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 equipments comply with specific standards and with the IEC 62271-1 standard “Common specifications for high-voltage switchgear and controlgear”, which defines the normal conditions for the installation and use of such equipment.
For instance, regarding humidity, the standard mentions:
The conditions of humidity are as follows:
- The average value of the relative humidity, measured over a period of 24 h does not exceed 90%;
- 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;
Under these conditions, condensation may occasionally occur.
NOTE 1: Condensation can be expected where sudden temperature changes occur in period of high humidity.
NOTE 2: To withstand the effects of high humidity and condensation, such as a breakdown of insulation or corrosion of metallic parts, switchgear designed for such conditions and tested accordingly should be used.
NOTE 3: Condensation may be prevented by special design of the building or housing, by suitable ventilation and heating of the station or by use of dehumifying equipment.
As indicated in the standard, condensation may occasionally occur even under normal conditions. The standard goes on to indicate special measures concerning the substation premises that can be implemented to prevent condensation.
Use under severe conditions
Under certain severe conditions concerning humidity and pollution, largely beyond the normal conditions of use mentioned above, correctly designed electrical equipment can be subject to damage by rapid corrosion of metal parts and surface degradation of insulating parts.
Fig. B15: SM6 metal enclosed indoor MV eqpuipment
Remedial measures for condensation problems
- Carefully design or adapt substation ventilation.
- Avoid temperature variations.
- Eliminate sources of humidity in the substation environment.
- Install an air conditioning system.
- 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.
- 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 with filters to restrict entry of pollution and dust.
- Regularly clean all traces of pollution from metal and insulating parts.
Ventilation
Substation ventilation is generally required to dissipate the heat produced by transformers and to allow drying after particularly wet or humid periods.
However, a number of studies have shown that excessive ventilation can drastically increase condensation.
Ventilation should therefore 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.
Guidelines for sizing the air entry and exit openings of substations are presented hereafter.
Calculation methods
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.
The required ventilation opening surface areas S and S’ can be estimated using the following formulas:
[math]\displaystyle{ \mbox{S}=\frac{1.8\times10^{-4}\mbox{P}}{\sqrt\mbox{H}} }[/math] and [math]\displaystyle{ \mbox{S}^'={1.10\times\mbox{S}} }[/math]
where:
S = Lower (air entry) ventilation opening area [m²] (grid surface deducted)
S’= Upper (air exit) ventilation opening area [m²] (grid surface deducted)
P = Total dissipated power [W]
P is the sum of the power dissipated by:
- The transformer (dissipation at no load and due to load)
- The LV switchgear
- The MV switchgear
H = Height between ventilation opening mid-points [m] See Fig.B16
Fig. B16: Natural ventilation
Note:
This formula is valid for a yearly average temperature of 20 °C and a maximum altitude of 1,000 m.
It must be noted that these formulae are able to determine only one order of magnitude of the sections S and S', which are qualified as thermal section, i.e. fully open and just necessary to evacuate the thermal energy generated inside the MV/LV substation.
The pratical sections are of course larger according to the adopted technological solution.
Indeed, the real air flow is strongly dependant:
- on the openings shape and solutions adopted to ensure the cubicle protection index (IP): metal grid, stamped holes, chevron louvers,...
- on internal components size and their position compared to the openings: transformer and/or retention oil box position and dimensions, flow channel between the components, ...
- and on some physical and environmental parameters: outside ambient temperature, altitude, magnitude of the resulting temperature rise.
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.
Example:
Transformer dissipation = 7,970 W
LV switchgear dissipation = 750 W
MV switchgear dissipation = 300 W
The height between ventilation opening mid-points is 1.5 m.
Calculation:
Dissipated Power P = 7,970 + 750 + 300 = 9,020 W
[math]\displaystyle{ \mbox{S}=\frac{1.8\times10^{-4}P}{\sqrt\mbox{1.5}}=1.32\mbox{m}^2 }[/math] and [math]\displaystyle{ \mbox{S}^'={1.1\times1.32}=1.46\mbox{m}^2 }[/math]
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 is negligible.
To avoid condensation problems, the substation ventilation openings should be located as far as possible from the switchboard
(see Fig. B 17).
Fig. B17: 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.
Always make sure the baffles are oriented in the right direction (see Fig. B18).
Fig. B18: Chevron-blade baffles
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 (standard versions are generally sufficient).
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.
- 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.
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 check the documentation or ask the manufacturer.
