In photovoltaic (PV) power generation systems, combiner boxes are a key device that collects direct current (DC) from multiple photovoltaic modules, either in series or in parallel, and transmits it to the inverter for conversion. Therefore, the wiring, connection and electrical safety design of combiner boxes are essential for the efficient operation and safety of the system. This article will explore the wiring and connection methods of combiner boxes and related electrical safety standards in detail to better understand how to design and install efficient and safe photovoltaic systems.
1. Basic concepts and functions of combiner boxes
Combiner boxes are commonly used in photovoltaic systems to combine currents from multiple photovoltaic strings into a single output, which is then evenly transmitted to the inverter. The voltage and current generated by each photovoltaic module are relatively low, but by connecting multiple modules in parallel with a combiner box, the overall current output of the system can be increased to meet the input requirements of the inverter. In addition, the combiner box also provides lightning protection, overcurrent protection, and protection of photovoltaic components and equipment.
2. Principles of combiner box wiring
1. Photovoltaic string collection
In a photovoltaic power generation system, each string consists of several photovoltaic modules connected in series. Due to factors such as sunlight intensity and temperature, the current of each module may be different. The combiner box combines the currents from these strings in parallel to achieve current balance and stable transmission in the system. The wiring should ensure that the positive and negative poles of each string are correctly connected to avoid cross-connection.
2. Internal wiring of the combiner box
The internal wiring inside the combiner box is critical, because proper layout can effectively prevent cable congestion and reduce the failure rate. The following principles should be observed when wiring:
-Simple and orderly: The internal wiring of the combiner box should be as direct as possible, avoiding line crossing or tangling to minimize electromagnetic interference and cable wear.
-Clear polarity marking: All input and output lines should have clear positive and negative polarity markings to prevent short circuits or equipment damage caused by incorrect connections.
-Cable fixing: All cables should be firmly fixed to avoid mechanical damage or displacement during transportation or installation.
3. Grounding and lightning protection wiring
Grounding and lightning protection design are key aspects of combiner box wiring, especially for outdoor photovoltaic systems. The grounding system can effectively prevent electrical equipment from being damaged by lightning strikes or surges. The metal shell of the combiner box and the internal lightning protection device must be properly grounded. Specific requirements include:
-Selection of grounding wire:The grounding wire should have sufficient cross-sectional area, generally using copper wire or galvanized steel wire to ensure that the grounding system can effectively conduct lightning current.
-Grounding resistance requirements:The grounding resistance of the system should comply with relevant standards, usually less than 4 ohms, to ensure effective current dissipation.
4. Cable selection and wiring standards
The cables used in photovoltaic systems should meet the following requirements:
-Matching voltage and current levels:The rated voltage and current of the cable should match the output parameters of the system to ensure safe energy transmission over time.
-Weather resistance and corrosion resistance:Since photovoltaic systems are usually installed outdoors, the cables should have excellent UV resistance, high temperature resistance and moisture resistance to avoid aging and damage caused by environmental factors.
-Correct cable layout:Avoid laying cables over long distances, as resistance increases with distance, resulting in power loss. Cables should be as short and straight as possible, avoiding excessive bending and tight bundling.
-Cable protection:Protective sleeves or trays should be installed to prevent mechanical damage and external environmental influences.
5. Notes on parallel connection
The main function of the combiner box is to aggregate the current from multiple photovoltaic strings in parallel, which requires consideration of how to avoid current imbalance between strings during the design process. To solve this problem, fuses or circuit breakers are usually installed on each string circuit so that the circuit can be cut off in time when the current in one string is too large to protect other strings from being affected.
3.Electrical safety standards for combiner boxes
1. Electrical isolation and protection
In photovoltaic systems, electrical isolation is essential to prevent fault propagation. Each string circuit in the combiner box should have independent circuit protection and isolation devices to prevent fault expansion. In addition, the housing of the combiner box must have good protection performance and reach a dust and water resistance level of at least IP65 to ensure normal operation under harsh environmental conditions.
2. Overcurrent and overvoltage protection
The combiner box must be equipped with overcurrent and overvoltage protection devices to prevent damage under abnormal conditions. Common protection measures include:
-Overcurrent protection: Install a DC fuse or circuit breaker to automatically disconnect the circuit when the current exceeds the safe value.
-Overvoltage protection:Use surge protectors or voltage clamping devices to prevent equipment from being damaged by lightning strikes or grid voltage fluctuations.
3. Lightning and surge protection
The combiner box is usually equipped with lightning protection devices, such as DC surge arresters, to protect the equipment from lightning strikes. The wiring of the lightning protection device must comply with the relevant grounding standards to ensure that the surge current can be quickly discharged through the grounding system to avoid damage to the system.
4. Electrical safety standards and compliance
The design, manufacture and installation of the combiner box should comply with relevant national and international electrical safety standards, such as:
-IEC 61439-2:This standard specifies the design, manufacture and testing requirements of the combiner box to ensure safety and reliability under various conditions.
-GB/T 29319:This is China’s photovoltaic combiner box standard, which specifies the technical requirements for electrical safety, insulation performance and environmental adaptability.
4. Installation and inspection of the combiner box
1. Selection of installation location
The combiner box should be installed in a well-ventilated area with suitable ambient temperature, avoiding direct sunlight or high temperature environment. They should also be installed off the ground to prevent immersion in water or exposure to moisture. For outdoor installations, the combiner box is usually installed in a shielded enclosure to prevent exposure to harsh environmental conditions.
2. Connection inspection and commissioning
After the combiner box is installed, it must be fully inspected and commissioned to ensure that all connections are correct and the protection devices are working properly. Common inspection steps include:
– Check whether the input line of each photovoltaic string is correctly connected to the corresponding port of the combiner box.
– Ensure that the surge protector, circuit breaker and fuse are properly installed and working properly.
– Use a multimeter or other test equipment to check the ground resistance, output voltage and current to ensure that the system operates within the normal range.
3. Regular maintenance and troubleshooting
Regular maintenance of the combiner box is the key to ensure the long-term and stable operation of the system. Maintenance tasks include:
– Regularly check the cables and terminals for signs of looseness.
– Regularly test the function of the surge protector and circuit breaker to ensure sensitive protection.
– Check the bushing for damage and verify that the waterproof seal is intact.
5. Conclusion
The combiner box is one of the core equipment in the photovoltaic power generation system. Its wiring, connection and electrical safety design directly affect the safety and efficiency of the system. In practice, adhering to the correct wiring standards, using cables and equipment that meet the standards, and implementing comprehensive safety protection can effectively improve system stability and extend equipment life. Regular inspection and maintenance can further reduce the failure rate and ensure the long-term reliable operation of the photovoltaic power generation system.