Structural design and electrical performance requirements of drawer type switchgear

Whether the overall structural design of the low-voltage switchgear is reasonable involves four aspects of electrical performance requirements:

1. Insulation coordination

2, resistance to thermal stability

3. Improve the protection ability of fault arc

4, temperature rise

1 Strengthen the insulation performance of the equipment, which helps to improve the overall resistance to overvoltage.

The insulation coordination of the drawer type switchgear is determined according to the environmental conditions of the equipment, installed in the position of the power supply system (overvoltage category), and the performance of the insulation material determines the minimum clearance and creepage distance. The group of insulating materials is divided into four categories according to the numerical range of the tracking index (CTI) of the material, the material combination is I600≤CTI; II400≤CTI<600; III175≤CTI<400E;IV100≤CTI<175E, low pressure There are many insulation parts in the drawer cabinet, such as insulators, busbar clamps, and functional boards made of polycarbonate engineering plastics, secondary connectors, etc., which affect the overall insulation performance of the cabinet. Statistics show that about 50% of accidents in China's electrical products are caused by insulation systems. Of course, if the insulation is properly selected and the quality meets the requirements in the normal operation of the system, the electrical performance is still fully satisfactory. After all, the voltage level is not high. If the level of contamination is 4, the selection of insulation should be considered as a key consideration. For example, the main busbar insulation in the cabinet adopts creepage distance and clearance higher than the standard performance, and measures such as insulation with high CTI value.

2 The dynamic thermal stability of the cabinet has always been considered by the customer in the first place. Especially in the case of high current, the structural strength of the cabinet is very high. Once a short circuit occurs, the cabinet should have the ability to withstand short-circuit current. As a designer of the low-voltage cabinet, first consider the fixing method of the horizontal main busbar and Place the position. For example: the low-voltage incoming current of a large power plant of a 300 MW unit is generally greater than 4000 A. The main busbar is 3X3 (100 x 10) according to the process requirements, but in practice it is not selected in this way. Generally, single-phase is arranged in two layers. Each layer has 2 80 x 10 rows and is fixed by special insulated busbar clamps. This structure design disperses the impact of short-circuit current on the cabinet and greatly improves the overall resistance of the cabinet to short-circuit current. The contact pressure of the bolt connection part is strengthened to ensure the torque and the rotation angle of the bolt connection, and also to resist the impact of the short-circuit current.

3 In order to improve the fault arc protection capability of the equipment, metal baffle isolation precautions are generally adopted inside the structure, and insulation isolation of the live conductor is added, and a pressure relief channel and an arc detecting device are added.

There are many factors contributing to the fault arc and will not be discussed here. The purpose of arc protection is specified in IEC 4397-1. The final requirements are: 1 to prevent access to dangerous parts of adjacent functional units; 2 to prevent solid foreign objects from entering a unit from one unit of the plant; High personal protection rating. To this end, the functional rooms of the cabinet are strictly separated, mainly divided into functional unit room, bus room (horizontal bus room and vertical bus room), cable room and other three parts, which not only prevent the spread of accidents, but also facilitate the electrification maintenance. The unique design of the low-pressure drawer cabinet is designed to limit the generation of arcs and limit the intensity and duration of the arc.

In order to prevent the fault arc generated in the switchgear from causing injury to nearby personnel, a pressure relief device is added to the top of the cabinet. When the fault arc ignites, the pressure generated forces the pressure relief device to open, and the hot gas is discharged to the top of the cabinet, thus ensuring personnel safety. This passive arc protection function is a measure that must be taken in the event of an arc, mainly to protect personal safety, but it is an indispensable important link in the design of the cabinet structure. Incidentally, the development of low-voltage switchgear for fault arc protection is particularly important after verification by IEC-1641 "Guidelines for fault arc test of enclosed low-voltage switchgear and control equipment".

The temperature rise of the 4-drawer switchgear is also an important indicator for the design of the cabinet structure. The temperature rise in the cabinet is related to the size of the casing, the installation form of the casing, the size of the ventilation slots and ventilation slots, the number of internal horizontal partitions, the effective loss of components and the number of drawer units. As a manufacturing plant, the design institute cannot change the various parameters of the drawing when the product design is finalized. In the actual design process, some parties or design institutes are equipped with too many drawers in order to save space and equipment. Although the installation can be completed, the temperature rise of the cabinet is neglected, especially in the summer. If the distribution room is not well ventilated, the temperature rise in the cabinet is often too high during the use process, resulting in insulation damage caused by equipment damage. Caused a short circuit accident. This should be brought to the attention of our designers and it is necessary to present it to the other party.

In addition to the temperature rise requirements, the design of the upper and lower ventilation slots should also consider whether the protection level is met. The IP54 cabinet should fully consider the derating factor, otherwise it will easily cause the temperature rise to be too high. In addition, the enterprise should also regularly warm the temperature. Lit test. Get enough data and make the necessary improvements and adjustments to the structure inside the cabinet.