Engineering, 2013, 5, 20-25
doi:10.4236/eng.2013.51b004 Published Online January 2013 (http://www.SciRP.org/journal/eng)
Design of Mv/Lv Substation Transformer
Sabir Messal ti1, Fares Z itou ni2, Issam Griche2
1University of M’sila, Faculty of Technology, M’sila , Algeria
2Universi ty of Setif, F aculty of Technology, Setif, Algeria
Received 2013
ABSTRACT
A substation is a part of an electrical generation, transmission, and distribution system. Substations generally have
switching, pr otec tion and co ntro l eq uipment, and transformers. T he lo w voltage distr ibutio n net works are s upp lied fro m
MV/LV substation transformer that represent the last step of bringing transformation to the low voltage and ensuring
the protection and monitoring of the network. There are three types of MV/LV substation: pole-mounted transformer
substation, the substation in an envelope and substation in masonry. This paper presents a general description for dif-
ferent MV/LV substation and a methodology to design the various elements making up the public distribution substa-
tion (choice of the emplacement, engineering, power transformer, choice of MV cells, circuit breakers, fuses, disconnect
switch, conductors,...etc).
Keywords: Substa tion Transformer; MV Cells ; Substation in Masonr y, Pole-Mounted Transformer Substation
1. Introduction
A substation is a part of an electrical generation, trans-
mission, and distribution system. Substations transform
voltage from high to low, or the reverse, or perform any
of several other important functions. Electric power may
flow through several substations between generating
plant and consumer, and its voltage may change in sev-
eral steps. Substations generally have switching, protec-
tion and control equipment, and transformers. Distribu-
tion circuits are fed from a transformer located in an
electrical substation, where the voltage is reduced from
the high values used for po we r transmi s sion [1].
Distribution networ ks are typically of two types, radial
or interconnected. A radial network leaves the station and
passes through the network ar ea with no nor mal connec-
tion to any other supply. This is typical of l ong rural lines
with isolated load areas. An interconnected network is
generally found in more urban areas and will have mul-
tiple connections to other points of supply. These points
of connection are normally open but allow various con-
figurations by the operating utility by closing and open-
ing switches. Operation of these switches may be by re-
mote control from a control center or by a lineman. The
benefit of the interconnected model is that in the event of
a fault or required maintenance a small area of network
can be isolated and the remainder kept on supply.
The MV/LV substations are a node of a network,
which includes a set of equipment designed to protect
and facilitate the operation of the electrical energy [1-3].
The MV/LV substations provide the interface between
the distribution MV and LV. The MV / LV adapts to all
modes of operation and why should fulfill the following
functions:
● Distribute the power and protect the LV departures;
● Isolate the MV/LV substation in case of default;
● Manage the MV network in case of default and the
position by remote control.
MV/LV substation transformer has two purposes:
- The interconnection between the lines of the same
voltage level, it helps to distribute the power of the
different lines from substations.
- The transformation of electrical power, processors
can switch be tween voltage levels to anot her.
So, we can classify the MV/LV substations over the
networks on which they are used as well as the functions
they perform. The realization of MV/LV substations
needs prior knowledge:
● Standards of reference and statutory instruments
(voltage, quality of supply, short-circuit power, ...,etc);
● Needs to be performing (power plant availability,
operation);
● Specific needs-related uses (vo lta ge variations tolera te d,
compensation of reactive power, immunity to
interference receivers, regulations related to facility
security);
● Constraints, installation and environment [2,3].
This paper presents the different MV/LV substation
transformer and a methodology to design the various
elements making up the publi c distr ibution substat ion.

S. MESSALTI ET AL.
21
2. The Birth of MV/LV Substation
Transformer
The decision of a MV or LV supply will depend on local
circumstances and considerations such as those men-
tioned above, and will generally be imposed by the utility
for the district concerned. When a decision to supply
power at MV has been made, there are two widely-fol-
lowed methods of proceeding:
● The power-supplier constructs a standard substation
close to the consumer’s premises, but the MV/LV
transformer is located in transformer chamber inside
the premises, close to the load centre.
● The consumer constructs and equips his own substa-
tion on his own pre mises, to which the power supplie r
makes the MV connection [3].
2.1. Project Studies
The consumer must provide certain data to the utility at
the earliest stage of the project. (maximum anticipated
power demand (kVA), layout plans and elevations show-
ing location of proposed substation, degree of supply
continuity required from the information provided by the
consumer), the power-supplier must indic a te :
● The type of power supply proposed, and define the
kind o f power-supply syste m: over headline or under-
ground-cable network, the service connection details:
single-line service, ring-main installation, or parallel
feed ers, and the power (kVA) l imit and fault cur rent.
● The nominal voltage and rated voltage (Highest volt-
age fo r eq uipment)
● Metering details which define: The cost of connection
to the power network and tariff details (consumption
and st anding charges).
2.2. Implementation
Before any installation work is started, the official agree-
ment of the po wer -supplier must be obtained. The request
for approval must include the following information,
largely based on the preliminary exchanges noted above:
● Location of the proposed substation;
● Single-line diagram of power circuits and connections,
together with ear thing-circuit propo sals;
● Full details of electrical equipment to be installed,
including performance characteristics;
● Layout of equipment and provision for metering
components;
● Arr angements for power-fa ctor im provem ent if required;
● Arrangements provided for emergency standby power
plant (MV or LV) if eventually required.
2.3. Commissioning
After testing and checking of the installation by an inde-
pendent test authority, a certificate is granted which per-
mits the substation to be put into service. When required
by the authority, commissioning tests must be success-
fully completed before authority is given to energize the
installation from the power supply system. Even if no
test is required by the authority it is better to do the fol-
lowing veri fication tests:
● Measurement of earth-electro de resistances;
● Continuity of all equipotential earth-and safety bond-
ing conductors;
● Inspection and functional testing of all MV compo-
nents;
● Insulation checks of MV equipment;
● Dielectric strength test o f transformer oil;
● Inspection a nd te sting of the L V installation;
● Chec ks on al l inter loc ks (mec hanic al ke y and e lectr ic-
al) and on all auto matic sequences;
● Checks on correct protective-relay operation and set-
tings;
● Personnel of the power-supply authority will energize
the MV equipment and check for correct operation of
the metering;
● The installation contractor is responsible for testing
and connection of the LV installation. When finally
the substatio n is oper ational, the po wer-sup pl y aut hor-
ity has operational control over all MV switchgear in
the substation;
● The power-supply personnel have unrestricted access
to the MV equipment.
The consumer has independent control of the MV
switch of the transformers only, the consumer is respon-
sible for the maintenance of all substation equipment,
and must request the power-supply authority to isolate
and earth the switchgear to allow maintenance work to
proceed. The power supplier must issue a signed permit-
to-work to the consumers maintenance personnel, to-
gether with ke ys of lo cked -off isolators, e tc. at which the
isolation has been carried out.
A study guide of MV/LV distribution substation in-
cludes:
● Te chnica l supply;
● Layout plans and elevations showing location of pro-
posed substation;
● A single-li ne diagram;
● Diagrams and plans of electrical equipment;
● Pole and foundation plans, different cuts of the
substation, the views of faces);
● Pla n of earthing system and li ghting of the substatio n.
3. Classification of Mv/Lv Substation
Transformer
Substations transformer may be classified according to
metering arrangements (MV or LV) and type of supply
(overhead line or underground cable), Substations trans-
former are classified into three type s:

S. MESSALTI ET AL.
22
● Substation in masonry: Installed in a dedicated enclo-
sure prefabricated or not, with indoor equipment
(switchgear and transformer);
● Pole mounted ( S ≤ 160 KVA) with dedicated outdoor
equipment (switchgear a nd tr a nsformers);
● Prefabricated substations.
3.1. Pole Mounted Substations
These substations are mainly used to supply isolated ru-
ral consumers from MV overhead line distribution sys-
tems . In thi s type o f substa tion, most oft en, the MV tra ns-
former protection is provided by fuses. Lightning arres-
ters are provided, however, to protect the transformer and
consumers as shown in Figure 1. As previously noted
the locatio n of the s ubstation must allo w easy acce ss, not
only for personnel but for equipment handling (raising
the transformer for example) and the manœuvring of
heav y vehicles [3,5 ].
The kinds of outdoor substation are common in some
countries, based on weatherproof equipment exposed to
the elements. The pole mounted substation is the easiest,
it is used in cases where the power transformer is less
than or equal to 160 KVA. This substation allows of
economic power customers, increasing the injection sites
(low power stations) and by shortening the low voltage
lines. The position includes a transformer connected
directly with fuses. The transformer is protected against
overvoltage by spark horned (anti-bird) [5] and against
over current by a low voltage circuit breaker. The
transformer feeds the LV through an LV circuit breaker,
which provides insulation and protection of the network.
The transformer is hooked up to a pole or a main line
taken in parallel on the main line. LV breaker is enclosed
in a sealed package, itself hangs on the pole, and in
addition to its automatic fault on low voltage, it can be
operated manually by means of an order brought down
the pole by a linkage. Insulated cables provide the
connection between the transformer and low voltage
cab inet o n the o ne ha nd, and be twee n the cab ine t a nd t he
line feeding the low voltage distribution network.
Figure 1 . Pole-mounted transformer substation.
In a Pole-mounted transformer substation, the maximum
number of departures low voltage is fixed at two. Depar-
tures are equipped with thermal-magnetic circuit breakers
D165 for a transformer of 100 KVA and D265 for a 160
KVA transf ormer.
3.2. Prefabricated Substations
Prefabricated substations provide a particularly simple,
rapid and competitive choice. The MV/ LV prefabricated
are made using: MV cells, transformer …etc, grouped in
an envelope whose role is the protection of internal
equipment against external influences and to ensure a
degree of protection against the p ublic.
The prefabricated substations are manufactured, wired
and tested at the manufacturer, bringing the transformer,
the LV and MV switchgear, connections and auxiliary
equipment limit the study time and achievement. The
transformer and switchgear are assembled in a metal case,
the start is done by airline or underground, these substa-
tions are very compact and their implementation is very
fast, one of the best-known b e nefits:
● Simplified ci vil engineering (platform only);
● Immediate availability;
● Limited space and time.
● The prefabricated substation is transported by truck. It
is placed on a concrete slab. The assembly is to con-
nect the cables arrival and departure [6-7].
3.3. Masonry Substation
Operating a distribution network sometimes requires
switching points in addition to the HV/MV substations,
in order to limit the effect of a fault on the net work. Dis-
tribution substations in masonry may be classified into
three types:
● Public Distrib ution substations (DP);
● Distribution substations with LV metering (S ≤ 630
KVA) ;
● Distribution substations with MV metering called
specific substations (a transformer whose power S>
630 KVA or several transformers).
Figure 2 . MV/LV Prefabricated s ub s t ation.

S. MESSALTI ET AL.
23
Figure 3 . Masonry Substation.
3.3.1. Public Distribution Substation (DP)
Public distribution substation designated for public use
(domestic customers, buildings, subdivisions,) to convert
permanently the medium voltage (30or 10 kV) and low
voltage (380 or 220V).
The public distribution substation consists of three
part s :
● Equipment for MV c onnection ;
● The distrib utio n tra nsformer M V /L V ;
● Table of LV departures (TDP public switchboard) as
the connection points of the network downstream of
low voltage distribution [8].
3.3.2. Distribution Subst ations with LV Metering
This substatio n has almost the same features as the public
distribution substation DP, the only difference is that in
the post delivery LV metering are two access panels and
counti n g, fo r the Pub lic distribution sub station contai ns a
single access and a distribution panel public. A substa-
tion with LV metering includes a single MV/LV trans-
former generally not exceeding 630 kVA.
3.3.3. Distribution Subst ations with MV Metering
This substation for customers of a large consumption
(S>630KVA or distribution substation with multiple
transformers), in which case the counting will be done
through a counting MV, there are two types of MV me-
tering distrib ution substatio ns (for QM or DM1) :
● Substation transformer with DM1 cell breaker (I>
45A);
● Substation transformer with switch fuse combined
QM (I < 45A). The circuit breaker protection is man-
datory when the current MV side is greater than 45A
(IEC 60265).
The line diagram of Distribution substations with MV
metering is give n in Figure 5 [9,10]:
● The distribution sub stations count MV includ es:
● 02 incoming panels, each with a load-break switch.
These are part of the loop and are connected to a bus
(IM);
Figure 4 . Line diagram of public distribution su bstation.
Figure 5 . Distribut ion sub stations with MV metering.
● A cell for counting and measuring CM;
● A cell for circuit breaker disconnect double DM2,
● One or more cells of individual protection by QM or
DM1.
4. Design of Public Distrubution Substation
Public distribution substations with prefabricated cells
metal is done with cells each fulfilling a function. The
public distrib ution substatio n includes:
● 02 incoming panels,
● Fuse-s witc h combination or fused switch;
● Fault detector for MV underground;
● LV circuit brea ker (Table of public distrib ution);
● Power Trans f orme r 100,160, 250, 400.630 KVA ;
● Cell power transfor mer (closing metal);
● Safety equipment;
● Boxes of deflector;
● Boxes end external and internal;
● MV cable and lugs;- LV cable with lugs;
● Spare fuses;
Terms of equipment and booking a DP position are
shown in Figure 6.
4.1. MV Cells
Devices (circuit breakers, switches, busbar, ..., etc ) are

S. MESSALTI ET AL.
24
integrated in metal casings that facilitate the installation
and operation, safe people, these envelopes are called
cells, they can achieve MV part transformer stations.
The different cells Schneider Electric 36 or 24 KV in
the composition of MV / LV sub station trans former are:
● IM, IMC, IMB switch;
● PM fused switch;
● QM fuse-s witch combination;
● DM1-A, DM1-D, single-isolation SF6 type circuit
breaker;
● DM2 double-isolation SF6 type circuit br e a ker ;
● CM, CM2 voltage transfor mers;
● SM disconnector;
● GBC-A, GBC-B current and/or voltage measurement s ;
● GIM intermediate bus unit;
● GBM connection unit,…[10 ].
4.2. Fault Current Indicator
The fault current detector is designed for the detection of
permanent fault s on MV u nderground networ ks , covering
all types of MV systems [10].
4.3. Choice of MV/LV Transformer
The MV/LV power transformers are usually step-down,
they used to supply low voltage from a medium voltage
network. They are characterized by the rated power Pn
( 50, 100, 160, 250, 400, 630, 800, 1000, 1250
1600KVA,..) and rated primary and secondary voltages
(30/0.4 KV for overhead networks and 10/0.4 KV for
under ground M V ne t works) . I n order to select an optimal
power (kVA) rating for a transformer, the following fac-
tors must be taken into account list the po wer of installed
power-consuming equipment Pi, the power factor, the
utilization factor (ku) ,the simultaneity factor (ks), the
rated power Sn transformer is give n by:
1cos
ii
n
iu s
ni
ii
PK K
S
ηϕ
=
⋅⋅
=⋅
∑ (1)
4.4. Choice of LV Circuit Breaker
The main role of a circuit breaker is to protect the
electrical s ystem during fault s, overlo ad and short cir cuit;
it is designed to open a circuit automatically when the
current flowing through it exceeds a pred etermined value.
To effectively function, the trigger circuit breaker must
take into account the evolution of the receptors [11]. The
LV circuit breaker can be fixed, removable, CLPG (general
protection cell), removable LV circuit breaker, or TDP
(Public Distribution Table).
The choice of the LV circuit breaker will be from the
secondary rated current.
4.5. Public Distribution Table
The table for public distribution (TDP) is the lower
voltage distrib ution station public ; it is used at the top of
LV circuit to p r ovide the following main functions:
● Power supplied of LV consumer;
● The distribution by multiplying the number of
departures (4 or 8 departures).
● Opening, isolation of each departure.
The table for public distribution includes a switching
device which can be a general switch or a circuit breaker
[12]
4.6. Choice of Cables and Connections
The connection between the MV cells and transformer
(MV side) will be through a MV cable, able to pass the
input current of the network, the connection of the
transformer (LV terminals) with LV circuit breaker will
be through a LV cable. LV cable length is calculated by
measuring the distance between the transformer and LV
circuit breaker. The number of cables per phase is
determined from the current supplied by the transformer
and cable characteristics. The optimal choice of the cable
must meet several criteria (rated current, operating
temperature, fashion poses, voltage drop, and short
circuit.
4.7. Choice of Fuses
Fuses are a very widely used means of protecting distri-
bution transformers, mainly due to their simplicity and
the correspondingly reduced cost of the equipment. The
rules regarding the selection of fuses, given by the man-
ufacturers and dependent upon the characteristics of each
fuse type, cover the following criteria: the transfor-
mer’s operating voltage, the switching currents, and
technology of fuses [10].
4.8. Materials for Operation and Safety
Safety equipment for a MV/LV substation transfor mer
consists essentially of:
● Insul at i ng sto o l.
● Insul at i ng mat.
● Insul at i ng glo ve s.
● Voltage detector.
● Earthing attac hments;
● Safety alarms and plaque and fire-extinguishing [3,8].
5. Conclusion
In this paper, we presented a synthesis of the various
MV/ LV substation transformer, we have explained
below the various documents constituting technical stud y,
a detailed study of the public distribution substations
(line diagram, the plan equipment, the choice of MV

S. MESSALTI ET AL.
25
cells and MV / LV transformer, choosing the LV circuit
breaker, the cables and the necessar y safety equipment).
Operating a distribution network requires switching
points in addition to the MV/LV substations, in order to
limit the effect of a fault on the network. To build a
MV/LV substation transformer, it is necessary to size a
number of parameters to achieve the appropriate wiring
diagram. The development of these components must
meet certain conditions dictated by the dual concern for
the safety of operation and lower operating costs. These
construction conditions can be classified into conditions
related to tension, conditions relating to the thermal
effects of the current, and conditions relating to the
safety of operating personnel.
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