The grounding system design for a photovoltaic grid-connected cabinet requires a holistic architecture that integrates protective grounding and lightning protection grounding. This requires a clear, coordinated protection logic to address both leakage and lightning strike risks. Protective grounding focuses on protecting electrical components within the cabinet from leakage, while lightning protection grounding dissipates surge currents generated by lightning strikes. Both systems share a common grounding grid, but with independent grounding paths to avoid mutual interference. For example, non-live metal components such as the cabinet's metal casing, doors, and internal brackets must be securely connected to the grounding busbar via dedicated grounding bolts to form a protective grounding loop. Meanwhile, lightning protection-related surge protectors (SPDs) and lightning arrester down conductors must be directly connected to the grounding grid's core nodes to ensure that surge currents can be quickly dissipated around sensitive components, preventing both electric shock and damage to equipment.
The key to protective grounding is ensuring reliable grounding of all metal components to eliminate the risk of potential differences during leakage. A photovoltaic grid-connected cabinet contains components such as DC circuit breakers, AC contactors, and metering modules. Leakage from aging insulation in the metal casings or terminals of these components could potentially cause the cabinet to become live. During design, a dedicated grounding busbar must be installed inside the cabinet. The busbar's cross-section must be adapted to the cabinet's maximum potential leakage current to ensure that current can be safely conducted to the ground. Furthermore, all metal components (including cabinet door hinges and screws) must utilize a "multi-point grounding" method. The cabinet door is connected to the cabinet grounding busbar via soft copper tape to prevent grounding interruption caused by loose doors. Internal component grounding terminals are crimped to the busbar using copper wires with cross-sectional areas that meet regulatory requirements to prevent overheating and melting in the event of excessive leakage current, thereby losing grounding protection.
Lightning protection grounding must prioritize rapid surge current discharge to prevent lightning energy from impacting core components within the cabinet. Photovoltaic systems are susceptible to direct and induced lightning strikes. If the high-frequency surge current generated by lightning cannot be discharged promptly, it can damage components such as inverters and circuit breakers. During design, suitable SPDs should be installed at the DC input and AC output of the grid cabinet. The SPD's ground terminal should be connected to the ground grid via the shortest path possible. The path length should be minimized to reduce impedance (due to the high-frequency characteristics of surge current, long paths generate significant inductive reactance, hindering current discharge). Furthermore, the grounding grid should adopt a low-impedance design, using galvanized angle steel or copper rods as grounding electrodes. In areas with high soil resistivity, a resistance-reducing agent can be added to ensure that the overall grounding grid resistance meets lightning protection standards. This allows surge current to be conducted to the ground within microseconds, avoiding the formation of high potential differences within the cabinet.
The grounding grid topology should balance current uniformity and coverage to enhance overall protection reliability. A single grounding electrode should not be used for the grounding grid of a photovoltaic grid-connected cabinet. A combined "ring grounding + radial grounding" structure should be employed. With the grid cabinet as the center, a ring grounding electrode made of galvanized flat steel should be laid, with radial grounding electrodes extending from the ring grounding electrode to form a three-dimensional grounding network. This structure evenly distributes leakage or surge current within the grounding grid, preventing localized high potentials from causing equipment breakdown. Furthermore, the ring grounding conductor effectively weakens the electric field induced by surrounding lightning, reducing the impact of induced lightning on components within the cabinet. This structure is particularly suitable for parallel installations of multiple grid-connected cabinets, enabling coordinated protection through a shared grounding grid and reducing the risk of grounding failure in individual cabinets.
The material and routing of the grounding line must be adapted to the complex outdoor environment to prevent grounding system failure. Photovoltaic grid-connected cabinets are often installed on rooftops or in outdoor distribution rooms, where grounding lines are susceptible to corrosion from rain, ultraviolet rays, and soil. During design, copper-core cables are preferred for grounding conductors (copper offers superior conductivity and corrosion resistance over aluminum). Outdoor buried installations should be protected with PE tubing to prevent acidic and alkaline substances in the soil from corroding the conductor insulation. Exothermic welding should be used to connect the grounding busbar to the grounding electrode, replacing traditional bolted connections to reduce contact resistance. (Bolted connections are prone to poor contact due to oxidation over time, resulting in increased grounding resistance.) Furthermore, all grounding connection points should be treated with corrosion protection (e.g., by applying anti-corrosion paint or wrapping with waterproof tape) to ensure continuity of the grounding loop during long-term use and prevent grounding interruptions due to corrosion.
Coordinated design of the grounding system with other PV system equipment is crucial for eliminating overall leakage and lightning strike risks. A photovoltaic grid-connected cabinet is not an isolated device and must form a unified grounding system with the inverter, PV panel rack, and distribution box. For example, the inverter's grounding terminal must be reliably connected to the grid cabinet's grounding busbar via a dedicated conductor to prevent potential differences between the two, which could lead to circulating currents. The PV panel mounting system must also be grounded to the grid cabinet's grounding grid. If the panel is struck by lightning, the current can be discharged through the path from the mounting grid to the grounding grid and then to the earth, bypassing the grid cabinet's internal components. Furthermore, the grid cabinet must also be reliably connected to the lightning protection grids of surrounding buildings (if located in close proximity) to form a larger grounding protection network, further enhancing its ability to withstand strong lightning strikes.
Regular inspection and maintenance mechanisms are crucial to ensuring the long-term effectiveness of the grounding system. During operation, grounding poles may corrode, dry out the soil, and increase grounding resistance, and grounding lines may break due to external forces. During design, ground resistance test points should be set up in the grounding grid to facilitate regular (e.g., quarterly) testing of the ground resistance using a ground resistance tester to ensure that the protective ground resistance is no greater than 4Ω and the lightning protection ground resistance is no greater than 10Ω (specific requirements must comply with local electrical regulations). Also, regular inspections should be conducted on the ground connection points for corrosion protection and on the insulation of wires for damage. Any problems identified should be repaired or replaced promptly to avoid increased risks of leakage or lightning strikes due to grounding system failure, thus ensuring the long-term safe and stable operation of the photovoltaic grid-connected cabinet.
