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How to select the switch when the altitude is higher than 3000 meters
Author:管理员    Posted on:2019-06-17 11:06:09    Browse (149)
Summary:When designing switching power supplies or other electronic equipment, there are many indicators that need to be considered and tested, and altitude is rarely mentioned or concerned, which is a rarely mentioned equipment performance......

When designing switching power supplies or other electronic equipment, there are many indicators that need to be considered and tested, and altitude is rarely mentioned or concerned, which is a rarely mentioned equipment performance parameter. Switching power supplies or electronic devices that are really used in high altitude areas may have a series of unique problems.

Altitude is the height difference between a place and sea level, and it is the vertical distance that a place on the ground is above sea level. my country has a vast territory and abundant resources, and the geographical and climatic conditions are very complex. Most of the northwestern regions are plateaus. The areas above 2000M above sea level in China account for about 33%, and the areas above 3000M above sea level account for about 16%. Many cities are located in areas above 2000M above sea level, such as: Lhasa 3658.0M, Xining 2261.2M and so on. With the implementation of the country's western development policy, the application of electronic equipment in high-altitude areas is increasing, so designers should pay attention.

The higher the altitude, the lower the air density and the lower atmospheric pressure, which increases the air viscosity coefficient and reduces the number of air molecules, resulting in a decrease in the amount of heat transferred. of. In this way, the heat transfer efficiency is reduced, and the heat dissipation performance of the electronic components becomes worse. At an altitude of 5000M, the heat release coefficient will drop by 21% compared to the value at sea level, and the heat transferred by convective heat dissipation will also drop by 21%. At an altitude of 10000M it will reach 40%. The reduction in heat transfer by convection heat dissipation will result in an increase in product temperature rise. Therefore, the heat dissipation of equipment at high altitudes is reduced. The equipment in the data center will generate heat during operation. Due to the reduction in the number of molecules in the local area, the temperature of the heating element is not easily dissipated, resulting in an excessively high local temperature rise of the equipment. If the heat dissipation is not timely, some components will be burned out.

In areas with higher sea waves, the thinner the air, the lower the strength of the insulating medium, which makes the equipment more likely to discharge, making the usual insulating distances insufficient. Condensation is prone to occur in high altitude areas, reducing the creepage distance of electronic equipment. The performance of insulating devices in data center electronics also degrades. For safety reasons, it is generally desirable that the insulation resistance of the insulating material be as large as possible. The insulating material mainly includes three types of gas insulating material, liquid insulating material and solid insulating material. In electronic information products, gas medium and solid medium are widely used to achieve the purpose of insulation, so the quality of the insulating medium directly affects the safety performance of the product. If the insulating material of the equipment is destroyed in the electric field due to exceeding its dielectric strength and loses its proper insulating properties, then the phenomenon of dielectric breakdown will occur, and the equipment will not be able to continue to work normally.

Table 1 lists the atmospheric pressure at different altitudes:

High altitude also reduces the breaking capacity of high and low voltage electrical appliances that extinguish arcs in the atmosphere. When the breaking capacity is unqualified, a product with a higher rated capacity should be selected. The DC motor is used in the plateau environment with low humidity and low air pressure, and it is easy to generate large commutation sparks. If the commutation spark is unqualified, a motor with good commutation performance should be selected or measures should be taken to reduce the commutation spark level, or even lower the level of the commutation spark. capacity usage.

Generally, areas with an altitude of more than 1000M must start to consider the impact on electronic equipment. Of course, the vast majority of equipment can work normally at an altitude of 3000M, but about 16% of the areas above 3000M above sea level in my country, must pay attention when designing or selecting switching power supplies or electronic equipment.

All electronic devices have a working altitude range, so how is the working altitude range obtained? It is impossible to run the product to different altitudes for testing. The pressure sensor specially used to measure the altitude can be used for the simulation test, and the working altitude value that the device can withstand can be obtained by applying different air pressure values to the device.

It can be seen that when introducing switching power supplies or other electronic equipment in high sea wave areas, it is necessary to fully consider the electronic equipment or to be able to work normally in high altitude areas.

Detail the problems and countermeasures caused by power supply or electronic equipment at high altitude

As altitude increases, it is characterized by:

a. Low air pressure or air density;

b. The air temperature is low and the temperature changes greatly;

c, the air is good and the humidity is small;

d. The irradiance of the sun is high;

e, less precipitation;

f. There are many windy days every year;

g. The soil temperature is low and the freezing period is long.

These characteristic properties have the following four influence laws, which are listed as follows:

1. The effect of reduced air pressure or air density

1. Influence on the strength of insulating medium The decrease of air pressure or air density causes the decrease of external insulating strength. In the range from altitude to 5000m, for every 1000m increase, that is, every 7.7~10.5kPa decrease in average air pressure, the external insulation strength decreases by 8%~13%.

2. Influence on the breakdown voltage of the electrical clearance For the designed products, since the electrical clearance has been fixed, the breakdown voltage will also decrease with the decrease of the air pressure. In order to ensure that the product has sufficient breakdown resistance when used in the plateau environment capacity, the electrical clearance must be increased.

3. Influence on corona and discharge voltage

a. High altitude and low air pressure reduce the partial discharge starting voltage of the high-voltage motor, the corona starting voltage is reduced, and the corona corrosion is serious;

b. High altitude and low air pressure reduce the internal air pressure of the power capacitor, resulting in a decrease in the initial voltage of partial discharge;

c. High altitude and low air pressure reduce the voltage in the inner cavity of the arrester, resulting in a reduction in the power frequency discharge voltage.

4. Influence on arc extinguishing performance of switchgear

The reduction of air pressure or air density reduces the arc-extinguishing performance of the switchgear for air-medium arc extinguishing, reduces the on-off capacity and shortens the electrical life.

a) The arcing time of the DC arc increases with the increase of altitude or the decrease of air pressure;

b) The arcing distance of DC and AC arcs increases with the increase of altitude or the decrease of air pressure.

5. The cooling effect of the medium, that is, the influence of the temperature rise of the product

A reduction in air pressure or air density causes a reduction in the cooling effect of the air medium. For electrical products with natural convection, forced ventilation or air radiator as the main heat dissipation method, the temperature rise increases due to the decrease of heat dissipation capacity. In the range from altitude to 5000m, for every 1000m increase, that is, every 7.7~10.5kPa decrease in average air pressure, the temperature rise increases by 3%~10%.

a. The increase rate of the temperature rise of static electrical appliances with the increase of altitude is generally within 0.4K per 100m, but for high-heating electrical appliances, such as electric furnaces, resistors, electric welding machines and other electrical appliances, the increase rate of temperature rise with the increase of altitude, Every 100m reaches more than 2K.

b. The increase of the temperature rise of power transformers with altitude is related to the cooling method. The increase rate per 100m is: oil-immersed self-cooling, 0.4% of the rated temperature rise; dry-type self-cooling, 0.5% of the rated temperature rise; oil-immersed forced air Cold, 0.6% of the rated temperature rise; dry forced air cooling, 1.0% of the rated temperature rise;

c. The increase rate of the temperature rise of the motor with the increase of altitude is 1% of the rated temperature rise per 100m.

6. Influence on the mechanical structure and sealing of the product

a. Changes in the physical and chemical properties of low-density, low-concentration, porous materials (such as electrical insulating materials, thermal insulation materials, etc.);

b. The evaporation of lubricants and the evaporation of plasticizers in plastic products are accelerated;

c. Due to the increase of the internal and external pressure difference, the gas or liquid is easy to leak from the sealed container or the leakage rate increases, and the electrical products with sealing requirements will indirectly affect the electrical performance;

d. Changes in the pressure of the pressurized container, resulting in easy rupture of the pressurized container.

2. The influence of air temperature decrease and temperature change (including daily temperature difference) increase

1. Compensation of product temperature rise by ambient air temperature in plateau The average air temperature and higher air temperature both decrease with the increase of altitude, and the thermal aging life of electrical insulating materials depends on the average air temperature. The decrease in the ambient air temperature in the plateau can partially or fully compensate for the increase in temperature rise in the operation of electrical products caused by the decrease in air pressure. The compensation value of ambient air temperature is 0.5K/hm.

2. The influence of daily temperature difference or temperature change on product structure The daily temperature difference of plateau air temperature is large. Large temperature changes make the product shell easily deformed and cracked, and the sealing structure is easily broken.

3. The influence of good air humidity reduction

1. The influence of good humidity on the external insulation strength

The average good humidity decreases with increasing altitude. When the humidity decreases, the external insulation strength of electrical products decreases, so the humidity correction of power frequency discharge voltage and impulse flashover voltage should be considered. The humidity correction is based on the average good humidity at zero altitude: 11g/m3, and the specific correction is in accordance with the relevant regulations in GB311.2.

2. Influence of good humidity on motor commutation and carbon brush wear

The reduction of good humidity increases the commutation spark of the commutator motor, and at the same time increases the wear rate of the motor carbon brushes.

4. Effects of increased solar irradiance, including ultraviolet irradiance

1. The influence of the increase of plateau thermal radiation

At an altitude of 5000m, the maximum solar irradiance is 1.25 times that of the corresponding value at a low altitude, and the thermal radiation has a heating effect on the object. For outdoor electrical products, the increase of solar thermal radiation causes a larger surface additional temperature rise, reduces the material properties of organic insulating materials, deforms the materials, and produces mechanical thermal stress and other effects.

2. The effect of increased ultraviolet radiation in the plateau

The increase rate of ultraviolet irradiance with the increase of altitude is much larger than that of the total solar irradiance, and it has reached 2 times of the corresponding value at low altitude when the altitude is 3000m. Ultraviolet rays cause accelerated aging of organic insulating materials, which makes the air easily ionized and reduces the outer dielectric strength and corona inception voltage.

It can be seen from the above four influences that the design of switching power supply or electronic equipment in the plateau environment should reduce these influences, improve the insulation coordination, and at the same time increase the electrical clearance, comprehensively consider the selection of materials and devices, from the structural design and selection of plateau To start with type devices and solve related technical problems, the main means of realization is to consider from the product design level.

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