Low-voltage switchgear completes the redistribution of electric energy in the low-voltage distribution system, and realizes the role of control, measurement, protection and conversion of the power grid. It plays a pivotal role in national economic construction, especially in petroleum, chemical, metallurgy, construction, mineral and other industries. Therefore, the quality level of low-voltage switchgear, especially the stability, reliability and safe operation of electrical performance, is closely related to the country's economic strength, scientific and technological development and people's living standards. Under the impetus of this general trend, the varieties of low-voltage complete switchgear have been renovated, the quality has been improved, and the electrical safety has been perfected.
1. Comparison of the structure of several mainstream drawer switchgear functional units
Low-voltage drawer switchgear is widely used in low-voltage switchgear because there is no knife switch to reduce the installation space of components and increase the number of circuits in the cabinet, and the drawers of the same specifications can be interchanged, so that the spare drawer can be replaced in time when the line fails, thus reducing the power outage time. Both production, variety, electrical performance are more than the fixed cabinet. At present, the market demand for several domestic mainstream drawer cabinet has three kinds, namely MNS, GCS, GCK (GCL). These three cabinet types are basically similar from the appearance, but their internal functional unit structure are differences. Customers should choose the cabinet type according to its characteristics and actual needs, the following is a comparison of the three types of cabinets in the form of a brief table on the 3C certification products that have been obtained by Chint sets of companies.
2, the overall structure of the electrical performance of the impact
The overall structure layout of the low-voltage drawer cabinet should be beautiful, generous, smooth lines, the division of the drawer function unit is reasonable, in line with the requirements of the electrical parameters.
The frame structure of "C" profile is simple and easy to install. At present, low-voltage drawer switchgear has been commonly used.
Whether the overall structure design of low-voltage switchgear is reasonable involves four aspects of electrical performance requirements.
1、Insulation fit
2、Resistance to dynamic thermal stability performance
3、Improve the protection ability of fault arc
4、Temperature rise
①Strengthening the insulation performance of the equipment helps to improve the overall resistance to overvoltage.
The insulation fit of drawer-type switchgear is determined by the minimum electrical clearance and creepage distance in accordance with the environmental conditions of the equipment, the location of the power supply system in which it is installed (overvoltage category), the performance of the insulation material, and other factors. The group of insulation materials is divided into 4 categories according to the range of values of the material leakage starting index (CTI), and the material combination is Ⅰ600≤CTI; Ⅱ400≤CTI<600; Ⅲ175≤CTI<400E; Ⅳ100≤CTI<175E, there are many insulating parts of the low-voltage drawer cabinet, such as: insulators, busbar clamps and functions made of polycarbonate engineering plastics Board, secondary connectors, etc., all of which affect the overall insulation performance of the cabinet. Statistics show that about 50% of the accidents in our electrical products are caused by the insulation system. Of course, if in the case of normal operation of the system, insulating parts as long as the appropriate choice, quality in line with the requirements, its electrical performance is still fully able to meet the requirements, after all, the voltage level is not high. If under the condition of 4 level fouling level, the selection of insulating parts should be considered as a key point, for example, the main busbar insulation in the cabinet adopts higher creepage distance and electrical gap than the standard performance, with high CTI value of insulating parts and other measures.
②Cabinet dynamic thermal stability performance is always the first consideration for customers. Especially in the case of high current, the structural strength of the cabinet is very demanding, once there is a short circuit, the cabinet should have the ability to withstand the short-circuit current, as a low-voltage cabinet designers, the first thing to consider the level of the main bus bar fixed way and position placement. For example: 300MW unit of large power plant low-voltage incoming current is generally greater than 4000A according to the process requirements for the main bus bar 3X3 (100X10), but in practice is not so selected, generally consider single-phase according to the upper and lower two-layer arrangement, each layer of two 80X10 row, and the use of special insulated bus bar clamps for fixed, such a structural design, dispersion of the short-circuit current on the impact of the cabinet, greatly improving the overall cabinet resistance. Greatly improve the overall short-circuit current resistance of the cabinet. Strengthen the contact pressure of the bolt connection parts, to ensure the torque and angle of the bolt connection, can also resist the impact of short-circuit current.
③ In order to improve the fault arc protection capability of the equipment, metal partition isolation preventive measures are commonly used inside the structure, insulation isolation of live conductors, additional pressure relief channels, arc detection devices, etc.
Fault arc generation factors are a variety of factors, not discussed here. IEC4397-1 in the purpose of arc protection to make specific provisions, the ultimate requirement is: ① to prevent touching the adjacent functional unit of dangerous parts; ② to prevent solid foreign matter from a unit of the set of equipment into the adjacent unit.
③ provide the highest possible level of personal protection. For this reason the functional rooms of the cabinet are strictly separated, mainly into three parts: functional unit room, bus bar room (horizontal bus bar room and vertical bus bar room), cable room, etc. This prevents the proliferation of accidents and facilitates powered maintenance, which is a unique feature of the structural design of low-voltage drawer cabinets, with the aim of limiting the generation of electric arcs and limiting the intensity and duration of the arcs.
In order to prevent the fault arc generated in the switchgear to cause injury to nearby personnel, the top of the cabinet is equipped with additional pressure relief devices. When the fault arc ignition pressure generated by forcing the pressure relief device to open, the burning gas exhausted to the top of the cabinet, thus ensuring the safety of personnel. This passive arc protection function is in the arc has occurred as a last resort to take measures, mainly to protect personal safety, but is an important link in the design of the cabinet structure is indispensable. By the way, the development of arc fault protection of low-voltage switchgear, and after the IEC-1641 <enclosed low-voltage switchgear and control equipment arc fault test guidelines>> verification, is particularly important.
④ The temperature rise of the drawer switchgear is also an important indicator to examine the design of the cabinet structure. The temperature rise inside the cabinet is related to the size of the enclosure, the installation form of the enclosure, whether with ventilation slots and the size of the ventilation slots, the number of internal horizontal partitions, the effective loss of components and the number of drawer units and other parameters. As a manufacturing plant, the design institute in the case of completion of product design finalization, can not change the drawing of various parameters, in the actual design process some A party or design institute in order to save space and the number of equipment, too much equipped with the number of drawers, although the installation can be completed, but ignored the cabinet temperature rise, especially in the summer, if the distribution room ventilation is not good, often in the use of the cabinet temperature rise too high and lead to Equipment damage caused by insulation broken ring, resulting in short circuit accidents. This point should draw our attention to the majority of designers, it is necessary to propose to each other.
