The rapid development of the photovoltaic (PV) industry has led to common practices of rushing project deadlines and grid connections. Consequently, a series of construction issues arise, including loosely connected wire harnesses, reversed wire harness connections, non-insulated cables, and string connections of components exceeding the design range. These construction problems, compounded by contractors outsourcing work to rush deadlines, result in a workforce with varying levels of skill and knowledge about electrical systems, laying the groundwork for potential operational hazards later on. Ensuring normal and stable equipment operation, pre-grid connection inspections become crucial.
Potential Issues Without Pre-Grid Connection Inspection of Combiner Boxes:
Abnormal Open Circuit Voltage:
Excessive string voltage due to connecting too many PV panels, raising the combiner box voltage above the system’s rated voltage, can degrade internal component performance over time, leading to component breakdown or even fires.
Significant open circuit voltage deviations across circuits can generate circulating currents, risking fuse holder and fusible core damage, and in severe cases, combiner box destruction.
Reversed DC Cable Polarity:
Inversion of the positive and negative poles in the string input lines could risk circuit damage upon closing the circuit.
Reversed polarity of DC output cables, when the combiner box’s output cables are inverted, results in short-circuiting different combiner box components. Since the components have been combined, the short-circuit current is significant, potentially causing fuses under the same inverter to blow and, in severe cases, destroy multiple combiner boxes in the same string.
DC Cable Ground Insulation Failure:
Ground insulation failure or short circuits in component cables create low-impedance points at the fault, attracting other strings’ currents through the combiner busbar, forming large current loops. These exceed the branch circuit breaker’s action current, leading to rapid fuse blowing and severe damage or destruction of the fuse holder due to overcurrent and high temperatures.
Loose Electrical Connections:
Inadequately tightened screws at internal connection points, often done manually on-site, can lead to overheating or damage over time due to loose or overly tight connections. Pre-grid connection torque checks of screws are essential to prevent sparking or damage from high current through loosely connected points.
Analysis of the causes
Case 1: A project designed string open-circuit voltage of 1400V, measured 1846V,far more than the maximum voltage of the string.
Case 2: All fuses are blown after grid-connection of the converter box, and the positive and negative poles of the output cable are found to be reversed through troubleshooting and measurement.
Case 3: The inverter of the convergence box reported abnormal insulation impedance of the square array during grid-connecting and when verified on-site, the strings were burnt.
Case 4: For a project, the breaker tripped frequently after the convergence box had been connected to the grid for a period of time. Upon on-site verification, it was found that the problem was due to the construction screws at the output end of the breaker not being tightened.
Combiner Box Installation and Wiring Standards:
Box Installation:
Vertical, upright installation is mandatory; inverted installation is prohibited. Wall-mounted or column-mounted installations are recommended, ensuring the wall or column can support the combiner box’s weight.
Install the combiner box’s support braces on the same horizontal plane to prevent long-term deformation.
Use M17/304 stainless steel screws for secure wall-piece installation.
Installation should facilitate easy door opening and closing for maintenance.
Note: Moisture during installation can damage the combiner box. Avoid installation during rain or high humidity. Bumps and scratches can lead to rust, affecting the box’s lifespan. Rust prevention treatment is necessary if the wall-piece’s paint layer is damaged.
Internal Wiring:
Preparation: Ensure the circuit breaker/isolator switch is in the “OFF” position and all fuse holders are open.
Positive and Negative Input Wiring:
Loosen the waterproof terminal nuts at the bottom of the combiner box.
Thread positive strings through white cable glands and negative strings through black ones, allowing extra cable length for bending and secure attachment inside the box.
Use a wire stripper to expose about 12mm of the copper core.
Apply marking numbers and use insulated crimp lugs, securing them with a specialized crimping tool.
Loosen the fuse holder’s screws with a Phillips screwdriver, insert the crimped cable’s copper core into the bottom of the fuse holder, and tighten the screws to a torque of 2.3Nm.
Tighten the cable gland nuts.
Note: PV strings carry high voltage. Accidental contact can cause fatal electric shock or severe burns. Cover PV modules or disconnect module connectors during wiring.
Pre-Grid Connection Check Preparation:
Ensure the circuit breaker is in the “OFF” or “TRIP” position (or the load isolation switch is in the “OFF” position) to disconnect the combiner box from the PV DC output side.
All fuse holders inside the combiner box should be open (or remove the fuse core using specialized pliers) to disconnect the DC combiner box from the PV string input side.
Verify cable connections against the wiring diagram and internal markings of the combiner box to ensure accuracy.