Electrical Installation Guide

Power Supply at Medium Voltage 

The term "medium voltage" is commonly used for distribution systems with voltages above 1 kV and generally applied up to and including 52 kV. For technical and economic reasons, the service voltage of medium voltage distribution networks rarely exceeds 35 kV. 

Networks which operate at 1000 V or less are referred to as low voltage (LV) networks. 

(Photo Credit Schneider Electric)

The connection of an electrical installation to a MV utility distribution network is always realized by means of a dedicated MV substation usually designed "Main substation". Depending on its size and specific criteria mainly related to the loads (Rated voltage, number, power, location, etc…), the installation may include additional substations designed "Secondary substations". The locations of these substations are carefully selected in order to optimize the budget dedicated to MV and LV power cables. They are supplied from the main substation through the internal MV distribution. 

Generally, most of the loads are supplied in low voltage by means of MV/LV step down transformers. Large loads such as asynchronous motors above around 1MW are supplied in MV. 

MV/LV step down power transformers are indifferently located either in the main substation or in the secondary substations. Small installations may only include a single MV/LV transformer installed in the main substation in most of the cases. 

A Main Substation Includes Five Basic Functions: 

Function 1: 

Connection to the MV utility network 

Function 2: 

General protection of the installation 

Function 3: 

Supply and protection of MV/LV power transformers located in the substation 

Function 4: 

Supply and protection of the internal MV distribution 

Function 5: 


For the installations including a single MV/LV power transformer the general protection and the protection of the transformer are merged. The metering can be performed either at MV level or at LV level. It is authorized at LV level for any installation including a single MV/LV transformer, provided that the rated power of the transformer remains below the limit fixed by the local utility supplying the installation. 

In addition to the functional requirements the construction of both main and secondary substations shall comply with the local standards and rules dedicated to the protection of persons. IEC recommendations should also be taken into consideration in all circumstances

According to the IEC there is no clear boundary between medium and high voltage. Local and historical factors play a part, and limits are usually between 30 and 100 kV (see IEV 601-01-28). The publication IEC 62271-1 "Highvoltage switchgear and controlgear; common specifications" incorporates a note in its scope: "For the use of this standard, high voltage (see IEV 601-01-27) is the rated voltage above 1000 V. However, the term medium voltage (see IEV 601-01- 28) is commonly used for distribution systems with voltages above 1 kV and generally applied up to and including 52 kV.".

Main Requirements for Power Supply at Medium Voltage and Typical Architectures

The characteristics of electrical equipment (switchgears, transformers, etc…) installed in the substations are fixed by the rated values of both voltage and current specified for the distribution network supplying the installation: 

  • Ur, rated voltage, rms value, kV 
  • Ud, rated power frequency withstand voltage, rms value, kV during 1mn 
  • Up: rated lightning impulse withstand voltage, peak value, kV 
  • Un, service voltage, rms value, kV 

As the rated voltage Ur indicates the maximum value of the "highest system voltage" of networks for which the equipment may be used, the service voltage Un really existing in the network, including its possible variations shall remain below the rated voltage. 

  • Rated normal current Ir, rms value, A 
  • Rated short-time withstand current Ik, rms value, kA 
  • Rated peak withstand current Ip, peak value, kA. 

Considering the previous requirements and basic usages, four typical architectures can be defined for an electrical installation connected to a MV utility distribution network: 

Fig. B1: Single MV/LV power transformer with metering at LV level 

Fig. B2: Single MV/LV power transformer with metering at MV level

Fig. B3: Several MV/LV transformers, all located in the main substation 

Fig. B4: Several secondary substations supplied by an internal MV distribution. 

Most of MV/LV transformers are located in secondary substations. Some of them when required are installed in the main substation

This article was extracted from " Schneider Electric - Electrical installation guide 2016" the full guide can be downloaded from their website at the below link. 

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