Choice of MV equipment

The electrical equipment must withstand both electrical and environmental constraints to which it will be submitted during its life time without any mechanical and dielectric degradation reducing its level of performance.

Standards and specifications

Depending on the devices, components and products included in the MV switchgear, different standards have to be considered for compliance, such as:

• IEC 62271-1, 62271-100, 62271-102, 62271-103, 62271-105, 62271-200.

Local regulations may also require compliance with national standards:

• ANSI/IEEE for USA
• EN for European Union
• GOST for Russia
• GB/DL for China.

Types of MV equipment

Substations shall be designed and built according to local standards and practices.

The following types of equipment may be used:

• Compartmented modular units supporting all types of single line diagram and layout
• Compact solution based on ring-main unit solution when the supply is provided by a ring.

A ring main unit includes two load break switches for the connection of the substation to the ring and a transformer protection unit. Some compact RMU designs are particularly suitable when harsh environmental conditions apply.

Modular metal-enclosed switchgear

Fig. B35

The IEC 62271-200 standard specifies requirements for "AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV".

Different categories of prefabricated metal enclosed switchgear are defined with respect to the consequences on network service continuity in case of maintenance on the switchgear.

For classification in categories, various aspects have to be taken into account:

• Definition of functional unit: "a switchgear component contained in a metallic enclosure and incorporating all the main and auxiliary circuit equipment required to perform a single function" - usually a modular unit
• Definition of compartment: "a switchgear component contained in a closed metallic enclosure. The manufacturer defines the content (e.g. busbar, cable connections, etc.)
• Accessibility to individual compartments (see Functional interlocks):
  • Controlled by interlocking
  • In accordance with procedures; for compartments which can be opened during normal operation
  • Using tools; for compartments which should not be opened during normal operation
  • Not accessible for compartments which must not be opened
• Loss of Service Continuity (LSC) (see Service continuity) defining the extent to which other compartments can remain energised when one compartment is open. Four LSC categories are defined:
  • LSC1, LSC2, LSC2 A, LSC2 B
• Definition of partition: "a switchgear component contained in a metallic enclosure and separating one compartment from another". There are two types of partitions :
  • PM: metallic partitions
  • PI: insulating partitions.

Metal-enclosed switchgear can be based on all modern switchgear technologies, such as:

• AIS (Air Insulated Switchgear)
• SIS (Solid Insulated Switchgear)
• GIS (Gas Insulated Switchgear)
• 2SIS (Shielded Solid Insulated Switchgear).

Fig. B35 – SF6 modular unit

Choice of MV switchgear panel for a transformer circuit

Three types of MV switchgear panel can be used:

• Load-break switch associated to MV fuses without coordination between the fuses and the breaking capability of the load break switch
• Load-break switch/MV fuses combination with coordination between the fuses and the breaking capability of the load break switch
• Circuit breaker

As explained in MV/LV transformer protection with circuit breaker, a circuit breaker with a dedicated protection relay ensures a better protection of the transformer than the MV fuses coordinated or not with a load break switch.

Note: The fuses used in the load-break switch / fuses combination have striker-pins which ensure tripping of the 3-pole switch as soon as at least one fuse blows.