Ventilation in MV Substations

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.

Remark concerning HV/LV outdoor prefabricated substation

• 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 change generated by the transformer heating reduces the impact of irradiance. This air flow change is a natural convection
• 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.
• For outdoor installations, any switchgear should be preferably installed in a thermal insulated enclosure protecting it from outdoor service condition (dust, humidity, solar radiation etc.) especially for very hot and cold climates.

General requirements

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. 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, (see Fig. B43), being the most used for MV installations, a guideline for sizing the air entry and exit openings of HV/LV substations is presented hereafter.

Fig. B43: Natural ventilation

Calculation methods

The proposed method is suitable for transformers installed in prefabricated enclosures or in dedicated rooms inside buildings. Fig. B44 using the same ventilation grids for air inlet and air outlet.

A number of calculation methods are available to estimate the required size of substation ventilation openings, either for the design of new substations or the adaptation of existing substations for which condensation problems have occurred.

The basic method is based on transformer dissipation by natural convection. The required ventilation opening surface areas S and S’ can be estimated using the following formulas, knowing or not the air flow resistance coefficient of the ventilation grids ξ (see Fig. B44):

1- Heat dissipation: Qnac=P-Qcw-Qaf

2- Opening surface areas : S and S'

• 2.1 The air flow resistance coefficient of the ventilation grid ξ is unknown

[math]\displaystyle{ S=1.8 \cdot 10 ^{-1} \frac {Q_{nac}}{\sqrt H} }[/math]     if air flow resistance is unknown

S ’= 1.1 * S     S and S’ are efficient net area

• 2.2 Openings with chevrons blade

[math]\displaystyle{ S = \frac {Q_{nac}} {K\cdot {\sqrt {H \cdot (\theta 2 - \theta 1)^3 }}} }[/math]     with

[math]\displaystyle{ K=0.222 \sqrt \frac {1}{\xi} }[/math]    (see Fig. B45)

The formula 2.2 is near the formula 2.1 if Δθ = (θ2 - θ1) = 15 K, and if ξ = 5, then K= f (ξ) = 0.1. This is equivalent to free opening, without ventilation grid. When K = 0.1 the formula 2.2 is equivalent to the formula given in IEC 60076-16 standard for transformers dedicated to wind turbine applications.

Qnac is the dissipation by natural air circulation [kW].

P is the sum of the power dissipated [kW] by:

• The transformer dissipation: no load losses+Load losses
• The LV switchgear dissipation
• The MV switchgear dissipation.

Qcw is the heat dissipation by conduction through the walls and ceiling [kW] (assumption Qcw = 0 in the example). The losses by conduction through the walls, the ceiling and the slab can be expected from 200 W for a thermal insulated housing up to 4 kW for a 10 m² prefabricated substations using concrete material.

Qaf is the heat dissipation by forced air circulation [kW] (assumption Qaf = 0 in the example)

θ1 and θ2 are the respectively air temperatures of inlet and outlet [°C]

ξ is the resistance coefficient of the pressure losses depending on the design of the ventilation grid.(see Fig. B44)

S is the lower (air entry) ventilation opening area [m²] as expressed by formulas 2.1 and 2.2.

S’ is the upper (air exit) ventilation opening area [m²] as expressed by formulas 2.1 and 2.2.

H = the height between ventilation opening mid-points [m].

(θ2 - θ1) is the air temperature rise which reflects the double of the transformer overheating (Loading guide IEC 60076-7) for oil immersed transformer and (Loading guide IEC 60076-11) for dry type transformer.

Fig. B44: Coefficient of pressure losses defined by air flow tests

Fig. B45: Impact of the ventilation grids

The transformer overheating is an extra temperature rise due to the installation of the transformer in a prefabricated enclosure or in a dedicated room inside a building. (see Fig. B46, B47.1, B47.2)

It is the extra temperature rise measured at the top of the oil (see Fig. B47.1) for liquid filled transformers or the extra average temperature rise of the winding (see Fig. B47.2) for dry type transformer.

The normal service condition of the power transformers are the following:

• Ambient temperatures
  • 40 °C Maximum temperature at any time
  • 30 °C Monthly average temperature during the hottest month
  • 20 °C Yearly average temperature
• Maximum temperatures and temperatures rises for oil filled transformer
  • Maximum temperature measured on the top of the oil: 100 °C
  • Maximum temperature of the winding: 105 °C
  • Maximum oil and winding temperature rise: 30-35 K, 40-45 K, 50-55 K, 60-65 K
• Maximum temperatures and temperatures rises for dry type transformer
  • Maximum temperature of the winding: 155 °C corresponding to class Fof insulation
  • Maximum average temperature rise of the winding: 100 °C.

Example of transformer overheating (see Fig. B46):

Assuming Δθt1 = tt1 - ta1 = 60 K for oil temperature rise of a liquid filled transformer installed outside any enclosure it will become Δθt2 = tt2 - ta2 = 70 K when installed inside an enclosure generating an over heating Δθt2 - Δθt1 of 10 K.

The air temperature rise inside the enclosure will reach the double of the transformer overheating: 10 K x 2 = 20 K. When these transformer overheatings

When these transformer overheatings are assessed by a type test according to IEC 62271-202 (HV/LV prefabricated substations) this overheating is the rated enclosure class. The overheating, combined with the average temperature, gives the limit load factor to maintain the expected transformer lifespan according to the IEC transformer loading guides (see Fig. B47.1 and Fig. B47.2).

For masonry substation the overheating of the transformer is expected unknown, as the calculation shall define the ventilation areas S and S’. So only the ambient temperature and load factor can be known. The following examples explain how assess the overheating of transformer then the temperature rise of air (θ2 - θ1) to use the formula 2.2.

Process to use graphes (see Fig. B47):

a) select the average ambient temperature in a given period of time for the substation site on the vertical axis; b) select the load factor of the transformer c) the intersection gives an expected overheating of the transformer corresponding to the maximum top oil temperature rise limit for liquilled filled transformers (see Fig. B47.1) or the average winding temperature rise for dry type transformers (see Fig. B47.2).

Fig. B46: Transformer overheating

Fig. B47: Load factor limits

Examples:

• Moderate climate 10 °C as yearly average using a 60-65 K respectively for oil and winding temperature rise of the transformer, can be used at full load. Expected overheating is 10 K then air temperature rise (θ2 - θ1) is expected at 20 K.
• Hot Climate 30 °C as summer average using 50-55 K respectively for oil and winding temperature rise transformer can be used with a load factor at 0.9. Expected overheating is 10 K then air temperature rise (θ2 - θ1) is expected at 20 K.
• Cold Climate -20 °C as winter average using 60-55 K respectively for oil and winding temperature rise transformer can be used with a load factor at 1.2. Expected overheating is 20 K then air temperature rise (θ2 - θ1) is expected at 40 K.
• Hot Climate 30 °C as summer average using a dry type transformer at 155 °C insulation thermal class can be used with a load factor at 0.9. Expected overheating is 10 K then air temperature rise (θ2 - θ1) is expected at 20 K.

For prefabricated substation, the overheating is known due the temperature rise class of the enclosure defined by type test. Any use with a defined enclosure class, limited by the maximum losses, will adapt the transformer load factor to the ambient temperature to assure the transformer lifespan.

The calculation methods use formulas reflecting specific cases of a general formula based on Bernouilli equation and stack effect due the transformer heating, assuring the natural convection inside the transformer compartment as required by the IEC 62271-202 standard.

Indeed, the real air flow is strongly dependent:

• On the openings shape and solutions adopted to ensure the cubicle protection index (IP): metal grid, stamped holes, chevron louvers etc. (see Fig. B44)
• On transformer temperature rise and overheating in °K (class) due to the use in an envelope as mentioned in Fig. B47.
• On internal components size and the whole layout as follow:
  • transformer and/or retention oil box position
  • distance from the transformer to the openings
  • transformer in a separate room using partition wall
• On some physical and environmental parameters as follow:
  • outside ambient temperature θ1 used in equation 2.2
  • altitude
  • solar radiation.

The understanding and the optimization of the attached physical phenomena are subject to precise flow studies, based on the fluid dynamics laws, and realized with specific analytic software. These could be separated in two categories as follow:

• Software used for thermal dynamic studies of the building especially used for energy management for building efficiency.
• Software used for air flow study especially when a component embeds it’s own air cooling system (Inverter, Grid Frequency Converter, Data centres etc.)

Example for HV/LV substation:

Oil immersed transformer 1250 kVA

Ao (950 W No load losses) Bk (11000 W Load losses)

Transformer dissipation = 11950 W

LV switchgear dissipation = 750 W

MV switchgear dissipation = 300 W

H the height between ventilation opening mid-points is 1.5 m.

ξ is 12 for chevrons louvers if α = 90° then K = 0.064

(θ2 - θ1) air temperature rise taken at 20 K for expected transformer overheating at 10 K.

Calculation:

Dissipated Power P = 11.950 + 750 + 300 = 13.000 KW

- Formula 2.1:

[math]\displaystyle{ S=1.8 \cdot 10 ^{-1} \frac {Q_{nac}}{\sqrt H} }[/math]

S = 1.91 m² and S' 1.1 x 1.91 = 2.1 m² (Net area)

- Formula 2.2: Chevrons Blade

[math]\displaystyle{ S = \frac {Q_{nac}} {K\cdot {\sqrt {H \cdot (\theta 2 - \theta 1)^3 }}} }[/math]

S = 1.85 m² and S' 1.1 x S = 2.04 m² (Gross area)

Three ventilations with the following dimensions (see Fig. B48 and Fig. B49) 1.2 m x 0.6 m, 1.4 m x 0.6 m, 0.8 m x 0.6 give a gross area S’ at 2.04 m².

Fig. B48: Example of layout for 13 kW of total losses Δθ2 - Δθ1= Air temperature rise = 20 K corresponding to transformer overheating at 10 K

Fig. B49: Example of HV/LV prefabricated substation tested with 1250 kVA liquid filled transformer, 19 kW of losses

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. B50).

Fig. B50: Ventilation opening locations

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. Always make sure the baffles are oriented in the right direction (see Fig. B44).

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 are 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.

Protection

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

Cleaning

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.

For cleaning procedures, please contact your Schneider Electric correspondent.