Cabinet design and practice of electrical equipment

□ Gu Jinsong 1 Ren Guishan 2 Li Hongyan 1 (1. The First Mining Area Management Service Company, Dagang Oilfield Company Mining Service Division, Dagang, Tianjin 300280, China; 2. Dagang Oilfield Production Technology Research Institute, Dagang, Tianjin 300280, China) The summary and application of design methods for electrical cabinets are presented. This paper analyzes and summarizes the difficulties in the design of electrical equipment cabinets, and identifies the “five key links” that need to be considered in the design process of electrical cabinets. It also includes the environmental design, safety design, electromagnetic shielding, heat dissipation design and appearance of electrical cabinets. A comprehensive analysis and study was conducted on the design and the design ideas and methods of the electrical equipment cabinet were clarified. The design method is applied to the design process of potentiostats and oil wells standardized switch boxes. Through various tests of the products, the factors that affect the operation of the products are analyzed.

The chassis and cabinet of electrical products are indispensable in the electronics industry. Its quality, shape and effect, market value, and economic benefits are closely related to the product's structural design. For a good electronic product, its structure design should have new, odd, brilliant, hi, and true appeal. New: There are incomparable novelties in similar products; odd: bold use of structures and shapes that others have not used; Yan: Unique bright and vivid eye-catching sensation in the environment. Hi: The product can better meet the physiological and psychological requirements of people. Truth: It can reflect the unity of content and form better than other products. The design of electrical equipment cabinets is one of the key links in the design of electrical equipment. It is a prerequisite for realizing the functions of electrical equipment and is also the basis for ensuring personal safety and equipment safety. In the process of vigorously enhancing the level of self-innovation, our products require all aspects of research and design. Therefore, optimizing the design of electrical cabinets plays an extremely important role.

Oilfield production requirements for electrical cabinets The cabinet products currently in use in Dagang Oilfield mainly include oil well control cabinets for oil and gas stations, frequency conversion cabinets, switch boxes, power distribution cabinets, reactive power compensation cabinets, fire alarm control boxes, and potentiostats. Power cabinets, server cabinets, and various types of cabinets for automatic control systems. The electrical cabinet should be protected against rain, dust, and small animals. It must also ensure that the heat is increased by heat. It is necessary to meet the opening requirements of the cabinet and ensure good electromagnetic shielding performance. And all metal parts in the cabinet should adopt anti-corrosion measures. Steel profiles and metal structural parts should be rust-removed, oil-removed, and phosphatized before painting, thoroughly remove rust and welding slag, and then spray anti-corrosion primer.

For outdoor electronic equipment, whether it is overhead, buried or ordinary outdoor placement, there is a corresponding IP, IK requirements.

IP means that all electrical equipment housing shells should have a degree of protection against external solid foreign matter entering the shell and touching live parts or running parts and preventing water from entering the shell. The code representing the degree of protection is composed of the character letter IP and two additional character numbers and supplementary letters appended. The first character and the digits added after the digits indicate that the appliance has the protection of the human body and the components inside the casing, and the second digit indicates the protection due to the harmful influence caused by the water in the casing.

The IK rating indicates the degree of protection of the enclosure against harmful mechanical impact from the outside, and the designation of the grade consists of the characterizing character IK and the attached characterizing number.

Electrical cabinets can be divided into indoor cabinets and outdoor cabinets according to the use environment.

The protection level of outdoor electrical equipment used in Dagang Oilfield is IP54, which means that dust can not be completely prevented from entering the shell. However, the amount of dust entering is not enough to affect the normal operation of electrical appliances; splashing water in any direction should have no harmful influence. The cabinet enclosure is subject to a collision rating of IK07. The design method for electrical equipment cabinets has a wide variety of environments. Different types of research objects have different environmental classifications. The environment in which electronic equipment is located can be broadly divided into natural environments, industrial environments, and special environments. There are three major categories of environmental use, of which the environmental factors include temperature, humidity, atmospheric pressure, salt fog, precipitation, and biological factors. The Dagang Oilfield is located in the warm semi-humid continental monsoon climate of the Northern Hemisphere. The general climate characteristics are as follows: The four seasons are distinct, with sufficient sunshine, moderate temperature, and rain and heat during the same period. The annual precipitation distribution is uneven, with the characteristics of spring drought, summer flood, and autumn hanging. The environmental protection design of electrical equipment cabinets refers to the selection of components and materials and protective measures, especially the environmental temperature, humidity and dust will have adverse effects and consequences on electronic equipment. Table i lists the main influences and results of various climatic conditions on components and materials in electronic equipment.

All should meet the safety requirements.

Enclosure protection 1. Enclosure protection of enclosures shall be in accordance with the provisions of GB4208-2008 and to Table 1. The main impacts of climatic conditions on components and materials in electronic equipment. Weather conditions Effect of temperature Softening of high temperature materials; Chemical decomposition and aging; Equipment overheating; Metals Different expansion; metal oxidation accelerated structure strength weakened; electrical properties of component materials change, even damage; component damage, fire, low melting point solder cracking cracks or solder joints off; grounding contact resistance increases, metal material surface resistance increases; Material becomes brittle; material shrinks differently; component performance changes; strength of the sealed rubber-hardened structure weakens, cable damage, wax harden; active parts get stuck, plugs, sockets, switch pieces, etc. contact poorly; battery capacity decreases; water vapor Deposition; Reduced insulation resistance, increased dielectric constant, increased dielectric loss; High moisture absorption of moisture; Some plastic parts uplift and deformation, changes in electrical properties, structural damage; Humidity metal corrosion; Structural strength weakened, active part was stuck , Increased surface resistance, poor electrical contact, chemical changes; other components The material is contaminated by corrosive substances; Salt spray metal corrosion; Insulation material resistance decreases; Some plastics expand, dielectric loss increases dust and sand into the active part; Bearings, switches and relays are damaged, contact is bad, and electrical noise is generated; moisture absorption decreases Insulation performance of components and materials. Daylight equipment damage due to overheating of components, ignition; photochemical effects. Accelerated aging and decomposition of organic materials, paint fading and exfoliation Gale damage to the antenna, structure structure less IP30, for waterproof and dustproof requirements It reaches IP54 level.

The capability shall meet the following requirements: The door glass of the cabinet shall reach the rest of the cabinet and other parts of the cabinet shall have special grounding screws to ensure the continuity of the protection grounding; it shall be set separately from the protection ground and insulated, at least to double Insulation or Reinforced Insulation and Dangerous Live Parts Therefore, the material and structure of the electrical cabinet must meet the requirements of the weather conditions such as temperature, humidity, salt spray, dust and sand, high wind, etc. The normal use condition of the electrical cabinet is that the ambient air temperature does not exceed + 55C, and in 24h its average temperature does not exceed +35C. The lower limit of the ambient air temperature is -10C; atmospheric conditions are clean air, relative humidity 20% 80%, no condensation. The cabinets are installed indoors. They are installed in places where the surrounding air is clean, non-conducting, without explosion dust, and without strong vibration and impact. If the place of use does not meet the normal conditions of use, it shall be designed and used according to the relevant product standards or according to the manufacturer's and user's agreement. When the cabinet is used in a place suitable for mold growth, the cabinet should have the ability to resist mold; when the cabinet is used at the seaside, the cabinet should have the ability to resist salt spray corrosion.

The waterproof of the electric cabinet shall be started from the structure of the cabinet, such as the appropriate outer edge of the cabinet top, the flanged structure of the door frame, the sealing strips at the joints, etc., and the high temperature resistant and anti-aging materials shall be used when selecting the sealing strips. A filter can be added to the vent to achieve dust protection in the electrical cabinet.

When electrical equipment is installed and used in accordance with regulations, it must ensure the safety of people and equipment and no danger shall occur. All equipment, devices and parts are isolated; 3, grounding wire should be yellow-green two-color wire.

2. The safety signs should mainly give the following information: Safety Fingerprints Safety Warning Signs Heat Dissipation Design Various Components Typical Cooling Methods 1 Components that Need Thermal Design The following components need thermal design: Power Diodes, Power Transistors, traveling wave tubes, thyristors, oscillating tubes, switch tubes, electronic modules, microcircuits, high-power resistors, transformers, capacitors, etc.

According to the use of environmental conditions, reliability requirements, equipment load size and heat value reasonable ventilation and heat dissipation design. When considering the heat dissipation method, the natural cooling method should be used as much as possible. Only when the natural cooling cannot meet the heat dissipation requirements, forced air cooling, water cooling, or other cooling methods can be used.

When necessary, cabinets can be equipped with natural inlet and outlet openings to ensure good ventilation and heat dissipation to meet reliability requirements. When setting ventilation holes, shutters, vents or exhaust pipes for ventilation and heat dissipation, consider dust, insects, rodents, etc., and take necessary protective measures.

When using forced convection to cool the equipment, the air duct shall adopt one of the following; bottom surface - front air inlet, top surface - rear air outlet (preferred) bottom surface - front air inlet, top air outlet; front central air inlet, rear Outer air in the middle; front central air inlet, top-back air outlet; front central air inlet, top air outlet.

According to the nature of the interference field, the shielding can be divided into three categories: electrical shielding, magnetic shielding and electromagnetic shielding.

The electric shield is made of a good conductor and the isolation is transmitted through the capacitive coupling. The electric shield must be well grounded, and the smaller the ground resistance, the better the shield effect.

The magnetic shielding material is related to the frequency of the interference source and the shielding object. At low frequencies, ferromagnetic materials with high magnetic permeability and low magnetic resistance are often used; at high frequencies, the permeability of the ferromagnetic material itself decreases, and the shielding effect deteriorates. When the shielding body is closer to the circuit, the coil Q is also reduced. Value, increase the loss, usually use good conductor material instead.

Electromagnetic shielding is mainly used for high frequency, and it uses the reflection on the metal interface and the absorption in the metal layer to suppress electromagnetic radiation interference. The shield is usually made of good conductors.

The reflection mainly depends on the ratio of the impedance of the wave to the impedance of the metal. The larger the ratio, the greater the reflection. As a result, the high-resistance field (electric field) is mainly reflected, and the low-resistance field (magnetic field) has almost no reflection. This is why low frequency magnetic field shielding is very difficult.

The transmission of electromagnetic waves in metallic materials will be attenuated, and the degree of attenuation will depend on the material's magnetic permeability and electrical conductivity. For an electric field, a material with a high conductivity has a large attenuation; for a magnetic field, a material with a high magnetic permeability has a large attenuation. Obviously, the thicker the material, the greater the degree of attenuation and the better the shielding effect.

Effect of openings on shields Due to heat dissipation, mounting buttons, switches, etc., circular, square, or rectangular holes need to be opened on the shield. As shown, the direction of the holes should be taken into account to ensure that the eddy currents are in the material. Uniform distribution. Obviously, the effect of d is good, and b and c cannot meet the shielding requirements and may become a slit antenna. The hole area is S and the shield area is A. When A>>S is satisfied, the diameter of the round hole or the side of the square hole is shorter than the wavelength, HP=4(S/A)3/2H0. The n holes constitute the array holes, and the measures for improving the shielding effectiveness of the openings are the largest for the array holes, and the second is the maximum size of the holes. When there is a conflict between heat dissipation and shielding, the ideal approach is to increase the depth of the hole, increase the maximum size of the hole, or reduce the maximum size of the hole, while reducing the hole spacing (increasing the number of holes).

In practice, the shielding effectiveness of the array holes is up to 30db/1GHZ. If a higher level is required, or if the contradiction between shielding and heat dissipation is outstanding, consider the use of a waveguide ventilation plate. The shielding effectiveness of the waveguide ventilation plate can be very high and the porosity is large, which is an ideal ventilation method. But must pay attention to its expensive price, under normal circumstances is not recommended.

The ventilation holes have good stability and consistency of shielding effectiveness, and basically do not need to consider the safety margin when designing.

In order to reduce radiation, the openings can be shielded in general. Shielding materials are broadly divided into 5 categories, wire meshes, perforated metal sheets, waveguide ventilation windows, aluminum mesh laminate mesh ventilation windows, and foamed metal ventilation windows.

The cabinet should be simple in structure, economically reasonable, beautiful in appearance, convenient in operation and maintenance, and painted in soft colors.

The structural layout of the cabinet should be concise, reasonable, easy to use and maintain, and meet the requirements of ergonomics; the cabinet is beautiful in appearance and the color of the cabinet is coordinated to meet the monitoring requirements of the operating personnel; the weld of the cabinet welding should be solid and reliable, no cracks, no obvious The lack of fusion, porosity and other defects. The outer surface should be polished smooth; the joints of the cabinets and the gap between the door and the superstrate should be well-proportioned, the difference in the width of the same gap within 1m should not exceed 1mm, and the difference between the widths of the gaps larger than 1m should not exceed 1.5mm; The fixed joints should be solid and reliable, there should be no loosening phenomenon, there should be anti-loose measures to remove the threaded joints Removable connection should be reliable, easy removal, disassembly does not affect the quality of re-assembly, and does not increase the re-assembly Difficulty.

Surface coating and protection requirements should be applied to the cabinet frame, side panels, door panels and other surfaces should have a good coating; coating layer adhesion should not be less than the GB/T9286-1998 level 1 performance requirements; The coating layer should not be dazzling, reflective, etc., and generally should be selected matte coating; the color of the cabinet is recommended to use neutral colors.

The doors of the cabinet should be opened and closed flexibly, and the locking should be reliable. The opening angle of the door should not be less than 120. When necessary, the limit mechanism of the door should be set; the opening direction of the handle of the mechanical door lock should be coordinated with the opening of the door; when the door is opened right, the operator faces the equipment operation surface and presses the inverse. When the hour hand goes outside, the rotation of the door lock is counterclockwise. When the door is left open, it is clockwise.

The degree of protection of the cabinet door lock should not be lower than the degree of protection of the cabinet.

Nameplates, signs, and logo product nameplates should be placed in front of the cabinet or under the machine's front panel. The nameplate of the unit running in the field should be placed on the cabinet head components. The nameplate of the product carried by the cabinet should be placed on the cabinet cabinet head. Below the components, the signs for the devices and components shall be placed in a conspicuous and unambiguous position; the safety signs shall be placed in the corresponding parts of the cabinet and comply with the enterprise name mark, which shall be placed in the lower part of the cabinet door, or integrated with 3 Together.

Case Analysis of the Cabinet Combined Cathode Protection Potentiostat Combined cathode protection Potentiostat integrates the thyristor control mode and inverter control mode organically, enabling independent operation in both modes and automatic switching operation under fault conditions. The operation is stable and reliable.

As the combined cathodic protection potentiostat uses two drawer units vertically arranged in the cabinet, the drawer contains electronic components such as thyristor and thyristor. Therefore, in the design of the cabinet, it is necessary to consider forced air cooling, electromagnetic shielding and Assembly difficulty.

The heat dissipation design installs two exhaust fans on one side of each drawer unit, and two ventilation holes on the other side. The fan is equipped with a detection circuit to detect the temperature in the drawer in real time. Based on the detection results, the fan selects whether the operation mode is one operation or two operations to ensure the normal operation of the device. The fan's operating mode also meets the energy saving requirements. Ventilation holes are provided on both sides of the cabinet, one side enters the air, and one side is out of the air to ensure air circulation inside the housing; electromagnetic shielding installs wire mesh at the ventilation holes of the drawer unit to reduce radiation and improve the shielding of the equipment. effect.

Oil well standardization switch box mechanical interlocking load disconnecting switch The operating handle and the load disconnecting switch are mechanically interlocked. When the operation isolation handle of the load disconnecting switch on the panel is closed, the operator cannot open the door and plays a role in safety protection; waterproof cabinet The door frame adopts the flanging structure and the sealing seams are adopted at the seams to allow the rainwater to flow down the frame without entering the cabinet. The bottom panel of the anti-small animal box has four round wire inlet holes, and the line holes are equipped with protective baffles. Prevent small animals from entering.

Based on the relevant standards and specifications of the electrical cabinet design, based on the actual requirements of the oil field production for the cabinet, the cabinet design scheme for each device is refined to set clear goals for each link, which will lay the foundation for the successful development of the equipment.

In the design process, some aspects of the considerations are mutually restrictive. At this time, the pros and cons of each factor need to be measured, and priority should be given to the requirements of safety technology. Through the relevant tests on the design and performance of the combined cathodic protection potentiostat and the well-standard switchgear cabinet, it is shown that the design of the electric cabinet needs a thorough and meticulous demonstration. The cabinet design is no longer superficial, and it is necessary to raise awareness and attention. .

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