Ventilation in MV Substations

From Electrical Installation Guide

Substation ventilation is generally required to dissipate the heat produced by transformers and other equipment, and to allow drying after particularly wet or humid periods.

However, a number of studies have shown that excessive ventilation can drastically increase condensation.

The following paragraphs highlight a number of recommendations and good practices to ensure proper ventilation of MV substations. More details to design a natural ventilation of a transformer can be found within the MV Technical Guide §Ventilation.

Remark concerning HV/LV outdoor prefabricated substation in special service conditions

  • Any installation of any transformer in a same room or in a same enclosure with HV and LV switchgears will impact the lifespan of the products
  • Any air flow generated by the transformer heating reduces the impact of irradiance. This air flow is the natural convection as required by the IEC 62271-202 standard.
  • Any separation of the transformer by a partition wall with the HV and LV switchgears compartment improve the service condition of the switchgears for moderate climates, and avoids exposing them to harsh environment, as an example wind farms near coastal areas.
  • For outdoor installations, any switchgear should be preferably installed in a thermal insulated enclosure protecting it from outdoor service conditions (dust, humidity, solar radiation etc.) especially for very hot and cold climates, and harsh environment.

Recommendations for HV/LV substation ventilation

General considerations

Ventilation should be kept to the minimum level required.

Furthermore, ventilation should never generate sudden temperature variations that can cause the dew point to be reached. For this reason, natural ventilation should be used whenever possible. Heating could be required when the application can be de-energized for a period; this is to maintain a minimum air flow. If forced ventilation is necessary, the fans should operate continuously to avoid temperature fluctuations.

When forced ventilation is not enough to assure the indoor service condition of the switchgear or when the installation surrounding is a hazardous area, HVAC unit will be necessary to separate completely the indoor service conditions to the outdoor service conditions.

Natural ventilation is the mostly used method for MV installations (see Fig. B42 and Fig. B43). A guideline for sizing the air entry and exit openings of HV/LV substations is proposed in the "MV Technical Guide" by Schneider Electric.

Fig. B42 – Two different examples of HV/LV substation designs with natural ventilation, according to the layouts described in Fig. B48
Fig. B43 – Example of HV/LV prefabricated substation tested with 1250 kVA liquid filled transformer

Ventilation opening locations

To favour evacuation of the heat produced by the transformer via natural convection, ventilation openings should be located at the top and bottom of the wall near the transformer. The heat dissipated by the MV switchboard could be neglected. To avoid condensation problems, the substation ventilation openings should be located as far as possible from the switchboards (see Fig. B44).

Type of ventilation openings

To reduce the entry of dust, pollution, mist, etc., the substation ventilation openings should be equipped with chevron-blade baffles when the transformer is installed in a same room with the switchboards, otherwise a use of higher efficiency ventilation grids is allowed, especially advised when total losses are above 15kW.

Temperature variations inside cubicles

To reduce temperature variations, always install anti-condensation heaters inside MV cubicles if the average relative humidity can remain high over a long period of time. The heaters must operate continuously, 24 hours a day all year long. Never connect them to a temperature control or regulation system as this could lead to temperature variations and condensation as well as a shorter service life for the heating elements. Make sure the heaters offer an adequate service life.

Temperature variations inside the substation

The following measures can be taken to reduce temperature variations inside the substation:

  • Improve the thermal insulation of the substation to reduce the effects of outdoor temperature variations on the temperature inside the substation
  • Avoid substation heating if possible. If heating is required, make sure the regulation system and/or thermostat are sufficiently accurate and designed to avoid excessive temperature swings (e.g. no greater than 1 °C). If a sufficiently accurate temperature regulation system is not available, leave the heating on continuously, 24 hours a day all year long
  • Eliminate cold air drafts from cable trenches under cubicles or from openings in the substation (under doors, roof joints, etc.).

Substation environment and humidity

Various factors outside the substation can affect the humidity inside.

  • Plants: avoid excessive plant growth around the substation, and closing any opening.
  • Substation waterproofing: the substation roof must not leak. Avoid flat roofs for which waterproofing is difficult to implement and maintain.
  • Humidity from cable trenches: make sure cable trenches are dry under all conditions. A partial solution is to add sand to the bottom of the cable trench.

Pollution protection and cleaning

Excessive pollution favours leakage current, tracking and flashover on insulators. To prevent MV equipment degradation by pollution, it is possible to either protect the equipment against pollution or regularly clean the resulting contamination.


Indoor MV switchgear can be protected by enclosures providing a sufficiently high degree of protection (IP).


If not fully protected, MV equipment must be cleaned regularly to prevent degradation by contamination from pollution.

Cleaning is a critical process. The use of unsuitable products can irreversibly damage the equipment.