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.

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

Fig. B1 – Installation including a single MV/LV power transformer with metering at LV level

Fig. B2 – Installation including a single MV/LV power transformer with metering at MV level

Fig. B3 – Installation including several MV/LV transformers, all located in the main substation

Fig. B4 – Installation including several secondary substations supplied by an internal MV distribution

The functional and safety requirements defined above are detailed in this chapter, in the following sub-clauses:

• Voltages and currents according to IEC Standards, different types of MV power supply, practical issues concerning MV distribution networks
• Procedure for the establishment of a new substation
• Protection against electrical hazards, faults and mis-operations
• Consumer substation with LV metering
• Consumer substation with MV metering
• Choice and use of MV equipment and MV/LV transformer
• Substation including generators and parallel operation of transformers
• Types and constitution of MV/LV distribution substations.

The methodology of selection of an architecture for a MV/LV electrical installation is detailed in chapter MV and LV architecture selection guide for buildings.