Temperature rise of outdoor box

For the ambient temperature, transformers and switchgears are directly installed in outdoor boxes or containers made of steel plates, which is completely different from the indoor environment of reinforced concrete structures. The temperature inside and outside the outdoor box is almost the same. Therefore, for this kind of indoor switchgear, it is necessary to be consistent with the outdoor application environment. The high temperature caused by summer sunlight radiation should be fully considered. Even if the external temperature is 35 degrees Celsius, the temperature inside the box will be 50 degrees Celsius, which is a test for operating mechanisms, electronic devices, etc., and long-term use will cause aging and shortened life.

For example, in photovoltaic power stations in the Middle East, 36kV ring network cabinets are installed in outdoor boxes. The IP43 protection level of the outdoor box obviously cannot prevent dust from entering the ring network cabinet. In addition, the outdoor environment is 50 degrees Celsius for a long time, and the temperature inside the box is very high, causing the switch to refuse to open in the closed position. There are several reasons for the analysis. Dust enters the mechanism, high temperature causes the lubricating oil to dry up, dust and lubricating oil mix to cause half-axle pollution, high temperature causes half-axle material to soften, etc., which ultimately leads to the inability to electrically open the switch. In this way, it is necessary to simulate the high temperature environment to test the switch, put the switch inside the oven, set the temperature to 70 degrees, and put the switch in the closed position. After 24 hours, the switch is tested for electric opening and closing, and the minimum control voltage opening and closing speed is measured.

For transformers, high-current switch cabinets, etc., the heat is based on the standard ambient temperature, such as 40°C. Then, inside the outdoor box, the ambient temperature is equivalent to 10°C more, so it must be used at a reduced capacity. Circuit breakers and switch cabinets generally have a reduction factor for different ambient temperatures and shell IP levels. Therefore, transformer manufacturers must also accurately specify application requirements, such as formulating corresponding reduction factors for the IP level and temperature level of the installation shell, otherwise it is difficult to ensure safe operation at full load.

For large-capacity transformers, they are all cooled by lower cross-flow fans. Fan cooling requires air inlet and outlet. The effect of the transformer installed inside the IP3x box and the IP54 box should be completely different, unless the internal space of the shell is very large and the external air inlet and outlet can be ignored.

When the transformer is heated, the fan at the bottom blows the heat to the top, reaching the top of the box, and combined with the external solar radiation heat, a huge hot air field is formed. At this time, if there is no air outlet on the top of the box, the hot air will continue to accumulate and expand downward, thus forming a large heating space inside the entire system, a vicious cycle, and the temperature rise will continue to increase, exceeding the heat resistance level of the transformer insulation epoxy material, causing cracking and phase short circuit.

Therefore, the outdoor box must be designed reasonably

1. Suppression of sunlight

(1) Use white panels to reduce the radiation coefficient.

(2) Use double-layer top panels and two-layer canopy.

(3) Use insulation material boards. Under direct sunlight at noon in summer, the use of two-layer top panels and insulation boards has different effects on the temperature at different heights in the switch cabinet. The use of insulation boards is about 10K lower than the temperature of 10 double-layer top panels.

2. Increase ventilation and heat dissipation

For general outdoor applications, the protection level of IP43 is sufficient to meet environmental requirements. IP4x prevents solid objects with a diameter greater than 1mm from entering, and insects cannot enter the shell. IPx3 prevents liquid spray at an angle of 60 degrees to the vertical direction, and can prevent rain from entering.

IP43 does not mean no ventilation, but it is less efficient than P3x. Ventilation can be achieved by separating functions. For solid objects, a ventilation protection box is added, and a mesh plate with a diameter less than 1mm is used to block it. For liquids, the 60-degree water spray requirement can be met by tilting the baffle and arranging them in an overlapping manner.

3. Use equipment to monitor temperature

(1) Wireless temperature measurement device, using PA66+50%GF V0 flame retardant, environmentally friendly/reinforced/heat-resistant/wear-resistant materials. 2.4GHz, strong penetration ability, stable and reliable communication. CT power supply, long service life, no need to be replaced regularly. It can work for a long time, monitor in real time, and effectively prevent emergencies.

(2) Dry-type transformer temperature controller. High-precision, low-temperature drift devices are used for accurate temperature measurement. There is a door cabinet alarm function and a humanized mute button function. After the fan is manually turned on, it can be turned off by timing/manually.

Therefore, for outdoor boxes, first, an insulation layer must be added to prevent the absorption of radiation and heating, and the ventilation must be good to ensure that there is inflow and outflow, hot and cold air circulation, and condensation can also be reduced.

For switchgears and transformers used in outdoor boxes, corresponding application environment requirements and corresponding derating parameters must be formulated to guide their use correctly.

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