The role of HV switchgear and the causes of high temperature

High-voltage switchgear is electrical cabinet equipment used in power systems. The function of high-voltage switchgear is to turn on, off, control and protect the power system in the process of power generation, transmission, distribution and energy conversion. The components in the high-voltage switchgear are mainly high-voltage circuit breakers, high-voltage disconnectors, high-voltage load switches, high-voltage operating mechanisms, etc.

There are many classification methods for high-voltage switchgear. For example, by installing circuit breakers, it can be divided into mobile high-voltage switchgear and fixed high-voltage switchgear, or divided into open high-voltage switchgear and metal-enclosed box according to the cabinet structure. High-voltage switchgear, metal-enclosed high-voltage switchgear and metal-enclosed armored high-voltage switchgear.

The actual temperature rise inside the switchgear, especially the busbar connection, is always higher than the data measured in the type test. The main reasons are as follows:

(1) The measured data of the type test is usually completed in the laboratory, the duration is not long, usually no more than 8 hours, there is no cumulative effect of temperature rise, and it cannot be equivalent to long-term operation and continuous heating equipment.

(2) Different metals have different expansion effects. The metal expansion coefficient of steel bolts is much smaller than that of copper and aluminum busbars, especially bolt equipment joints. During operation, with the change of load current and temperature, the degree of expansion and contraction of aluminum or copper and iron will be different, resulting in creep, that is, the metal is slowly plastically deformed under stress. The creep process also has a lot to do with the temperature at the joint.

High temperature switchgear temperature rise test device

Practice has proved that when the working temperature of the joint exceeds 80 °C, the joint metal will expand due to overheating, and the position of the contact surface will be staggered, resulting in micropore formation and oxidation. When the load current is reduced and the temperature returns to the original contact position, it is not possible to directly contact the metal during the original installation due to the oxide film covering the contact surface. The increased contact resistance of each temperature change cycle increases the heat of the next cycle, and the increased temperature further deteriorates the operating conditions of the joint, creating a vicious circle.

(3) The fastening bolt at the joint is improperly compressed. Some installers or repairers believe that the tighter the attachment bolt, the better. In particular, the elastic coefficient of aluminum busbars is small. When the pressure of the nut reaches a certain critical pressure value, if the strength of the material is poor, and then increase the improper pressure, it will cause the contact surface to deform and bulge. The contact resistance increases, which affects the conductor contact effect.

(4) The conductivity of the selected conductor material does not meet the requirements, and most of the conductor raw materials are not pure enough.

(5) Other factors on site, such as improper installation and maintenance process, such as improper handling of the busbar contact surface during processing, connection, and installation, unevenness, flatness, and uncoated special electric grease. The contact area decreases, the contact resistance increases, and heat is generated.