work.

AdaptabilityThis criterion particularly interests thedistributor and indicates the degree ofevolution that is possible (possibilitiesand ease) for the protection systemwith regards to modifications to thenetwork topology.

Of all these criteria, selectivity is theone that leads to the most variedsolutions from country to country. Theydepend on two initial choices made bythe energy distributors:■ of which neutral earthing layout isused, from which we define notably theprotection systems against earthingfaults (see previous section).■ and that of which selectivity principlewill be used, of which the mostcommonly used, called ampere-chronometric selectivity, is based onthe association of the fault currentrating (amperemetric selectivity) and avalue of the tripping delay time(chronometric selectivity). However,several different techniques can existwithin the same network, in SouthAfrica for example E.S.C.O.M. useswithin one network, ampere-chronometric selectivity, pilot wire

between HV/MV and MV/MVsubstations and biased differential forHV/MV transformers. Lastly, distanceselectivity technique is mostly used byGerman distributors.

telecontrol systemUnder the one term of telecontrol aregrouped all of the elements tied in withoperation of the network.

A telecontrol system defines all of theseelements and their relative operationalorganization. In this way the telecontrolsystem must enable the operator (thedistributor) to account for 3 situations■ normal operation,■ the instance of a fault,■ maintenance operations (live anddead).

Lastly, the operating tools installed inthis system are going to make aconsiderable contribution to the qualityof service that is obtained.

These tools range from the push-buttoncontrol of a MV device up to the MVnetwork control centre, from theammeter on a MV cubicle up to aautomatic remote reading of the loadcurve for a MV feeder, etc...


This chapter is a reminder of the mainsubstations installed on MV networks,and the main technologies used in MVequipment. It is concluded by twolayouts showing their applications inreal terms.

substations on MVnetworksA substation or installation is a physicalentity defined by its position and itsfunction within electrical networks.

The role of a substation is essentially toperform the transition between twovoltage levels and/ or to supply the enduser.

The HV/MV substation in a publicdistribution systemThis installation is present in any of acountry’s electrical structures; it ispositioned between thesubtransmission network and the MVnetwork.

Its function is to ensure transition fromHV (≈ 100 kV) to MV (≈ 10 kV).

Its typical layout (see fig. 15) involvestwo HV inputs, two transformers HV/MV, and 10 to 20 MV feeders. Thesefeeders supply overhead lines and/ orunderground cables.

The MV/MV substation in a publicdistribution systemThis installation performs two functions:■ to ensure the demultiplication of MVfeeders downstream of HV/MVsubstations (see fig. 15). In this case,the substation does not include atransformer. It is made up of 2 MVinputs and 8 to 12 MV feeders. Thistype of substation is used in severalcountries like Spain, Belgium, SouthAfrica.■ to transfer between two MV voltagelevels. Such MV/MV substations docontain transformers. They arenecessary in countries that use twosuccessive voltage levels in their MVnetworks, for instance in Great Britainwhere the MV network is broken downinto two levels 11 kV and 33 kV. Theirtypical layout ressembles that of theHV/MV substation.

The MV/LV substation in a publicdistribution systemPositioned between the MV networkand the LV network, this installation

performs the transfer from MV(≈ 10 kV) to LV (≈ 100 V).The typical layout of this substation isof course a lot more simple than the

fig. 15: various layout types for substations used on public distribution networks

4. MV public distribution

HV incomers

HV/MV transformers

MV bus sectioncircuit breaker

overhead and/or underground MV feeders

MV incomers

overhead and/or underground MV feeders

MV incomers LV feedersMV/LV transformerprotection

MV incomers

private MV feeders

general MV protectionand metering system

delivery substation for a MV customer

MV/LV substation

MV/MV substation

HV/MV substation

private LV feeders


Amongst all the various types ofdevices, two are particularly used in MVswitchgear, the circuit breaker and theswitch. They are virtually alwayscombined with other devices (protectionand telecontrol units, sensormeasurement units,...) that make uptheir associated equipment.■ MV circuit breakerIts main function is protection, but alsoperforms a control function and,depending on its type of installation, aswitching function (draw-out, glossary).MV circuit breakers are nearly alwaysmounted in a MV cubicle.■ MV switchIts main function is command, but alsooften performs a role of switching. Inaddition, it is combined with MV fusesto ensure protection of MV/LVtransformers (30% of all MV switchesused).

MV cubicles have metal enclosures thatmeet specifications laid down in thepublication IEC 298 that differentiatesfour types of switchgear, each typecorresponding to a level of protectionagainst fault propagation within acubicle.This protection system involvespartitioning of the cubicle into threebasic compartments (see fig. 17):■ the switchgear compartmentcontaining the device (MV circuitbreaker, MV switch,...),■ the MV busbars for electricalconnections between several MVcubicles grouped together on a board,■ MV cable connections compartment,often with space for measurementsensors.

Often a fourth compartment completesthis assembly, it being the controlcompartment (or LV box) containingprotection and control units.

In addition to this classification thedistinction should be noted betweenFixed and Draw-out (see glossary) thatapply to the MV device and cubicle.This distinction, that depends on the

fig. 16: various functions of MV devices that are used in public distribution(contactors are essentially used in industry).

isolator switch circuit breaker switch isolator draw-out circuit breaker

fuse MV devicesfunction

protection

isolatingcommand

■

■ ■ ■ ■

■

■ ■■

■

However, if the user is supplied withMV, a standard layout may beproposed (see fig. 15). Installation ofsuch a substation is, however, evidentlydependant on the distributor’sagreement since they may have theirown specific requirements (metering,operating conditions,...)

other MV installationsOutside of the substations alreadydescribed, other MV installations existthat are mainly positioned on overheadnetworks. Often single function, theyare used■ either as protection, as in the case offuses and reclosers (glossary).■ or for operation, as in the case ofremote control switches.The MV remote control switch is part ofnetwork remote control systems. Itallows rapid reconfiguration operationswithout the operator having to travel.

MV switchgear (see appendix 1)MV switchgear enables the threefollowing basic functions to beperformed:■ isolation, which consists of isolating apart of the network in order to work on itin complete safety,■ control, that consists of opening orclosing a circuit under its normaloperating conditions,■ protection, that consists of isolatingpart of a network with a fault in it.

It essentially takes three forms:■ separate devices (see fig. 16)(directly fixed on a wall and protectedfrom access by a door),■ metal enclosures (or MV cubicles)containing these devices,■ MV boards that are made up ofseveral cubicles.

The use of separate devices isincreasingly rare; only several countriessuch as Turkey or Belgium, still use thistechnology.

previous installations. In particular, themost common MV device used is theswitch and no longer the circuitbreaker.

These substations are made up of fourparts:■ MV equipment for connection to theupstream network,■ the MV/LV distribution transformer,■ the LV feeder board as connection tothe downstream network (in LV),■ and inceasingly frequently a pre-fabricated outer enclosure (in metal orincreasingly frequently in concrete) toenclose the previous elements.

The delivery substation for a HV orMV customerThese installations perform the transferfrom public distribution to privatedistribution. They enable connection:■ to the HV subtransmission networkfor a high-consumption customer(≈ MVA) via a HV/MV substation,■ to the MV network of a medium-consumption customer (≈ 100 kVA) viaa MV/LV substation.The choice of the connecting voltage tothe public distribution network for acustomer depends essentially on:■ the quality of the LV network,particularly its power capacity (electricalcapacity);■ the distributors policy, particularly onthe rates offered, since for thecustomer this defines the cost savingadvantages of electrical energy,compared with other sources ofenergy: oil, gas,...In practice, it is the power subscribed toby the customer that determineswhether he is connected to LV or MV,with very different values from countryto country. Thus, in France a customeris supplied MV from 250 kVA whereasin Italy this threshold is nearer severaltens of kVA. On the other hand in theUnited States where a customer can besupplied LV up to 2500 kVA. For userssupplied with HV, the layout of thesubstation is specifically designed.


fig. 17: various compartments of a MVcubicle, surrounded with a metal enclosure,and their main elements.

ease of operation (as a function of timetaken to change a device), is onlyindirectly involved in the idea of MVnetwork safety.

IEC 298 defines the following four typesof switchgear for MV cubicles:■ "BLOC" switchgear divided to agreater or lesser extent intocompartments;■ "COMPARTIMENTED" switchgear, ofwhich only the outer enclosure isobliged to be metal, has the threecompartments separated with metal orinsulated partitions;■ "METAL CLAD" switchgear also hasdistinct compartments, but of which thepartitions are obliged to be metal;■ "GIS" switchgear (Gas InsulatedSwitchgear) that is hermetically sealedand within which the compartments nolonger play an important safety role.The GIS is essentially made up ofcircuit breakers.

This technology is also applied toswitches within a RMU (ring Main Unit).It provides the three standard functionsto be performed of a MV/LV substationthat is connected to an open loopnetwork (two connecting switches tothe network plus a tee-off fuse switch ora circuit breaker as protection of theMV/LV transformer).

The table in figure 18 shows theswitchgear that is most often used as afunction of the type of substation,whereas the table in figure 19 showsthe current importance and trends ofvarious MV breaking techniques.

fig. 18: main MV switchgear applications.

switchgearcircuit breaker

control

connection and measurementmeasurement transformer

incomerfeeder

busbars

air oil SF6 vacuum

MV switch

MV circuit breaker

■ ■ ■ ■ ■

■ ■ ■ ■ ■ ■

fig. 19: MV device breaking techniques, their relative importance and the development oftheir use.

substation public HV/MV

public MV/MV

public MV/LV

HV/MV customer

MV/LV customerswitchgear

separate device

bloc

compartmented

metal clad

GIS

RMU

CB

S

CB

S

CB

CB or S

CB or S

S

S

CB

CB

CB or S

CB or S

CB

CB

S S

S = with switch CB = with circuit breaker


French andNorth-American layoutsWith these two typical examples it isaimed to show in real terms the variouselements that have been set out in thischapter and to highlight the diversity ofsolutions that exist across the world.Needless to say, other layouts exist,even in these two countries.

The overhead single-line layout usedby the EDF (France) (see fig. 20)This layout applies the followingprinciples:■ at the HV/MV substation, neutralearthing is via an impedance limitingthe phase-earth shorting current to300 A at 20 kV,■ three-phase MV lines, undistributedneutral,■ radial layout (antenna type).

This concept enables detection at MVfeeder level of any earthing faultswithout any other MV protection devicedownstream of this substation.

The result is protection and controlsystems that are easy to design, tooperate and to modify.

Personal safety is guaranteed.

However, the quality of service that isobtained is only average due to the factthat each MV feeder, at the HV/MVsubstation, is controlled by one singleprotection device: when the protectiondevice is tripped, the whole networkdownstream of this MV feeder is cut off.Solutions exist to make up for this weakpoint. These days they are based onthe use of additional equipment such asremote controlled switches, in thefuture reclosing circuit breakers will beused in the network.

The overhead single-line layoutfound in North-America (see fig. 21)This design is sometimes seen incountries influenced by North-America(e.g. Tunisia). It is based on thefollowing principles:■ maximum MV distribution, byreducing the length of LV feeders inorder to reduce losses;■ MV neutral distribution with regularearthing (e.g. every 300 meters);■ three-phase MV lines on mainstructure, with three-phase, two-phaseor one-phase shunting for the MV/LVconnections.

fig. 20: overhead MV distribution layout (EDF - France).

reclosers, sectionalisers, see glossary).However, in certain cases, theprotection is limited to cut-out typefuses (see glossary), the financialinvestment is thus restricted, but to thedetriment of performance and safety(risk of fire).

This sort of design reduces the costs oflines, the losses and the surges due tofaults. However, it requires a very highquality of neutral earthing.In order to obtain a satisfactory degreeof personal safety, it is necessary toinclude numerous MV devices (fuses,

HV/MV substationHV/MV transformer

overhead MV distribution, three-phase neutral undistributed

three-phase circuit breaker with automatic reclosing

MCOS : manually controlled three-phase overhead switch

RCOS : remote-controlled three-phase overhead switch

MV/LV transformer

MCOS

MV/LV

RCOS

branch

mai

n gr

id

MV/LV

MV/LV

MV/LV

MCOS

MCOS

MCOS

RCOS


fig. 21: North-American overhead MV distribution layout (each phase shown).

The design of the protection andcontrol systems is complex in terms ofthe selectivity between the differentprotection devices.

In addition the operation andmaintenance of such networks ismore difficult than for that

concerning grid of EDF-type layouts:■ highly trained staff are required(switchgear maintenance, protectiondevice setting,...),■ considerable stocks of spare partsmust be held (different rated fuses,...).

This solution is especially justified in

countries spread over a large area andwith a low load density (e.g.: UnitedStates and Canada in rural areas).

HV/MV transformer

circuit breaker (3 independant single phasesor 1 three-phase)

neutral conductor

two fuses as branch protection (2 phases + neutral)

recloser : overhead multiple reclosing circuit breaker (3 single-phased or 1 three-phased)

MV/LV transformer (see inset below)

sectionaliser

mai

n gr

id

MV/LV

branch

MV overhead distributionmain grid : three phases and neutral,branch : one, two or three phase with neutral

HV/MV substation

MV/LV

MV/LV

MV/LV

Nb

Nb

Nb

North-American MV/LV transformer substation

MV

LV LV


Lastly in the 1980”s, digital technologyhas, due to the processing power ofmicroprocessors, enabled dataprocessing units to be produced thatcan:■ generally back-up various protectiondevices,■ replace the cubicle’s relays(automatic control),■ provide electrical parametermeasurements to the operator.

These versatile devices are:■ flexible (protection selection is simplyprogrammed),■ parameter-adjustable (wide choice ofsettings),■ reliable (equipped with self-monitoring or watch-dog devices andself-testing),■ cost saving (their cabling andinstallation time are reduced).

In addition, due to their powerfulalgorithms and their digitalcommunication, they enable theperformance of additional functionssuch as logical selectivity.

Using this ability to communicate, acomplete network control system isnow possible (similar to technicalmanagement of an industrial plant).

In terms of sensors, and in particularthose of current, there is an increasingtrend towards wide measurement bandsensors in place of current transformers(1 or 5 A). This sort of sensor, thatworks on the Rogowski principle (aircored coil), is the type currently onoffer. It offers distributors optimisedsolutions (reduction of variables andease of choice) and greatly improvedperformance (greater linearity of theresponse curve) over traditional currenttransformers.

fig. 22: SEPAM, protection and control unitand its three Rogowski coil current sensors(Merlin Gerin).

5. protection and control of MV networks

The advent of digital technologiesbased on microprocessors has greatlymodified solutions used in design ofprotection and control systems. Thischapter presents the latestdevelopments and future prospects forthese increasingly complex functionsthat are operated on MV networks. Italso shows the importance of a newsubject, that being electromagneticcompatibility (EMC).

MV protection devicetechnologyThe role of a protection device, or MVrelay (see appendix 1), is:■ to continually monitor variousparameters for part of the network (line,cable or transformer),■ to act in the case of a fault,■ and increasingly frequently totransmit information for operation of thenetwork.

To this end it analyses the electricalvalues that are supplied to it bymeasurement device sensors, andgives operating commands to theswitching circuits.

Having been limited to electromagnetictechnology, MV protection devices aretoday undergoing fundamentaldevelopment with the use ofmicroprocessors.

Thus, the equipment available today isbased on three technologies:electromagnetic, analog and digital.The oldest is electromagnetictechnology, the relays are simple andspecialised (current, voltage, frequencycontrol) but are not very accurate, theirsettings are liable to change with time.

The more recent electrical analogtechnology (transistor) has improvedprecision and reliability.


electromagneticcompatibility (EMC)The electromagnetic compatibility(EMC) is defined as the ability of adevice, a piece of equipment or asystem to function in a satisfactorymanner in its electromagneticenvironment, without affecting thisenvironment; this environment possiblycontaining other more or less sensitivedevices.

In order to develop its new products,taking account of the development ofdigital techniques and the necessarycohabitation of MV equipment (highvalues of voltage and current,particularly when being switched) andelectronic protection and controldevices (low voltage level and highsensitivity to electromagnetic radiation),Merlin Gerin has had to increase thescope of and then develop applicationsin the field of electromagneticcompatibility (EMC). In addition, inorder to satisfy the distributors’requirements (operational safety), it hasbeen necessary to carry out morestringent tests than those defined inrecent standards that are currently valid(see appendix 1) that define theacceptable interference level.

■ Thus, for measurement devices,the IEC 801-3 standard detailstests across a frequency range of27 MHz - 500 MHz and three levels(1, 3,10 V/m) whereas test conditions inMerlin Gerin laboratories areconsiderably more severe: the range offrequencies covered is from 10 kHz to1 GHz; in addition, from 27 MHz to1 GHz, devices can be tested in fieldsof up to 30 V/m. (also see Merlin Gerin"Cahier Technique" n°149)■ And for MV equipment, certain testsare carried out under real conditions oncomplete boards (MV switchgear andprotection devices).

However, although electromagneticcompatibility (EMC) is taken intoaccount in all development andmanufacturing phases of equipment, inorder to ensure perfectly operationalequipment, it must also be consideredduring on site cabling and installationphases.

fig. 23: example of remote control of a MV network, with the various links required forinformation exchange.

MV control applicationsTowards centralized operationRemote control is the grouping of allthat is necessary to control remotly MVnetwork into one or several points (seefig. 23). These points are fixed ormobile (taken in a vehicle) controlstations. They are also called,dependant on distributors, controlcentres, dispatching or SCADA(supervisory control and dataacquisition).

To account for the specific needs ofMV network control, these controlcentres are different from those usedon transmission and subtransmission

networks. The number andgeographical dispersion of the points ofremote control, the simultaneousmanagement of several different controlcentres, the number andlevel of qualifications of the operatorsrequires adapted solutions:■ ergonomics and user-friendliness ofthe work stations,■ control assistance tools,■ control centre configuration tools(glossary),■ management of the varioustransmission systems used.

In practice remote control covers thefunctions of remote-data transmission,remote supervision, telemetering and

radio wavestelephone network

telephone network (specialized line)

telephone network

radio link

overhead MV deviceMV/ LV substationHV/ MV substation

fixed control centre

mobile remote-controlunit


control of the network in real time, or forits analysis at a later date.

All this information together with itsdisplay mode, is defined at the designstage of the telecontrol system. Inparticular, the mimic diagram displaysare created as a function of the actualinstallation and the operators needs. Inaddition they are displayed in real time.The operator can then visualise:■ the operational layouts (electricalnetwork, substation, MV cubicle);■ installation status (status of MVdevices,...);■ values of operational data (currents,voltages, powers,...);■ values of MV protection devicessettings;■ details concerning alarms, with theirchronology of occurence;■ ...

fig. 24: several functions, here grouped according to the direction of transmissions betweenoperator and network, are required for remote control.

Remote controllingRemote opening andclosing of powerequipment is a basic example ofremote controlling. The practicalapplication is in the form of remotelycontroled MV switches and circuitbreakers. Other functions can beremote controlled: settings, automaticcontrol,...

There must be a maximum degree ofcertainty concerning remote controlcommands. This is obtained using apowerful communications network thatallows access to information in realtime. Thus a switching command for aMV device is transmitted via a doubleremote control (TCD), and confirmedby return of a double transmissionsignal (TSD).Remote control processes alsointegrate requests for validation andconfirmation before execution of aswitching command.

Remote controlIn MV distribution this is a cost savingfactor in terms of operation of thenetwork. Indeed, without having totravel, the operator can continuallymonitor and control the operation of hisnetwork. As an example: following afault it is possible to rapidly change thenetwork operating layout so as tominimise the proportion of it that is cutoff, this being done by remoteconsultation of fault position indicators(see glossary) that are installed atdifferent points on the MV network,followed by remote control of MVswitches. This results in a considerablereduction in the amount of undistributedenergy, but also in network optimizationwith possibilities of optimummanagement of the load distribution.

The network loading can also beanalysed.

In particular, by logging the load curveverification and optimization of energyconsumptions can be performed.

Finally, to increase efficiency, theoperator can have rapid access to themost relevant information via anautomatic pre-processing method suchas a sorting operation, graphicalrepresentation operation, calculationoperation,....

remote controling. These functionscan be divided into two groupsdependant on the direction oftransmission between the operator andthe network:■ remote supervision, of devices by theoperator,■ remote controlling, by the operator ofthe devices.

Remote supervisionThis in itself groupssignals concerningthe position of various MV equipment,their possible tripping on fault,instantaneous or weightedmeasurements from various parts ofthe electrical network, and any otherinformation indicating the up-to-datestatus of the network. It enables forexample, the automatic andcontinuous printing, in sequentielorder, of all the events necessary to

remote control

remote controlling (command transmission)

remote supervision(information transmission)

■ remote control of network's MV devices■ remote adjustment of settings.

+

=

■ remote data transmission■ telemetering


Automatic energy supplymanagementThis management, which aims toimprove the quality of service via thecontinuity of supply on the network,has its main application in thepermutation of various electrical energysupplies. This application is based onautomatic control.

The double shunt type topology, asoperated by the EDF on some of itsnetworks, is an example of this.

MV telecontrolarchitecturesThe advent of digital technologies hashad a considerable impact on thesolutions employed in MV telecontrol.In particular, the ability to use compactand reasonably priced digital protectionand control units enables, withcentralized operation control, the usetoday of local intelligent systems. Thisdevelopment offers the followingadvantages:■ it reduces the disadvantages ofintelligent systems concentrated in onesingle point. Indeed, a fault at this pointwould be catastrophic for the wholeelectrical network operation.■ it offers the advantage of a bettermaintainability and an increasedoperational flexibility.

Electrical networks, whatever theirlayout, adapt completely to thisdevelopment. Thus, inspite of thediversity of operational methods, it islogical that we see development ofprioritizing MV control functions. Withthis aim, a MV control system defines:■ functions to be performed,■ their prioritized level,■ their geographical location.

It can be studied in terms of four levels(see fig. 25):■ level 0: MV devices and sensors,■ level 1: protection and control of anMV cubicle,■ level 2: local control of a substationor installation,■ level 3: remote control of a MVnetwork.(see EDF application in appendix 3).

fig. 25: the various priority levels of MV control-command functions

local installation

control

protection and control

of MV cubicle

MV devices and sensors

remote-control of

electrical network


Together this constitutes a MV controlarchitecture, whose operation relies onnumerous exchanges of informationbetween the various priority levels. Thisinformation is essentially:■ remote data transmission,■ telemetering,■ remote control.

The exchange of such information canbe continuous or as events occur(network incident, switchingcommands,...); it requires high-performance communicationsnetworks.

communications networksAll of these exchanges are groupedtogether within the remote datatransmission function, as defined by thefollowing parameters:■ the organization,■ the transmission equipment involved,■ the communication protocol.

All these parameters are such as toensure that any message transmitted iscorrectly received (free of errors).

Remote data transmissionorganizationThe simplest solution is tocommunicate between twotransmitters-receivers. However, withlinks between only two points, the limitof this system’s application possibilitiesis very quickly reached. When severalunits are involved in telecontrol, asingle point-to-point link becomesinsufficient, and this is where the ideaof multi-point comes in. In this case,two organization types are possible:■ master-masterAll the units within this organization cantake the initiative to communicate.■ master-slaveThe control unit with the highest prioritylevel in the architecture is generally themaster. It is responsible for themanagement of all transmissions, towhich end it interrogates all slave unitsin turn on a continuous basis orfollowing an incident. The slave unitsrespond to the interrogation andexecute orders given by the masterunit.In terms of control for electricalnetworks, the most frequently used andthe safest organization is that ofmaster-slave type.

As for data transmission, it is of seriestype. This means that pieces of binary(0/1) coded information are sent oneafter the other using the sameequipment. Above all the advantages ofthis type of transmission are simplecabling and good immunity to externaldisturbances.

Transmission methodsInformation transmission requires theuse of one or several pieces ofequipment.

In the case of control of electricalnetworks, the method used is:■ paired wires, coaxial cable,(specialized telephone links or nationaltelephone networks)■ radio waves (radio links),■ power cable (power line carrier).

Optic fibre is still in the experimentalstage since, inspite of its greatadvantage of being insensitive toelectrical disturbances, its installationcost remains a considerable hurdle tobe overcome. As for power line carriersystems, to date they have only beenused for network load management, forexample sending of signals to changethe applied price-rate by the EDF andsending of load shedding commands inthe United States. However, they are inthe experimental stage for otherapplications, for example remoteelectricity-meter reading orreconfiguration of a network following afault.

In fact, at the moment, no one type ofmethod is predominantly used; thechoice depending on various criteria:■ amount of information to betransmitted,■ frequency of exchange (number andperiodicity),■ speed required for exchanges,■ type of information,■ transmission distance,■ geographical constraints (e.g.mountain region),■ cost of exchanged information.

In practice, an electrical energydistributor will always use variousmethods:■ specialized lines (paired wires) forcontrol of important installations (HV/MV substation, MV/MV substation),■ radio-electrical or telephone links forcontrol of secondary installations(remote controlled MV/LV substationand MV pole-mounted switch).

Transmission protocolsProtocol is the language used forinformation exchange between thevarious protection and control unitswithin an architecture. It defines thestructure of the exchanged messages,both in terms of requests forinformation and for responsemessages. These protocols can beunique to one equipment manufacturer(or to several manufacturers) orstandardized, conforming to variousstandards. As far as public distributionis concerned, within the concept ofarchitecture that has been presentedabove, which is becoming generalized,distributors are trying to standardizeprotocols between levels 2 and 3.However, internal switchgeartransmission protocols remain thechoice of the manufacturer.

Communicating in the chosen protocolfor an architecture is a condition whicha piece of equipment must satisfy if it isto be integrated within that architecture.


6. conclusion

The noticeable feature of currentinstallations, remains their country-dependant diversity:■ diversity in electrical layouts and theirprotection system,■ diversity in basic technical choices,■ diversity in operating methods.

However, two major long termdevelopments are envisaged by alldistributors: development towards a MVsystem and development towardsautomatic control of MV networks.

Development towards a MV systemAs has been seen in the above pages aMV electrical distribution network, isformed by the overlapping of twonetworks:■ the energy network, whose objectiveis to carry energy to user units. It isdefined by the single line layout, and ismade up of electrotechnical equipment,transformers, cables,...■ the information network, whoseobjective is to process data to obtainmaximum safety and overall availabilitywithin this energy network. Defined byprotection and control systems, thisnetwork is made up of protection andcontrol units, which are interlinked via ahigh-performance communicationsnetwork and are distributed:■■ on the MV equipment level,■■ on the substation or installation level,■■ on the electrical network level itself.

Thus, from electrical network design tooperation, "Medium Voltage Man"becomes "System Man".

Development towards automaticcontrol of MV networksFollowing the agricultural and industrialrevolutions, that of communications hasirreversibly created new needs and newsolutions. The next stage will be theuse of expert systems to automaticallyanalyse and operate networks. Thisdevelopment is already undergoingtrials by certain distributors, forexample TEPCO in Japan.However, an obstacle to futuredevelopment is the state of existingnetworks. Indeed, the latter were notdesigned with a view to automaticoperation: their complex, inconsistant

layouts do not facilitate rationalanalysis.Distributors are well aware of thisobstacle. Thus, their new long-termobjectives involve moving towardssimplification and rationalization ofnetwork layouts, which requires long-term and costly investment. Withoutwaiting for this future stage, they arecontinually trying solutions that areadapted to their current MV networklayouts.In the same way, manufacturers usethe most recent technologies to benefitdistributors.

Of course Man must always remainmaster of such Systems. And whilstdata processing, born from computing,already offers distributors betterknowledge and understanding of theirelectrical networks, future years willbring innovative solutions which willcontribute to achieving the mainobjective: satisfying the requirements ofelectrical energy consumers with anoptimum quality of service.


■ with respect to Ring Main UnitIEC 129,265,298, and 420,UTE C 64-130, C 64-131,and C 64-400,VDE 0670,BS 5227,

■ with respect to protection devicesIEC 68,IEC 225,IEC 655,NF C 20-455,NF C 63-850.

■ with respect to electromagneticcompatibility:■■ in terms of susceptibility todisturbances,IEC 801 - chapters 1 to 4,NF C 46-020 to 023,■■ in terms of disturbance emissionEN55 022,NF C 91-022.

The diversity of equipment mentionedin this "Cahier Technique" makes itimpossible to give all of their respectivestandards.

As an example here are a fewstandards:

■ with respect to MV switchgearIEC 56 and 694,IEC 470 for contactorsUTE C 64-100 and C 64-101,VDE 0670,BS 5311,ANSI C37-06 for circuit breakers.

Logical selectivityThis selectivity is obtained by means ofa "logical stand-by" command emittedby the first protection device situatedjust upstream of the fault which shouldbreak the circuit, transmitted to otherprotection devices situated furtherupstream. It enables the number ofselectivity steps to be increased withoutincreasing the upstream trip time.Pilot wires are required betweenprotection units. This technique,developed by Merlin Gerin, is detailedin "Cahier Technique" n° 2.

appendix 1: some MV product standards

appendix 2: various selectivity techniques

generalWhen a fault occurs on an electricalnetwork, it may be detectedsimultaneously by several protectiondevices situated in different areas.

The selectivity of the protection systemgives priority of operation to the closestdevice situated upstream of the fault.Thus, interruption of supply is limited toas small a part of the network aspossible..

However, a protection system alsoallows for contingencies. Thus, whenthe system is designed, provision ismade for the incorrect functioning of aprotection device, in which case adifferent device, situated upstream ofthe latter, should function to limit theeffects of the fault.

Each of these protection devicesinstalled in series on the networkrepresents a selectivity step.

In a MV network, the number ofselectivity steps between HV/MV andMV/LV transformers is generally limitedto between 3 and 5 depending on thecountry. Indeed, above this safety canno longer be guaranteed since thereaction time and the fault currentsbecome very dangerous.

the various techniquesTo give this selectivity in a MVprotection system five main techniquescan be used: amperometric,chronometric, differential, distancerelated and logical.

Amperometric selectivityThis is obtained by adjusting tripthreshold current values.

Chronometric selectivityThis is obtained by adjusting tripthreshold timing values.

Differential selectivityThis is obtained by dividing the networkinto independant zones, with detectionfor each of the zones of any differencein the sum of currents entering the zoneand the sum of currents leaving thezone. This technique requires wiringbetween protection devices situated atthe various extremities of the monitoredzone.

Distance related selectivityThis is obtained by dividing the networkinto zones, with the protection deviceslocating in which zone the fault hasoccured by calculating downstreamimpedances.


appendix 3: EDF architecture and Merlin Gerin equipment

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Grouping and reference point

PRR : grouping and reference point

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SM6 panelboard : miniature switch cubicle

overhead and/ or underground network MV/ LV remote-controlled substations

RM6 panelboard : Ring Main Unit switchgear

TCL11MSsubstation computer

Isis 2000 :mimic diagram status logger

Vigi PAprotection and control-command unit

F24G draw-out panelboardHV/ MV source substation

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level 3: network remote control

level 2: local control of an installation

level 1: protection and control-command of a MV cubicle

level 0: MV devices and sensors

Communications networks:

internal switchgear network (currently line to line)remote transmissions by radio and/ or telephone networksremote transmission via specialised telephone lines

M2S : remote-controlled overhead switch

control interfaces : internal internal external


appendix 4: references

Real. ERI - Lyon - photo. : IPVIPV - 03-92 - 3500 - Imprimeur : Léostic

Merlin Gerin "Cahiers Techniques"■ Network protection using the LogicalSelectivity System"Cahier Technique" n°2 (F. Sautriau)

■ EMC: Electromagnetic Compatibility"Cahier Technique" n°149 (F. Vaillant)

Various publicationsNumerous documents cover certainthemes touched on in this "CahierTechnique". However, whilst less ofthem cover all of the themes, we haveconsidered it of more use to indicate tothe reader organizations that distributetechnical reports written for variouscongresses:

■ "journées d’etude technique S.E.E."Société des Electriciens et desElectroniciens (= Society of Electriciansand Electronics Specialists)■■ quality and cost saving for electricalsupply,■■ Medium Voltage rural networks:developments and future prospects.

Address:Société des Electriciens et desElectroniciens48, Rue de la Procession75724 PARIS Cedex 15

■ C.I.R.E.D.Congrès International des RéseauxElectriques de Distribution(= International Congress on ElectricalDistribution Networks).

Address:Institution of Electrical Engineers,Savoy Place,London WC2R OBLUnited Kingdom.

■ UNIPEDEUnion Internationale des Producteurs etDistributeurs d’Electricité.(= International Union of ElectricityProducers and Distributors).

Address:39 avenue Friedland75008 PARIS

mv public distribution networks throughout the world public distribution networks throughout ......

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