Achieving reliable grounding for the withdrawable switchgear unit requires a comprehensive approach encompassing structural design, material selection, mechanical interlocking, conductive circuit optimization, and standardized installation to ensure operator safety and stable equipment operation.
Structural design is fundamental to grounding reliability. The withdrawable unit's chassis is typically made of metal, achieving initial grounding through contact with the cabinet's rails. To enhance dynamic grounding stability, some products incorporate movable contact plates on both sides of the chassis. Regardless of the withdrawable unit's position, these plates maintain dynamic contact with the cabinet rails, forming a stable conductive path. Furthermore, the withdrawable unit's metal frame must be directly connected to the cabinet's grounding busbar to prevent grounding failure due to loosening in the intermediate links, ensuring the continuity of the grounding path.
Material selection directly impacts grounding performance. The grounding conductor should be made of copper with excellent conductivity, and its cross-sectional area must meet short-circuit current carrying requirements to prevent melting due to overheating. For example, the minimum cross-section of the grounding busbar is typically no less than 50×5mm, and bolts must be used to ensure connection strength. For grounding contacts of removable components, wear-resistant and arc-resistant alloy materials, such as silver-plated copper contact fingers, should be used to reduce contact resistance and extend service life. Simultaneously, the contact surface should be kept smooth to prevent localized overheating caused by tip discharge.
Mechanical interlocking devices are crucial for ensuring grounding safety. The operation of the removable unit must form a mandatory interlock with the grounding switch. For example, the grounding switch can only be closed after the isolating plug on the main circuit power supply side is disconnected; conversely, the isolating plug cannot be inserted if the grounding switch is not disconnected. This design prevents live grounding accidents caused by misoperation. Furthermore, some products further strengthen the interlocking logic through program locks or electromagnetic locks to ensure that the operating sequence complies with safety regulations.
Optimized design of the conductive circuit can improve grounding stability. The grounding path of the removable unit should be as short and straight as possible, reducing the number of intermediate joints. For example, the panel or frame of removable circuit breakers, transformers, etc., should be reliably connected to the grounding busbar, and the grounding wire should be easily removable when the removable component is withdrawn. The panels or frames of fixed circuit breakers, instrument transformers, etc., must also be reliably connected to the grounding busbar to ensure that all accessible metal parts are at the same potential.
Standardized installation procedures are essential for grounding reliability. During installation, it must be ensured that the grounding conductor is securely connected to the cabinet's grounding busbar, and that the contact resistance meets standard requirements. For example, the connection between grounding conductors should use bolts no smaller than M12, with connection holes at both ends of the cabinet, and M12 bolts should be used to connect to the lead-in wire of the substation's grounding grid. Furthermore, the grounding conductor must have good corrosion resistance to prevent increased contact resistance due to environmental corrosion.
Environmentally adaptable design can extend the lifespan of the grounding system. In humid or polluted environments, measures such as increasing the creepage distance and using rust-proof bolts should be taken to improve the grounding system's corrosion resistance. For example, the surface of the metal parts of the withdrawable unit can be coated with a conductive or semi-conductive shielding layer and reliably contacted to the metal casing through connectors to ensure electrical continuity. Meanwhile, the grounding system must meet the requirements of different voltage levels. For example, the air insulation clearance for a 12kV metal-clad withdrawable switchgear should be no less than 125mm, and for a 40.5kV metal-clad withdrawable switchgear, no less than 300mm.
Through structural design optimization, material upgrades, enhanced mechanical interlocking, simplified conductive circuits, standardized installation, and improved environmental adaptability, the withdrawable unit of the metal-clad withdrawable switchgear can achieve reliable grounding. These measures not only improve equipment safety but also reduce maintenance costs, providing solid support for the stable operation of the power system.
