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Researchers Lu Huiqiang and Xiao Peng from XJ Electric Co., Ltd., Hangzhou Taim Electric Co., Ltd., Zhejiang Tianzheng Electric Co., Ltd., Zhejiang Yongji Electric Co., Ltd., and Wenzhou Shengpu Electric Co., Ltd., wrote an article in the first issue of "Electrical Technology" magazine in 2019, pointing out that the low standardization of low-voltage switchgear, as an important component of the power distribution network, has become a major factor restricting the development of the industry. Standardized design, improving the versatility and interchangeability of equipment from various manufacturers, can not only improve the overall quality of low-voltage switchgear but also increase user satisfaction.
After decades of development, my country's low-voltage complete set technology has made great strides, significantly narrowing the gap with international advanced technologies in terms of capacity, safety, and reliability. In recent years, my country has continuously increased investment in its power grid, resulting in significant progress in basic distribution network construction. However, overall electricity consumption levels still lag behind international advanced levels; for example, uneven development between urban and rural areas and the urgent need to improve power supply quality remain issues.
Improving power transmission and distribution capacity and building a technologically advanced and environmentally friendly power distribution network are the top priorities for my country's power industry. As a crucial component of the power distribution network, the manufacturing level of low-voltage switchgear directly impacts the stability and quality of my country's power distribution system. The development of low-voltage switchgear has reached a bottleneck; its low standardization and weak industry versatility have become major factors restricting the industry's development.
1. Current Status of Standardization of Low-Voltage Switchgear
Low-voltage switchgear products comply with the GB7251.1—2013 standard "Low-voltage switchgear assemblies" (IDTIEC614391, 2011). In power systems, from the generation system to the transmission system and then to the distribution network, as well as in the process of power conversion, low-voltage switchgear products play a role in switching, controlling, and protecting electrical equipment. They are mainly suitable for various occasions such as building construction, infrastructure construction, energy, and industrial control.
Currently, the four main types of low-voltage switchgear commonly used in China are: ① MNS type drawer-type switchgear; ② GCK type low-voltage withdrawable switchgear; ③ GCS type low-voltage withdrawable switchgear; ④ GGD type AC low-voltage distribution cabinet.
During the localization of low-voltage switchgear production, through continuous exploration and improvement, its structure has become increasingly compatible with the operating habits of Chinese users, and its practicality and operability have also improved significantly. The low-voltage switchgear industry has relatively low entry barriers, and with economic and social progress, more and more manufacturers of complete sets of equipment have emerged. While open design and personalized production have contributed to the industry's prosperity, the problems that have arisen have also become increasingly prominent. Poor versatility, low standardization, and inconsistent production quality among manufacturers, coupled with the lack of a sound elimination mechanism, all pose significant hidden dangers to the entire industry.
In addition, the low profit margin of low-voltage switchgear is also a major factor restricting the industry's development. The long distance between manufacturing plants and project sites often results in high after-sales and maintenance costs, leading to passive after-sales service, poor user satisfaction, and significant damage to the reputation of manufacturers and even the entire industry.
Low-voltage switchgear is partially standardized in its initial design, such as a uniform cabinet height of 2200mm, standard cabinet widths of 600mm, 800mm, and 1000mm, and drawer widths and depths. However, the internal component distribution, functional unit structure, dimensions, and primary and secondary interfaces are designed by individual manufacturers based on their own capabilities or design habits, resulting in extremely low universality within the low-voltage switchgear industry. If equipment malfunctions, users need to contact the original manufacturer for repair or replacement. If the original manufacturer has been deregistered, a new supplier is required to measure the original cabinet dimensions on-site to ensure successful cabinet integration.
Through the analysis of numerous issues, the need for standardization in low-voltage switchgear has become increasingly apparent. Standardized design enhances the versatility and interchangeability of equipment from various manufacturers, not only improving the overall quality of low-voltage switchgear systems but also enabling on-site after-sales service from other equipment manufacturers when the distance is far. This improves after-sales efficiency and user satisfaction while reducing after-sales costs.
2. Standardized Design of Low-Voltage Switchgear
The standardized design of low-voltage switchgear should adhere to the principles of safety and reliability, robustness and durability, unified standards, universal interchangeability, reasonable grading, and wide applicability. It should consider both the manufacturer's capabilities and user experience satisfaction, improving the overall standardization of low-voltage switchgear through improvements and adjustments. Depending on the specific equipment requirements, the standardization design primarily focuses on the cabinet, primary interfaces, and secondary interfaces of the low-voltage drawer cabinet.
2.1 Standardization of cabinets
1) The cabinet material needs to be standardized. Different materials used in different assembly methods result in inconsistent cabinet appearance and structure. Therefore, material standardization is fundamental to equipment interchangeability. Currently, there are two options:
①MNS and GCK use C-shaped profiles made of aluminum-zinc coated sheet, while GCS uses 8MF profiles welded from cold-rolled steel sheet;
②MNS, GCK, and GCS cabinets all use C-shaped profiles, but the C-shaped profile module used in MNS and GCK is 25mm, while the module used in GCS is 20mm. The door panel material should consider the cabinet's appearance and corrosion resistance requirements; cold-rolled steel sheet with powder coating for corrosion protection is recommended. The thickness of the main frame structure material should be 2.0mm, and the thickness of non-load-bearing sheet metal parts such as partitions and baffles should be 1.5mm.
2) Standardization of the mounting hole positions at the bottom of the cabinet. Equipment installation requires fixing to the foundation. Due to inconsistencies in the mounting holes used by different manufacturers, it is currently necessary to contact the equipment manufacturer for foundation drawings before foundation construction to construct cable trenches and fixing channels, facilitating smooth equipment installation later. Therefore, standardizing the mounting holes allows owners to directly construct according to the standard drawings, reducing communication steps, saving construction time, and lowering installation risks.
2.2 Functional Unit Standardization
Standardization of functional units includes standardization of structure and dimensions. Structure standardization encompasses the installation structure of the incoming line frame circuit breaker, the feeder drawer circuit structure, and the feeder fixed partition structure. Dimensional standardization includes determining the busbar installation location, the installation height of the incoming line frame circuit breaker, the installation height of the double-frame feeder cabinet circuit breaker, and the height occupied by different drawer units. Furthermore, the height and thickness of the cabinet doors need to be standardized to improve the overall aesthetic appeal of the cabinet.
Table 1 Drawer Unit Specifications
2.3 Primary Interface Standardization
Primary interface methods include: ① incoming line method; ② interface method between the incoming line cabinet and the transformer; ③ main busbar system. Common incoming line methods for low-voltage switchgear include top-entry copper busbar cabinet, side-entry copper busbar cabinet, top-entry cable cabinet, and bottom-entry cable cabinet. The interface method between the incoming line cabinet and the transformer should be determined as either top busbar cable tray entry or direct side connection.
If the top-mounted cable tray is used for cable entry, the location, size, and protection method of the cable entry hole need to be determined on the top plate of the cabinet. If the direct parallel side cable entry method is used, the side sealing plate of the low-voltage cabinet needs to be equipped with a copper busbar cable entry hole. The common method is to extend the copper busbar of the low-voltage cabinet 200mm out of the cabinet body and connect it inside the transformer.
The main busbar system is recommended to be a three-phase five-wire system. The current specification should be determined based on the system's rated current. Common system current specifications include 800A, 1250A, 1600A, 2000A, 2500A, and 3200A copper busbars. According to survey data, most manufacturers use galvanized rounded corner copper busbars for ease of processing and connection. Considering the system's safety and stability, the short-time withstand current value of the busbar system should be determined based on the rated current.
Table 2 Main Busbar Specifications Comparison Table
2.4 Standardization of Secondary Interfaces
The secondary interface should specify the communication interface method for circuit breakers and smart meters. Currently, RS485 communication is commonly used, with shielded cables connecting the communication port to the secondary terminals. Incoming line cabinet instruments must be able to measure parameters such as current, voltage, frequency, power, power factor, and reactive power, and should also be equipped with circuit breaker status inputs. Bus tie cabinet and feeder cabinet instruments must meet the measurement requirements for parameters such as circuit current and voltage. An instrument room should be provided at the top front of the incoming line cabinet, with reserved installation positions for electricity meters, junction boxes, and data acquisition devices. For applications with high metering requirements, a separate metering room should be provided and secured with lead.
Instruments, opening/closing buttons, and indicator lights should be arranged according to standards, and the quantity of instrument components should meet uniform requirements. The wire diameter requirements for secondary circuits are as follows: control circuits use 1.5mm² wires; measurement circuits use 2.5mm² and 1.5mm² wires for current and voltage respectively; metering circuits use 4mm² and 2.5mm² wires for current and voltage respectively; and communication circuits use 1mm² shielded wires. Metering circuits should be color-coded according to phase sequence; other circuits do not require color-coding according to phase sequence.
Table 3 Cable Diameter Comparison Table
in conclusion
Standardization of low-voltage switchgear is a systematic issue. Low-voltage equipment cabinets are complex, with numerous components and a wide range of applications, and there is still a long way to go. Standardization requires the joint efforts and promotion of the entire industry. For the healthy development of the industry, advancing the national "Made in China 2025" strategy and the path of standardization is imperative.
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