Discussion on Electrical Design of Low-Voltage Distribution Cabinets

Modern low-voltage distribution cabinets primarily consist of two parts: the panel and the enclosure. During the installation of the cabinet panel, the principle of "neat, aesthetically pleasing, safe, and easy to maintain" should be adhered to. Cabinets can be classified based on material (e.g., wooden, steel) and installation method (e.g., surface-mounted, recessed). With the continuous development of China's power industry, requirements for the automation level and reliability of low-voltage distribution cabinets are constantly increasing.

1 Brief Description of Modern Low-Voltage Distribution Cabinet Design and Function

The modern low-voltage distribution cabinet is a critical link connecting the power grid and end-users. Its operational flexibility and reliability are essential for improving power supply quality. These cabinets effectively enhance the flexibility of grid power supply and reduce fault outage times. Unlike traditional cabinets, modern ones feature increasingly higher levels of automation and informatization. Their characteristics generally include the ability to effectively partition feeder load, perform targeted reactive power compensation, conduct real-time monitoring of power system operating parameters, and offer comprehensive protection functions, among others. These advantages significantly improve the reliability and power quality of the distribution network, optimizing parameters like voltage and power factor during grid operation. Furthermore, through advanced control system design, they effectively protect against abnormal conditions such as overvoltage, overcurrent, and harmonics, while also possessing excellent dynamic response capabilities.

2 Analysis of Specific Design Key Points

2.1 Circuit Design of Low-Voltage Distribution Cabinets

The circuit design for modern low-voltage distribution cabinets includes the design of electrical schematic diagrams and the selection of conductors. Considerations for this aspect are as follows:

• Design of Electrical Schematics: The electrical schematic is one of the most fundamental aspects of low-voltage  distribution cabinet design. During the design process, designers should      comprehensively consider the load characteristics, magnitude, and the      features of the supply circuits. For instance, the cross-sectional area, type, material of conductors for each circuit, and the performance of   protective devices should be determined based on factors like load rating      and short-circuit current. Additionally, since it is unlikely that all loads  within the cabinet will operate at rated power simultaneously, the maximum  power demand of the cabinet should be determined using parameters like the  demand factor, aligned with the actual load characteristics.

• Conductor Selection:  Based on practical application requirements and material properties,   busbars in modern low-voltage distribution cabinets are primarily made of   copper or aluminum. During conductor selection, designers must fully  account for the influence of skin effect and proximity effect (specific to  AC power supply) on the current-carrying capacity of conductors.  Simultaneously, attention should be paid to the heat dissipation and  wiring layout of the cabinet's circuits. Furthermore, coordination between   the circuits and protective devices must be emphasized during design,      carefully considering potential interference between different parts.

2.2 Structural Design of Low-Voltage Distribution Cabinets

Taking a low-voltage distribution cabinet for rural grids as an example, the heat dissipation performance of the enclosure must be fully considered during the structural design process. During operation, factors like external sunlight, high summer temperatures, and increasing electrical loads can easily lead to excessive internal temperatures, potentially affecting the normal operation of internal components. Considerations for the structural design include:

• Enhancing Heat Dissipation through Structural Design: On one hand, while ensuring the enclosure's ingress protection (IP) rating, ventilation can be improved by increasing the inlet and  outlet air openings. On the other hand, measures like installing meshes or baffles can be added to prevent rain, debris, etc., from entering the cabinet.

• Selecting Components with Higher Ratings: Since the operation of internal electrical components is  significantly affected by temperature, selecting components with ratings exceeding the actual operating parameters can mitigate the adverse effects of high internal temperature rise.

• Ensuring Rational Layout of Internal Components: Considering the different operating parameters and characteristics of components within the cabinet, components that generate significant heat or have high cooling requirements should be prioritized  in the layout.

Once the placement of internal components is determined, the design of the wiring diagram can commence. In this diagram, each circuit is divided according to its power source. Components within each circuit are arranged from left to right according to the actual connection sequence, while rows are arranged top to bottom based on the sequence of operation. Furthermore, appropriate text labels should be provided for each circuit in the diagram. The basic requirements for the electrical design and installation of low-voltage distribution cabinets are shown in Table 1.

Table 1 Basic Requirements for Electrical Design and Installation of Low-Voltage Distribution Cabinets

2.3 Component Selection for Distribution Cabinets

Components for low-voltage distribution cabinets generally involve various parts such as electrical parameter display instruments, secondary measurement instruments, and switching devices. Taking the low-voltage circuit breaker in a practical cabinet as an example: surface-mounted cabinets generally have better heat dissipation, whereas recessed cabinets, due to limited air circulation, are less conducive to heat dissipation. In recessed installations, the internal temperature can rise significantly due to the influence of the circuit breaker. Therefore, the operating temperature of the circuit breaker inside a recessed cabinet is typically higher than the ambient temperature. Consequently, when designing this type of low-voltage cabinet, designers should refer to manufacturer-provided data on breaker temperature rise and current, fully considering the impact of ambient temperature and enclosure installation method on the operating current of the circuit breaker, and make appropriate corrections to its rated current value.

Furthermore, when installing display and control elements on the exterior of the cabinet, principles such as "orderly arrangement, reliability, safety, and ease of operation" should be followed. Simultaneously, ensure tight connections and cleanliness at all busbar joints and connection points. For the design and installation of secondary wiring within the cabinet, proper identification of components and wires is crucial. Standardized and tidy installation should make the function, routing, and hierarchy of internal components and wires clear at a glance, facilitating subsequent operation and routine maintenance of the low-voltage distribution cabinet.

3 Summary

As the automation level of modern low-voltage distribution cabinets continues to increase, their functions and structures are becoming increasingly complex. Therefore, in the actual design process, attention must be paid to all stages, including load analysis, component selection, and the formulation of protection measures. It is essential to thoroughly understand common issues and their root causes in low-voltage distribution cabinets to effectively enhance the level of electrical design.