Researchers of Xuji Power Co., Ltd. Gao Qingyong, Fang Xiangcheng, Chen Weiming, Liu Yanmin, Wang Xiaolei wrote in the first issue of "Electrical Technology" in 2018 that the electric vehicle charging facility is directly installed outdoors, along with the temperature of the outdoor environment. The humidity changes rapidly, and condensation will occur on the surface of the charging facility housing.
The mechanism of condensation and the damage to electrical equipment were analyzed. The anti-condensation design schemes of four outdoor charging facilities were proposed. The anti-condensation effect of the four design schemes was verified by the high-temperature chamber simulated condensation test. It provides the best anti-condensation solution for charging facilities, guarantees the safety and reliability of internal electrical equipment operation, and has important guiding significance for similar outdoor product anti-condensation design.
In recent years, the global energy crisis and the environmental crisis have become more prominent. The state has vigorously advocated the development of new energy vehicles, and electric vehicle technology has made remarkable progress. The development of the electric vehicle industry requires efficient, convenient, safe and stable charging services. Charging facilities are the cornerstone of the development of new energy vehicles.
In 2020, the National Development and Reform Commission issued the "Guidelines for the Development of Electric Vehicle Charging Infrastructure (2015-2020)", which proposes that by 2020, more than 12,000 new centralized charging and replacing stations will be installed, and more than 4.8 million decentralized charging piles will be distributed. It can meet the overall goal of 5 million electric vehicle charging needs [1].
As of 2016, the “six vertical, six horizontal and two ring†expressway fast charging networks have been formed by the State Grid Corporation, with an average spacing of no more than 50 kilometers. And announced on January 19, 2017, this year plans to build 29,000 charging piles, which will form a “fast, nine-way, nine-way, two-ring expressway network†during the year; by 2020, the expressway fast-fill network will cover 202. A city, 36,000 kilometers, "completely solve the problem of charging electric vehicles" [2].
The charging station has a wide distribution area. In the Central Plains, due to the small temperature difference and low humidity, the condensation time is relatively small, but it is easier to form condensation in the coastal zone and the northwest region due to high humidity or large temperature difference between day and night. dew.
Condensation mainly has the following hazards to electrical equipment: 1) condensation drops onto internal metal parts, causing parts to rust [3]; 2) condensation drops onto electrical components, causing problems such as short-circuit and explosion of components; 3) electrical components Long-term operation in a high-humidity environment will shorten the life of components; 4) It is more likely to form mold and other undesirable phenomena in a humid environment.
During the operation of the equipment, any of the above problems will cause greater safety problems and shorten the operating life of the product. Therefore, it is necessary to solve the condensation problem caused by humidity and ensure that the equipment operates in a reasonable humidity environment.
1 condensation production principle
The air is mainly composed of dry air, water vapor and dust. Normal humidity refers to the amount of water vapor in the air and can be expressed in terms of relative humidity, absolute humidity, and saturated humidity. Saturated humidity is the maximum amount of water vapor that can be contained in a unit volume of air at a certain temperature; absolute humidity is the mass of water vapor in a certain volume of air, in g/m3; and relative humidity is absolute humidity and saturated humidity. The ratio.
The saturated humidity is related to the air temperature. The higher the temperature, the more water is contained. 30% to 60% relative humidity is suitable for general electrical equipment [4]. If the absolute humidity of the air is kept constant, the air temperature is lowered, and when the temperature is lowered to a certain value, the humidity in the air will reach saturation, and the temperature will continue to decrease, and the moisture in the air will be precipitated. This phenomenon of liquid water precipitation is called "condensation". The dew point temperature is the temperature at which the relative humidity of the air reaches 100% under the premise that the moisture content and the atmospheric pressure remain unchanged [5].
Condensation under laboratory conditions mainly includes two cases. One is that during the heating phase, when the surface temperature of the casing is lower than the ambient temperature during the heating process, when the air on the outer surface of the casing encounters the surface of the product below the dew point temperature, the moisture will condense on the outer wall of the casing to form condensation.
The other is that during the cooling phase, the external environment is cooled first, so the inner wall of the casing is lower than the internal air temperature. If the inner wall temperature of the casing reaches the dew point temperature of the internal air, condensation will form on the inner wall of the casing.
Usually the problem of condensation that we have to solve is mainly the second one, to avoid condensation on the inner wall and affect the performance of internal electrical components. Condensation is the result of temperature and humidity. Generally, the following two conditions are more likely to produce condensation: 1) high ambient humidity and large temperature difference; 2) the cable trench at the bottom of the cabinet is not blocked, and the cable trench has water;
China has a vast territory and huge climate differences. The humidity in the coastal environment is large, and the temperature difference in the northwest is large. Such areas are generally more prone to condensation [6].
2 electrical equipment anti-condensation design
At present, the methods of dealing with problems such as moisture and condensation in the electrical industry include installing electric heaters, fans, dehumidifiers, etc. inside the cabinet, which have their own advantages and disadvantages. The best dehumidification scheme should be selected according to the specific product characteristics.
2.1 heating dehumidification (humidity controller + heater)
By heating the air, the heater increases the moisture content of the air and reduces the relative humidity of the air. The density of the air after heating is small, and it rises to the upper side, and the cold air drops to the lower side, so that the entire space is filled with hot gas, and the air is heated. When the rear volume is increased, the air is exhausted from the air outlet to the outside [7].
In fact, the heater does not really dehumidify, but only heats the air inside the device, thereby reducing the relative humidity inside the product. The humidity in the air is only evaporated by superheat and becomes a form of "superheated steam". If the necessary measures are not taken in time, the superheated steam is condensed, and the humidity inside the equipment will return to its original state. It is equal to a dehumidification scheme that is always in circulation and is suitable for equipment containing vents.
2.2 dehumidifier dehumidification
At present, the dehumidifier usually uses a fan to pump the air inside the product casing into the dehumidifier. The application of the semiconductor thermoelectric element produces a cold and heat effect under a certain voltage. The local manufacturing condensation conditions cause the humid air in the casing to condense into water and directly discharge the shell. The outside of the body, thereby reducing the humidity inside the casing, preventing condensation from occurring, and the dehumidification effect is remarkable. The semiconductor dehumidifier consists of a PN junction, a fan, a drain pipe, etc. The dehumidification process is based on the principle of thermoelectric refrigeration. The Peltier effect is mainly applied to convert electric energy into heat energy through two types of semiconductors, N-type and P-type, and the opposite is Shell effect.
In the cooling mode, the direct current flows from the N-type semiconductor to the P-type semiconductor, and the temperature of the thermoelectric heat to the intermediate cold end is lowered, and the heat in the environment is absorbed. When electrons pass through the conductor from the low energy end of the P-type semiconductor to the high energy end of the N-type semiconductor, heat is absorbed from the environment, and heat is conducted to the other end via electrons, releasing heat when the electrons flow in the opposite direction. A plurality of thermoelectric pair connections constitute a thermoelectric device [8]. The refrigeration model and dehumidifier of the thermoelectric pair are shown in Figure 1.
Figure 1 Dehumidification principle and structure diagram of dehumidifier
The dehumidifier can condense the water vapor in the air in the space into "liquid water" and discharge it to the outside of the equipment. The original air humidity will not recover and can achieve the purpose of real dehumidification. Dehumidifier condensation dehumidification technology has become increasingly mature, and can be equipped with remote control, communication, low temperature heating and other functions according to user needs [9], which is more suitable for relatively closed space.
2.3 fan dehumidification
The fan can not be directly dehumidified. If the device has ventilation holes, it is easier to bring the outside humid air to the inside of the charger. The humid air encounters the low temperature wall of the charger and accelerates the formation of condensation. The fan needs to cooperate with the temperature controller and the heater. The fan can let the air flow, allowing the heat to be released to the air more quickly and evenly [10].
When the humidity controller detects the ambient humidity to the set value, the heater and the fan start, and the heat is quickly diffused into the interior of the cabinet to increase the overall temperature inside the cabinet. The fan is generally integrated with the heater and placed in the lower part of the heater, but This scheme generally does not require ventilation holes in the lower part of the fan.
2.4 metal coating anti-condensation
After the surface of the metal is coated with the silica gel desiccant, when the surface temperature of the metal is lower than the dew point temperature, the water is firstly adsorbed into the metal surface coating layer, and the condensation occurs after the adsorption is saturated. Therefore, coating the surface of the metal with a silica gel can effectively delay the generation of condensation in a certain period of time [11], but it does not achieve the purpose of dehumidification, and increases the processing and maintenance costs.
3 analysis of anti-condensation test results of charging facilities
Charging facilities are generally divided into AC charging piles and DC charging machines. DC charging machines are generally designed with ventilation holes because they have internal power conversion units. There are no power components in AC charging piles. Generally, they can be used as fully enclosed or with a small number of ventilation holes. Semi-enclosed space, this time a total of 4 chargers were used for testing twice. The first test conditions are shown in Table 1.
Table 1 charger status table
The test environment is a 30 m3 high and low temperature environmental chamber, and the test equipment is used as a temperature and humidity meter [12]. The test method is as follows: open the front door of the charger during the test, set the temperature of the high and low temperature box to 45 ° C, the humidity is 95%, after running for 30 minutes, turn off the front door of the charger and run it again for 10 minutes, turn off the high and low temperature environment box, let it cool naturally, after 5 hours See Table 2 for the internal conditions of the high and low temperature environmental tank charger.
Table 2 charger test results
The second test has ventilation holes on the left and right sides of the charger, and other conditions are the same as the first test. The test results are shown in Table 3.
Table 3 charger test results
After two test comparisons, the following conclusions were drawn:
1) In the relatively closed space, the "dehumidifier" is similar to the "humidity controller + heater + fan" effect, the effect is slightly better than the "humidity controller + heater"; in the relatively closed space, the heater heats the air to reduce the relative temperature There may be repeated cycles of operation, so a dehumidifier solution is recommended for relatively closed spaces.
2) In the open space, the three effects are compared: “humidity controller + heater + fan†> “humidity controller + heater†> “dehumidifierâ€.
4 Conclusion
The principle of product condensation and the method of product dehumidification were studied. The actual product was tested in high and low humidity chambers, and it was found that it is necessary to increase the dehumidification components to prevent condensation. The following conclusions are drawn on the selection of dehumidification components:
1) No heating element charging pile can open the ventilation hole or only a small number of ventilation holes in the upper part. It is not recommended to open the ventilation hole in the lower part; 2) The dehumidifier should be applied to the charging pile in the relatively enclosed space, and the dehumidifier power is determined according to the actual space of the product. 3) ventilating charging pile (machine) should use the "temperature controller + heater + fan" heating scheme, the heater power can be determined according to the actual space of the product.
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