I. Causes and hazards of moisture in cabinets

(1) Causes of moisture in the cabinet

1) The environment is closed. The ring main unit or switch room itself is a closed environment. Due to poor ventilation conditions, it is easy to accumulate moisture and generate condensation. In order to reduce the occupied area, many large buildings today build switch rooms and power distribution rooms in the basement. . Since the indoor environment and cable trenches are closed and dark, and there is a lack of ventilation windows, it is not conducive to air convection. The moisture accumulated in the room is not easy to escape, which will greatly increase the relative humidity in the opening and closing room.

2) Temperature difference changes. As the temperature changes caused by the heating of the electrical equipment in the cabinet, coupled with changes in the ambient temperature, water vapor will condense into condensation when it encounters the cold cabinet surface or wall.

3) Air humidity. Harsh humid and hot environments can easily lead to condensation or even water accumulation on the surface of electrical equipment, severe degradation of insulation performance, corrosion damage, and even serious failures. Relevant studies have shown that general electrical equipment is suitable for normal operation in an environment with a relative humidity of 40% to 60% [1-2]. When the relative humidity exceeds 80%, moisture will have adverse effects on electrical equipment.

(2) Hazards caused by moisture in electrical equipment

1) The harm of condensation. Condensation on the surface of the insulating material, especially when the insulation surface is dirty, will reduce the creepage of the insulation surface and increase the number of free electrons on the surface of the medium, causing the insulation resistance value to decrease. When the resistance value drops to a certain level, under the action of the conduction current Will cause partial discharge.

2) The harm of ozone. When the water vapor in the cabinet undergoes partial discharge, the water molecules will be ionized into hydroxide ions. The oxygen ions and the oxygen in the cabinet will combine to form ozone. Ozone has strong oxidizing properties. The bimolecular chain structure of the epoxy resin insulation material will be damaged by ozone. The insulation layer of electrical equipment is also destroyed.

3) Hazards of electrochemical corrosion. Each metal has a critical relative humidity, which is between 50% and 80%. When the humidity reaches the critical relative humidity, an adsorbed electrolyte film will be produced on the metal surface. The existence of this liquid film makes Metal changes from chemical corrosion to electrochemical corrosion [3]. Due to the change in corrosion properties, the corrosion ability is greatly enhanced. When the atmosphere contains pollutants and the humidity exceeds 80%, the corrosion rate will be greatly accelerated, which will seriously damage equipment components.

Condensation, ozone, and electrochemical corrosion often combine to accelerate the degradation of insulation performance and damage equipment components. The discharge phenomenon on the insulation surface can not only generate heat and ozone, but also cause the hydrophobic performance of the insulation material to further decrease. Water droplets will move or deform under the action of electrostatic force, causing the wet area of the insulation surface to expand, thus expanding the discharge area. Due to partial discharge along the surface, discharge traces in the shape of leaf meridian are formed on the insulation surface, which gradually leads to insulator flashover. happened.

II. Moisture-proof measures for electrical equipment in cabinets

(1) Raise the height of the civil construction of the new switch office. The ground civil work of a newly built switch room or ring main unit should be raised (cushioned) to a height of about 40cm or more. This can effectively improve the impact of moisture formed by groundwater on the opening and closing floor and cable trench, and can also prevent the backflow of rainwater. This is one of the effective measures to maintain a dry operating environment for switches.

(2) Seal the cable holes tightly. Cable holes must be tightly sealed and long-term water accumulation in cable trenches must be prevented. This can prevent moisture from the cable trench at the bottom of the cabinet from entering the switch cabinet, and prevent moisture and dirty corrosive gases from entering the cabinet. There are numerous examples of cabinets getting damp due to cable holes not being sealed tightly. Insufficient sealing of cable holes is an important cause of moisture in the cabinet.

(3) A heater is installed in the switch cabinet. Using a heater to keep the air inside the cabinet dry is also a common way to reduce the humidity of the air inside the cabinet. Usually, a heater (or environmental monitoring system) capable of automatic control is installed in the switch cabinet. By detecting the equipment environment, the heating device is automatically operated to adjust the air humidity in the cabinet. In addition, after the air in the cabinet is heated, it produces a gas expansion effect, which can also prevent moisture and corrosive gases from outside the cabinet from intruding into the cabinet, so that the equipment inside the cabinet can operate in a good air environment.

(4) Maintain effective ventilation. Both the switch room and the ring main unit are designed with ventilation windows. For switch rooms with closed environmental conditions, it is necessary to install ventilation fans. The ventilation fans are conducive to the exchange of fresh air indoors and outdoors, and can effectively reduce the concentration of turbid gases in the switch room. . When maintaining effective natural ventilation in the opening and closing room, it should be noted that the vent should be as far away from the switch cabinet as possible. Usually, there are two ventilation louvers on the upper and lower walls of the opening and closing room. According to the design requirements, each louver leaf is inclined downward at a certain angle. When looking straight at the window, it is impossible to see through, thus preventing the intrusion of rain and dust. The shutters should also be equipped with protective wire mesh to prevent small animals from getting into the opening and closing space.

(5) Install an industrial dehumidifier. It is advisable to install a 3-5kW industrial dehumidifier underground or in a closed humid environment to control the indoor air humidity to the ideal standard. Indoor relative humidity should be controlled below the critical relative humidity as much as possible, and the humidity can be controlled at 50-75%. Although using a dehumidifier is very effective, its power consumption is a significant drawback.

(6) Inspection of equipment inside the cabinet. Dampness in the cabinet does not occur within a few days, but is caused by the long-term accumulation of moisture in the cabinet. It is also related to local climate characteristics (seasonal humidity, etc.). The absence of signs of moisture in the cabinet for a while does not mean that it will not occur at a certain time. Moisture: There have been cases where the ring main unit to be put into operation many days after the new construction was completed showed signs of moisture. When the cabinet is slightly damp, fine dewdrops will appear on the inner wall of the cabinet, and moisture vapor is easily visible inside the pressure indicating instrument of the sulfur hexafluoride gas chamber. When these phenomena occur, it means that the cabinet is already in a hidden danger of getting damp. Take moisture-proof measures with reference to the above matters to prevent further deterioration of moisture in the cabinet. It can be seen that it is also very important to do a good job of inspection of indoor electrical equipment such as ring main units and switching stations. Through management methods such as regular inspections, focused attention, and timely rectification, we ensure that the equipment in the cabinet operates reliably in a good environment.

III. Conclusion

Dampness of electrical equipment in cabinets is an important cause of failure and is extremely harmful to electrical equipment. The quality of construction is closely related to preventive measures, so acceptance checks must be done. In addition, it is necessary to combine equipment periodic inspection work to detect moisture problems in time and deal with them in a timely manner to reduce further damage to electrical components caused by moisture factors.

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