Principles of Anti-Condensation Heaters Used in Medium Voltage Switchgears
There is information pollution and lack of information in the sector related to the use of anti-condensation heaters in medium voltage switchgear. Given the cost and dimensions of anti-condensation heaters, the continuity of the system and the importance of the personnel and the investment are unfortunately unclear. We will look for answers to the following three questions in this article;
- Why should the anti-condensation heaters be used?
- How should the anti-condensation heaters be used?
- Which type of anti-condensation heaters should be used?
Why should the anti-condensation heaters be used?
The relationship between the temperature of the air and the humidity in the air is one of the most important issues especially in electrical applications. Using thermodynamic properties of moist air, the thermodynamic branch, which deals with the processes and conditions in moist air, is called “Psychometry”. Psychometric diagrams and its values illuminate the necessity of anti-condensation heaters.
“Humidity” by Psychrometry; It is the maximum amount of water vapor contained in the air. This sentence also tells us that this water kept in the gas in the air cannot be kept under certain conditions. As the air gets warmer, the amount of water vapor it can keep in it increases. On the contrary, when the air starts to cool, the point of temperature that can no longer hold the water contained in it is called the “Condensation” (Dew) point. If the temperature of the air in a switchgear falls below the critical dew point, the vapor inside the switchgear starts to turn into water droplets.
The table below shows the maximum amount of water in grams that can be kept in 1 m³ of air at different temperatures.
|Temperature (°C)||Retainable Water Vapor|
|30°C||30 gr per cubic meter|
|20°C||17 gr per cubic meter|
|10°C||9 gr per cubic meter|
According to the above table, we can see that 13 g of water has turned into tiny dew drops in a switchgear where the air temperature decreases from 30 ° C to 20 ° C. These dew drops are over switchgear sheets, insulators, cutters and other electrical equipment. Everybody in the industry knows how unpleasant this is. Dew droplets, combined with dust and dirt on isolated surfaces, unfortunately reduce the creepage distance to extremely hazardous values, resulting in partial discharges or direct insulation failures.
Considering the malfunctions occurring in the medium voltage switchgears statistically, if the materials are selected correctly and not counting external factors such as lightning, one of the two most important factors are short- or long-term insulation errors caused by moisture.
Fortunately, keeping the humidity within reliable limits that will not cause malfunctions is only possible with a properly selected anti condensation heater.
Researches show that increasing the temperature within the switchgear by only 3-4 ° C greatly reduces the risk. According to some approaches, it is considered safe to keep humidity below 65% and some to below 80%. The fact is that, regardless of the temperature and humidity value, the anti-condensation heater must be used in all conditions and the temperature of the switchgear should be kept above the dew point.
The failure to fully understand this issue leads to widespread acceptance of the following two wrong approaches.
Misconception: Anti condensation heaters are only needed in cold.
To find the answer to this error, we can reach the values in Table-2 from the psychometric diagram.
|Temperature °C||Humidity (%)||Dew Point (°C)|
|A Winter Day||7°C||%85||4,65°C|
|A Summer Day||23°C||%85||20,3°C|
As you can see, the dew point will be 4.65 ° C on a winter day when the humidity is 85% and the air temperature is 7 ° C. The air temperature will start to condense only when it decreases to only 2.35 ° C within 1 hour.
On a summer day when the humidity is 85% and the air temperature is 23 ° C, the dew point is 20.3 ° C. Condensation begins when the air temperature decreases to only 2.7 ° C per hour.
These two conditions indicate that the risk of condensation is independent of the air temperature. Even when the values in Table-1 are taken into consideration, the amount of water condensing in the hot environment is higher.
In this case, hot or cold, summer or winter without the need of the anti-condensation heater is a permanent need. When Table-2 is considered, if a heater is activated below 20 ° C, condensation will not be prevented for summer day. In this case, the heater must be switched on continuously or the thermostat must be operated in a very wide temperature range.
As a result, the cost of the heater to save or to save energy consumption of the heater is a very serious error that can lead to even higher failure costs.
Misconception: When the current is passed through the conductors in the switchgear, such as the copper busbar, the switchgear will become hot, so there will be no need for heating.
The highest temperature rise in the switchgear occurs in copper bars and breaker contacts. Let’s think of a switchgear with a 1250-A circuit breaker. According to IEC 61439, the temperature increase can be maximum 105 ° C. However, if 200 A current is flowing from this system, the temperature increase is maximum (200² / 1250²) * 105 ° C = 2.7 ° C because it is proportional to the square of the current, which may be below or above 2.7 centigrade according to the selected busbar cross section. It is important to note that the temperature at which the conductors passing through the current can increase the temperature of the air in the switchgear, which is much greater than the volume, can only slightly increase. Moreover, it will be in switchgear that are under energy, but which do not have enough power conductors. This claim can sometimes keep the air temperature inside the switchgear on the dew point, but it is obvious that it does not reduce the risk for each condition. As a result, it is necessary to use a separate heater in each switchgear.
How should the anti-condensation heaters be used?
Based on the above information and field experience over the years, the following practice recommendations have been introduced regarding the use of anti-condensation heaters.
- Use one or more anti-condensation heaters in each medium-voltage switchgear and a separate thermostat if used.
- The thermostat must not be used at all or set for a very large work area.
- In anti-condensation heaters, aluminum body, wire winding heaters with high cooling surface should be preferred.
- Under normal conditions, a heater of 100 W per m³ is required with an Elektres brand (with high cooling surface, aluminum body, wire wound heaters) heater. The humidity in the 1 m³ switchgear is reduced from 95% to 80% in 1 hour with a 100 W heater (~ 15% reduction). An Elektres brand heater can achieve a 12% decrease in 100 W per 1 hour on a 2 m³ switchgear. If this switchgear will operate at a place that reaches 95% humidity values, it is correct to increase the heater power to 150 W. In this case, considering the site conditions and the switchgear volume, the heating power requirement for each switchgear can be determined.
- The anti-condensation heaters must be installed at the bottom of the switchgear. The heaters should be operated at a height of maximum 20 cm from the floor, as the air to be heated will rise and usually the humidity input to the switchgears is from the bottom of the switchgears.
- The active heating surfaces of the anti-condensation heaters must be mounted so that they do not touch the switchgear construction. Because the high surface area construction reduces the temperature of the heater surface by contact, it significantly reduces the efficiency.
- A control can be established to monitor the heater circuit as a precaution against the risk that the heater is switched on for any reason.
- Since the anti-condensation heater surface temperature will be high, it should be installed at a minimum distance of 10 cm from the materials to be affected by the temperature.
- The thermostats that can be used with the anti-condensation heater should be installed at least 20 cm away from the heater.
- A common misconception is that the heater is installed in the cabinet and the thermostat is installed in the LV cabinet. Since the thermodynamic values between these two environments will be different, the thermostat and the heater it controls should be in the same environment.
Which type of anti-condensation heaters should be used?
As with all the resistive products in the sector, the anti-condensation heaters have a power value (Watt) complexity. In other words, all anti-condensation heaters with 100 W in the label can provide the same effects, can they perform the same way?
Leaving the label values aside should be analyzed in detail whether the product serves the intended purpose of use and whether it is safe.
Wire wound with a high cooling surface through cooling channels aluminum housinganti condensation heater (Elektres Brand heaters)
Rezistans sektöründe kullanılan, bir sıvıyı daldırarak ya da katı bir kütleyi temas yoluyla ısıtmaya yarayan paslanmaz çelik gövdeli rezistans ısıtıcı.
Although the housing is aluminum, the PTC semiconductor is used as a resistance element, in fact the heater made from an electronic circuit element material.
The following evaluations, tests and analyzes were carried out on 100 W samples for each of the heater types described above.
- Physical and electrical design
- Surface area and surface efficiency
- Reduction performance in Switchgear
- Distribution of humidity drop in switchgear
- Humidity reduction rate
Physical and electrical design
Due to its low specific gravity and high thermal conductivity, it is very advantageous to use aluminum with increased surface tension in ambient heating. While the thermal conductivity of aluminum is 120-220 W / m.C, for Ni-Cr alloy tube used in tube heaters this value is 12-17 W / mC. In addition, it is not enough that the body is only aluminum, the cooling surface can not be increased if the heater body made of aluminum cast.. The most accurate solution is the extruded aluminum body.
In addition, if the aluminum is covered with anodic oxidation, corrosion is prevented. Research shows that the eloxal coating significantly increases the aluminum strength and thermal conductivity.
Electrical insulation is also an important issue for anti-condensation heaters. In general, it has to provide a minimum of 2 kV / 1-minute insulation. Common malfunctions in heaters are caused by leakage insulation. This is due to the incorrect selection of the powder used forinsulation. Due to the cost of SiO2 (Silicon Oxide) powder produced using heaters are more at risk of insulation. This powder can be a solution in the insulation of large volume products not in small volume. This is because the iron content in SiO2 is 7 times higher than the iron content in MGO.
As resistance elements in the heaters, stainless resistance wire should be used from the alloy whose resistance value does not increase. In this way, even if the temperature rises, the power value does not decrease and remains constant.
Surface area and surface efficiency
The anti-condensation heaters are an electrical heater that heats the air for its intended use. For such heaters, the larger the surface area in contact with the air, the higher the efficiency of the heater. In line with the designs of the Elektres brand aluminum heaters, have increased the surface area thanks to cooling channels and have an eloxal coated housing which is produced by extrusion method. Tube heaters can be used to heat liquids under normal conditions or to heat a mass through contact. In this case, it is acceptable low surface area and low surface efficiency . Also the housing material is not aluminum, it is stainless steel which has lower thermal conductivity. Although the body of PTC heaters is aluminum, the surface efficiency is low.
Here, for the surface efficiency; Let us consider the ratio of the surface area of a heater to the cubic area it occupies. That is, the higher the area per unit cm³ the higher the surface (cm2) efficiency. There is a need for higher surface area in a smaller volume to ensure insulation distances inside the switchgear.
|Surface Area (cm²)||Cubic Volume (cm³)||Surface Efficiency (cm²/cm³)|
According to the values in Table – 3, aluminum heaters provide an unquestioned success and high efficiency in terms of surface efficiency. At the same time, the lifetime is more longer due to the low power per surface area.
Reduction performance in Switchgear
If our thesis on the above surface efficiency is correct, the performance of the three types of anti-condensation heaters in a switchgear should also yield similar results. Apart from physical design, let’s see how the electrical design approach will affect the order above (Chart – 1).
Three types of 100 W anti-condensation heater were operated in the same volume (2 m³) in the same time period (60 minutes) and the temperature and humidity values in the switchgear were measured each 10 minutes. With this test, we determined the humidity reducing ratio of heaters at the end of one hour.
As shown in graph – 2, aluminum heaters have a very good dehumidification performance of 13% in 1 hour. The tube heater and PTC heater decreased 8% and 1.67% respectively. The most important point here is that there is no difference between the PTC heaters being switched on or not. The electrical designs of PTC heaters, which are worse than their physical designs, have produced this result. The PTC resistance material is a semiconductor material and cannot be used for heating purposes. Due to increasing the resistance value against to temperature rise, the power decreases.
Distribution of humidity drop in switchgear
Heaters will reduce the humidity in the switchgear more or less. However, it is important whether this decrease is homogeneous.. The heater is successful if the reducing humidity difference is low between top and bottom of the panel.
Here, our thesis is that the pipe heater, which is not suitable for air heating as it has low surface area, can not perform its function homogeneous. Therefore, the air near the heater is warmer than the air at the top of the panel. To test this thesis, we made 3 different measuring points into the switchgear. The first is the bottom measuring point 450 mm away from the heater, the middle measuring point at 900 mm distance and the top measuring point at 1350 mm distance. Again, a switchgear of 2 m³ was tested for 60 minutes and the humidity values taken from these three points at the end of 60 minutes are shown in Graph – 3 below.
At the end of 60 minutes, the difference in humidity between the lower and upper points in the test with the aluminum heater was 21%, whereas the difference was 33% with the tube heater. The result of the test is that the aluminum heater is more successful in homogenous dehumidification.
Humidity reduction rate
As well as the moisture reduction function within a switchgear of a anti-condensation heater, the rate of moisture reduction is also very important. As we mentioned in our article, the condensation point can be reached even when the ambient temperature sometimes decreases only 3-4 ° C. For example, a rapid decrease in air temperature with the sun falling can quickly shift the inside of the switchgear to the dew point. In such a case, the rate of humidity reduction is critical.
We analyzed our test results for aluminum heater and tube heater to measure humidity reduction rates. We cannot evaluate the PTC heater since it has no effect. We cannot evaluate the PTC heater since it has no effect. The humidity at the time we tested was approximately 85%. A moisture reduction of 6% was chosen to reduce the overall acceptance to below 80% humidity.
As can be seen clearly in Graph 4, the aluminum heater performs a selected 6% humidity decrease in 16 minutes while this time is 34 minutes for the tube heater. The use of an aluminum heater will be a more accurate solution thanks to the high humidity reduction rate, which is even more critical in the MV switchgears installed in geographies where high temperature changes occur during the day.
Based on these results, let’s list the characteristics of the anti-condensation heater we need to use;
- – The housing of heater shall be manufactured by extrusion for ensure increase cooling channels and shall be anodized to prevent corrosion.
- – Stainless steel resistance wire shall be used as resistance element.
- – “MGO” (magnesium oxide) insulation powder shall be used as insulation material.
- – Porcelain terminal which is not affected by temperature shall be used.
- – The resistance value shall not increase against to the temperature rise, so the declared power value at the operating voltage shall remain constant.
- – The insulation level shall be a minimum of 2 kV / 1 minute.
- Essentials of Meterology – C. Donald Ahrens
- Meteorology Today – C. Donald Ahrens
- Condensation in Switchgear and Anti-Condensation Heater – Jim Bowen – Powell
- Space heater – sizing and application principles – TechTopics No. 84 – Siemens Industry, Inc.
- Psikrometrik Diyagram Ve Uygulamaları – Celalittin Kırbaş – Mak. Müh. – Mmo 2013 Kocaeli Şubesi
Electrical Engineer – Factory Manager
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