Chapter A General rules of electrical installation design

B22
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B - Connection to the MV public
distribution network
(1) Polychlorinated biphenyl
4 The consumer substation
with LV metering
4.1 General
A consumer substation with LV metering is an electrical installation connected to a
utility supply system at a nominal voltage of 1 kV - 35 kV, and includes a single
MV/LV transformer generally not exceeding 1,250 kVA.
Functions
The substation
All component parts of the substation are located in one room, either in an existing
building, or in the form of a prefabricated housing exterior to the building.
Connection to the MV network
Connection at MV can be:
b Either by a single service cable or overhead line, or
b Via two mechanically interlocked load-break switches with two service cables from
duplicate supply feeders, or
b Via two load-break switches of a ring-main unit
The transformer
Since the use of PCB (1) -filled transformers is prohibited in most countries,
the preferred available technologies are:
b Oil-immersed transformers for substations located outside premises
b Dry-type, vacuum-cast-resin transformers for locations inside premises, e.g.
multistoreyed buildings, buildings receiving the public, and so on...
Metering
Metering at low voltage allows the use of small metering transformers at modest cost.
Most tariff structures take account of MV/LV transformer losses.
LV installation circuits
A low-voltage circuit-breaker, suitable for isolation duty and locking off facilities, to:
b Supply a distribution board
b Protect the transformer against overloading and the downstream circuits against
short-circuit faults.
One-line diagrams
The diagrams on the following page (see Fig. B21) represent the different methods
of MV service connection, which may be one of four types:
b Single-line service
b Single-line service (equipped for extension to form a ring main)
b Duplicate supply service
b Ring main service
4.2 Choice of MV switchgear
Standards and specifications
The switchgear and equipment described below are rated for 1 kV - 24 kV systems
and comply with the following international standards:
IEC 62271-1, 62271-200, 60265-1, 62271-102, 62271-100, 62271-105
Local regulations can also require compliance with national standards as:
b France: UTE
b United Kingdom: BS
b Germany: VDE
b United States of America: ANSI
Type of equipment
In addition of Ring Main Units discussed in section 1.2, all kinds of switchgear
arrangements are possible when using modular switchgear, and provisions for later
extensions are easily realized.
Schneider Electric - Electrical installation guide 2008