Instructions for use of MV equipment

The purpose of this chapter is to provide general guidelines on how to avoid or greatly reduce MV equipment degradation on sites exposed to humidity and pollution.

Normal service conditions for indoor MV equipment

All MV equipment are intented to be used in the normal services conditions as defined in IEC 62271-1 standard "Common specifications for high-voltage switchgear and controlgear".

For instance, regarding humidity, the standard mentions:

• The average value of the relative humidity, measured over a period of 24 h does not exceed 95 %;
• The average value of the water vapour pressure, over a period of 24 h does not exceed 2.2 kPa;
• The average value of the relative humidity, over a period of one month does not exceed 90 %;
• The average value of water vapour pressure, over a period of one month does not exceed 1.8 kPa.

As indicated in the standard, condensation may occasionally occur even under normal conditions. Either switchgear designed for such conditions shall be used and/or special measures concerning the substation premises can be implemented to prevent condensation, such as suitable ventilation and heating of the station.

Use under severe conditions

Under certain severe conditions concerning humidity and pollution, largely beyond the normal conditions of use mentioned above, electrical equipment can be subject to damage by rapid corrosion of metal parts and surface degradation of insulating parts. Examples of suitable measures of protection against condensation and pollution are listed below.

Remedial measures for condensation problems

• Carefully design or adapt substation ventilation.
• Avoid temperature variations.
• Eliminate sources of humidity in the substation environment.
• Install an Heating, Ventilation, Air Conditioning unit (HVAC)
• Make sure cabling is in accordance with applicable rules.

Remedial measures for pollution problems

• Equip substation ventilation openings with chevron-type baffles to reduce entry of dust and pollution especially when the transformer is installed in the same room with switchgears or controlgears.
• Install the transformer in a different room to use more efficient ventilation grids if any,
• Keep substation ventilation to the minimum required for evacuation of transformer heat to reduce entry of pollution and dust
• Use MV cubicles with a sufficiently high degree of protection (IP)
• Use air conditioning systems or air forced cooling with filters installed in air inlet to restrict entry of pollution and dust.
• Regularly clean all traces of pollution from metal and insulating parts.
• Instead of using AIS equipment Fig. B36, use equipment that is insensitive to the environment such as GIS or 2SIS type (see Fig. B37).

Fig. B36 – SM6 Modular Unit

Fig. B37 – PREMSET. Shielded Solid Insulated MV equipment