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.

In this chapter, networks which operate at 1000 V or less are referred to as low voltage (LV) networks.

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. Only LV loads are considered in this electrical guide.

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: Metering.

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.