MV public Distribution Network throughout the world (no 155)

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                                          MV public distribution
                                          networks throughout
                                          the world

Christian Puret

A graduate Engineer from the
E.N.S.E.R.G. (Institut National
Polytechnique de Grenoble) and the
I.A.E. in Paris, he joined Merlin Gerin
in 1977.
His first assignment gave him
expertise in the field of
programmable logic controllers.
He was then put in charge of
customer training for the
Merlin Gerin Group.
In 1986, he rejoined the Medium
Voltage Division in which he now
holds a position involving strategic
marketing. He is in charge of
integration of developments in
protection systems and telecontrol
in MV equipment, and more
specifically in those destined for
public distribution networks.

E/CT 155 first published March 1992
Configuration : an operation, in both         Cut-out type fuse : MV striker-type          Sectionaliser : MV switch equipped
digital protection and telecontrol            fuse, that performs the two functions of     with a fault passage counter, it is
systems, that involves assigning, via         protection and isolation. Isolation          installed downstream of a MV
either a built-in or a loaded software        occurs when the fuse melts by ejection       overhead feeder protected by a
package, a standard piece of                  of the striker which in turn automatically   recloser. Its automatic control counts
equipment to a specific application.          triggers switch-over of the fuse             the number of fault current passages
The latter operation, loading a software      cartridge.                                   (corresponding to the number of times
package, is carried out using a tool :                                                     the recloser is activated), and when a
                                              Fault passage indicator : a device
the configurator, normally a personal                                                      preset number has been reached it
                                              fitted to MV networks that indicates,
computer PC. This allows for example:                                                      opens the circuit breaker. Selectivity
                                              fault passage either locally or at a
s definition of functions that the                                                         can thus be obtained by installing
                                              distance. In order to improve quality of
equipment will perform,                                                                    several sectionalisers in series on a
                                              service, the operating company makes
s devising of connections with the                                                         MV feeder, the last sectionaliser
                                              every effort to reduce the interruption in   (which is furthest from the recloser)
                                              supply to that part of the network in        opening on the second instance of
s creation of mimic diagrams and
                                              which the fault exists. To achieve this      current fault, the preceding
labelling of alarms for users.
                                              they must know which portion of the          sectionaliser opening on the third
Draw-out (part of an assembly)                network is affected by the fault. With       occasion and so on. This device is
(IEC 50 - chapter 441, NF C 01- 441) :        this aim, the operating company installs     used in North-American type
withdrawable part of an assembly that,        fault passage indicators. Analysis of the    distribution networks.
whilst remaining mechanically attached        information provided enables the faulty
to the assembly, can be moved to the          area to be closed off, followed by
position or one of the positions that         reconfiguration of the network (typical
gives an insulating distance or a metal       remote control application)
separation between open contacts.
This insulating distance or metal             Prediction : a new science with the
separation always intervenes in the           objective of event prediction, it is based
main circuit. It may or may not also          on reasoning and scientific deduction.
intervene in auxiliary circuits.              Recloser : MV circuit breaker
Fixed (fixed device),                         incorporating a multishot autoclosing
(IEC dictionary of electricity) :             relay, installed on an overhead MV
a device that is designed to be               feeder and coordinated with protection
mounted on a fixed support and                devices (fuses) placed on the feeder
intended to be connected to external          (upstream and downstream). It is used
circuits using fixed electrical               in North-American type distribution
conductors.                                   networks.

Cahier Technique Merlin Gerin n° 155 / p. 2
MV public distribution networks                                                  In a country, the Transmission and
                                                                                 Public Distribution networks ensure the
throughout the world                                                             transfer of electrical energy from points
                                                                                 of production to consumer units.
                                                                                 The points of production are power
                                                                                 stations that generate electrical energy
                                                                                 from various primary energy sources
                                                                                 (nuclear, hydro-electric, coal....)
                                                                                 The points of consumption in
                                                                                 MV - Medium Voltage -, are
table of contents                                                                substations, from which the energy is
                                                                                 delivered to customers (subscribers).
                                                                                 This takes place via the “MV
1. Different types of electrical       The transmission and              p. 4    distribution system “wich is the object of
networks                               interconnection network                   this “Cahier Technique“ report.
                                       The subtransmission network       p. 5    In this “Cahier Technique“ report, after
                                       The MV network                    p. 5    having described the various types of
                                       The LV network                    p. 6    distribution networks and the
                                                                                 distributor’s role, the reader who is not
                                       Type of electric current          p. 6
                                                                                 familiar with MV will find details on:
                                       Network planning                  p. 6    s topologies of MV networks,
2. The distributor                     Reason of existence               p. 7    s substations,
                                       His role                          p. 7    s protection and telecontrol devices.

                                       His development                   p. 8    Comment: In this “Cahier Technique“
3. MV network topologies               Criteria in choosing a topology   p. 10   report, the term MV applies to any
                                                                                 voltage from a few kV to 40 kV.
                                       Items that depend on              p. 10
                                       the chosen topology
                                       Various MV network layouts        p. 10
                                       Neutral earthing layouts          p. 12
                                       Protection system                 p. 13
                                       Telecontrol system                p. 14
4. MV Public distribution              Substations on MV networks        p. 15
                                       Other MV installations            p. 16
                                       MV switchgear                     p. 16
                                       French and                        p. 18
                                       North-American layouts
5. Protection and control              MV protection device technology   p. 20
of MV networks                         Electromagnetic compatibility     p. 21
                                       MV control applications           p. 21
                                       MV telecontrol architectures      p. 23
                                       Communications networks           p. 24
6. Conclusion                                                            p. 25
Appendix 1: some MV product standards                                    p. 26
Appendix 2: various selectivity techniques                               p. 26
Appendix 3: EDF architecture and Merlin Gerin equipment                  p. 27
Appendix 4: references                                                   p. 28

                                                                                 Cahier Technique Merlin Gerin n° 155 / p. 3
1. different types of electrical networks

Producing electrical current in power                corresponding voltage levels can be                   the transmission and
stations is not enough in itself, it must            different from country to country.
also be brought to the end-user.                     However, in general the number of
                                                                                                           interconnection network
In order to link production and                      voltage levels is limited to three; indeed            The geographical distance between
consumption, which in turn can be                    in 1983 the IEC publication 38                        production sites and consumer centres,
translated into financial benefit, a                 formulated recommendations for                        the irregularity of consumption and the
country’s electrical structure is                    voltage levels for 50 and 60 Hz                       impossibility of storing electrical energy
generally broken down into several                   networks.                                             create the need for an electrical
levels that correspond to different types            However, in order to gain better                      network that is capable of directing and
of electrical networks. (see fig. 1).                understanding of this split, the following            transmitting it across large distances.
It should be noted that there is no                  paragraphs present each grid with:                    These lines can stretch across
standard structure that exists                       s its object,                                         thousands of kilometers , for
worldwide, and that the split into                   s its voltage level,                                  example in the French network there
several networks with their                          s its structure.                                      exists 20 000 km.

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

Cahier Technique Merlin Gerin n° 155 / p. 4
The object of this network is threefold:    the subtransmission                                 the MV network
s a "transmission" function with the aim
of carrying electricity from producing
                                            network                                             This level of a country’s electrical
                                            The object of this network is essentially           structure will be covered in more detail
power stations to the main consumer
                                            to carry electricity from the transmission          in the chapters that follow. Thus, here
                                            network to the main consumer centres.               we only give a few distinguishing
s a "national interconnection" function
that manages the product distribution       These consumer centres are:
by relating the production to the           s either from the public sector with                The object of this network is to carry
geographical and time-dependant             access to the MV network,                           electricity from the subtransmission
nature of demand;                           s or from the private sector with access            network to points of medium
s an "international interconnection"        to high-consumption customer (greater               consumption (greater than 250 KVA
function - that manages energy flux         than = 10 MVA) supplied directly                    in France).
between countries dependant on              with HV. In any country the number of               These consumer points are:
programmed exchanges or as back-up.         such consumers is very small (e.g. 600              s either in the public sector, with
In general only very few customers with     in France). They are essentially                    access to MV/LV public distribution
a high consumption are connected to         industries such as iron and steel,                  substations,
these networks.                             cement, chemicals, rail transport,....              s or in the private sector, with access to

                                            The structure of these networks is                  delivery substations for medium
These networks are essentially of
                                            generally of overhead type (sometimes               consumption users. The number of
overhead type structure.
                                            underground near urban areas). In this              these customers (e.g. 160 000 in
The voltages are normally between 225                                                           France) is only a small proportion of the
                                            respect environmental concerns (care
and 400 kV, and sometimes 800 kV                                                                total number of customers supplied
                                            for the environment and protection of
(e.g.: 765 kV in South Africa). Use of                                                          directly with LV. They are essentially
                                            certain natural sites) are often raised in
such high voltages is tied in with cost                                                         from the tertiary sector, such as
                                            opposition to the construction of lines.
saving objectives. Indeed for a given                                                           hospitals, administrative buildings,
                                            As a result it is more and more difficult
power, the line losses by Joule effect                                                          small industries,...
                                            and expensive for subtransmission
are inversely proportional to the square
                                            networks to reach high population                   The structure is of overhead or
of the voltage:
                                            density areas.                                      underground type.
p = k/U2, with
U= network voltage,                         Voltage in these networks is between                Voltage in these networks ranges from
k = constant - dependant on the line.       25 kV and 275 kV.                                   a few to 40 kV (see fig. 2).
In addition the transmitted power           The protection systems are of the same              Operation of these networks, can be
values are such that using low voltages     kind as those for transmission                      carried out manually or, more
would require totally unrealistic cable     networks, the control centres being                 frequently, by remote control from fixed/
cross-sections. Use of high voltages is     regional.                                           mobile control centres. However, in
thus imposed in spite of the drawbacks                                                          order to account of specific needs for
involved in higher equipment insulation
costs, the easiest solution being the
use of overhead lines.                      % 100
                                                                   5               2              5                              5
In any case, the choice of transmission
voltage is above all a technico-economic
one, dependant on the power to be                80
transmitted and the distances to be
covered.                                         70
                                                                                  70                             80
The safety aspect is fundamental for             60
these networks. Indeed any fault at this         50
level leads to important supply failures                                                          95                            95
for all consumer units. In 1965 in the           40
United States, 30 million people were
without electricity for 12 hours due to
such a fault.                                    20
                                                                  35              25                             10
These networks’ protection systems               10
must be very high-performance. As for                                                                            10
their operation on a national level, this
                                                               Belgium          France       Great Britain       Italy         Japan
is the task of a control centre from
which the electrical energy is
permanently monitored and managed.                      < 12 kV             12 < U < 17 kV             17 < U < 24 kV           > 24 kV
                                            fig. 2: proportion of various voltage levels within national MV networks as a function of the
                                            lengths of lines of each country.

                                                                                                Cahier Technique Merlin Gerin n° 155 / p. 5
control of MV networks, these control         type of electric current                    network planning
centres are different to those used on
                                              Energy transmission on these various        The installation and evolution of the
transmission and subtransmission grid.
                                              networks is accomplished using electric     structure of an electrical supply network
The number and the geographical
                                              current.                                    for a country corresponds to planning
dispersion of remote control units, the
                                              Direct current or HVDC (high voltage        operations.
management of several simultaneous
control centres, the number and level of      direct current) links are used for          For transmission and subtransmission,
qualifications of the operators all           exchanges between countries                 these operations are generally
require appropriate solutions:                exclusively on a transmission network       centralised, since:
ergonomics and user-friendly type work        level. The choice of this technique         s the decisions leading to a
stations, control assistance tools,           enables optimization of the use of          modification in the structure of such
control centre configuration tools, and       transmission cables, particularly by        networks, for example the introduction
management of the various                     cancelling the "skin" effect. Such          of a new MV/LV substation, requires to
transmission devices that are used.           intercontinental and even continental       take into account numerous technical
                                              links exist, for example a link between     and economical parameters;
                                              Italy and Sardinia via Corsica (300 MW/     s the number of these parameters
the LV network                                200 kV).                                    involved and their possible interactions
The object of this network is to carry        In other cases, particularly public MV      means that help from computerized
electricity from the MV network to            networks, links are through alternating     tools is required, such as use of a data-
points of low consumption (less than          current. Indeed, in these networks, the     base and of expert systems.
250 KVA in France) in the public sector       use of direct current would not be          For MV and LV networks, the planning
with access to LV customers.                  profitable:                                 is, however, often decentralized.
It represents the final level in an           s losses reduced on short networks
electrical structure.                         (less than 100 km),
This network enables supply to a very         s equipment made more expensive
large number of consumers (26 million         (requirement of numerous direct/
in France) corresponding to the               alternating converters).
domestic sector.                              In addition, alternating current is very
Its structure, whether overhead or            well adapted to voltage changes
underground is often influenced by the        (transformers) in the course of its
environment.                                  transmission as electrical energy.
Voltages in these networks are                With very few exceptions (Saudi
between 100 and 440 V.                        Arabia), and outside of the American
                                              continent where 60 hertz is used
Such networks are often operated
                                              throughout, the current frequency
                                              is 50 hertz.
                                              Of particular note is the case of Japan
                                              where half of the country is on 60 hertz,
                                              and the other half on 50 hertz.

Cahier Technique Merlin Gerin n° 155 / p. 6
2. the distributor

reason of existance:                          The role of the MV distributor is not                   transmission and distribution; it
                                              clear-cut: it often covers LV - Low                     manages around 80% of the electricity
to supply electricity                         Voltage - distribution and in some                      distributed in Italy.
The electrical energy distributors exist      cases he is also in charge of                           s in France the situation is similar with
to supply electrical energy to                transmission, for example:                              the E.D.F.
consumers taking into account several
                                              s in Japan, nine regional private
objectives such as:
                                              companies are each responsible for the
s continuity and quality of service,
                                              production, transmission and
s safety of people and goods,                                                                                     transformers
                                              distribution for their area,                                                = 5%     MV switchgear
s flexibility and ease of operation,
                                              s in Germany around a thousand                                                       = 5%
s commercial competitiveness.
                                              companies are involved in the
                                              distribution of electricity. Around 1/3
his role                                      have their own production facilities.
If electricity supply is satisfactory in      s in Great Britain production is the
industrialized countries, the degree of       responsibility of two companies (NP -
electrification still remains variable in     National Power - and PG - Power Gen).
certain other countries.                      The NGC (National Grid Company) are
Varying objectives depending on the           in charge of transmission, and the
degree of electrification...                  regional distribution is looked after by
For countries that are not 100%               around twelve Regional Electricity
electrified, the priority objective remains   Companies. This structure is a result of                  lines
                                              a privatisation bill for British distributors             + poles
the improvement of this degree of                                                                       + total installation
electrification. To this end, investment      voted in 1990.
                                                                                                        = 90%
is mostly in construction of networks         s in Italy a law founded the E.N.E.L. in
and installations (see fig. 3).               1962. This is a public service                          fig. 3: break-down of costs of MV overhead
However, the investment capabilities,         responsible for the production,                         lines.
sometimes reduced, can lead to
solutions based on simplification of the
networks’ structure to the detriment of
the system performance. In the same                      Net consumption / country                     Net consumption / inhabitant
way, sometimes a lack of availability                    TWh                                                                  KWh
and competence of the operators can           3000                                                                                           30000
lead to over-simplified operation.
Varying situations in industrialized
countries                                                                                                                            11760
In countries that are 100% electrified,       1000                                                                                           10000
there are considerable differences in                                                                                595
uses of electrical energy:                                                                                  5320
                                                                          5200                                             4900
s national electrical energy                                                                   3190
                                               300                                                       294                                 3000
consumptions show great differences                                                2790
(see fig. 4). These differences are due                       1860                        183
to the size of country, to its economic                                          108
growth (GNP) and to the weight of the          100                                                                                           1000
industrial sector (example 40% of the
French consumption).
s per capita consumption can vary by a                               27
                                                30                                                                                           300
factor of 10 between certain countries                   19
(see fig. 4). These differences are
mostly due to the pricing policy of
distributors, but also to climatic
                                                        Portugal Denmark          Spain       Italy      France      Japan United states
                                              fig. 4: net consumption per country and per inhabitant.

                                                                                                      Cahier Technique Merlin Gerin n° 155 / p. 7
These examples would thus appear to                   his development: supply of                      A permanent fault implies an
show that the number of parties                                                                       interruption lasting between several
involved, particularly in MV distribution,
                                                      high quality energy                             minutes to several hours ; it requires
can vary considerably from country to                 Increasingly, the energy distributor is         human intervention.
country (see fig. 5).                                 required to supply a high quality
                                                                                                      Overhead networks, that are naturally
                                                      electricity product. In order to do this
In the case of MV distribution the                                                                    considerably more exposed than
                                                      he must:
distributor generally has complete                                                                    underground grid, require specific
                                                      s reduce the number and the length of
responsibility for the network, from the                                                              solutions to problems encountered
                                                      interruptions in supply to his customers,
HV/MV substation to the MV/LV                                                                         such as:
                                                      s minimise the consequences of them,
transformer substation. In addition the                                                               s tree branches falling on overhead
                                                      s avoid disruptions such as voltage and
distributor’s job now includes a                                                                      lines;
                                                      frequency fluctations (see fig. 6).
commercial aspect with respect to the                                                                 s birds landing on the line or its
sale of "electricity" as a product in the                                                             supports;
form of a kWh. He must thus                                                                           s faults due to lightning, wind, frost,
continually improve the quality of the                parameters       nominal           tolerances   snow;
product to satisfy the demands of his                                  values                         s vandalism.
various customers, and remain                         Frequency        (50 Hz)           ± 1Hz        As a consequence, the type of failures
competitive in relation to other sources              MV Voltage       (12 to 24 kV)     ± 7%         encountered differ between overhead
of energy. This objective leads                       LV Voltage       (230 or 400V)                  and underground networks:
distributors to consider several price                overhead                           ± 10%        s on overhead networks, the faults are
levels dependant on the quality of kWh                underground                        ± 5%         mostly momentary (80 to 90%) and
being sold.                                                                                           one-phased (75%) since they are often
Equally, the electrical network                       fig. 6: examples of a distributors "quality"    due for example to storms, to a line
represents an important investment for                constraints (EDF France).                       fallen to the ground or to shorting
the distributor. He has to make this                                                                  across an insulator.
investment as profitable as possible                                                                  s on underground networks, the faults
                                                       The type of faults depend on the
and it is for this reason that it is                                                                  are mostly permanent (100%) and
                                                       type of network
increasingly frequent that distributors’                                                              multi-phased (90%) since they are very
requirements include the idea of energy                For customers, the consequences of             often the result of severing a cable.
management.                                            such phenomena depend above all on
                                                       the type of fault.                             A need for information
Lastly, distributors play an important                                                                The importance of understanding the
social and political role, a role which                A fault can be:
                                                                                                      incidents occuring on the network
                                                       s momentary or permanent in duration;
can have a bearing on his choices, or                                                                 increasingly justifies a need for
                                                       s one-phase or three-phase dependant
at least on their priorities, as in the                                                               information that distributors satisfy by
following two examples:                                on the type of its cause.
                                                                                                      using statistical studies.
s supplying new customers may                          A momentary fault often means a brief
                                                                                                      This analytical work aims to:
require extension of the network,                      interruption of the order of several
                                                                                                      s classify and code the incidents,
s the price per kWh may be limited in                  100 ms, essentially related to the             s determine their origins and causes,
line with government economic policy.                  operating time of a recloser.                  s statistically calculate their frequencies,
                                                                                                      s search for correlations,
                                                                                                      s study the comparative performances
                                                                                                      of the various topologies,
                                                                                                      s analyse the results according to
country                    number of distributors                        the most important           operating methods and installed
                           total         distributing 80%                                             equipment involved.
                                         of national
                                                                                                      These statistics provide a tool for
                                                                                                      helping distributors in the design,
Germany                    600           20                              R . W . E.                   operation and maintenance of public
Saudi arabia               5             5                               S.C.E.C.O.                   networks.
Spain                      200           6                               Hydro Electrica              In addition, in order to be able to decide
France                     200           1                               E.D.F.                       on the best solutions, the quality of
                                                                                                      service must be able to be quantified
Great Britain              15            10                              Regional Electricities Cie
                                                                                                      and measured, and no longer just be
Italy                      150           1                               E.N.E.L.                     approached in a subjective manner.
Japan                      9             9                               Tokyo Electric Power Co      In order to achieve this, new tools have
                                                                                                      been designed (based on mathematical
                                                                                                      models), particularly including the idea
fig. 5: electrical energy distributors in several countries.                                          of "undistributed energy". Notably, to
                                                                                                      measure the cost of low quality in

Cahier Technique Merlin Gerin n° 155 / p. 8
MV networks, the E.D.F. use the              Advances in protection and telecontrol             control centres:
formula:                                     equipment with digital technology                  s there still exists control centres in
A*N*N*P + B*N*P*T, with                      (microprocessor) and expansion of                  which:
N = number of permanent breaks per           communication networks, offer the                  s the status of various networks is
feeder,                                      perspective of innovative solutions in             indicated by manually displaced
P = average power per feeder in kW,          terms of event prediction (see                     symbols on large charts several meters
T = average length of interruption per       glossary).                                         square in area,
failure,                                                                                        s and the instructions relating to
                                             Lastly, the practice of live-line repair
A and B = economic value coefficients                                                           operations are hand written in log-
                                             work and remote control of the
(in 1990, for the EDF in France                                                                 books;
                                             networks are also points that favour
A = 6 FF/ kW and B = 13.5 FF/ kWh).                                                             s in new centres all these operations
                                             increased quality of service, by
However, measurement of the quality                                                             are performed on computer terminals,
                                             reducing the number of interruptions
of service can require inclusion of even                                                        with:
                                             and their duration.
                                                                                                s all information available on screens in
more parameters. The complexity of
                                             Of course, all of these developments               real time (network layouts,
the calculations and simulations to be
                                             require that the workforces should                 geographical description),
carried out, justifies the development of
software tools that are increasingly         adapt quickly, following the example of            s events logged automatically (data

powerful to help with the decision.          the current changes in work practices in           logging).
To measure the reliability of energy
supply to a residential LV customer,
distributors prefer to use the criteria of   %
"degree of unavailability": this is the      number of
                                             LV customers                                                                  1986        1995
total annual time during which an
average customer has his supply                      60
interrupted due to a fault on the
electrical network (HV, MV or LV).
Lastly, it is important to note that for a
LV customer, numerous incidents are
due to the MV network (60% according                  40
to an EDF study) (see fig. 7).
Networks, equipment and operators,                    30
are all evolving
It should not be forgotten that a
network’s performance depends above
all on its topology. However, throughout
the world, present networks are simply                10
the result of years of laying of
structures one on top of the other as
needs have increased. In addition a                    0
                                                               0           1          2          3          4         5          >5    hours
network ages and is constantly in need
                                                                                                                    duration of interruption
of maintenance and renovation work to
retain its performance level and to          fig. 7: degree of unavailability of electrical energy on one LV network (EDF - France).
avoid incidents, the sources of
"undistributed energy".
To answer these needs, the
manufacturers thus propose
"maintenance-free" or reduced
maintenance equipment; equipment for
which maintenance, modification and
addition type operations do not
adversely effect the continuity of
In addition, energy distributors are not
willing to undertake preventive
maintenance, particularly device
monitoring by recording and analysing
incidents that occur on the network
(use of "disturbance recorders" and

                                                                                                Cahier Technique Merlin Gerin n° 155 / p. 9
3. MV network topologies

The topology of an electrical network is       items that depend on the                          s the choice of an overhead or
defined here as all of the principles                                                            underground distribution system has a
involved in carrying electrical energy in
                                               chosen topology                                   big influence on the installation costs
public distribution (layout, protection,       The choice of a topology fixes the main           and the quality of service (e.g.: costs of
operation).                                    design elements of a distribution                 a trench / vulnerability to momentary
                                               system, such as:                                  faults...). For MV, in industrialized
In practice for a distributor, defining a
                                               s the rated power and the maximum                 countries, this choice can be broken
topology means fixing a certain number
                                               value of earthing currents, e.g. for MV,          down into three cases:
of physical factors, whilst taking
                                               the EDF limits the value of these                 s highly populated urban area with an
account of criteria dependant on
                                               currents to 300 A at 20 kV overhead               underground distribution system,
objectives aimed for and technical
                                               and to 1000 A underground;                        s highly populated suburban area with
constraints. Since these factors are
                                               s the rated voltages, e.g.:for MV Japan           underground or part-underground part-
closely interrelated, choice of a certain
                                               supplies at 6.6 kV, Great Britain at 11           overhead distribution system;
topology is always the result of
                                               and 33 kV and France mostly at 20 kV;             s scarcely populated rural area with
technico-economic compromises.
                                               s voltage surge ratings and
Here, the graphical representation of a                                                          overhead distribution.
                                               coordination of isolation as well as
topology will be by simplified single-line     protection systems against atmospheric            However it should be noted that
layouts.                                       voltages surges;                                  historically, due to high initial costs,
                                               s the earthing connection layouts, as             numerous urban areas have overhead
                                               well as the number of distributed wires,          distribution systems, as in Japan and
criteria in choosing a                         s the maximum feeder lengths (tens of             the United States.
topology                                       kilometers at MV);
The choice of a topology depends on            s the type of distribution: overhead or
meeting objectives:                            underground (see fig. 8);
                                                                                                 various MV network layouts
s to ensure the safety of people and           s the type of operation: manual,                  The choice of layout is important to a
goods,                                         automatic, remote controlled.                     country: particularly for MV networks
s to attain a pre-defined level of quality                                                       since they are of great length. Thus, for
                                               It is important to note that:
of service,                                                                                      example, the total MV structure in
                                               s the choice of short circuit current has
s to produce the desired profitability.                                                          France is around 570 000 km long, that
                                               repercussions on the rating of the
                                                                                                 of Italy being 300 000 km long and that
However, it must also meet certain             equipment used in the network;
                                                                                                 of Belgium being 55 000 km long.
requirements:                                  s the choice of voltage rating(s) is
s to correspond to the housing density         always a compromise between the                   Several topologies exist:
and/or to consumption, known as the            installation and operating costs of the           s lattice type, closed loop topology,
load density, it plays an ever increasing      network;                                          s simplified lattice type, open loop
role. Calculated in units of MVA/km2,          s the choice of insulation rating of              topology,
this density enables the expression of         equipment is generally in line with               s open loop topology,
various geographical zones in terms of         international and/ or national standards;         s radial topology.
load concentration. Certain distributors
distinguish two types of consumption
zones by defining:
                                                                  0                 25                 50                 75             100%
s low load density zone:
< 1 MVA/km2,
s high load density zone:                      Canada
> 5 MVA/km2.
                                               United States
s to account for the geographical
spread, terrain and construction               Denmark
                                               Great Britain
s to satisfy environmental constraints,
particularly climatic (maximum and             Netherlands
minimum temperatures, frequency of
storms, snow, wind, etc.) and respect                                     underground                                       overhead
for surroundings.
                                               fig. 8: proportion of lengths of overhead (lines) and underground (cables) on MV networks, for
                                               several countries.

Cahier Technique Merlin Gerin n° 155 / p. 10
Other topologies are also used, for                 This layout is particularly used for MV               unit on this structure can be supplied
example the double shunt found on                   distribution in rural areas. Indeed, it               via two possible electrical paths, whilst
French MV networks.                                 enables easy and low-cost supply to                   only one of those paths is activated at
Although none of them are "standard"                low load density (≈ 10 kVA) consumer                  any one time, back-up is provided by
in MV, distributors lean towards two                units with a wide geographical                        the possibility of using the other loop. In
base topologies: radial and open loop.              dispersion (≈ 100 km2).                               such a layout, there is always an open
Each of these two topologies will thus              Very often a radial layout is used with               point in the loop, which leads to a
be covered in more detail and defined               an overhead type distribution system.                 similar operation to two antenna
in terms of:                                                                                              layouts.
                                                    Its strengths and weaknesses are
s its operating principle,                                                                                The typical line layout is, of course, a
                                                    summarized in the table in figure 10.
s its typical line layout,                                                                                loop on which are connected consumer
s its typical application,                          Open loop layout                                      units (see fig. 9) which can be public
s its strengths and weaknesses.                     Its operating principle uses two lines of             distribution substations MV/LV, and/ or
                                                    supply. This means that any consumer                  delivery substations for a MV customer.
Radial layout
This layout can also be called
Its operating principle is based on using           technology                strengths                         weaknesses
a single supply line. This means that all           radial                    s simplicity                      s   quality of service
consumer units in such a structure only                                       s operation
have one possible electrical feed path.                                       s installation costs.

It is of arborescent type (cf. fig. 9).             open loop                 s   simplicity                    s operation with more

This arborescence originates from                                             s   quality of service            frequent switching
                                                                                                                s installation costs
supply points, that are MV/MV or
HV/MV distribution substations.                     fig. 10: comparative table of the two base MV network layouts.

                                   HV/ MV
                                                          HV/ MV transformers

  radial layout                                                                                                            open loop layout
                                                                                                 supply path for all substations except n°1

                                                                                                                                MV/ LV
                                                                  supply path for
                                                                  substation n°1

 MV/ LV                                                                       substation n°1
 transformer                                                                  MV/ LV                                            MV/ LV
                                                                              transformer                                       transformer
                                                                                                   open point

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

                                                                                                          Cahier Technique Merlin Gerin n° 155 / p. 11
Each point (between 15 and 25 points           The five neutral earthing layouts                   The differences between the five
per loop) is connected to the loop by          used in MV throughout the world                     categories
two MV switches. All these switches            Here again, no one standard neutral                 It has been previously stated that the
are closed , except for one of them that       earthing layout exists. However, it is              choice of neutral earthing layout
forms the opening in the loop and              possible to bring together all the                  influences the performance of the
defines the feed path for each                 various cases met around the world                  network and the design of its protection
consumer unit. This opening can be             into five categories (see fig. 12):                 system. Indeed, the main differences
displaced within the loop, particularly        s direct distributed neutral earthing,              between the five categories lie in the
during reconfiguration operations              s direct non-distributed neutral earthing,          behaviour of the network in an earth
following a fault.                             s neutral earthing via an impedance,                fault situation.
Very often this layout is used in              s neutral earthing via a designated
                                                                                                   These differences translate in real
association with an underground type           circuit,                                            terms to the degree:
distribution system.                           s neutral insulated from earth.                     s of ease of detection of these faults,
It is typically used in highly populated       As has already been said, none of the               s of security achieved for people,
urban areas, and its strengths and             categories is dominant throughout the               s of impact on the requirements of
weaknesses are detailled in the table,         world: some solutions are specific to               electrotechnical equipment.
figure 10.                                     some countries, and several categories              The distributed neutral earthing layout
                                               can be found within one single country,             that allows single phase distribution
Double shunt layout                            or even within the network of one single
This rather uncommon layout is mostly                                                              should, however, be considered
                                               electricity distributor.                            seperately. This possibility can be
used by the EDF in the Paris region, it
                                               But in the end, the choice of a MV                  considered in certain countries on the
is shown in figure 11.
                                               neutral earthing layout is always the               basis of its reduced installation costs.
The operating principle is the following:      result of a compromise between                      However, the more complex protection
s the MV network is doubled up,                installation and operating costs.                   devices require stricter maintenance.
containing two circuits A and B, both
normally live,
s every MV/LV substation
s is connected to both MV cables ("A"
and "B"), but it is only actually live to       HV/ MV                                                                HV/ MV
one of the cables (MV switch closed on          substation                                                            substation
cable "A"),
s is equipped with a simple local
automatic control device,
s on failure of cable "A", the automatic
control detects the lack of voltage in the
cable, checks the presence of voltage
in cable "B" and commands the closing
of one MV switch and the opening of
the other MV switch.
                                                                                                                   circuit "A"
neutral earthing layouts                                                                                                                  circuit "B"
The choice of neutral earthing layouts
(or neutral MV systems) defines
amongst other things the voltage surge
ratings and earth fault currents that
could be found on a network. It must be                                                                        U
noted that these two parameters are                                                                                   1
                                                                                                    inlet A
contradictory, in view of the fact that
obtaining a low fault current level leads
to a high voltage surge and vice versa.                                                             inlet B           1
These values thus pose electrical                                                                                     0
constraints that the electrotechnical                                                                                            tf       tf
equipment must be capable of                                                                        network           1
withstanding. However, in choosing the                                                              supplied          0
                                                                                                    by...                 A           B         A
connecting layout, we simultaneously
select the possible solutions for
                                                                                                    t f = 5 or 25 s                                 t
protection of the electrical network and
influence the operating methods.               fig. 11: double shunt distribution layout, used by EDF - France. In the inset, automatic control
                                               sequence of Merlin Gerin permutator, conforming to EDF - France specifications.

Cahier Technique Merlin Gerin n° 155 / p. 12
Independantly of this particular case,
the table in figure 13, a summary of the
strengths and weaknesses of these
categories, shows why no one of these
categories is predominantly used
throughout the world.

protection system                                           neutral             neutral            neutral             neutral           neutral
The electrical structure of a country                       distributed         earthed            earthed via         earthed via       insulated
                                                            with numerous       directly and       an impedance        a designated      from earth
corresponds to a group of electrical                        earthing points     undistributed                          circuit
networks.                                     country
                                              Australia     s
An electrical network can itself be           Canada        s
broken down into areas.                       Spain                             s                  s                   s                 s
Each of these areas is generally              France                                               s
protected by a circuit breaker in             Japan                                                                                      s
conjunction with detector devices             Germany                                                                  s
(measurement sensor: current or
voltage transformer...), protection and
                                              fig. 12: various MV neutral earthing layouts, and their applications throughout
control devices (protection relays), and
                                              the world.
trip devices (actuators).
All these elements together make up a
chain of protection (see fig. 14) that
ensures isolation of the faulty part of
                                              neutral earthing method         strengths                          weaknesses
the network in the event of a fault.
Its role is to ensure safety by protecting    direct earthing                 authorises one-phase               s requires numerous high quality
                                              and distributed                 and three-phase                    earthing points (safety)
against insulation faults between
                                                                              distribution                       s requires a complex protection
phases or between phase and earth,                                                                               system
and against prolonged overloading. The                                                                           s leads to high values of earth
chain of protection must especially                                                                              fault currents
reduce the consequences of a short            direct to earth                 eases detection of                 leads to high values of earth
circuit, such as the risks of fire,           and undistributed               earthing faults                    fault currents
explosion of mechanical damage,...            insulated                       limits earth fault                 leads to surge overvoltage
The network’s protection system is                                            currents
made up of all of these chains of             designated                      favours auto-extinction            requires complex
protection, integrating their installation                                    of earth                           protection systems
with the organization of the operation                                        fault currents
between them. This protection system          impedant
organization, including the trip times of     (compared with neutral          limits earth fault                 requires more complex
the associated circuit breakers, defines      direct to earth)                currents                           protection systems
the maximum duration of a current fault       (compared with neutral          reduces surge                      leads to higher earth fault
in the different parts of the electrical      insulated from earth)           overvoltage                        currents
The effectiveness of a protection             fig. 13: a summary of strengths and weaknesses of the five MV neutral earthing methods.
system depends on several criteria:
reliability, selectivity, rapidity,
sensitivity, adaptability.
Reliability                                   measurement        protection                            breaker
This criterion describes the level of           sensor              unit                               device
quality concerning safety of people and
property, particularly in terms of
dangers of electrocution by increased
earth potential. Although a protection
device is rarely activated, when there is
a fault it must act effectively, throughout                         electrical network
its many years of service. This criterion
directly influences the network               fig. 14: MV chain of protection, and photograph of a SF set (Merlin Gerin), an example of
performance, thus, for example, any           complete integration.

                                                                                                   Cahier Technique Merlin Gerin n° 155 / p. 13
interruption in supply must be "justified"     between HV/MV and MV/MV
since it entrains a loss of operation for      substations and biased differential for
the customer ... and the distributor.          HV/MV transformers. Lastly, distance
Sensitivity                                    selectivity technique is mostly used by
This criterion also has a bearing in           German distributors.
terms of security and costs: it describes
the ability to detect weak fault currents      telecontrol system
without being sensitive to transitory
                                               Under the one term of telecontrol are
phenomena due to the network
(operations) or to surrounding                 grouped all of the elements tied in with
                                               operation of the network.
electromagnetic effects, thus before
there is any risk to people or property,       A telecontrol system defines all of these
and that without tripping prematurely.         elements and their relative operational
                                               organization. In this way the telecontrol
                                               system must enable the operator (the
This criterion has a particular bearing
                                               distributor) to account for 3 situations
on costs of operation, since it indicates
                                               s normal operation,
to what degree it is possible to maintain
                                               s the instance of a fault,
the network operation whilst one part of
                                               s maintenance operations (live and
it is not working as it should do. In
practice, this leads to shutting off the
faulty element, and only the faulty            Lastly, the operating tools installed in
element (see appendix 2: different             this system are going to make a
selectivity techniques).                       considerable contribution to the quality
                                               of service that is obtained.
This criterion, as in the preceding case,      These tools range from the push-button
has an effect on costs: it enables             control of a MV device up to the MV
reduction of the damage due to                 network control centre, from the
electrical arcs and short circuit currents,    ammeter on a MV cubicle up to a
it particularly reduces the risk of fire       automatic remote reading of the load
and the costs of repair work.                  curve for a MV feeder, etc...
This criterion particularly interests the
distributor and indicates the degree of
evolution that is possible (possibilities
and ease) for the protection system
with regards to modifications to the
network topology.
Of all these criteria, selectivity is the
one that leads to the most varied
solutions from country to country. They
depend on two initial choices made by
the energy distributors:
s of which neutral earthing layout is
used, from which we define notably the
protection systems against earthing
faults (see previous section).
s and that of which selectivity principle
will be used, of which the most
commonly used, called ampere-
chronometric selectivity, is based on
the association of the fault current
rating (amperemetric selectivity) and a
value of the tripping delay time
(chronometric selectivity). However,
several different techniques can exist
within the same network, in South
Africa for example E.S.C.O.M. uses
within one network, ampere-
chronometric selectivity, pilot wire

Cahier Technique Merlin Gerin n° 155 / p. 14
4. MV public distribution

This chapter is a reminder of the main       The MV/LV substation in a public                   performs the transfer from MV
substations installed on MV networks,        distribution system                                (≈ 10 kV) to LV (≈ 100 V).
and the main technologies used in MV         Positioned between the MV network                  The typical layout of this substation is
equipment. It is concluded by two            and the LV network, this installation              of course a lot more simple than the
layouts showing their applications in
real terms.
                                             substation                                          HV incomers
substations on MV
A substation or installation is a physical
entity defined by its position and its                                                       HV/MV transformers
function within electrical networks.
The role of a substation is essentially to
perform the transition between two                                                             MV bus section
voltage levels and/ or to supply the end                                                       circuit breaker
The HV/MV substation in a public
distribution system
This installation is present in any of a
country’s electrical structures; it is                                             overhead and/or underground MV feeders
positioned between the
subtransmission network and the MV           MV/MV
network.                                     substation                                         MV incomers
Its function is to ensure transition from
HV (≈ 100 kV) to MV (≈ 10 kV).
Its typical layout (see fig. 15) involves
two HV inputs, two transformers HV/
MV, and 10 to 20 MV feeders. These
feeders supply overhead lines and/ or
underground cables.                                                                overhead and/or underground MV feeders

The MV/MV substation in a public             MV/LV
distribution system                          substation
This installation performs two functions:
s to ensure the demultiplication of MV
feeders downstream of HV/MV
substations (see fig. 15). In this case,
the substation does not include a
transformer. It is made up of 2 MV                                   MV incomers     MV/LV transformer                     LV feeders
inputs and 8 to 12 MV feeders. This                                                  protection
type of substation is used in several
countries like Spain, Belgium, South         delivery
Africa.                                      substation for
s to transfer between two MV voltage         a MV customer
levels. Such MV/MV substations do
contain transformers. They are
necessary in countries that use two
successive voltage levels in their MV
networks, for instance in Great Britain
                                                                  MV          general MV protection   private MV                  private LV
where the MV network is broken down                               incomers    and metering system     feeders                     feeders
into two levels 11 kV and 33 kV. Their
typical layout ressembles that of the
                                             fig. 15: various layout types for substations used on public distribution networks
HV/MV substation.

                                                                                                Cahier Technique Merlin Gerin n° 155 / p. 15
previous installations. In particular, the          However, if the user is supplied with                Amongst all the various types of
most common MV device used is the                   MV, a standard layout may be                         devices, two are particularly used in MV
switch and no longer the circuit                    proposed (see fig. 15). Installation of              switchgear, the circuit breaker and the
breaker.                                            such a substation is, however, evidently             switch. They are virtually always
These substations are made up of four               dependant on the distributor’s                       combined with other devices (protection
parts:                                              agreement since they may have their                  and telecontrol units, sensor
s MV equipment for connection to the                own specific requirements (metering,                 measurement units,...) that make up
upstream network,                                   operating conditions,...)                            their associated equipment.
s the MV/LV distribution transformer,                                                                    s MV circuit breaker
s the LV feeder board as connection to                                                                   Its main function is protection, but also
the downstream network (in LV),                     other MV installations                               performs a control function and,
s and inceasingly frequently a pre-                 Outside of the substations already                   depending on its type of installation, a
fabricated outer enclosure (in metal or             described, other MV installations exist              switching function (draw-out, glossary).
increasingly frequently in concrete) to             that are mainly positioned on overhead               MV circuit breakers are nearly always
enclose the previous elements.                      networks. Often single function, they                mounted in a MV cubicle.
                                                    are used                                             s MV switch
The delivery substation for a HV or
                                                    s either as protection, as in the case of            Its main function is command, but also
MV customer
                                                    fuses and reclosers (glossary).                      often performs a role of switching. In
These installations perform the transfer                                                                 addition, it is combined with MV fuses
                                                    s or for operation, as in the case of
from public distribution to private                                                                      to ensure protection of MV/LV
                                                    remote control switches.
distribution. They enable connection:                                                                    transformers (30% of all MV switches
                                                    The MV remote control switch is part of
s to the HV subtransmission network
                                                    network remote control systems. It                   used).
for a high-consumption customer
                                                    allows rapid reconfiguration operations              MV cubicles have metal enclosures that
(≈ MVA) via a HV/MV substation,
                                                    without the operator having to travel.               meet specifications laid down in the
s to the MV network of a medium-
consumption customer (≈ 100 kVA) via                                                                     publication IEC 298 that differentiates
                                                                                                         four types of switchgear, each type
a MV/LV substation.                                 MV switchgear (see appendix 1)                       corresponding to a level of protection
The choice of the connecting voltage to             MV switchgear enables the three                      against fault propagation within a
the public distribution network for a               following basic functions to be                      cubicle.
customer depends essentially on:                    performed:                                           This protection system involves
s the quality of the LV network,                    s isolation, which consists of isolating a
                                                                                                         partitioning of the cubicle into three
particularly its power capacity (electrical         part of the network in order to work on it           basic compartments (see fig. 17):
capacity);                                          in complete safety,                                  s the switchgear compartment
s the distributors policy, particularly on          s control, that consists of opening or               containing the device (MV circuit
the rates offered, since for the                    closing a circuit under its normal                   breaker, MV switch,...),
customer this defines the cost saving               operating conditions,                                s the MV busbars for electrical
advantages of electrical energy,                    s protection, that consists of isolating             connections between several MV
compared with other sources of                      part of a network with a fault in it.                cubicles grouped together on a board,
energy: oil, gas,...
                                                    It essentially takes three forms:                    s MV cable connections compartment,
In practice, it is the power subscribed to          s separate devices (see fig. 16)                     often with space for measurement
by the customer that determines                     (directly fixed on a wall and protected              sensors.
whether he is connected to LV or MV,                from access by a door),
with very different values from country                                                                  Often a fourth compartment completes
                                                    s metal enclosures (or MV cubicles)                  this assembly, it being the control
to country. Thus, in France a customer              containing these devices,
is supplied MV from 250 kVA whereas                                                                      compartment (or LV box) containing
                                                    s MV boards that are made up of                      protection and control units.
in Italy this threshold is nearer several           several cubicles.
tens of kVA. On the other hand in the                                                                    In addition to this classification the
United States where a customer can be               The use of separate devices is                       distinction should be noted between
supplied LV up to 2500 kVA. For users               increasingly rare; only several countries            Fixed and Draw-out (see glossary) that
supplied with HV, the layout of the                 such as Turkey or Belgium, still use this            apply to the MV device and cubicle.
substation is specifically designed.                technology.                                          This distinction, that depends on the

      MV devices     isolator              switch                circuit breaker       switch isolator       draw-out          fuse
function                                                                                                     circuit breaker
isolating                       s                                                               s                     s
command                                             s                     s                     s                     s
protection                                                                s                                           s                s

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

Cahier Technique Merlin Gerin n° 155 / p. 16
          connection and measurement
          measurement transformer                    substation public            public            public                 HV/MV         MV/LV
                                              switchgear        HV/MV             MV/MV             MV/LV                  customer      customer
                                              separate device                                       S                                    S

                                switchgear    bloc                                CB or S           S                      CB            CB or S
                                breaker       compartmented                       CB or S           S                      CB            CB or S

                                              metal clad           CB             CB                                       CB

busbars                                       GIS                  CB                                                      CB

                                              RMU                                                   S                                    S
fig. 17: various compartments of a MV
cubicle, surrounded with a metal enclosure,
                                              S = with switch           CB = with circuit breaker
and their main elements.
                                              fig. 18: main MV switchgear applications.

ease of operation (as a function of time
taken to change a device), is only
indirectly involved in the idea of MV                             air                oil                     SF6                       vacuum
network safety.
                                               MV switch                s   s                s                         s       s
IEC 298 defines the following four types
of switchgear for MV cubicles:
s "BLOC" switchgear divided to a               MV                                            s                     s       s       s         s   s
greater or lesser extent into                  circuit breaker
s "COMPARTIMENTED" switchgear, of
which only the outer enclosure is             fig. 19: MV device breaking techniques, their relative importance and the development of
obliged to be metal, has the three            their use.
compartments separated with metal or
insulated partitions;
s "METAL CLAD" switchgear also has
distinct compartments, but of which the
partitions are obliged to be metal;
s "GIS" switchgear (Gas Insulated
Switchgear) that is hermetically sealed
and within which the compartments no
longer play an important safety role.
The GIS is essentially made up of
circuit breakers.
This technology is also applied to
switches within a RMU (ring Main Unit).
It provides the three standard functions
to be performed of a MV/LV substation
that is connected to an open loop
network (two connecting switches to
the network plus a tee-off fuse switch or
a circuit breaker as protection of the
MV/LV transformer).
The table in figure 18 shows the
switchgear that is most often used as a
function of the type of substation,
whereas the table in figure 19 shows
the current importance and trends of
various MV breaking techniques.

                                                                                                    Cahier Technique Merlin Gerin n° 155 / p. 17
French and                                     This sort of design reduces the costs of            reclosers, sectionalisers, see glossary).
                                               lines, the losses and the surges due to             However, in certain cases, the
North-American layouts                         faults. However, it requires a very high            protection is limited to cut-out type
With these two typical examples it is          quality of neutral earthing.                        fuses (see glossary), the financial
aimed to show in real terms the various        In order to obtain a satisfactory degree            investment is thus restricted, but to the
elements that have been set out in this        of personal safety, it is necessary to              detriment of performance and safety
chapter and to highlight the diversity of      include numerous MV devices (fuses,                 (risk of fire).
solutions that exist across the world.
Needless to say, other layouts exist,
even in these two countries.
The overhead single-line layout used
by the EDF (France) (see fig. 20)
This layout applies the following                                   HV/MV substation
principles:                                                                                                       HV/MV transformer
s at the HV/MV substation, neutral
earthing is via an impedance limiting
the phase-earth shorting current to
300 A at 20 kV,
s three-phase MV lines, undistributed
s radial layout (antenna type).
This concept enables detection at MV                                                                              circuit breaker with
feeder level of any earthing faults                                                                               automatic reclosing
without any other MV protection device
downstream of this substation.
The result is protection and control
systems that are easy to design, to
operate and to modify.
                                                  overhead MV distribution,
Personal safety is guaranteed.
                                                  three-phase neutral undistributed
However, the quality of service that is
obtained is only average due to the fact
that each MV feeder, at the HV/MV
substation, is controlled by one single
protection device: when the protection                                                                            MCOS : manually
device is tripped, the whole network                                   h                    MCOS                  controlled three-phase
                                                                 branc                                            overhead switch
downstream of this MV feeder is cut off.
Solutions exist to make up for this weak
point. These days they are based on
the use of additional equipment such as
                                                            MCOS                MV/LV
remote controlled switches, in the                                                                    MCOS
                                                                                                                  RCOS : remote-
future reclosing circuit breakers will be                                                                         controlled three-phase
used in the network.                                                                        RCOS
                                                                                                                  overhead switch
The overhead single-line layout
found in North-America (see fig. 21)                           MV/LV
This design is sometimes seen in

countries influenced by North-America
                                                                           in g

(e.g. Tunisia). It is based on the                                                                        MCOS

following principles:
s maximum MV distribution, by                         MV/LV
reducing the length of LV feeders in
order to reduce losses;                                                                 MV/LV
s MV neutral distribution with regular                                                                             MV/LV
earthing (e.g. every 300 meters);                                                                                  transformer
s three-phase MV lines on main
structure, with three-phase, two-phase
or one-phase shunting for the MV/LV
                                               fig. 20: overhead MV distribution layout (EDF - France).

Cahier Technique Merlin Gerin n° 155 / p. 18
The design of the protection and                    concerning grid of EDF-type layouts:                countries spread over a large area and
control systems is complex in terms of              s highly trained staff are required                 with a low load density (e.g.: United
the selectivity between the different               (switchgear maintenance, protection                 States and Canada in rural areas).
protection devices.                                 device setting,...),
In addition the operation and                       s considerable stocks of spare parts
maintenance of such networks is                     must be held (different rated fuses,...).
more difficult than for that                        This solution is especially justified in

                                                                                                           HV/MV transformer

                                                                                                           circuit breaker
                                                                                                           (3 independant single phases
                                                                                                           or 1 three-phase)

    MV overhead distribution                                                                            neutral conductor
    main grid : three phases and neutral,
                                                                                                        two fuses as branch protection
    branch : one, two or three phase with neutral
                                                                                                        (2 phases + neutral)

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

                                                                                                        MV/LV transformer
                                                                                                        (see inset below)

                                           MV/LV                                  Nb

                                                                                                               North-American MV/LV
                                                                                                               transformer substation

                                                                         Nb                     MV/LV                    MV

                                                              Nb                    MV/LV
                                                                                                                    LV        LV

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

                                                                                                        Cahier Technique Merlin Gerin n° 155 / p. 19
5. protection and control of MV networks

The advent of digital technologies             Lastly in the 1980”s, digital technology
based on microprocessors has greatly           has, due to the processing power of
modified solutions used in design of           microprocessors, enabled data
protection and control systems. This           processing units to be produced that
chapter presents the latest                    can:
developments and future prospects for          s generally back-up various protection
these increasingly complex functions           devices,
that are operated on MV networks. It           s replace the cubicle’s relays
also shows the importance of a new             (automatic control),
subject, that being electromagnetic            s provide electrical parameter
compatibility (EMC).                           measurements to the operator.
                                               These versatile devices are:
                                               s flexible (protection selection is simply
MV protection device
technology                                     s parameter-adjustable (wide choice of
The role of a protection device, or MV         settings),
relay (see appendix 1), is:                    s reliable (equipped with self-
s to continually monitor various               monitoring or watch-dog devices and
parameters for part of the network (line,      self-testing),
cable or transformer),                         s cost saving (their cabling and
s to act in the case of a fault,               installation time are reduced).
s and increasingly frequently to               In addition, due to their powerful
transmit information for operation of the      algorithms and their digital                 fig. 22: SEPAM, protection and control unit
network.                                       communication, they enable the               and its three Rogowski coil current sensors
To this end it analyses the electrical         performance of additional functions          (Merlin Gerin).
values that are supplied to it by              such as logical selectivity.
measurement device sensors, and                Using this ability to communicate, a
gives operating commands to the                complete network control system is
switching circuits.                            now possible (similar to technical
Having been limited to electromagnetic         management of an industrial plant).
technology, MV protection devices are          In terms of sensors, and in particular
today undergoing fundamental                   those of current, there is an increasing
development with the use of                    trend towards wide measurement band
microprocessors.                               sensors in place of current transformers
Thus, the equipment available today is         (1 or 5 A). This sort of sensor, that
based on three technologies:                   works on the Rogowski principle (air
electromagnetic, analog and digital.           cored coil), is the type currently on
The oldest is electromagnetic                  offer. It offers distributors optimised
technology, the relays are simple and          solutions (reduction of variables and
specialised (current, voltage, frequency       ease of choice) and greatly improved
control) but are not very accurate, their      performance (greater linearity of the
settings are liable to change with time.       response curve) over traditional current
The more recent electrical analog              transformers.
technology (transistor) has improved
precision and reliability.

Cahier Technique Merlin Gerin n° 155 / p. 20
electromagnetic                              MV control applications                            networks. The number and
                                                                                                geographical dispersion of the points of
compatibility (EMC)                          Towards centralized operation                      remote control, the simultaneous
The electromagnetic compatibility            Remote control is the grouping of all              management of several different control
(EMC) is defined as the ability of a         that is necessary to control remotly MV            centres, the number and
device, a piece of equipment or a            network into one or several points (see            level of qualifications of the operators
system to function in a satisfactory         fig. 23). These points are fixed or                requires adapted solutions:
manner in its electromagnetic                mobile (taken in a vehicle) control                s ergonomics and user-friendliness of
environment, without affecting this          stations. They are also called,                    the work stations,
environment; this environment possibly       dependant on distributors, control                 s control assistance tools,
containing other more or less sensitive      centres, dispatching or SCADA                      s control centre configuration tools
devices.                                     (supervisory control and data                      (glossary),
In order to develop its new products,        acquisition).                                      s management of the various
taking account of the development of         To account for the specific needs of               transmission systems used.
digital techniques and the necessary         MV network control, these control                  In practice remote control covers the
cohabitation of MV equipment (high           centres are different from those used              functions of remote-data transmission,
values of voltage and current,               on transmission and subtransmission                remote supervision, telemetering and
particularly when being switched) and
electronic protection and control
devices (low voltage level and high
sensitivity to electromagnetic radiation),
Merlin Gerin has had to increase the
scope of and then develop applications
in the field of electromagnetic
                                                                       radio waves                                                 fixed control
compatibility (EMC). In addition, in            mobile
                                                                       telephone network
                                                remote-control                                                                     centre
order to satisfy the distributors’
requirements (operational safety), it has       unit
been necessary to carry out more
stringent tests than those defined in
recent standards that are currently valid
(see appendix 1) that define the
acceptable interference level.
s Thus, for measurement devices,
the IEC 801-3 standard details
tests across a frequency range of                                                                                          radio
27 MHz - 500 MHz and three levels                                                                                          link
(1, 3,10 V/m) whereas test conditions in
Merlin Gerin laboratories are
considerably more severe: the range of                telephone network
frequencies covered is from 10 kHz to                 (specialized line)
1 GHz; in addition, from 27 MHz to
1 GHz, devices can be tested in fields
of up to 30 V/m. (also see Merlin Gerin
"Cahier Technique" n°149)
s And for MV equipment, certain tests
are carried out under real conditions on
complete boards (MV switchgear and
protection devices).
However, although electromagnetic
compatibility (EMC) is taken into
account in all development and
manufacturing phases of equipment, in
order to ensure perfectly operational
equipment, it must also be considered
during on site cabling and installation      HV/ MV substation                       MV/ LV substation                   overhead MV device

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

                                                                                                Cahier Technique Merlin Gerin n° 155 / p. 21
remote controling. These functions                control of the network in real time, or for   Remote controlling
can be divided into two groups                    its analysis at a later date.                 Remote opening and
dependant on the direction of                     All this information together with its        closing of power
transmission between the operator and             display mode, is defined at the design        equipment is a basic example of
the network:                                      stage of the telecontrol system. In           remote controlling. The practical
s remote supervision, of devices by the           particular, the mimic diagram displays        application is in the form of remotely
operator,                                         are created as a function of the actual       controled MV switches and circuit
s remote controlling, by the operator of          installation and the operators needs. In      breakers. Other functions can be
the devices.                                      addition they are displayed in real time.     remote controlled: settings, automatic
Remote supervision                                The operator can then visualise:              control,...
This in itself groups                             s the operational layouts (electrical         There must be a maximum degree of
signals concerning                                network, substation, MV cubicle);             certainty concerning remote control
the position of various MV equipment,             s installation status (status of MV           commands. This is obtained using a
their possible tripping on fault,                 devices,...);                                 powerful communications network that
instantaneous or weighted                         s values of operational data (currents,       allows access to information in real
measurements from various parts of                voltages, powers,...);                        time. Thus a switching command for a
the electrical network, and any other             s values of MV protection devices             MV device is transmitted via a double
information indicating the up-to-date             settings;                                     remote control (TCD), and confirmed
status of the network. It enables for             s details concerning alarms, with their       by return of a double transmission
example, the automatic and                        chronology of occurence;                      signal (TSD).
continuous printing, in sequentiel                s ...                                         Remote control processes also
order, of all the events necessary to                                                           integrate requests for validation and
                                                                                                confirmation before execution of a
                                                                                                switching command.
                                                                                                Remote control
                                                                                                In MV distribution this is a cost saving
                                                                                                factor in terms of operation of the
                                                                                                network. Indeed, without having to
                                                                                                travel, the operator can continually
                                                                                                monitor and control the operation of his
                                                                                                network. As an example: following a
                                         remote control                                         fault it is possible to rapidly change the
                                                                                                network operating layout so as to
                                                                                                minimise the proportion of it that is cut

                                                                                                off, this being done by remote
                                                                                                consultation of fault position indicators
                                                                                                (see glossary) that are installed at
                                                                                                different points on the MV network,
                                                                                                followed by remote control of MV
                                                                                                switches. This results in a considerable
                                                                                                reduction in the amount of undistributed
    remote supervision                                            remote controlling            energy, but also in network optimization
    (information transmission)                                    (command transmission)        with possibilities of optimum
                                                                                                management of the load distribution.
                                                                                                The network loading can also be

                                                                                                In particular, by logging the load curve
                                                                                                verification and optimization of energy
                                                                                                consumptions can be performed.
                                                                                                Finally, to increase efficiency, the
                                                                                                operator can have rapid access to the
     s   remote data transmission                                 s remote control of           most relevant information via an
     s   telemetering                                             network's MV devices          automatic pre-processing method such
                                                                  s remote adjustment
                                                                                                as a sorting operation, graphical
                                                                  of settings.
                                                                                                representation operation, calculation
fig. 24: several functions, here grouped according to the direction of transmissions between
operator and network, are required for remote control.

Cahier Technique Merlin Gerin n° 155 / p. 22
Automatic energy supply                            MV telecontrol                                          Electrical networks, whatever their
management                                                                                                 layout, adapt completely to this
This management, which aims to
                                                                                                           development. Thus, inspite of the
improve the quality of service via the             The advent of digital technologies has                  diversity of operational methods, it is
continuity of supply on the network,               had a considerable impact on the                        logical that we see development of
has its main application in the                    solutions employed in MV telecontrol.                   prioritizing MV control functions. With
permutation of various electrical energy           In particular, the ability to use compact               this aim, a MV control system defines:
supplies. This application is based on             and reasonably priced digital protection                s functions to be performed,
automatic control.                                 and control units enables, with                         s their prioritized level,
The double shunt type topology, as                 centralized operation control, the use                  s their geographical location.
operated by the EDF on some of its                 today of local intelligent systems. This                It can be studied in terms of four levels
networks, is an example of this.                   development offers the following                        (see fig. 25):
                                                   advantages:                                             s level 0: MV devices and sensors,
                                                   s it reduces the disadvantages of
                                                                                                           s level 1: protection and control of an
                                                   intelligent systems concentrated in one                 MV cubicle,
                                                   single point. Indeed, a fault at this point             s level 2: local control of a substation
                                                   would be catastrophic for the whole                     or installation,
                                                   electrical network operation.                           s level 3: remote control of a MV
                                                   s it offers the advantage of a better
                                                   maintainability and an increased                        (see EDF application in appendix 3).
                                                   operational flexibility.

                                                                                       trol of
                                                                           remot al network


                                                                                                                           n and
                                                                                                                 pro tectio icle
                                                                                                                   of MV

                                                                                                                                                 es and se
                                                                                                                                     MV    devic

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

                                                                                                          Cahier Technique Merlin Gerin n° 155 / p. 23
Together this constitutes a MV control         As for data transmission, it is of series   Transmission protocols
architecture, whose operation relies on        type. This means that pieces of binary      Protocol is the language used for
numerous exchanges of information              (0/1) coded information are sent one        information exchange between the
between the various priority levels. This      after the other using the same              various protection and control units
information is essentially:                    equipment. Above all the advantages of      within an architecture. It defines the
s remote data transmission,                    this type of transmission are simple        structure of the exchanged messages,
s telemetering,                                cabling and good immunity to external       both in terms of requests for
s remote control.                              disturbances.                               information and for response
The exchange of such information can           Transmission methods                        messages. These protocols can be
be continuous or as events occur               Information transmission requires the       unique to one equipment manufacturer
(network incident, switching                   use of one or several pieces of             (or to several manufacturers) or
commands,...); it requires high-               equipment.                                  standardized, conforming to various
performance communications                                                                 standards. As far as public distribution
                                               In the case of control of electrical        is concerned, within the concept of
networks.                                      networks, the method used is:               architecture that has been presented
                                               s paired wires, coaxial cable,
                                                                                           above, which is becoming generalized,
communications networks                        (specialized telephone links or national    distributors are trying to standardize
                                               telephone networks)                         protocols between levels 2 and 3.
All of these exchanges are grouped             s radio waves (radio links),
together within the remote data                                                            However, internal switchgear
                                               s power cable (power line carrier).
transmission function, as defined by the                                                   transmission protocols remain the
following parameters:                          Optic fibre is still in the experimental    choice of the manufacturer.
s the organization,                            stage since, inspite of its great           Communicating in the chosen protocol
s the transmission equipment involved,         advantage of being insensitive to           for an architecture is a condition which
s the communication protocol.                  electrical disturbances, its installation   a piece of equipment must satisfy if it is
                                               cost remains a considerable hurdle to       to be integrated within that architecture.
All these parameters are such as to            be overcome. As for power line carrier
ensure that any message transmitted is
                                               systems, to date they have only been
correctly received (free of errors).           used for network load management, for
Remote data transmission                       example sending of signals to change
organization                                   the applied price-rate by the EDF and
The simplest solution is to                    sending of load shedding commands in
communicate between two                        the United States. However, they are in
transmitters-receivers. However, with          the experimental stage for other
links between only two points, the limit       applications, for example remote
of this system’s application possibilities     electricity-meter reading or
is very quickly reached. When several          reconfiguration of a network following a
units are involved in telecontrol, a           fault.
single point-to-point link becomes             In fact, at the moment, no one type of
insufficient, and this is where the idea       method is predominantly used; the
of multi-point comes in. In this case,         choice depending on various criteria:
two organization types are possible:           s amount of information to be
s master-master                                transmitted,
All the units within this organization can     s frequency of exchange (number and
take the initiative to communicate.            periodicity),
s master-slave                                 s speed required for exchanges,
The control unit with the highest priority     s type of information,
level in the architecture is generally the     s transmission distance,
master. It is responsible for the              s geographical constraints (e.g.
management of all transmissions, to            mountain region),
which end it interrogates all slave units      s cost of exchanged information.
in turn on a continuous basis or               In practice, an electrical energy
following an incident. The slave units         distributor will always use various
respond to the interrogation and               methods:
execute orders given by the master             s specialized lines (paired wires) for
unit.                                          control of important installations (HV/
In terms of control for electrical             MV substation, MV/MV substation),
networks, the most frequently used and         s radio-electrical or telephone links for
the safest organization is that of             control of secondary installations
master-slave type.                             (remote controlled MV/LV substation
                                               and MV pole-mounted switch).

Cahier Technique Merlin Gerin n° 155 / p. 24
6. conclusion

The noticeable feature of current             layouts do not facilitate rational
installations, remains their country-         analysis.
dependant diversity:                          Distributors are well aware of this
s diversity in electrical layouts and their   obstacle. Thus, their new long-term
protection system,                            objectives involve moving towards
s diversity in basic technical choices,       simplification and rationalization of
s diversity in operating methods.             network layouts, which requires long-
However, two major long term                  term and costly investment. Without
developments are envisaged by all             waiting for this future stage, they are
distributors: development towards a MV        continually trying solutions that are
system and development towards                adapted to their current MV network
automatic control of MV networks.             layouts.
                                              In the same way, manufacturers use
Development towards a MV system               the most recent technologies to benefit
As has been seen in the above pages a         distributors.
MV electrical distribution network, is
formed by the overlapping of two              Of course Man must always remain
networks:                                     master of such Systems. And whilst
s the energy network, whose objective
                                              data processing, born from computing,
is to carry energy to user units. It is       already offers distributors better
defined by the single line layout, and is     knowledge and understanding of their
made up of electrotechnical equipment,        electrical networks, future years will
transformers, cables,...                      bring innovative solutions which will
s the information network, whose
                                              contribute to achieving the main
objective is to process data to obtain        objective: satisfying the requirements of
maximum safety and overall availability       electrical energy consumers with an
within this energy network. Defined by        optimum quality of service.
protection and control systems, this
network is made up of protection and
control units, which are interlinked via a
high-performance communications
network and are distributed:
s on the MV equipment level,
s on the substation or installation level,
s on the electrical network level itself.
Thus, from electrical network design to
operation, "Medium Voltage Man"
becomes "System Man".
Development towards automatic
control of MV networks
Following the agricultural and industrial
revolutions, that of communications has
irreversibly created new needs and new
solutions. The next stage will be the
use of expert systems to automatically
analyse and operate networks. This
development is already undergoing
trials by certain distributors, for
example TEPCO in Japan.
However, an obstacle to future
development is the state of existing
networks. Indeed, the latter were not
designed with a view to automatic
operation: their complex, inconsistant

                                                                                          Cahier Technique Merlin Gerin n° 155 / p. 25
appendix 1: some MV product standards

The diversity of equipment mentioned           s with respect to Ring Main Unit           s with respect to electromagnetic
in this "Cahier Technique" makes it            IEC 129,265,298, and 420,                  compatibility:
impossible to give all of their respective     UTE C 64-130, C 64-131,                    s in terms of susceptibility to
standards.                                     and C 64-400,                              disturbances,
As an example here are a few                   VDE 0670,                                  IEC 801 - chapters 1 to 4,
standards:                                     BS 5227,                                   NF C 46-020 to 023,
                                               s with respect to protection devices       s in terms of disturbance emission
s with respect to MV switchgear
                                               IEC 68,                                    EN55 022,
IEC 56 and 694,                                                                           NF C 91-022.
IEC 470 for contactors                         IEC 225,
UTE C 64-100 and C 64-101,                     IEC 655,
VDE 0670,                                      NF C 20-455,
BS 5311,                                       NF C 63-850.
ANSI C37-06 for circuit breakers.

appendix 2: various selectivity techniques

general                                        the various techniques                     Logical selectivity
When a fault occurs on an electrical           To give this selectivity in a MV           This selectivity is obtained by means of
network, it may be detected                    protection system five main techniques     a "logical stand-by" command emitted
simultaneously by several protection           can be used: amperometric,                 by the first protection device situated
devices situated in different areas.           chronometric, differential, distance       just upstream of the fault which should
                                               related and logical.                       break the circuit, transmitted to other
The selectivity of the protection system
                                                                                          protection devices situated further
gives priority of operation to the closest     Amperometric selectivity                   upstream. It enables the number of
device situated upstream of the fault.         This is obtained by adjusting trip         selectivity steps to be increased without
Thus, interruption of supply is limited to     threshold current values.                  increasing the upstream trip time.
as small a part of the network as
                                               Chronometric selectivity                   Pilot wires are required between
                                               This is obtained by adjusting trip         protection units. This technique,
However, a protection system also              threshold timing values.                   developed by Merlin Gerin, is detailed
allows for contingencies. Thus, when                                                      in "Cahier Technique" n° 2.
the system is designed, provision is           Differential selectivity
made for the incorrect functioning of a        This is obtained by dividing the network
protection device, in which case a             into independant zones, with detection
different device, situated upstream of         for each of the zones of any difference
the latter, should function to limit the       in the sum of currents entering the zone
effects of the fault.                          and the sum of currents leaving the
                                               zone. This technique requires wiring
Each of these protection devices
                                               between protection devices situated at
installed in series on the network
                                               the various extremities of the monitored
represents a selectivity step.
In a MV network, the number of
selectivity steps between HV/MV and            Distance related selectivity
MV/LV transformers is generally limited        This is obtained by dividing the network
to between 3 and 5 depending on the            into zones, with the protection devices
country. Indeed, above this safety can         locating in which zone the fault has
no longer be guaranteed since the              occured by calculating downstream
reaction time and the fault currents           impedances.
become very dangerous.

Cahier Technique Merlin Gerin n° 155 / p. 26
  appendix 3: EDF architecture and Merlin Gerin equipment

, ,  , , 
  ,,   , 
 ,,,  ,
       , ,
 ,,,,, , ,
,  , , , ,
, , , ,  ,

                                                   Grouping and                      PRR : grouping and reference point
                                                   reference point

                                                                     control interfaces :
                                                                                     internal          internal                  external


      TCL11MS             Isis 2000 :
      substation          mimic
      computer            diagram
                          status logger

      Vigi PA
  1   protection and
      control-command unit
  0                                                                      SM6 panelboard :
                                                                         miniature switch
                                                                                                       RM6 panelboard :
                                                                                                       Ring Main Unit

                                                                         cubicle                       switchgear

                                          F24G draw-out                      overhead and/ or underground network
      HV/ MV source substation            panelboard                         MV/ LV remote-controlled substations
                                                                                                                                M2S :
                                                                                                                                overhead switch

       level 3: network remote control
                                                                                    Communications networks:
       level 2: local control of an installation
                                                                                    remote transmission via specialised telephone lines
       level 1: protection and control-command of a MV cubicle                      remote transmissions by radio and/ or telephone networks
       level 0: MV devices and sensors                                              internal switchgear network (currently line to line)

                                                                                                      Cahier Technique Merlin Gerin n° 155 / p. 27
appendix 4: references

Merlin Gerin "Cahiers Techniques"
s Network protection using the Logical
Selectivity System
"Cahier Technique" n°2 (F. Sautriau)
s EMC: Electromagnetic Compatibility
"Cahier Technique" n°149 (F. Vaillant)
Various publications
Numerous documents cover certain
themes touched on in this "Cahier
Technique". However, whilst less of
them cover all of the themes, we have
considered it of more use to indicate to
the reader organizations that distribute
technical reports written for various
s "journées d’etude technique S.E.E."
Société des Electriciens et des
Electroniciens (= Society of Electricians
and Electronics Specialists)
s quality and cost saving for electrical
s Medium Voltage rural networks:
developments and future prospects.
Société des Electriciens et des
48, Rue de la Procession
75724 PARIS Cedex 15
s C.I.R.E.D.
Congrès International des Réseaux
Electriques de Distribution
(= International Congress on Electrical
Distribution Networks).
Institution of Electrical Engineers,
Savoy Place,
London WC2R OBL
United Kingdom.
Union Internationale des Producteurs et
Distributeurs d’Electricité.
(= International Union of Electricity
Producers and Distributors).
39 avenue Friedland
75008 PARIS

                                               Real. ERI - Lyon - photo. : IPV
Cahier Technique Merlin Gerin n° 155 / p. 28   IPV - 03-92 - 3500 - Imprimeur : Léostic

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