[Influence of structural design of low-voltage withdrawable switchgear on electrical performance]: Preface Low-voltage switchgear completes the redistribution of electrical energy in the low-voltage power distribution system, and realizes the functions of control, measurement, protection, and conversion of the power grid. All places that use electric energy need to be equipped with low-voltage complete sets of equipment, which are used in the construction of national economy, especially petroleum, chemical, metallurgy, construction, minerals, etc ...

Preface

   The low-voltage complete switchgear completes the redistribution of electrical energy in the low-voltage power distribution system, and realizes the functions of control, measurement, protection, and conversion of the power grid. Every place that uses electric energy needs to be equipped with low-voltage complete equipment, which plays a pivotal role in the construction of the national economy, especially in various industries such as petroleum, chemical, metallurgy, construction, and minerals. Therefore, the quality level of low-voltage complete switchgear, especially the stable, reliable, and safe operation of electrical performance, is closely related to the country's economic strength, scientific and technological development, and people's living standards. In the late 1980s, with the urban and rural transformation, expansion, and infrastructure construction, the electrical equipment market expanded year by year, and manufacturers increased. The large investment in national power and energy also stimulated the development of the industry. Driven by this megatrend The variety of low-voltage complete switchgear is constantly renovated, the quality is constantly improved, and the electrical safety is also constantly improved.

   I. Comparison of functional unit structures of several mainstream drawer switch cabinets

   The low-voltage withdrawable switchgear reduces the installation space of components due to the absence of a knife switch and increases the number of circuits in the cabinet. At the same time, drawers of the same specifications can be exchanged. In the event of a line failure, the spare drawer can be replaced in time to reduce the power outage Therefore, it is widely used in low-voltage equipment. Both the output, variety, and electrical performance surpass the fixed cabinet. At present, there are three types of domestic mainstream drawer cabinets in the market, namely MNS, GCS, GCK (GCL). These three cabinet types are basically similar in appearance, but their internal functional unit structures are different. Customers should choose according to their characteristics and actual needs when choosing a cabinet type. Below is a comparison of the three cabinet types in the form of a brief table on the products that Chint has obtained 3C certification.

   Second, the impact of the overall structure on electrical performance

The overall structural layout of the low-voltage drawer cabinet should be beautiful, generous, and smooth, and the functional unit of the drawer should be reasonably divided to meet the requirements of electrical parameters.

   The frame structure of "C" profile is simple and easy to install. At present, low-voltage drawer switch cabinets have been widely used.

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

   1.Insulation coordination

   2, dynamic and thermal stability

   3.Improve the protection ability of fault arc

   4, temperature rise

   ① Strengthening the insulation performance of the equipment will help improve the overall resistance to overvoltage.

   The insulation coordination of the drawer switchgear is based on the environmental conditions of the equipment, the location of the power supply system (overvoltage category), and the performance of the insulation material to determine the minimum electrical clearance and creepage distance. The group of insulation materials is divided into 4 categories according to the value of the tracking index (CTI) of the materials. The material combination is Ⅰ600≤CTI; Ⅱ400≤CTI <600; Ⅲ175≤CTI <400E; Ⅳ100≤CTI <175E, low voltage. There are many insulation parts of drawer cabinets, such as insulators, bus clips, functional boards made of polycarbonate engineering plastics, secondary connectors, etc., all of which affect the overall insulation performance of the cabinet. Statistics show that about 50% of the 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, the electrical performance can still fully meet the requirements under normal system operation conditions. After all, the voltage level is not high. In the case of Class 4 pollution level, the selection of insulation materials should be taken as an important consideration. For example, the main bus insulation in the cabinet adopts creepage distances and clearances that are higher than the standard performance, and measures with high CTI insulation.

   ② The dynamic and thermal stability of the cabinet has always been considered by customers. 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 the short-circuit current. As a designer of a low voltage cabinet, the fixing method of the horizontal main busbar should be considered first. Location. For example: the low-voltage incoming current of large power plants of 300MW units is generally greater than 4000A. The main busbar is 3X3 (100X10) according to the process requirements, but it will not be selected in practice. Generally, single-phase layout is considered as two upper and lower layers. 2 layers of 80X10 rows, and fixed with special insulated bus clamps. This structure design disperses the impact of the short-circuit current on the cabinet and greatly improves the overall short-circuit resistance of the cabinet. Strengthen the contact pressure of the bolted joints, ensure the torque and corner of the bolted joint, and also resist the impact of short-circuit current.

   ③ In order to improve the equipment's fault arc protection capability, metal partition isolation measures are generally used in the structure to isolate the insulation of live conductors, add pressure relief channels, and arc detection devices.

   There are many factors that cause fault arcs, which will not be discussed here. The purpose of arc protection is specified in IEC4397-1. The final requirements are: ① to prevent access to dangerous parts of adjacent functional units; ② to prevent solid foreign objects from entering a neighboring unit from one unit of the complete set of equipment; ③ to provide as much as possible High level of personal protection. To this end, the functional rooms of the cabinet are strictly separated, and are mainly divided into three parts: the functional unit room, the busbar room (horizontal busbar room and vertical busbar room), and the cable room. This not only prevents the accident from spreading, but also facilitates live maintenance. This is The unique design of the low-voltage drawer cabinet structure is designed to limit the arc generation and limit the intensity and duration of the arc.

   In order to prevent the faulty arc generated in the switching equipment from causing harm to nearby people, a pressure relief device is added on the top of the cabinet. The pressure generated when the fault arc ignites forces the pressure relief device to open, and hot gas is discharged to the top of the cabinet, thereby ensuring personnel safety. This passive arc protection function is a measure that must be taken when an arc has occurred. It is mainly to protect personal safety, but it is an indispensable important link in the design of the cabinet structure. By the way, the development of low-voltage complete switchgear for protection against fault arcs, and the verification of IEC-1641 "Guidelines for Closed Low-voltage Complete Switchgear and Control Equipment Fault Arc Tests", are particularly important.

   ④ The temperature rise of the drawer switch equipment is also an important indicator for examining the structural design of the cabinet. The temperature rise in the cabinet is related to parameters such as the size of the casing, the installation form of the casing, whether there is a ventilation slot and the size of the ventilation slot, 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 various parameters of the drawing when the product design is finalized. In the actual design process, some Party A or the design institute have excessively equipped drawers to save space and equipment. Although the installation can be completed, the temperature rise of the cabinet is ignored, especially in summer. If the ventilation room is not well ventilated, the temperature rise inside the cabinet is too high during use, which will cause equipment damage and cause insulation breakage. Cause a short circuit accident. This point should attract the attention of our designers, and it is necessary to raise it to the other party.

   In addition to the design of the upper and lower ventilation slots, in addition to meeting the temperature rise requirements, consideration should also be given to whether the protection level is met. The IP54 cabinet should fully consider the derating factor, otherwise it will easily cause excessive temperature rise. In addition, for enterprises, the temperature should be regularly Liter test. Obtain sufficient data and make necessary changes to the structure of the cabinet