Chapter A General rules of electrical installation design

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D - MV & LV architecture selection guide
6 Choice of architecture
fundamentals
6.4 Number of MV/LV transformers
Main characteristics to consider to determine the number of transformers:
b Surface of building or site
b Total power of the installed loads
b Sensitivity of circuits to power interruptions
b Sensitivity of circuits to disturbances
b Installation scalability
The basic preferred configuration comprises a single transformer supplying the total
power of the installed loads. Certain factors contribute to increasing the number of
transformers (> 1), preferably of equal power:
b A high total installed power (> 1250kVA): practical limit of unit power
(standardization, ease of replacement, space requirement, etc),
b A large surface area (> 5000m²): the setting up of several transformers as close as
possible to the distributed loads allows the length of LV trunking to be reduced
b A need for partial redundancy (down-graded operation possible in the case of a
transformer failure) or total redundancy (normal operation ensured in the case a
transformer failure)
b Separating of sensitive and disturbing loads (e.g.: IT, motors)
6.5 MV back-up generator
Main characteristics to consider for the implementation of an MV back-up generator:
b Site activity
b Total power of the installed loads
b Sensitivity of circuits to power interruptions
b Availability of the public distribution network
The preferred basic configuration does not include an MV generator. Certain factors
contribute to installing an MV generator:
b Site activity: process with co-generation, optimizing the energy bill,
b Low availability of the public distribution network.
Installation of a back-up generator can also be carried out at LV level.
Schneider Electric - Electrical installation guide 2008